Isolated Fuel Pump Circuit.

[Jack]

After having my 87 pin in the RPM relay burn out yesterday I decided to draw up an Isolated Fuel Pump Circuit similar to Special T Auto's Fuel Pump saver mod.

My grounds are clean, it was probably was just a lose connection at the RPM relay pin.
Plus I'm tried of the number 7 fuse melting.
I replaced the fuse block with all new connections last year and it already started to melt again.

Here is the mock up.
23104

[Elvis]

Why did the fuseholder melt again ?

bad crimp ?

too much current to the pump ?

I'm a fan of knowing or finding out why shit happens.

I had the same problem - my crimping was bad the first time
even though I had expensive tools.

[Bitsyncmaster]

Normal fuel pump current is around 7 amps. The second RPM relay pin drives the WUR heater (about 1 amp) and the lambda relay coil (about 100 ma.). So a 20 amp fuse should not even get warm with normal current. I would measure the current in that fuse #7. You can unplug the WUR connector and find what is drawing to much current. A voltmeter can find bad connections but takes a little more work measuring voltage drops over point to point wiring.

My WUR heater burned out with a wire melting. I've seen a few others post the same photos of theirs.

[Jack]

When I pull fuse 7 ill check to see how the crimp looks.
Even the original crimp looked okay other then the melted plastic around it.
So I have my doubts that its a crimp causing the fuse to melt.
Back in 2012 I checked the voltage through the pump 11.65 did not check the amps though.
I had also opened the WUR the wires were okay at that point.
Going to have to check it all again.

Was it a 5 amp inline fuse you added to the WUR or a 2.5 amp?

[David T]

When I pull fuse 7 ill check to see how the crimp looks.
Even the original crimp looked okay other then the melted plastic around it.
So I have my doubts that its a crimp causing the fuse to melt.
Back in 2012 I checked the voltage through the pump 11.65 did not check the amps though.
I had also opened the WUR the wires were okay at that point.
Going to have to check it all again.

Was it a 5 amp inline fuse you added to the WUR or a 2.5 amp?

If you are melting the #7 fuse the problem is either the CPR is bad or the fuel pump is bad or they are BOTH bad. You should also check the plug in the wiring harness up inside the washer bottle bucket. The fuel pump circuit runs through it and they can get corroded. Sometimes to the point that the fuel pump won't run. You should also make sure you have an inertia switch with a white splotch of paint on it and it is mounted on the side of the footwell, not the Lambda counter.

[Bitsyncmaster]

When I pull fuse 7 ill check to see how the crimp looks.
Even the original crimp looked okay other then the melted plastic around it.
So I have my doubts that its a crimp causing the fuse to melt.
Back in 2012 I checked the voltage through the pump 11.65 did not check the amps though.
I had also opened the WUR the wires were okay at that point.
Going to have to check it all again.

Was it a 5 amp inline fuse you added to the WUR or a 2.5 amp?

I have my WUR fused with a 3 amp fuse. When I installed the new fuse block I just wired it into a spare location in the fuse block.

[DeLorean03]

You might be interested in this. I did this same idea a year ago:

http://dmctalk.org/showthread.php?5911-Dedicated-fuel-pump-circuit

[Jack]

Okay so I replaced the rpm relay today and did some test.
WUR is fine removing it for the circuit dropped around .5 amps.
Pulling the Main Fuel Relay from the circuit dropped around .75 amps.
Total AMPS on FUSE #7 picked at 9.60

Didn't test the fuel pump alone yet but it cant be pulling more than 8.35 amps.

Before I do how many amps do yall see total on fuse #7?
Also how much over 7 AMPS would be okay for a fuel pump or is anything over 7 a "bad" pump?

Thank's

[Bitsyncmaster]

Okay so I replaced the rpm relay today and did some test.
WUR is fine removing it for the circuit dropped around .5 amps.
Pulling the Main Fuel Relay from the circuit dropped around .75 amps.
Total AMPS on FUSE #7 picked at 9.60

Didn't test the fuel pump alone yet but it cant be pulling more than 8.35 amps.

Before I do how many amps do yall see total on fuse #7?
Also how much over 7 AMPS would be okay for a fuel pump or is anything over 7 a "bad" pump?

Thank's

With those readings the 20 amp fuse should not have overheated. So your dealing with something that increased current draw (intermitant) or the fuse had bad crimp or bad pin connection. John's "power pump" draws around 10 amps with the engine running (14.5 volts).

[Jack]

With those readings the 20 amp fuse should not have overheated. So your dealing with something that increased current draw (intermitant) or the fuse had bad crimp or bad pin connection. John's "power pump" draws around 10 amps with the engine running (14.5 volts).

Yeah that's what I have the "power" pump.
So it's not even able to draw the full 10 Amps.
I'll recheck my contact points. Is there anything else that could cause a 10 amp pump to only draw 8.5 ish?
Sorry I not very familiar with ohms law, just the basics stuff.

[Bitsyncmaster]

Yeah that's what I have the "power" pump.
So it's not even able to draw the full 10 Amps.
I'll recheck my contact points. Is there anything else that could cause a 10 amp pump to only draw 8.5 ish?
Sorry I not very familiar with ohms law, just the basics stuff.

I had his power pump and measured the current with the engine running so battery voltage was 14.5 volts or more. To test for a bad connection in the pump circuit start by measuring the voltage over the two pump terminals. My car had two bad connections on the pump circuit. One was the fuse and the other was that two pin connector up near the pump. I also made a new ground wire for the inertia switch to use frame ground.

[Jack]

So you were able to not bypass the inertia switch by running a dedicated ground for it ?

[Bitsyncmaster]

So you were able to not bypass the inertia switch by running a dedicated ground for it ?


I ran the new ground for two reasons. First was because it shared the ground with the binnacle. That causes an offset for the binnacle ground (hence you read low voltage on the gauge). Second I got a voltage drop of 0.4 volts. With the new ground that drop when to 0.1 volt.

[David T]

I ran the new ground for two reasons. First was because it shared the ground with the binnacle. That causes an offset for the binnacle ground (hence you read low voltage on the gauge). Second I got a voltage drop of 0.4 volts. With the new ground that drop when to 0.1 volt.

The grounding system on the whole car is problematic especially as it ages and connections get corroded. Add to that the way the British switch the ground side instead of the "hot" side of the load. That can really cause some strange symptoms when you loose a good ground connection because of all of the possible back-feed paths. Bill R had the best idea, run a dedicated ground bus and pick up all of the grounds. That is exactly what Lotus did on later cars. All cars with fiberglass bodies suffer from this.