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89 GT fuel pump relay and ECM confusion

15K views 6 replies 2 participants last post by  wbrockstar 
#1 ·
Hello everyone, my first post here although I've used this resource many times. I have an '89 GT I've owned since 1992, it's a daily driver with almost 300K on the clock and along the way I've had to fix many things, never the short block though, but this time I have a question that's got me stumped. This all started over a no prime/no fuel pump situation a few weeks ago. Leaving a large antique it would only turn over, the pump would only make noise past run and into start, on the key, but before that starter engaged. After fooling with what I could without tools, came home on a bouncy rollback. After doing nothing more than checking a few connections, fusible links and spark, the relay clicked and the pump primed. Motor started and I changed the rely and fuel filter, had the filter already so... The next day I run errands and stop and start the car 10 times without problem UNTIL my last stop at Taco Bell. On the way through the lot were the T.B. is, I went over 4 speed bumps, thin but high, narrow abrupt ones, I was in a hurry, bang bang, bang bang. Well I come out to my car and the same thing as the day before, I call a friend, get my groceries home and start reading here. After some reading and going over the checklist I thought I might try the inertia switch since jolting the car seemed to affect it. Back to the T.B.. eliminate the inertia switch, ground the ECM, may or may not have helped more on that later. and my relay clicked and pump primed. Car starts and I go home, yea! All week, no problems, stop and start everywhere until I was leaving the flea market, same problem again. With a multi meter I find power on two of the 4 connectors in the pump relay plug. Looking in the bottom of the relay itself I have 2 large spades and 2 narrow and power in the plug on one of each. Jumping the larger connectors ran the pump but no start. Jumping the narrow connectors as well and the cars started drove home and back to reading here. So everything points to a bad ECM or rather the solid state switch that powers the primary power to the relay which in turn puts the power through to the pump, if my understanding is right. Replaced the relay again and still no start so as the work week loomed, I rigged up some spades on heavy speaker wire with some 12v toggles and replaced my impromptu jumpers under the seat and been driving it like that for a week, all the while thinking it must be the ECM, where I'm stumped right now. About 15 years ago my car wouldn't start, had fuel, must've had spark because the consensus was it had jumped time but before I took the front apart, I went to see a Mustang guy I knew who told me to go home and take out the little plug on the pigtail near the TFI module and it will start. Sure enough he was right and he said I had a bad ECM. At the time I couldn't source another ECM and had to send mine away to be repaired. With that plug out and the ECM in the shop states away, I drove my car like that for about 3 months. No computer in the car and no spark advance but it ran well enough I just drove around like granny until the ECM came back and everything put back together. My confusion lies here, if the ECM controls the the opening, or closing, of the fuel pump relay, how did I drive the car then with the ECM removed?? Back to grounding the ECM thing, I once had a no start situ where at work, our electronics guy who's a very smart person, suggested I "clear the computer" by taking the ground cable off the batt and touching it to the positive side, he said things can get "buzzy" on a computer board and that I was draining any residual power. The car started. In the end , I replaced the stator but until I figured that out, I would going down the road and the car would shut off after a half hour or so. I would have to get out and ground the positive side and it would start right up again. I showed a Ford mechanic, at Ford, and he could only scratch his head but I swear it! So when I think I have a computer bug, I'll clear it like I said above but my problem right now is the relay. BTW, same electronic guy just this week wanted to look at my relay and said it was bad, he took it apart and a strand of wire almost hair sized coming form the coil was off it's connection, so he solders it back and we test it on my battery and it's fine. I install it and no start back to jumpers/switches. A long enough read for sure but I wanted to cover everything. I'm a handy guy and know my way around the tool box but electronics, instead of simple wiring is a little over my head. Anyone have any insight? I really did drive without the computer, for months. Thanks for reading if you've gotten this far, sorry for the length.
Jim
 
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#2 ·
You can try swapping the ac wot cutoff relay for the fuel pump relay,to verify if the fp relay is working correctly.Its located in front of the passenger strut tower.Both relays have the same part # so theyre interchangeable.
The diagram below shows what happens when you turn the key on.The eec relay is actually what energizes the fp relay when the key is turned on.The fp relay is energized,the pump primes for 2-5 seconds then shuts off,then the ecm waits until it detects engine rotation (via the pip sensor pulse)and once it sees this pulse the fp relay is energized again by the ecm ground pulse,at pin 22.So in otherwords,pin 22 is a ground pulse thats turned on/off by the ecm.When it wants the fuel pump to run,it grounds pin 22 & when it doesnt want the fuel pump to run it ungrounds pin 22.I would test the tan/lt green wire,between the fp relay & ecm,for continuity(ohms) Remove the fp relay from its harness & disconnect the harness from the ecm too for this test.Connect the red lead of an ohmmeter to the tan/lt green wire at the fp relay harness & connect the black lead to pin 22 at the ecm harness.The meter should read 5.0 ohms or less,otherwise the wire has an open.The link below details the ecm pinout layout.
Since the eec relay is responsible for energizing the ecm,fuel injectors,solenoids & the fuel pump relay, you should definitely test it.Its located above the ecm.Removing the dash speaker helps access the relay sometimes.

It wouldn't hurt to remove the ecm for inspection.After you remove its rear cover,inspect the 3 blue capacitors for leakage,buldging & broken/eroded posts.Theyre normally the main component that goes bad.
The only other things I can think of would be a faulty ignition switch or a shorted wire thats intermittent.As you can see in the diagram below,the ignition switch provides ignition on power to some very important components,like the eec relay,coil,tfi module,etc so keep that in mind when trouble shooting.



http://www.veryuseful.com/mustang/tech/engine/images/88-91_5.0_EEC_Wiring_Diagram.gif
 

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#3 ·
Thanks for your reply and those diagrams, I'm a bit more enlightened . Today after work I discussed everything with with the electronics guy and showed him the diagram in your first thumbnail. After a quick study of the sequence of events and the wiring he first bench tested the relay which proved good. The to the car, relay was plugged in and no start. We the measured voltage coming out of the relay plug. EEC relay was sending 12v into the pump relay and the smaller 12v ignition connection also was sending 12v but the green wire/22 pin connection from the ECM had 5.8v instead of 0. He explained how the pulse ground from the 22 pin should pull the coil in the relay to turn it on but there wasn't enough of a voltage difference to do it. He tapped on the relay with a screwdriver handle and the fuel pump came on. He said the "rap' was enough to acuate the coil the rest of the way and send power through to the pump. Now at this time, with the key on and relay installed the pump keeps running and no start, as I've read here with bad ECMs and pump problems. I told him it wont start without the 22 pin connector and 12v ignition connector jumped, the two small ones. He said maybe the loop need to be closed for a sensor or something, computers are strange lalala, he's not a mechanic but power, switches, caps, coils etc are his bread and butter. So in short, my EEC seems good but 22 pin is not grounding to operate the pump relay and he wants to see inside my ECM which has a bad component. This all makes sense to me I guess, it seemed intermittent 2 weeks ago but not now and by a very uneducated guess, I want to theorize that I was able to drive without my ECM before because the 22 pin connection WOULD have 0 volts when it wasn;t plugged into anything. Do I have anything figured out? I will also take your advice and check continuity through the 22 pin wire to relay connector unless the point is moot now. Out of curiosity what route does the wire take? Up and around the firewall then under the seat oe right over the trans tunnel more directly. BTW, I did check my ignition switch and tighten the folded tabs that hold it together, it seemed alright, nothing outwardly burnt. So, once again, 5.8 volts coming form the ECM 22 pin where there should be 0. This weekend I'll remove the ECM and see if runs like it did before when I had it fixed, if so I'll let my guy take it and see if he can fix it before I order one. I hope I'm making a little bit of sense and opinions are welcome and needed. Thanks in advance for any further input.
Jim
 
#5 ·
Hi again, was able to do the tests you linked to today, pretty sure it's my ECM. Have continuity from pin 22 to fuel pump relay connector Lt br/Lt gr and the also tested from test port fuel trigger and good body ground and found no continuity. It's been running pretty normal with my toggles but that is an emergency fix. Tried to start it with the ECM unplugged and spout plug removed, just for kicks, and it would not. I swear on a stack of bibles I drove that car for about 3 or 4 months with the ECM removed and away getting fixed halfway across the country. Is that not unheard of??? Anyway, thanks for your help and this great site. Will order an ECM now and get back with the results in time. Thanks again.

Jim
 
#7 ·
If the ecm is still connected,you can do a few quick tests.The following info is from 89Coupe @ AFM

. With key off, attach the red lead on the meter to a constant 12v source, and the black lead to the Fuel Pump relay ground(pin 22 wire)Turn key on. The meter should show battery voltage for a few seconds, then go off. Thats the ECM controlling the relay as it should. If the meter doesnt register voltage, then its one of 3 things. Bad ECM or faulty wiring between ECM PIN 22 and the Fuel pump relay. Or the ECM is just not powering up. To check to see if the ECM powering up, just check for 5v at the TPS orange wire. 5v there, and the wiring from PIN 22 to the Fuel Pump relay ground is good, then the ECM is powered up, but the problem is still in the ECM. It can fail, and still operate the engine just fine.

I got my ecm from O'Reillys,but I know how much in demand the A9L,A9P,A3M are.You can find them on Ebay at times & some of the ones on Ebay are not for an actual ecm,but a repair service for your own ecm.You send your ecm to them & they repair it then send it back to you.You can remove the rear cover off of yours and inspect the board.The signal return line & the 3 blue capacitors are normally what goes out.If so,you can buy the capacitors @ Radio Shack & replace them yourself if thats the only thing wrong with the board.The signal return wire can be fixed by soldering in a new piece of wire,in place of the gold trace.
 
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