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.

 

The fuel pump for that model should draw about 11.5 Amps when it's running at standard flow/pressure. If you have an in-tank pump (you probably don't) this will bring it up again, but should still be comfortably below the 25A threshold. If it's drawing more than 25A as you describe then there is something drastically wrong.

You need to get a multimeter and measure current, but if that's what the pump is drawing then it needs to be replaced. High current draw is often a precursor to complete failure.

 

Thanks Dave928S. The car has two fuel pumps, both external, one behind the fuel tank in the rear, and one in the rear passenger wheel well. Both pumps are new bosch pumps. Would too little fuel pressure cause this, causing the pump to increase flow/draw. A continuity check indicated no shorts in this circuit. I will measure amps within the circuit.

 

For grins have you removed the gas cap during one of your tests? New pumps or not - fighting a vacuum will draw more current.

 

Strainer in the tank restricted?

Incorrect part number pump/pumps? (I.E...two 044 pumps)

 

Thanks GregBBRD. I removed and cleaned out the fuel tank and replaced the in tank strainer, and replaced the fuel lines from the tank to both pumps, as well as the fuel filter after the 2nd pump. The 2nd pump part # 930-608-113.

 

That second part number is a pump for a Porsche 911 Turbo.

With two external pumps running it's no wonder you're cooking relays and fuses.

If the motor is a standard '78 Euro with K-Jet (correct?), then it will run just fine on the standard single pump for that model, with the accompanying single fuel filter.

My guess ... sounds like someone might have fitted an extra pump to compensate for a running problem, caused by incorrect adjustment of pressure and flow settings, as a consequence of lack of knowledge of the K-jet system and how to correctly adjust those settings.

Get back to the basics ... correct pump, correct filter, correct flow and pressure settings.

 

My 78 is also a dual pump setup, it is factory configuration. I currently have 2 aftermarket pumps, (el cheapos) and don't have that problem, no hot relays or fuses...something is wrong, like Greg said, double check those pump part #'s....

 

Most of the early CIS cars have two pumps. Either two on the outside or one on the inside and one on the outside.

That "partial" 930 part number is correct for the second external pump.

Get a VOM that will measure 10amps+ and measure how many amps each pump is drawing.

 

I stand corrected on my previous post. PET clearly shows that second pump and the part number. Follow Gregs advice.

 

Thanks all for the advice. I'm working on getting a VOM that will measure 10+ amps. Any idea how many amps the pumps should be producing under normal operating?

 

My k-jet 83 has only an external pump, and spent 10 years in Saudi at up to 120F, no problems. When my second pump relay went out I saw a spark at the pump fuse - poor contact - fixed with a blade fuse adaptor. Pump pulled ~8A. You can get small digital ammeter that plugs into a blade fuse socket, go to 25A - very convenient if you have blade fuses.
jp 83 Euro S AT 57k

 

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.

 

I know the 2 pumps are an original config, but the logic defies me.My single pump was originally an S4 model, but it always provided the spec system press (75psi), and delivery volume(1360/30secs), even in Saudi. I would verify your delivery rate is normal (not higher than spec), and think about ditching one pump. I dont believe the relay should run warm, but at 14A I am not surprised - see if a new relay runs cooler. Check all the fuse contacts, and the relay sockets, and see if the volts at the pump are not degraded from that at the relay.
jp 83 Euro S AT 57k