Fuel pump not priming but runs

[andy-w]

Ok here is a good one for you electrical gurus.

Two days ago the car broke down due to a snapped wire. I have since fixed the snapped wire but now the fuel pump does not prime when you turn the key. The relay to the ecu powers up and i can connect to the megasquirt but the relay for the fuel pump does not energise.
However, this is where it gets odd, if i connect both pins 85 on each relay together then the fuel pump powers up but runs constantly. I can start and run the car like this but now as soon as i turn the key the fuel pump is constantly running so i have to start it quickly.

I am completely stumped as prior to the wire snapping it would do the usual run for two seconds and then stop. Afaik i have not intentionally changed anything else.

I have ordered two new relays just in case and no settings have been changed in the Megasquirt.

Open to any suggestions or pointers..!

[Discopotatoes]

The fuel pump gets triggered by the ecu, you may have a welded terminal in one of the relays

[Loubaruch]

According to my information both pins 85 on the relays are connected to " request to run" commands from the ECU i.e. 0 volts.

I would not advise shorting these together as you could possibly cause your ECU to fail.

It seems as though the fuel pump relay is not getting its "request to run" 0volts from the ECU.

This could be:

1. Wiring
2. A possible immobiliser problem if your fuel supply is immobilised (some are not)

A better temporary fix to get the fuel pump running continuously would be to remove the relay and apply +12 volts to Pin 30 of the fuel pump relay.

Best of luck!

[andy-w]

Loubaruch said:

According to my information both pins 85 on the relays are connected to " request to run" commands from the ECU i.e. 0 volts.


It seems as though the fuel pump relay is not getting its "request to run" 0volts from the ECU.

This would make sense as i am seeing a constant 12v at pin 85 of the fuel pump relay.

I guess there will be more digging tomorrow.

[Sardonicus]

andy-w said:

This would make sense as i am seeing a constant 12v at pin 85 of the fuel pump relay.

I guess there will be more digging tomorrow.

Andy the MS grounds the fuel pump relay for a few seconds until it detects the motor is cranking i.e RPM signal this is not a high load ground/earth probably about 1 amp only so if you had a wiring malfunction/short etc there is a good chance that this stage in the ECU/MS is damaged easy to check with a test lamp, just giving you a worse case scenario if all else fails, check whats right before looking for whats wrong if that makes sense

Edited by Sardonicus on Saturday 4th March 23:40

[andy-w]

Sardonicus said:

ndy the MS grounds the fuel pump relay for a few seconds until it detects the motor is cranking i.e RPM signal this is not a high load ground/earth probably about 1 amp only so if you had a wiring malfunction/short etc there is a good chance that this stage in the ECU/MS is damaged easy to check with a test lamp, just giving you a worse case scenario if all else fails, check whats right before looking for whats wrong if that makes sense

Edited by Sardonicus on Saturday 4th March 23:40

Makes complete sense as i am seeing a constant 12v going to pin 85 of the fuel relay.

Time to start looking at the ecu i guess..!

Edited to add i checked with the ms stim. The stim fuel pump led comes on when i turn the rpm to zero but the led on the MS does not turn on.
Might be getting somewhere here.

Edited by andy-w on Sunday 5th March 10:01

[andy-w]

I have checked with the stim and have 4vwhen the rpm is high snd the fuel pump light is of. When i drop the rpm to zero the fuel pump light comes on and the volts drop to 2 so it is dipping. I guess it should go to zero..?

[andy-w]

Sorted.

Changed the transistors and diode on the ecu for the fuel pump driver and all is well again. Easy fix in the end.

Cheers for the initial pointers Sardonicus.

Cant wait to get it back out and on boost...!!!