Fuel pump won't run with 12v from car, but does from external power.

[Jake.]

Trying to get my 172 Ph1 started but the fuel pump won't run.
When I turn the ignition on I can measure 12v for a second or so at the fuel pump plug. But the pump doesn't prime or do anything.
If I bridge the fuel pump relay, like the bottom left hand side relay socket in this pic (not my pic)

then I can measure a constant 12v at the fuel pump plug - but the fuel pump still doesn't do anything.
BUT if I connect an external power supply (from a battery charger) to the pump if runs fine.

How can this be possible? What am I missing here?

I've bought another fuel pump and FP hanger assembly but still the same problem.
I'm pulling my hair out with frustration!

 

[Dan@SJM]

Bad earth

 

[Blerg]

As dan said, bad earth. You may find you have a bad earth on the pump loom, when the relay kicks in it will give 12v using the earth through the relay but once the resistance builds up it will drop out. Maybe confirm this by splicing an earth directly at the pump and see what happens

 

[Jake.]

Thanks Blerg. I tried connecting an earth directly (via a jump lead from the battery negative terminal) but it didn't make any difference. However, using the negative from the fuel pump plug and a live from battery positive (jump lead again) it did work.
Strange that there's 12.5v across the fuel pump plug wires but there not enough something (Amps? Watts?) to run the pump.

Should I run another positive wire to the fuel pump? If so, where from? I mean, where is that wire powered?

Thanks.

 

[GrahamS]

Evening chaps,

I've now got this car. Spent a bit of time looking at it and the pump. As said before the pump works when on an independent power source.

When you turn the ignition on there is 12v between the live and ground on the plug. As soon as you plug the pump in the 12v dies almost instantly and the pump doesn't buzz into life at all. If you run the pump on independent power in the car it also doesn't run (don't know if the ECU) knows when the pump isn't plugged in on the loom.

Earth isn't corroded at all.

Any ideas @AlexW @MicKPM

 

[GrahamS]

Bad earth

As dan said, bad earth. You may find you have a bad earth on the pump loom, when the relay kicks in it will give 12v using the earth through the relay but once the resistance builds up it will drop out. Maybe confirm this by splicing an earth directly at the pump and see what happens

Not a bad earth!
Sorted this problem today. Spent a while looking at the wiring diagrams and found that pin 3 of the coil shares the same 12v as the feed for the fuel pump. When both plugs were disconnected there was 4v at the coil pin and 3.9v at the pump. As soon as you connected either of the plugs or both at once the voltage disappeared.

Did a bit of research and looked into voltage drop. To cut a long story short I had to learn about how to diagnose voltage drop today which took a while. I ended up finding the drop appearing between the 30a fuse and the pin which goes into the fuel pump relay. I inspected the wired and found it was blown in one place, cut this section out and sure enough it was completely ruined. Replaced the wire and hey presto it's back up and running.

 

[Dan@SJM]

Funnily enough I had one in Thursday that had done the exact same thing.