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TOPIC: 78 Fuel Pump Relay and Fuse

78 Fuel Pump Relay and Fuse 26 Jun 2017 07:19 #19834


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We are putting a '78 euro 928 back on the road and have worked thru many repairs. While the car is running, the fuel pump relay gets very hot, and is hot from the relay back through both sides of the fuse. We took the 25a fuse out and installed an external fuse holder to eliminate the current fuse block for the circuit, but it still remains very hot. After a few minutes of the engine running, the relay melts/fails, but the fuse does not pop. All known terminals have been cleaned on the circuit.

Car starts and runs fine until the relay gives out.

We have yet to measure amps in this circuit. Don't know the specs on this.

Thanks for any advice.
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78 Fuel Pump Relay and Fuse 26 Jun 2017 17:18 #19836


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I would check the wiring before trying to test Amperage on the circuits. It sounds like there could be some other issues going on, but I would want to check and make sure that the wiring isn't shorting somewhere. Also, I know you said 78, I have a 79 Euro, and mine did not have the internal tank pump. Does yours? I have heard that if the in-tank pump fails it can cause excessive load on the external pump since it's making the external pump work harder until it eventually fails. Also, if you don't have the in-tank pump, check the wiring at the rear. My 79 Euro has the wiring for the in-tank pump there but it is not used. I had to cap it off to make sure that it wouldn't short out on the fuel pump/filter cover.

Hope this helps.
You can be a mechanic or technician...The choice is in HOW you fix it, not THAT you fix it.
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78 Fuel Pump Relay and Fuse 26 Jun 2017 20:09 #19837


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Thanks for the reply. Correct, the car is a 78 with two external pumps, one behind the tank and one in the passenger rear wheel well. I had replaced both pumps and had the tank out to clean it, and replaced the in tank strainer. I didn't notice any other wiring for an in tank pump. We tested for contunuity at the pumps and fuse panel and didn't notice any shorts in the circuit. I've noticed the old and new fuel pump in the passenger rear wheel well is loud, not sure if it's related.
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78 Fuel Pump Relay and Fuse 28 Jun 2017 13:29 #19843


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There are only 2 options that I know of, either an in tank pump along with an external pump or an in tank strainer with just the one external pump. What should be in the passenger side wheel well should be the fuel accumulator and a secondary fuel filter. It's kind of like a pressure check valve for the CIS fuel system. It is supposed to hold the pressure in the lines so that there is pressure to start the car. To my knowledge, the pumps prime to that point and when they overcome the held pressure then they will continue to flow up to the fuel distributor. If you have a pump in the wheel well, again, to my knowledge that is incorrect. If one is loud it is most likely bad or hooked up incorrectly. I would check the flow to make sure that the pumps are not working against each other. Truly the person that would know best is Jim Doerr with 928 classics in Saginaw, MI. He's the OB Wan when it comes to the early cars. He has #1 and #6 pre production press cars. I picked his brain plenty when I first got my 79 euro basket case. I can still try and work through some of the stuff though. I've had plenty of headaches with the CIS cars.

Also, now that I am thinking about it. even if the pumps are plumbed correctly, they may still be fighting each other since one may be pushing more flow than the other. If the one in the wheel well is pushing more than the one feeding it it will cavitate the first pump in line. If the first pump in line is pushing more than the one it is feeding it can possibly overwork the second pump.

Hope this helps.
You can be a mechanic or technician...The choice is in HOW you fix it, not THAT you fix it.
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78 Fuel Pump Relay and Fuse 07 Jul 2017 20:30 #19863


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I believe I answered this on rennlist as well, but the two pumps are correct, and in the correct location for an early car. Some early cars, like mine and yours, have the pumps as you describe, some early cars have an in tank pump and one in the fender well. Either you have the wrong pumps installed, as one is higher volume than the other, or one or both are failing, or you have a restriction in the fuel line, or a very poor wiring connection. Please update your progress. Good luck!

Ron
1978 5sp #30 - Rescue
1979 5sp - Rescue
1982 5sp. - Rescue
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78 Fuel Pump Relay and Fuse 11 Jul 2017 20:16 #19873


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Update- I THINK the problem has been resolved. We purchased a multimeter that measures >25amps. This had a detachable hook that looped around the wire, very easy to use. First measured at the fuse panel and amps were 24.9. This caused the fuse and relay to be very hot and melt but didn't quite pop the fuse. Next measured amps at the first pump at the rear of the car, it measured approx 16.3 amps. Next measured the pump in the rear passenger wheel well, it measured approx 8.6 amps. 16.3 + 8.6 = 24.9. Next verified the part numbers on the pump, the rear pass 930-608-11300, and the rear was bosch 058-025-498, which was an incorrect pump part number for this car! I then verified with pelicanparts the correct part number for the 1st external pump at the rear, 911-608-102-00. After installing this pump, total amps at the fuse panel now measure approx. 14.1 amps at idle. This also fixed the loudness of the pump in the rear pass wheel well to now be quiet while running, instead of the loud whine previously. The fuel pump relay is warm while running, so if anyone has any comments as to the temp of the relay that is normal while running, that would be helpful.
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