ECM not sending fuel pump command (Page 1/2)
Dre0415 NOV 30, 09:31 AM
Hello everyone,
I recently purchased an 85 fiero gt that hadn't moved under its own power in over 12 years. The car is definitely a disaster. Recently I've been trying to undo all the Jerry rigging that was done by previous owners and putting the car back to factory condition. Anyways when I had first purchased the car I had issues with fuel pump so I replaced it. And gave it 12 volts to test and it worked great. So I began digging into wiring diagrams found fuel pump relay tested all wires and found that the green and white wire wasn't getting a 12 volt signal from the ecm. I checked connuity between ecm and relay and it was perfect so I know the wire isn't broken I also feed the wire manually 12 volts from ecm side and the relay clicked and began functioning. Does this mean my ecm is bad or is there some other signal the ecm needs before it commands fuel pump. The reason I'm asking is because this car had a classic steal stopper in it that I ripped out. Not sure if that somehow could cut fuel to fuel pump or not. I will also say the engine runs perfect when fuel pump is manually feed 12 volts. So I know atleast most of the connections to the ecm are good.

Thanks for any input.
theogre NOV 30, 10:49 AM
ECM grounds the injector(s) to turn on.

If noid light or things don't get that ground then very likely... in no order...
Iffy wiring to ECM Engine or Both.
ECM is bad. Example: Injector driver(s) can get crap solder joint and dead while rest of ECM seems to work "100%" to you.
Ignition problems won't "send signal" to ECM. ECM needs that Before injection starts. See my Cave, HE Ignition & Ground "Myth" notes

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Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave

Dre0415 NOV 30, 10:56 AM
Injectors work perfectly fine this isn't an injection problem it's a fuel pump issue. The green and white wire to the ecu never receives 2 second 12 volt signal to prime fuel pump. Would this be a bad ecu? Like I said before engine runs perfectly fine on its own as long as fuel pump is manually given 12 volts.
A_Lonely_Potato NOV 30, 11:04 AM
ive had this twice. both times replacing the ECM fixed it. the green wire got melted on my exhaust manifold and blew the ECM. if theres nothing else wrong, you can crank till oil pressure builds and the fuel pump will kick on from the oil pressure sender circuit, and you can drive the car. the second time mine blew was because i didnt properly fix the wiring the first time, and it melted again. properly insulated it and no issues since.
theogre NOV 30, 12:09 PM
Sorry... I read that backward somehow.

ECM can die not "turning on" whatever like injection above.

While green wire seems good when tested... the wire can be cooked like A_Lonely_Potato said or other damage that kills the ECM and new one dies and often soon too.
Old ECM often don't have short protection on 12v or 5v lines and die in odd ways depending just what shorted out.
(Even many PCM and BCM on new models don't have short protection on most lines.)

Wires Doesn't have to touch hot things... exhaust cooks anything close to it. More so when have exhaust leaks.
So a Wire will often Pass ohms etc but still shorted to each other or ground wire.
A_Lonely_Potato NOV 30, 10:16 PM

quote
Originally posted by theogre:
Wires Doesn't have to touch hot things... exhaust cooks anything close to it. More so when have exhaust leaks.
So a Wire will often Pass ohms etc but still shorted to each other or ground wire.



yup. be sure to check the wire along its length for any damage. it may have gone crispy with another wire and they melted together. quick check would be to check continuity between the green wire on either end to ground, but still physically check the whole thing!
sanderson231 DEC 01, 01:27 PM
The wires are fine if putting a 12V jumper to ECM plug wire that goes to the fuel pump relay coil and causes the fuel pump to run. Check for 12V at the corresponding prong on the ECM when the key is turned on. It should show 12V for about 2 seconds and then switch off. If it doesn't then it probably is the ECM. The reason I say probably is that power from the ignition switch must go through the ignition module to get to the ECM.

Putting a 12V jumper to ALDL connection G should cause the fuel pump to continuously run. This bypasses the fuel pump relay. Once the engine is up a running the fuel pump relay is redundant. Power should go through the oil pressure switch directly to the the fuel pump also bypassing the relay. If the engine dies when the ALDL jumper is removed, there is a problem with the oil pressure switch circuit.

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formerly known as sanderson
1984 Quad 4
1886 SE 2.8L
1988 4.9L Cadillac
1988 3800 Supercharged

theogre DEC 01, 03:31 PM

quote
Originally posted by sanderson231:
The reason I say probably is that power from the ignition switch must go through the ignition module to get to the ECM.

Nope
ECM get power in 2 ways:
Key On in I-switch via 1 or more fuses.
Standby via 1 Fusible Link that has connector to reset ECM codes.

ICM has 1 "ground" for ECM + Ref and Control lines. See my Cave, Ground "Myth" notes
ECM has several "Grounds" and not all are Real grounds like Tan wire that is Return Path for "1 wire" O2 sensor.
Now most brands have 2 or more wires to O2 sensor(s) w/ 2 for the sensor and often 2 for heating that because many a too far down the pipe for exhaust to heat them fast. Many others have even more complicated O2 sensing and not talking about wide band O2 costing Hundreds of $ to replace. (Is part of why Cat Theft cost big time because many cat have O2 sensors in them...)

F-Pump power has nothing to do w/ ICM.
A_Lonely_Potato DEC 01, 06:29 PM

quote
Originally posted by sanderson231:

The wires are fine if putting a 12V jumper to ECM plug wire that goes to the fuel pump relay coil and causes the fuel pump to run.



gotta disagree with you there. my fuel pump ran when i applied power to the green wire at the ECM side, before i had fixed the wiring. OP, check the wiring! even if readings seem normal, check it!
phonedawgz DEC 01, 10:07 PM
Is your ECM getting power and is it running? Key on, does the Check Engine light come on? It should. If it is not coming on look to see if you have power on both the unswitched and the switched inputs to the ECM. As mentioned above the unswitched (constant) power wire is an orange wire that runs from the power distribution post next to C500, through a fusible link, through the ECM reset connector, then across the engine harness to the ECM. So if the ECM reset connector is left open, the ECM will not get power and of course, will not function.