ECM dead?

[uptomyearsinit]

Hi,
How do I know if my ECM needs to be replaced? My 84 will not start. The problem is that the fuel pump is not being energized when I turn the ignition on. Howevew, I can energize it manually through the ALCL. When I attempted to jump the A & B terminals on the ALCL to read the codes, the check engine light did not come on. If no code 12 is displayed, does this mean that the ECM is not operating? All the fuses are good. I do not want to purchase a new ECM and then find out that is not what I need because electronics are normally not returnable. Can anyone help?
Thanks

 

[bossvette]

I don't know about codes there should be a relay for the fuel pump you could "hot wire" to see if the engine will run. Corvette computers are on E-bay all the time

 

[rrubel]

Not sure why you're not seeing a code 12, but if the fuel pump isn't energising when you turn the key check the relay under the hood. If it's in the same place as 85 & 86, then it's against the firewall by the brake booster. I have a 4+3 and there are two relays mounted to the same bracket - the front one is the fuel pump and the back one is part of the transmission. Last year at Carlisle, I had the same non-running pump problem and traced it to bare wires at the relay that were shorting it. Bit of electrical tape later and I was back in business.
[RICHR]

 

[Top]

Rich,

On my 86 vert the front relay is the OD and the back one is the fuel pump. Wonder if yours or mine got moved by somebody, the wires are long enough anyway. Curious.......

Oh well things to ponder........Cheers!!

 

[uptomyearsinit]

Thanks folks,
I found a relay in the location that the Haynes manual describes. However, when I put battery power to the coil and the contactor snaps shut, I hear a click coming from the cruise control servo. Do I have the wrong relay? In addition, when I checked the fuel pump manually as described in the Haynes, I found that gas is reaching the throttle body. However when I turn the ignition over, no gas comes out of the injector. Do the fuel injectors operate automatically when the ignition is turned? Or do they rely on the ECM to send a signal to them. In other words, if there is no fuel pressure, will the injectors still open up? I can dribble gas directly into the throttle body and the car will fire long enough to burn it off. I at a loss. Does the fuel pump relay even need the ECM to operate? I apologize for my meddle of thoughts, I am just trying to give as many details as possible to try to inspire a resolution.
Once again thanks for the help!

 

[mattyg92]

i have seen a lot of problems with the fuel sending unit assembly.
the connector in the fuel pump/sending unit case that is mounted in the tank, the harness that feeds the fuel pump sender/ the wires have been broken or shredded away. at least i have seen this on 2 vehicles so far. it is easy to check. pull out the rubber grommet around the gas cap and unbolt the 4 bolts to check.
the sending unit assymbly so far is only available from GM.

hope this helps

 

[geekinavette]

The ECM only energizes the fuel pump for a few seconds when you turn the ignition on (and during cranking I think), after that the fuel pump relay relies on the oil pressure switch to keep the relay coil energized. The ECM triggers the injector drivers from the ignition pickup...a bad ignition module can cause a no-fuel issue, but won't affect the operation of the fuel pump.

You can use the ALDL to turn the fuel pump on, but I don't remember which pins to use...can look it up if you want.

The fact that your CEL isn't working and you have no fuel pump when you turn the ignition on smells like an ECM problem.

Bill

EDIT: I just reread your initial post...I missed that you already used the ALDL to run the fuel pump. Does it start up when you do this?

 

[uptomyearsinit]

The ALDL did energize the fuel pump. However, turning the ignition did not. The ECM was not flashing a code 12. I looked at the wiring a bit closer though and found a short in an orange wire that was attached to the positive lead at the battery. I can only assume that this was the direct battery feed to the ECM because when I fixed the short, the injectors started working and the car started. Unfortunately, I had already went out and purchased a new ECM before I found the short. Oh well, one less thing to replace at a later date and the car is running again. Thanks everyone for the help!

 

[geekinavette]

Good deal!

I believe you're correct, that is indeed the battery feed to the ECM. As I remember it has a fusible link right at the battery, make sure that link is intact.

Bill

 

[uptomyearsinit]

Hi Bill,
I ended up cutting the link out of the circuit and tying the two ends togeather with a wire nut. Hope it dosen't result in a burnt ECM I guess I should just replace the fusible link.

 

[geekinavette]

I guess I should just replace the fusible link.

YES! That's the only protection that circuit has. Replace the fusible link ASAP!

 

[bossvette]

YES! That's the only protection that circuit has. Replace the fusible link ASAP!

I agree, its OK to bypass to get the car home if needed, but something caused the link to fry and you should try to find it or you will be

uptomyearsinit

for sure:L :L

 

[Don Juan]

Hello everyone,

My corvette is a 1992 and I’m having a bit of difficult with it; it is doing everything your car did to you.

I hope you have your car runing ok now!!!



I'm also trying to figure out if I could change the ECM from an early model one (1990-91);
For instance, the 1990.91 model didn’t have ASR and the 1992-93 did, so the 92-93 are a pain when problems arise regarding (ASR, SYS and Security) flashing on and off.

So, if I change the ECM to 1990-91 model, would my car work ok?

And would the ASR be disabled?

Thank you

 

[Hrtbeat1]

Negative! The '90 and '91 are speed density type fuel injection systems the '92-'96 are mass air flow type systems. The difference is the speed density uses tables to determine the amount of air entering the engine, while the mass air flow systems actually meter the amount of air entering the engine.

I would first check all the fusible links at the battery (actually there is a battery terminal block they connect to.) The SYS and Security lights would point me in the direction of either a bad ECM or a bad prom, but check the fusible links first.



G

 

[WhalePirot]

Isomething caused the link to fry and you should try to find it or you will be

for sure:L :L

Then, you may need the '84 ECM from my car that resides in my garage cabinet. I'd prefer you fix the link, though.

 

[John Robinson]

I would guess that your car has a TPS switch. I am sending this as another way the fuel delivery is controlled.

Throttle Position Sensor (TPS)​

​

Copied from 1993 Service Manual​

​

The Throttle Position sensor (TPS) is a potentiometer connected to the throttle shaft on the throttle body. It is a potentiometer with one end connected to 5 volts from the ECM and the other to ECM ground. A third wire is connected to the ECM to measure the voltage from the TP sensor. As the throttle valve angle is changed (accelerator pedal moved), the voltage output of the TP sensor also changes. At a closed throttle position, the voltage output of the TP sensor is low (approximately .5 volt). As the throttle valve opens, the output voltage should be approximately 5 volts.

By monitoring the output voltage from the TP sensor, the ECM can determine fuel delivery based on throttle valve angle (driver demand). A broken or loose TP sensor can cause intermittent burst of fuel from the injectors and cause an unstable idle, because the ECM detects the throttle is moving.

If the TP sensor circuit is open, the ECM will set a DTC 22. If the TP sensor circuit is shorted a DTC 21 will be set. A problem in any of the TP sensor circuits will set either a DTC 21 or 22. Once a DTC is set, the ECM will use a default value for TP sensor, and some vehicle performance will return.

A personal note, when my TPS failed it DID NOT set a code other than to tell me the ECM was bad.

 

[Don Juan]

Negative! The '90 and '91 are speed density type fuel injection systems the '92-'96 are mass air flow type systems. The difference is the speed density uses tables to determine the amount of air entering the engine, while the mass air flow systems actually meter the amount of air entering the engine.

I would first check all the fusible links at the battery (actually there is a battery terminal block they connect to.) The SYS and Security lights would point me in the direction of either a bad ECM or a bad prom, but check the fusible links first.



G

I thank you guys for your information it raly helps a bunch;

Now what I've done is bought a "remanufactored" computer and this computer was worst than my original one, it wouldn't fire up at the begining, later after it started; but, it would idle; finaly, I returned the computer and now i have no computer at all; I have to start fresh.

Do you think the computer I bought was defective or did i not folowed the proper proscedures when instaling a diferent computer?

Is there a proper proscedure to folow when a new computer is intalled?

Now, I need a computer to sart with.

 

[WhalePirot]

speed density uses tables to determine the amount of air entering the engine

This is not my understanding. The SD ECM uses temperature and MAF inputs to compute the amount of air entering, while the MAF sensor, a heated wire, cooled by airflow, assists in the same calculation.


Refference: http://books.google.com/books?id=O2...ts=kDD1BU1Evd&sig=Q7rOB-mtMWqt101C2MBFVgaC6Bc

 

[Hrtbeat1]

The SD ECM uses temperature and MAF inputs to compute the amount of air entering

Not to be nit picky but without a MAF sensor where are those inputs coming from? It uses tables, specifically the Volumeteric Efficiency vs RPM, and VE vs RPM vs Temp. It calculates the amount of air based heavily on the VE tables. In tuning a SD system these are the tables that are adjusted the most to acheive the proper AFR.

I've tuned several of these type systems and the VE tables are the tables that I adjust. I'm no expert tuner or anything, and I could be wrong (been known to happen quite often) but after I've adjusted the VE tables the motor seems to have more power, runs better, and uses less fuel to do so. No more stinky rich exhaust. It takes quite a bit more adjustment, and data logging to get to the same point, but when I'm done the BLM's and INT's are very close to hovering right around 128 through out the RPM range.

Interesting reading there. I didn't know the LT5 motors were all SD.

 

[WhalePirot]

Hey, we're all learning here, I hope.