my girl is acting up again. I have new pump, regulator , interrupt switch, relay and ECU. I am still losing voltage to the pump somehow after backtracing and cleaning the entire circuit. I had to run a jumper to the rear harness through a separate switch/relay to start her and keep her running. she will not start w/o it as the pump is getting 9 volts through the harness. for awhile i could just use the bypass to get her started then turn it off and let the ecu run it...now i have to use the bypass all the time.
i have swapped the ecu out with a known good one and it makes no difference.

does the ECM control the voltage to the pump relay or does it just send full 12.8 v once you power the ecm by turning key to on position?

Is there any possibility of damaging the ecm by running my bypass?

Short of replacing the harness from the ecm back to the pump I can't see any other solution to this as there is nothing left to replace.

 

The ECM does not vary the voltage to the pump on OBDI, it switches it on and off through the fuel pump relay. On my 94 the connector to the fuel tank was resistive causing low voltage to the pump. Other areas to check would be the fuel pump relay and the socket it plugs into, the inertia switch, and the grounds.
I believe your 90 has the low pressure tank pump and the frame mounted high pressure pump.
Happy New Year
rikard

 

Single in-tank pump. It must be a short in the line from the ecu to the interrupt to the rear harness. I have pulled/cleaned every connection, even the main firewall harness. As long as running the bypass will not interfere with how the ecu operates i'm gonna just let it ride.

thanks for the reply.

 

On the single pump system check the selector switch/junction located on the dash panel on the drivers side. On a friends 93 the connector had come loose and on another 94 with dual tanks the switch was bad. The panel comes off after removing a couple of screws under the trim strips and the light switch.
Happy New Year
rikard