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90 f350 with a 4.9 (it has been swapped from a blown idi motor, don't ask too many questions). It's a project truck that I have been working on for some time now. It has been running and I'm at the point where I've been just working out bugs in it. The fuel pump relay hasn't been a issue for the last 2 years that I've been messing with this truck, it has fired up every time I've tried. It has sat for a month or so without me touching it and it decided it didn't want to start. I figured out that the fuel pump isn't getting power so I naturally look at the fuel pump relay. I tried swapping to a new relay and that didn't work. Using a test light, I have power from the yellow and red wire but no output to the fuel pump. Now if I ground out other wire (green/brown maybe) with my test light, the relay clicks and I can hear the fuel pump turn on. I think that wire goes to the ecm but I'm not really sure where it goes and what to test or look for. Is it a bad ground somewhere, or a bad ecm, or something else? The last thing I did to it was a new steering gearbox and it started up just fine. I haven't touched it for about a month and this problem just popped up. Any guidance on this is appreciated
Well, which processor are you using?
Your 1990 diesel has no EEC4 ECU / processor to ground the relay
Do you have a 1990 truck EVTM?
That will show both gas and diesel fuel system schematics
The EEC 4 processors have capacitor issues that affect the fuel pump circuit when the capacitors leak electrolyte
Just about every processor from that era will be affected by now
Not sure which processor you are running but checking the caps is a must
Might want to just throw a series of switches into the truck to control things
If you did a complete swap, maybe fix what you've got
Yes it was a efi swap complete with the ecm that came from the donor with a m5od tranny. I don't know which specific computer it is but I got engine, tranny, and wiring/ecm together. I know the diesel cab wiring is different and have all that sorted out. It was running fairly well, I only had 2 codes left to work out. Smog pump code and a spout one if I remember correctly. Im suspecting the computer but I'm not really sure what to test before throwing parts at it
The computer applies a ground to the fuel pump relay, via Pin 22, for 1-2 seconds when cycling the key from Off to Run position. When you advance the key to Start, and the engines starts to turn over, the PIP pulses from the distributor trigger the computer to keep applying that ground. With the engine running the same PIP pulses keep the the fuel pump relay energized via the computer.
Verify the computer ground is getting to the fuel pump relay coil circuit. You can also use a jumper on the diagnostic plug on Pins 2 & 6 to manually keep the fuel pump relay energized when the key is in the Run position.
courtesy of Subford
If you find power is on the output side of the fuel pump relay, check the inertia switch to see if it is tripped. I believe on a 1990 that switch is near the parking brake assembly under the dash. If it is tripped no power will get to the fuel pump(s).
Then you need to verify continuity between the fuel pump relay coil connection and Pin 22 on the computer. If the computer is not well ground you can have issues like this and a host of others.
Overall block diagram of the engine control circuits:
Take the EEC plug off, then test for connectivity from pin 22 to relay. Or, ground pin 22 from EEC plug see if relay clicks.
That will say if you have wiring or eec issue.
I got a little bit of time to pull it out and look at it. I still haven't tested wires for voltage or ground yet but this is what I found in the ecm. I didn't see any capacitor leaking but a few questionable resistors. I could try replacing the resistors but I don't know about that black rectangle. There is some color under it and I'm not sure if that means it's bad or if i just need to clean it up or if I should just leave it alone. Or should I just go looking for another ecm (after I check the wires of course)
Resistor looks bad Not sure about that black rectangle, it has some color under it but im not sure if thats fine Another questonable resistor
Also, when I look up ecu the only numbers I can match for the parts stores is the f0tf-ab. Is that the only number I can match up to verify its the correct one?
So I did test the pins for power and ground and they tested ok. I got a new ecu and it run now. I think it actually sounds better than before. Now the only concern i have is that my old computer gave me 2 digit codes and this one gives me 3 digit codes. Nothing too concerning popped up, a lean code is the only major thing. My old computer gave me a smog code (smog pump was deleted) and a spout code, but they don't appear on the new ecu. Should I worry about it possibly not being the correct computer or just since it appears to run fine, just call it fixed
