my 91 civic has developed an interesting fuel gauge problem in the past few months and upon investigating the wiring under the rear seat I found some rather interesting things.

My voltage at the wiring harness is very low. For the fuel pump I'm getting approximately 9.6 volts and for the fuel sender wire I'm getting about 6.7 volts. In the service manual it clearly states to check for battery voltage which it isn't (battery is 12.3 volts). I initially thought it was the grounds but I checked the resistance of the grounds and I got zero resistance (which is good). I probed the (+) wires on the 5P harness and touched the other end of the probe to a known good ground and I'm still getting low voltage.

Fuel sender BTW does read correctly based on the resistance range table.

What could possibly cause low voltage at the 5P connector at the fuel tank? Looking at the wiring diagrams it would appear that everything goes back to the main relay (which I repaired bad solder joints on about 2 years ago). Car has run reasonably well except that lately when its cold out it really bogs like its running out of gas or something (which is likely caused by the low voltage on cold mornings perhaps not pushing enough fuel). My fuel gauge also started developing issues.

Rather than trying to back probe and back probe wires has someone on HT seen this problem? Could it be that despite me fixing the main relay's bad solder joints that perhaps the relay's contacts are no good?

I know the wires under the carpet are likely fine b/c the car is 100% stock and has never been abused or modded in any way.

If this makes any difference I did replace the ignition switch about 2 years ago b/c the contacts inside were all worn out so I know that its okay too.

Suggestions?
Thanks

 

if your main relay is bad you would get no fuel to the motor at all

 

THe strange thing is that the car runs fine when warm out and even when cold after it runs for about 5 minutes it drives fine.

Why its getting such low voltage I can't figure out. I guess I could always pull the main relay and check the voltage at the connector and go from there...