Electrical conundrum and fuel pump

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drdanger
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I have an electrical conundrum and a half with power supply to the fuel pump. Yesterday I was driving from Worcester to London in my 1990 280 GE, stopped for about 2 min when I got back to the car it restarted and then promptly cut off and died. I will spare you the next 8 hours of figuring out the problem and eventually going back home on the back of the AA truck. The basics is that there is no power supply to the pump when the circuit is closed only (under load)

The facts are: Relay is irrelevant I have shorted it for testing so there is constant feed when the ignition is switched on. The fuel pump works fine with external power and the car starts on the button.

Now here comes the conundrum - there is 12V on the positive and negative wires (when disconnected from the pump), as soon as they are connected the feed dies. This doesn't have to be a pump, in fact any resistance (ie test lightbulb) I can literally watch the voltmeter go from 12V to 0 as soon as resistance is in the circuit. To add to confusion, it doesn't have to be the same earth wire, the result is the same from any earth (chassis, body etc)

Any ideas would be appreciated. 

Arnie
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Re: Electrical conundrum and fuel pump

This sounds like a broken circuit. Either the relevant fuse has gone. Sometimes they develop a hairline crack which allows voltage to be measured but the impedance is too high fo sustain the voltage under current. Or, similarly, you have a broken wire or connector. Check the 12v and ground separately

Roly
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Re: Electrical conundrum and fuel pump

I've experienced this type of fault on a heater fan switch. Check the circuit with a load and meter from the main fuse at each salient point to locate the place where the circuit is broken. I you can't find it then you can always run a new cable from the last known good point to the pump. You have proven that the fault is not with the earth cable.

drdanger
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Re: Electrical conundrum and fuel pump

Thanks for input guys. I have traced the fault between the fuse box and the connector under the bonnet. Circuit performs fine under load directly from the relay terminals but at the connector it fails. Not an easy place to find a problem. Is there any other connections on the way from the fuse box to the connector?

P.S. There are two wires coming out of the positive side of the relay circuit (black with a stripe and sripes across, one thick one one thin one) - only the thick one seems to go to the connector, where does the thin one go? Warm up valve?

G-Restorations
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Re: Electrical conundrum and fuel pump

 

 

We've just had a 280 in with us, similar symptoms. One fault we had was a broken earth wire to the fuel pump. It goes to ground on the end of the chassis rail to the left of the towing bracket.  This is a common bad earth on 460s because its out in the weather 

Good luck!!  Gordon

 

drdanger
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Re: Electrical conundrum and fuel pump

Is there another earth wire to the pump apart from the two from the loom? I was not aware of this?

Arnie
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Re: Electrical conundrum and fuel pump

I think Gordon was saying that the earth wires in the loom, are connected to a ground point at the rear of the chassis, which may need cleaning.