Low voltage at fuel pump

[robplus4]

Low voltage at fuel pump

I drove the car the night before (ran great). Try to start it the next morning and I didn’t hear the fuel pump running. Check the voltage at the pump (2.53 volts). I jumped rpm relay, still low voltage. Checked voltage at pin 30, got 12.4 volts. Can any tell me what’s between the rpm relay and the fuel pump.
Note: I ran power from the battery to the pump and it work
Thanks for any.

Robert

[Bitsyncmaster]

I did a write up of finding voltage drops to the fuel pump but I think it was lost on the old .com.

My drops were at:

1) One side of the fuse. (Ageing crimp)
2) The two pin connector near the pump. (Corroded pins)
3) The ground on the inertia switch. (Bad design, made that a frame ground).

I recommend you all remove that two pin connector (it gets wet). Just cut it off and splice the wires.

[DMCMW Dave]

I recommend you all remove that two pin connector (it gets wet). Just cut it off and splice the wires.

I'd replace it with another connector of some type, otherwise servicing the fuel pump in the future is a huge PITA.

[David T]

I'd replace it with another connector of some type, otherwise servicing the fuel pump in the future is a huge PITA.

There is a connector up inside where the windshield washer bucket is and it can get corroded. Figure out which side the problem is on. Check the power at the plug against a known good ground. If you have battery voltage there then your problem is on the ground side.
David Teitelbaum

[Bitsyncmaster]

I'd replace it with another connector of some type, otherwise servicing the fuel pump in the future is a huge PITA.

Not a problem with old type pump. Does the new one have the ring terminals on it?

[Exolis]

Also try disconnecting the Fuel Pump and Relay, measure resistance between Relay Pin and Fuel Pump Pin. Also do the same for the ground wire. Do visual inspection on connection points to check for corrosion as well. Is there any mal/abnormal-function of Radiator or Lights?

[robplus4]

Not a problem with old type pump. Does the new one have the ring terminals on it?

I'm not sure, I got from special t auto about 2 months ago. It's not the old long neck.
I think when I get chance i think i well cut off the connector and hard wire it to take it out of the loop.

Thanks, i'll let you know.
Robert

[DMCMW Dave]

Not a problem with old type pump. Does the new one have the ring terminals on it?

Depends on how you define PITA I guess. :hmm: With a connector it's a simple job to lift up the pump and check inside, without you have to take the nuts off the wires, drop them a few times, and hope that the rubber cover isn't hard or cracking when you bend it.

[Bitsyncmaster]

Depends on how you define PITA I guess. :hmm: With a connector it's a simple job to lift up the pump and check inside, without you have to take the nuts off the wires, drop them a few times, and hope that the rubber cover isn't hard or cracking when you bend it.

I guess that could be a pain but I only pulled my pump when it went bad. I made new wires with ring terminals when I removed the connector so the wires are long enough to pull the pump if you don't want to pull the ring terminals.

[DMCMW Dave]

I guess that could be a pain but I only pulled my pump when it went bad. I made new wires with ring terminals when I removed the connector so the wires are long enough to pull the pump if you don't want to pull the ring terminals.

Good idea, most people don't seem to think of that.

[robplus4]

Not a problem with old type pump. Does the new one have the ring terminals on it?

I cut out the fuel pump connector and hard wire it, still no power. Jumped pin 30 (with a jumper) and now the pump work. Put rpm relay back in and it doesn't work. It sound like the rpm relay cropped out. I'am I right that the relay is bad?

Thank you for your help.

Robert

[Bitsyncmaster]

I cut out the fuel pump connector and hard wire it, still no power. Jumped pin 30 (with a jumper) and now the pump work. Put rpm relay back in and it doesn't work. It sound like the rpm relay cropped out. I'am I right that the relay is bad?

Thank you for your help.

Robert

If you jumped the two large wires (brown to white/purple) and your pump ran but with the RPM relay installed is does not run when the engine is cranking then the RPM relay is most likely bad (very common failure). The RPM relay runs when it sees ignition pulses so it still could be a wiring problem there.

[sdg3205]

Encountered this today.

I have Dave's solid state relay and the car lives on a trickle charger.

Last week the car ran great, but went to start it today and the pump wasn't priming. Jumped the RPM relay pins and it still didn't run the pump.

12.5v at the battery, but only about 9v at the pump. I'm also only getting 9v at the rpm relay pins (purple/white and brown). I jumped/bypassed the inertia switch too with no positive result.

Typically it's the degradation of voltage and ground in the front harnesses but if i'm correct, i'm loosing voltage earlier.

[Bitsyncmaster]

Encountered this today.

I have Dave's solid state relay and the car lives on a trickle charger.

Last week the car ran great, but went to start it today and the pump wasn't priming. Jumped the RPM relay pins and it still didn't run the pump.

12.5v at the battery, but only about 9v at the pump. I'm also only getting 9v at the rpm relay pins (purple/white and brown). I jumped/bypassed the inertia switch too with no positive result.

Typically it's the degradation of voltage and ground in the front harnesses but if i'm correct, i'm loosing voltage earlier.

Most likely your fuse is causing the drop of voltage. I had a bad fuse pin on my car which dropped about one volt.

[sdg3205]

The fuel pump ground above the pedals, is it the ground on the wiper relay box?

[DMCMW Dave]

For future reference - It's not your issue (since you see the low voltage at the RPM relay) but there is a connection in the front body next to the windshield washer bottle (remove the bottle for access). I've seen one or two where that gets corroded and causes low voltage at the pump.

[sdg3205]

Yes, I cleaned that in 2010. It probably needs to be cleaned again.

Am I right about the ground location in post 15 Dave?

[David T]

If you only have 9 volts at the RPM relay your problem is between the battery and the RPM relay. The fuse and it's connectors would be the first thing to check.

[sdg3205]

I'll check that Dave thanks. Can u confirm the location of the fuel pump ground?

[content22207_2]

Inertia switch is grounded at a junction behind the central A/C plenum: http://dmctalk.org/showthread.php?13342-DeLorean-Ground-Schematic

I will try to find a picture on one of my machines tonight, otherwise I can snap one tomorrow.

Bill Robertson
#5939

[content22207_2]

Ground junction behind the central A/C duct:

https://scontent.xx.fbcdn.net/hphotos-xfa1/v/t1.0-9/309893_282221628489761_225725998_n.jpg?oh=5de2e24b db99245085422e3825135a89&oe=5759BAF5

12 gauge wires are from inertia switch, from windshield wiper motor, and to the rear bulkhead bolt. 16 gauge wires are instrument gauges, binnacle illumination, courtesy lights (driver side plunger), turn signal flasher, and wiper delay unit.

Bill Robertson
#5939

[sdg3205]

Thanks bill, but where does it actually ground to a frame, bracket etc?

I backed the female spade connectors out the back of the fuse block for fuse 7. Everything looks good. I checked voltage between each terminal at fuse 7 and the rpm relay just in case and both yield 7.5v

[content22207_2]

12 gauge wire runs to the rear bulkhead bolt (one of the 12 gauge wires from various junctions), then to the frame under the coolant expansion tank (8 gauge wire), then to the battery from the trailing arm bolt (4 gauge cable).

Bill Robertson
#5939

[content22207_2]

Ground junctions are crimped and soldered together, with a heat shrink cap. There's a readily accessible one in the relay compartment if you want to see what they look like.

Ground junction behind the central A/C duct is rather difficult to access. I tied my ground bus into it by braille:

https://scontent.xx.fbcdn.net/hphotos-xaf1/v/t1.0-9/379450_294229573955633_1943786923_n.jpg?oh=5e9b7c5 28d72a9e39afb2a0d61c59d2c&oe=575E70CB

https://scontent.xx.fbcdn.net/hphotos-xfa1/v/t1.0-9/386449_294229657288958_417856207_n.jpg?oh=381be1d7 07e02376b34fe8d85f5c7de0&oe=5791CAFA

Bill Robertson
#5939

[sdg3205]

Not only am I getting 12.5v to fuse one (from the accessory relay) but just 7v at fuse 7. My frequency valve (also powered by the main fuel relay) is NOT buzzing.

Now, at the fuel pump harness, if I take 12v from the harness and a ground to the frame I get 12.5v, so it's like he entire main fuel relay circuit has a crappy ground.

[sdg3205]

So perhaps the main fuel relay has failed?

[DMCMW Dave]

Not only am I getting 12.5v to fuse one (from the accessory relay) but just 7v at fuse 7. My frequency valve (also powered by the main fuel relay) is NOT buzzing.

Now, at the fuel pump harness, if I take 12v from the harness and a ground to the frame I get 12.5v, so it's like he entire main fuel relay circuit has a crappy ground.

How is the battery connection to the frame? The one on the bushing bolt? I've seen that do pretty strange things. Check the big ground at the bulkhead feed through as well.

[sdg3205]

I had that apart in February to run the hervey style additional ground from that bushing bolt to the transmission. I wonder if that has something to do with it? I thought that would only improve things. Starter certainly cranks like a champ.

I will certainly check the bulkhead ground. Thanks Dave

[sdg3205]

Can I temporarily swap in a new relay in place of the main fuel relay to see if the relay is toast? I know the main fuel relay is "heavy duty."

[content22207_2]

Make a wire and temporarily jumper across it. Relay terminals accept 1/4" male quick connects.

Bill Robertson
#5939

[content22207_2]

Doesn't the fuel relay just go to Lambda? Fuel pump itself is Brown to the RPM Relay, then Purple/White to the pump, then Black/Purple to the Inertia Switch, then Black to the rear bulkhead bolt.

Bill Robertson
#5939

[sdg3205]

Fuse 7 is tied to a 12v off ignition that runs to the main fuel relay, then to the RPM relay. We jump that brown lead to white/purple when we want to run the pump. I'm just looking at the schematic now.

So the pump side from the RPM relay is white/purple.

[sdg3205]

Well, Dave M. in another fuel/RPM relay thread just confirmed a bad main fuel relay would not cause a pump problem, so..... yeah. Time to check the grounds i guess.

[Bitsyncmaster]

When trouble shooting voltage drops, you need to start with one location to use for ground. I use the "-" battery post and have made a long 16 AWG wire with alligator clips on each end. Now with that ground attached to your meter, you can probe around and find the voltage drops of ground and or 12 volt circuits.

You can probe the little top exposed metal on your fuse to see if there is a voltage drop on the fuse or fuse block. You can also probe the ground locations in the relay box area and find bad ground connections that way. Of course you need to do your probing with the load powered (jumper the RPM relay for fuel pump load) to find voltage drops on bad connections.

[content22207_2]

Have you checked the connectors under the windshield washer bottle yet? Both positive and negative wires pass through those connectors.

Bill Robertson
#5939

[Elvis]

A bad crimp on one of the fuse connectors could cause a voltage drop, too...

[sdg3205]

Fixed. It was in fact the molex connector at the washer bottle. Not sure what the problem was, but i sanded the pins and bam, power to the pump.

Also turns out my washer pump was dead so i swapped that out since everything was apart.