Dedicated fuel pump circuit.

[DeLorean03]

Hey everyone,

So I am getting rid of the wiring that goes from the RPM relay --> washer pump --> fuel pump. I can't stand how frail this wiring seems to be. I am going to run a set of new wires - a red power wire to the RPM relay and a black ground wire to the horn bracket. I am fed up with how beat up my wires to the fuel pump are, and I want eliminate any power issues I may be having there.

So here's the question - I have 16 gauge wire. It looks nearly identical in terms of diameter as the stock purple/white wire that goes from the RPM relay --> washer bottle harness --> fuel pump molex connectors. My understanding is the max current load for this gauge of wire is 13A - 22A dependent on conditions relative to the wiring. Now, my understanding is the pump pulls 5A-7A - so I would think that this wire should be sufficient.

Am I missing anything in regards to my numbers and conclusion? Can 16 gauge wiring handle the fuel pump's current draw with no issues?

Thank you for your time.

[Ron]

Note factory fuse is 20 Amps.
For 12V at 20 Amps, 16 gauge is good for 10 feet TOTAL circuit length.

http://dmctalk.org/showthread.php?5894-Wire-Selection-Chart&p=84421#post84421 (http://dmctalk.org/showthread.php?5894-Wire-Selection-Chart&p=84421#post84421)

[Bitsyncmaster]

Wire gauge selection is not just how much current the wire can safely carry. Most of time with high current loads it's more of voltage drop over the wire. You can calculate the voltage drop if you know the current and length of the wire.

Use a table or calculator to get the resistance of your wire.

http://hyperphysics.phy-astr.gsu.edu/hbase/tables/wirega.html

Here you will see a 1000 feet of 16 AWG wire is 4.016 ohms. So if you use 16 AWG for your fuel pump and it's 10 foot from the RPM relay to the pump and 10 foot from the pump to the inertia switch then that 20 foot of wire will be 0.08032 ohms. Now use ohms law "E = I x R to get the voltage drop at 7 amps.

16 AWG drop would be 0.56224 volts.

I think there is more than 20 foot of wire but you get the idea.

[DeLorean03]

I would strongly assume this was created in response to my thread about current load and gauge wiring, so thank you for that. I totally forgot to attach the link I had for the chart I was using as a reference.

So, the pump does pull 6A-7A - correct? If so, then 16 gauge wiring should clearly be fine for the wire I am running from the terminal of the RPM relay to the pump.

I have verified that what I am doing is safe from two vendors - I just want to make 6A-7A is truly what the pump demands.

[DMCMW Dave]

Example (Red): A 12V system with a 20 Amp circuit can use 16 gauge wire up to 10 feet long (TOTAL length of circuit).

As Dave M mentions there are two figures at play - ampacity is the ability of a certain size wire to carry a current without melting, and resistance is the resistance the wire adds to the circuit based on its gauge and length. The two figures are somewhat related. And you don't want to design to be running right at the edge.

The fun part about this is "total length of circuit". This means the length both wires + and - added together. It gets a bit more complicated since the return path in most cars is the frame, and in this car it's often a black wire that is shared among many circuits. The return path on the fuel pump goes through the inertia switch (often resistive in itself) and then into the harness where it is shared with some other circuits on its way to the back of the car. This sharing does not effect the voltage drop (other than via the added tie points) but it does impact the ampacity.

In DeLorean03's case he is planning to run a hot wire from the RPM relay to the pump. That is probably 10-12 feet of wire. If he were to run a dedicated neg wire back to the bulkhead, that would be another 10-12 feet so he could calculate based on about 25 feet total length of circuit. Easy to figure out.

His ground path will probably be another wire connected to the front frame at one of the ground points (bypassing the inertia switch - save that for another discussion). The ampacity (current carrying capacity) of the return path (the frame) is not an issue at all, but the resistance of that ground connection and the connection from the frame to the battery are a bit more difficult to determine.

[DeLorean03]

In DeLorean03's case he is planning to run a hot wire from the RPM relay to the pump. That is probably 10-12 feet of wire. If he were to run a dedicated neg wire back to the bulkhead, that would be another 10-12 feet so he could calculate based on about 25 feet total length of circuit. Easy to figure out.

His ground path will probably be another wire connected to the front frame at one of the ground points (bypassing the inertia switch - save that for another discussion). The ampacity (current carrying capacity) of the return path (the frame) is not an issue at all, but the resistance of that ground connection and the connection from the frame to the battery are a bit more difficult to determine.

Based on this and other posts I've read, I feel confident that 16 GA should be sufficiently adequate for what I am planning on doing. The wire to the RPM relay looks to be very, very close to 16 GA - if not an exact match. There's no way the stock purple/white wire is 12 GA. I might be able to go up to 14 GA. The uninsulated female connectors I retrieved for the RPM relay can handle 16GA; 14 GA I don't know about.

I am planning to use 10-12 feet of wire going to the RPM relay and I would guesstimate 3-4 feet going to the horn bracket.

Where does the RPM purple/white wire run through the car exactly ? I know it goes from the RPM relay --> windshield washer bottle connector --> fuel pump. That seems WAY longer than what I plan on doing. I plan on going from RPM relay --> center console --> across from psgr side to drivers side through the driver's footwell --> up the fuel tank to the pump.

If I have to go get 12 GA wire and uninsulated female disconnects, that's fine. I just can't help but wonder why I should when it looks like 16ga factory stock wiring from 31 years ago seems to work just fine (short of blowing fuse #7).

[Ron]

I would strongly assume this was created in response to my thread about current load and gauge wiring, so thank you for that. I totally forgot to attach the link I had for the chart I was using as a reference.
Yes...I just put it here for future reference.


======

Not to to take away from anything said so far (I agree), but, when it is a question of replacing, rather than designing (already done in this case), wire choice is easy -- Larger than existing is a waste and smaller is a risk, so you choose a wire that has the same or greater rating. If you can't tell what rate it is, or need to add length, note the 20A fuse is chosen for wire protection, and refer to the chart. (If the replacement wire is rated as high as, or higher than, the fuse originally used to protect the entire circuit, you are covered...!)

Most connectors that accept 16 gauge will accept 14 gauge... get the 14 and forget about it ;-)

[DMCMW Dave]

across from psgr side to drivers side through the driver's footwell --> ).

You might be able to sneak it through the same hole that the wiring for the A/C low pressure switch goes, (and the heater/freon lines) and it's very close to the pump at that point. Just seal it up good and avoid chafing points.

Some future dealer tech is going to hate you. :what_the:

[DeLorean03]

Some future dealer tech is going to hate you. :what_the:

Not if I never sell the car ;) ....

So here's what I'm going to do:

1. Using all 12 Gauge wire

2. Examining the fuel pump circuit, I am going to run a hot wire with an in-line bladed 20A fuse from the battery directly to terminal 30 (terminal that is labeled "Hot/Live wire at all times). That will take some of the stress off fuse #7.

3. Running a wire from terminal 87 to the pump.

4. Running a ground from the pump to either the horn bracket or the ground that is located on the flat part of the frame near the cooling fans.

With 12 GA wiring, that gives me about 15 feet of wire to play with, BUT the power for the fuel pump will be taken directly from the battery, sparing fuse #7, so there should be a bit more of give there.

This will all also be with DMCH new fuel pump/sender combo.

I feel safe and confident with this setup. If 16 GA wiring could handle this (barely) with everything on fuse #7, then 12 Gauge wiring with a separate 20A fuse running the fuel pump circuit to keep fuse #7 safe with a possible maximum draw of 10A (assuming DMCH's fuel pump/sender combo demands as much current as John's powerpump), everything should be relatively safe.

Someone say something if you see a flaw with my idea/setup.

This is my way of keeping maximum uninterrupted power draw to my fuel pump as well as trying to keep fuse #7 safe. I'm tired of watching it try to melt down. Something needs to be done to fix this problem; this is my possible solution to both uninterrupted power draw and minimizes the stress on fuse #7...

Oh, and don't worry Dave - I am not cutting or hacking anything. This will be completely reversible if someone decides to reverse it after I die (: ... It'll also be documented and photographed.

[Bill Robertson's Sock Puppet #1]

IMHO Fuse #7 melts because it is already close to amperage limits in the best of circumstances, and fuse blade/terminal corrosion that is a very real threat for DeLoreans due to battery location pushes it over the edge.

There is a venturi tube that pulls battery gases into the passenger rear wheel well that does a very good job of keeping outgassing under control, but it only works when the car is moving. I suspect that is why DeLoreans that sit for protracted periods of time have so many more fuse box issues than DeLoreans that are driven regularly (I'm talking thousands of miles per year, not regular trips of a mile or two apiece).

Simply starting the engine and letting it idle while the car sits still definitely does absolutely nothing to vent battery gases.

[Bitsyncmaster]

Fuse #7 powers the RPM relay, FV and Lambda ECU. The RPM relay output powers the fuel pump and WUR heater and one relay coil.

So rewiring the RPM relay input power to it's own fuse will eliminate the FV and Lambda ECU power. My guess is the FV and Lambda ECU power is about 2 amps.

The WUR has been know to short circuit (did it on my car). That may be one reason that the #7 fuse can melt.

[DeLorean03]

Just to be clear, the rest of the RPM relay - the other 4 prongs - will be plugged into the corresponding female socket. I will just be removing the two factory wires from the RPM female socket, insulating them with electrical tape, and plugging in my own two wires into it. So fuse #7 will still power the FV and Lamba ECU, while my introduced fuse that is part of the wire from the battery to terminal 30 powers the fuel pump separately.

[DeLorean03]

Fuse #7 powers the RPM relay, FV and Lambda ECU. The RPM relay output powers the fuel pump and WUR heater and one relay coil.

So rewiring the RPM relay input power to it's own fuse will eliminate the FV and Lambda ECU power.



Could you please clarify what you mean here, Dave?

The way I'm looking at what I'm planning, running a wire directly to the battery with a fuse extension in it to the RPM relay's terminal 30, and then running a wire from terminal 87 to the fuel pump would isolate the fuel pump to its own fuse, thus reducing the strain on fuse #7.

The rest of the RPM's male spades will be plugged into the female wiring harness socket as normal, so there should be no eliminating the FV and Lambda ECU. Fuse #7 should still power those two devices, while my introduced fuse runs the fuel pump directly.

I just want to make sure we're on the same page here, and that I am not "losing" the FV and Lambda ECU. The Lambda ECU and FV should still receive power through fuse #7. The fuel pump gets its power directly from the battery through a 12GA wire through the fuse I am introducing from the battery to terminal 30, and then from terminal 87 through the 12GA wire I am introducing directly to the fuel pump.

For those who are wondering, I am going to get the ground for the fuel pump either off the driver's side horn bracket or the ground that is part of the driver's side radiator bracket.

[Bitsyncmaster]

The change you plan to do will power the pump from your new circuit but it will also power the WUR heater and one relay coil. The WUR heater is maybe 1 amp and the relay coil is about 100 ma.

[DeLorean03]

Gotcha Dave, but it should, in essence, have no adverse effects or cause the car to run differently than normal. That's my understanding.

[Bitsyncmaster]

Gotcha Dave, but it should, in essence, have no adverse effects or cause the car to run differently than normal. That's my understanding.

Correct. You will remove the fuel pump power and the WUR power from fuse 7. And the heavy wire and ground will improve the fuel pump voltage. However you will not have the fuel pump power turn off if in an accident since your bypassing the inertia switch.

[DeLorean03]

You are correct, Dave. Truth be told, when I bought the car, the inertia switch had already been bypassed; it has never been functional in my car. Not to say I shouldn't revert the original setup, but well, I'd like to think I work total this car.

I'll start a thread with my progress. I feel like I'm about 35%-40% done with this project so far!

[DeLorean03]

So, as we all know, fuse #7 gets HEAVILY taxed by the electrical system. It is responsible for powering the RPM relay, the frequency valve, and the lambda ECU - among other current-drawing goodies. Naturally, fuse #7 gets pushed to its limits in a stock DeLorean. I don't know the current that DMCH's new pump/sender combo pulls yet, but with a stock DeLorean, fuse #7 gets pushed close to its max of 20A - not good. We all know, have heard of or seen, or even own a DeLorean where fuse #7 has gone into meltdown and had to be bypassed with a fuse extender or some other such method. Well, I look to resolve two reasons for this strain on fuse #7 - inferior gauge wiring and distributing the load to a direct circuit.

This all started when I was having some random intermittent lack of power issues while driving around Dayton. From time to time - completely and totally randomly - my car if going from 0-15MPH, would just randomly stall. If I was going say - 20mph and over - the car would buck - almost as if it was about to die, but the car would hang on and I could keep going. It started in September 2012, went away for all of Oct and Nov 2012, and then decided to resurface in Dec 2012. I tracked down nearly anything I could - lambda relay, rpm relay, air/fuel mixture - finally it came down to the connectors to the fuel pump and terminal 87 of the RPM relay.

I cleaned up the connectors going to the pump, and then I inspected the female socket to the RPM relay, and I saw brown burn marks around terminal 87 of the connector. So out came that wire, tightened the female end of it, and plugged the RPM relay back in. So far all has been good - but then I noticed something that alarmed me. I have run John's baffle setup with fuel pump filter for a long time. Well, the fuel pump filter had COME OFF, and the pump was completely unfiltered.

At that point, I pretty much KNEW I was sucking contaminants up into my fuel pump, and my car stalling was the pump trying to die. Thankfully, I never had the pump freeze up on me, and as of today I have a new hose and filter (thank you DMCMW!) installed on my pump along with the fuel pump/sender combo ready to install from Toby at DMCNW - once I come back from Christmas vacation with Jessica.

So, all of this has described my situation and train of thought leading up to today: I want fuse #7 safe. I don't want ANYMORE problems with my fuel pump. It is time to come up with something to help both keep the pump running and fuse #7 safe. So, after studying the fuel pump schematic a bit and talking to Casey, Dave Swingle, and Toby Peterson in exhaustion, I decided to do the following - eliminate the fuel pump from fuse #7. To do this, I have the following plan:

1. Run a dedicated 12 gauge wire directly from the battery to terminal 30 of the RPM relay. This 12 gauge wire will have a fuse extender spliced in the middle of it with a 20A fuse protecting the circuit.

2. Run a second 12 gauge wire from terminal 87 of the RPM relay to the fuel pump directly. Remember, the wire that goes from the battery to terminal 30 of the RPM has a 20A fuse in the wiring, so the wire from terminal 87 to the fuel pump does not need a fuse.

3. Run a dedicated 12 gauge ground wire from the fuel pump to the frame. I am either going to put it on the driver's horn bracket or the bolt that anchors the driver's side of the radiator to the frame. I still haven't decided on that one yet. Either ground point such take less than 3 feet of wiring.

Per Dave Swingle's suggestion, my wire from the pump to the electrical relay compartment is running from the area behind the center console where the A/C pipes run to the front of the car. This makes the length of the wire MUCH shorter than my original plan (ask if you want to know). The wire reaches from the pump to the relay compartment with less than 12 feet of 12 gauge wiring.

With the backing of Dave, Toby, Casey Barlow, and Dave (Bitsync) - I have been extremely confident in doing this mod. Here are some pictures of my progress so far:

15510

So here the journey begins: the 12 gauge wire in the fuel pump area. I attached a mounting ring to the side for the fuel pump. You may have noticed there's no "break a way" point in this wire. That is because DMCH's fuel pump/sending unit combo will be installed Jan 2013, and Toby sent me the appropriate female connector along with the unit. So when the time comes, I'll splice the female connector to my wiring, install the new fuel pump/sender combo and "Bob's your uncle" so to speak (BYU = everything is all good).


15511

So, at first, I tried to wiggle the 12 gauge wire up and around various areas of the car to get it to the passenger side of the back of the center console - no such luck. I decided since the only purpose this rubber boot seems to serve is a "junction point" for the car that a small hole to run the wire through won't be an issue. Besides, the rubber will help support the wire and keep it from rubbing up against sharp edges of the underbody, possibly causing the insulation of the wire to get hacked and slashed over the years.

15512

Here is the view from inside the car with your face nearly in the passenger footwell looking up towards the windshield. As you can see, the wire passed through the rubber boot with no issues, and it is away from the heater hoses - thereby keeping this wire safe from the heat of the hoses.

15513

And here is the wire with plenty of slack. As of posting this picture, the wire has been hidden by running it alongside the center console of the passenger side, and it enters the electrical relay area through the small hole between the wood shelf and the center console.

15514

Here is the dedicated wire I made. It is a 12 gauge wire with the ring terminal that the anchoring battery bolt will run through, and the fuse extender (20A) is spliced on the end of the wire.

15509

Lastly, here is where I stopped for the night. Unfortunately, the 12 gauge wire is too big to run through the RPM relay female socket, and even if I did run it through the female connector and plug in the wire separately to the RPM relay male connectors, the insulation of the female connector to my red 12 gauge wire would catch the female molex connector of the RPM relay - thereby not allowing the rest of the male spades to connect to the female connections of the RPM relay. Only real solution: disconnect each wire out of the female molex connector and place them on the male spades of the RPM relay. At first, I was a bit "mmmmmmm... no" about this, but there are plastic spikes all around the male/female terminal connection points of the RPM relay, assuring me that the terminals will not come into contact with anything in the electrical compartment. I wouldn't be surprised if I put some electrical tape around the junction point of the male/female spades just to be safe, or I could use some of that "liquid electrical tape" stuff. We'll see - bottom line, as long as it's safe - I'm good.

So there's my progress and what I have gotten done tonight. I felt after 2.5 hours, it was time to end for tonight - let my mind/body rest as it is easily to get caught up and start making errors and miscalculations as you work. To me, these kinds of things are best spread across 2-3 evenings. I just work better like that than trying to do something like this for 4-6 straight hours. Yes, I know that's a long time for something like this. I am very methodical as I go. I prefer to do things "right" rather than "again" - as we all know those are the two kinds of ways to do things on automobiles.

So, what do I have left?

1. Strip the end of the battery wire and plug, crimp appropriate connector, and connect it to terminal 30 of the RPM relay.
2. Same as above for dedicated pump wire but have it go to terminal 87 of the RPM relay.
3. Take out the other 4 wires from the female molex connector and connect them each individually to the RPM relay.
4. Run my dedicated ground wire to either the horn bracket or the bolt that anchors the driver's side portion of the radiator to the frame.
5. Hook both wires up to the fuel pump.

Some may ask, "Well you're about to install the new fuel pump/sender combo - why do this twice?" Well, I want to make sure my idea is sound before merging it with the fuel pump/sender combo. I'd rather know it will work first before bringing it in conjunction with another kit (combo unit) I have yet to install.

Anyways, I know - it is a long post - nearly a book. But I like to be descriptive with everything, from the idea and inspiration to do this, to the process to coming up with this solution, to carrying it out step by step, to the fruition of it working.


That's that - for now (: ...

[jmpdmc]

Here is a brief mention of the new pump/sender current for reference: http://dmctalk.org/showthread.php?3824-NEW-Integrated-pump-sender-combo!/page3&highlight=PUMP%2FSENDER+AMPS It seems comparable to a stock pump.


Jeff

[Bitsyncmaster]

Here is a brief mention of the new pump/sender current for reference: http://dmctalk.org/showthread.php?3824-NEW-Integrated-pump-sender-combo!/page3&highlight=PUMP%2FSENDER+AMPS It seems comparable to a stock pump.


Jeff

So it looks a little higher than the stock units. When your engine is running the currents go up. I had a table of the pump current with different voltage on the old forum.

Also to note that you can get wire in 300 volt or 600 volt insulation. Big difference in the diameter of the wire. There also is a connector (PIN) that will crimp with 12 AWG. The normal pin is for 14 to 16 AWG.

[DeLorean03]

Here is a brief mention of the new pump/sender current for reference: http://dmctalk.org/showthread.php?3824-NEW-Integrated-pump-sender-combo!/page3&highlight=PUMP%2FSENDER+AMPS It seems comparable to a stock pump.


Jeff

Excellent - thank you! I am running John's 957 pump, which I read somewhere on here pulls 10A or so. It is nice to know my current may actually go down a lil' bit with the upcoming pump/sender combo!

[DeLorean03]

So I wrapped up this project/mod tonight. Took the car out for a good drive - all seems well and sound! Here's the remaining photos showcasing the steps.


15553

Here is the RPM relay - nothing special. Just was photo-documenting the configuration of the wires. Wouldn't want to put these on the wrong way o_O.

15554

Here is the RPM relay with the new wires installed. Due to the insulation of the newly crimped wires, there was no way the spades of the RPM would reach the unaltered terminals left in the female socket. The only choice I could make really was to remove the female connector. I have wrapped the connectors in about 5-7 strands of electrical tape, and I plan to put velcro on the RPM relay body and the electrical compartment to keep it from "wondering around."

15555

These are the new 12 gauge wires that will go directly to the fuel pump's terminals. Yes, lots of excess wire; it was trimmed accordingly with a "little play" in it for safety purposes.

15556

Here is where I put the ground for the fuel pump; this is the inner-most bolt of the horn bracket on the driver's side. It is a 13mm bolt with a washer on it. I utilized this bolt under the advice of Dave Swingle and Casey Barlow. The yellow is the insulation of the ring connector I used to establish a ground at this bolt.

15559

This is a picture of the ground wire. It goes behind the driver's side radiator bracket, away from the coolant pipe and past the master cylinder. It does have a little play in it just to be safe. These kinds of things I never want to have "just enough" wire.

15558

This is a picture of the new wires installed, ready to go! Of course, the old terminals were individually wrapped in electrical tape about 14 times as well as the old terminals at the RPM relay (old brown and purple/white wires)

15552

You would never know (: ....

Remember, I also installed a wire with a fuse extender housing a 20A fuse directly from the battery to terminal 30 of the RPM relay. This is - in essence - a fuse I'm adding to the system which I cordially call "Fuse #7A" ;). Now, the fuel pump has its own circuit - no more stressing out fuse #7 - it is now responsible for the FV and lambda ECU only.

I took the car for about a 1 hour drive - basically errand running. Everything went smoothly - so far - no issues! Due to my trouble-shooting, I did uninstall Bitsync's SS RPM relay temporarily. Once I am satisfied with my results (more driving), it will be re-installed as his SS relay is far superior to the analog RPM relay and probably gives off WAY less heat.

So, there you go, gang. Nothing extremely fancy - just added a new fuse, upgraded the 16 gauge wire to 12 gauge, and took some stress off fuse #7. Anyone who wants to do this as well, please, go for it. I will be driving my car down to Ken Koncelik's for the winter for storage, and I'll check fuse #7 to see how hot or cool it is with this mod.

If anyone wants to do this as well and has questions, please feel more than welcome to contact me. I will be glad and happy to answer any questions regarding this mod.

Mods, if you feel this is worthy of a "resource" to help de-stress fuse #7, please feel free to move it to the appropriate "resources" or "how-to" file!

Thank you Bitsync, Dave Swingle, Toby Peterson, and Casey Barlow for your time, help, and attention. I definitely appreciate all who helped me with this mod!

And thank you Ron, for providing me the amperage/length of wire chart - that definitely helped persuade me to use 12 gauge ;).

I can measure the wire I have left of both red and black to give everyone an idea of how much wire I used for this mod. Take care everyone!

And to cover my butt - if you choose to do this too - you DO SO AT YOUR OWN RISK. I'm not responsible for anything that happens to you or your car as a result of following my instructions.

[Bitsyncmaster]

Good work. I don't think our OEM wires are ever 16 AWG. It's more like 18, 14 and 12 AWG. Europe wire standards are based on strand count but I have done measurements and calculations to convert to AWG.

Glad your testing without the SS relay. I've stopped selling the starter relay because when people have installed a new starter they have blown them (bad starters).

The test of your wiring is to measure the voltage over the two terminals of the fuel pump when the engine is running. Then compare that voltage with the voltage you read at the engine "jump" terminal. That difference is the voltage drops of the wiring and connectors.

[Chris4099]

One suggestion. Have you thought about grounding your RPM relay through the inertia switch? That way you restore a critical safety feature but don't have much of a load going through that switch as well. Basically if you are in an accident, the inertia switch trips and kills ground to the RPM relay. The relay should then shutoff and kill power to the pump.

I don't have my schematic in front of me, but you might even be able to utilize the old fuel pump wiring. Connect the two old pump leads together so now the wire running back to the RPM relay is grounded via the inertia switch. Reconnect the wire to the RPM relay but on the ground terminal instead. This is assuming nothing else is connected to the pump power lead which I can't verify at this moment.

[Bitsyncmaster]

One suggestion. Have you thought about grounding your RPM relay through the inertia switch? That way you restore a critical safety feature but don't have much of a load going through that switch as well. Basically if you are in an accident, the inertia switch trips and kills ground to the RPM relay. The relay should then shutoff and kill power to the pump.

I don't have my schematic in front of me, but you might even be able to utilize the old fuel pump wiring. Connect the two old pump leads together so now the wire running back to the RPM relay is grounded via the inertia switch. Reconnect the wire to the RPM relay but on the ground terminal instead. This is assuming nothing else is connected to the pump power lead which I can't verify at this moment.

I've thought of the same thing. You would probably need to have a pullup on the RPM relay ground pin so the ground pin would not float if the inertia switch opened. I would try a 1 meg ohm pull up from the RPM power pin to the ground pin. If that works it would only have about a 100 ua. drain on the battery. I would have tried this but then you need to run a wire back to the relay or move the inertia switch to the relay compartment.

Another option is the switch can turn of the key on signal into the RPM relay.

[nkemp]

This all started when I was having some random intermittent lack of power issues while driving around Dayton. From time to time - completely and totally randomly - my car if going from 0-15MPH, would just randomly stall. If I was going say - 20mph and over - the car would buck - almost as if it was about to die, but the car would hang on and I could keep going. It started in September 2012, went away for all of Oct and Nov 2012, and then decided to resurface in Dec 2012. I tracked down nearly anything I could - lambda relay, rpm relay, air/fuel mixture - finally it came down to the connectors to the fuel pump and terminal 87 of the RPM relay.

Resurrecting an old thread but I had similar problems as described above. Some times the car would die and I'd roll of to the side of the rode and would get a flatbed ride back home. Then it would start the next day as if nothing had happened. I replaced the inertia switch with a Ford Mustang inertia switch and now I have no more bucking and stalling (knock on stainless). I went with the Mustang part since I will only give the original parts so many tries before switching.

YMMV,
Nick

[DeLorean03]

Good stuff, Nick. My inertia switch had been completely bypassed by the DPO (dreaded previous owner), so that's how I knew it was "one or the other". Just about drove me nuts.