Odd Fuel Supply Problem
#1
03-03-2009, 02:59 PM
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Odd Fuel Supply Problem
I have a '92 XJ40 that won't start.
To cut a long story short the car will start and run if I run a wire directly from the positive battery terminal to the "power in" terminal on the fuel pump relay. This is how the car's wiring should be according to the Haynes manual ~ a direct wire from battery positive to the relay ~ no other components in between. From this I take it that the relay, the signal input to the relay and the fuel pump are all operating properly.
Without my additional wire from battery to relay, when I put a volt meter on the relay "power in" terminal it reads 12v with the ignition off (as it should because of the car's direct wire). But when I turn the ignition on the 12v goes to zero, also If I do a continuity check between the battery and relay the ohmeter shows an open circuit.
It's as if the wire between the battery and the relay is intact but there is a very high resistance connection somewhere. With no current draw I can read 12v at the relay even across the high resistance, but operate the relay and introduce the fuel pump as a load and the full 12v is dropped across the high resistance with nothing left to power the pump.
I've delved under the dash to try to trace the wire (brown/black) between the battery and relay, but there are so many wires and I do not want to have to unwrap the wiring loom.
Is anyone sufficiently familiar with this circuit and wiring to be able to describe it's routing and identify the connectors I need to look at?
Thanks ~ Gary
To cut a long story short the car will start and run if I run a wire directly from the positive battery terminal to the "power in" terminal on the fuel pump relay. This is how the car's wiring should be according to the Haynes manual ~ a direct wire from battery positive to the relay ~ no other components in between. From this I take it that the relay, the signal input to the relay and the fuel pump are all operating properly.
Without my additional wire from battery to relay, when I put a volt meter on the relay "power in" terminal it reads 12v with the ignition off (as it should because of the car's direct wire). But when I turn the ignition on the 12v goes to zero, also If I do a continuity check between the battery and relay the ohmeter shows an open circuit.
It's as if the wire between the battery and the relay is intact but there is a very high resistance connection somewhere. With no current draw I can read 12v at the relay even across the high resistance, but operate the relay and introduce the fuel pump as a load and the full 12v is dropped across the high resistance with nothing left to power the pump.
I've delved under the dash to try to trace the wire (brown/black) between the battery and relay, but there are so many wires and I do not want to have to unwrap the wiring loom.
Is anyone sufficiently familiar with this circuit and wiring to be able to describe it's routing and identify the connectors I need to look at?
Thanks ~ Gary
#2
03-03-2009, 03:31 PM
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07-03-2009, 01:26 AM
#4
07-03-2009, 06:15 AM
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ggryder, it is possible that part of your wiring harness has suffered what is called fatigue failure. What I mean by this is the wire has been vibrating as you drive the car. This constant motion has strained the wires to the point that they have started breaking. So, what you have is a bunch of broken strands of wire still covered by the insulation that have been essentially cut. Due to this sort of event, the wires are still capable of barely touching, but when any sort of load is applied, they can't handle the load (think of it like trying to run 100 amps through 22 gauge wire).
Unfortunately, you have 2 options for this fix. 1) disassemble the wiring harness and replace the wire between the battery and relay (you can also simply run a new wire in parallel and then attach it to the wiring harness using tie straps) or, 2) replace the wiring harness all together (expensive, but will garantee no future problems with other components).
I see this sort of thing all the time with older vehicles. Sometimes you will also get water intrusion under the end of a wire and this will cause the wire to corrode inside the insulation, leading to a high resistance connection. In this case, you can cut back the wiring an inch or so to remove the high resistance point and then reattach the wire to the factory pin. But, that is a hit and miss situation.
Unfortunately, you have 2 options for this fix. 1) disassemble the wiring harness and replace the wire between the battery and relay (you can also simply run a new wire in parallel and then attach it to the wiring harness using tie straps) or, 2) replace the wiring harness all together (expensive, but will garantee no future problems with other components).
I see this sort of thing all the time with older vehicles. Sometimes you will also get water intrusion under the end of a wire and this will cause the wire to corrode inside the insulation, leading to a high resistance connection. In this case, you can cut back the wiring an inch or so to remove the high resistance point and then reattach the wire to the factory pin. But, that is a hit and miss situation.
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