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ECM Testing

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Old 07-11-2018, 11:06 AM
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RacerX10
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Default ECM Testing

I have a LT1 motor and 6 speed from a 93 Corvette in my 56 Bel Air. I've just had an issue crop up where the car will run for 15-20 minutes and die.

When it dies, the fuel pump won't run. The check engine light doesn't come on when the key is first turned on. The engine will turn over but obviously won't start without the fuel pump running. I can force-run (with 12v) the fuel pump and the engine still doesn't run.

These things lead me to believe the ECM is crapping out after things get warmed up. The car has had a new fuel pump and fuel pump relay installed.

So, just to reiterate .. when the car has sat for a while (and cooled off), when I turn the key on, the check engine light comes on and I hear the fuel pump running for a few seconds as expected. Turn the key to start and it fires right up.

After the car runs for a while (15-20 minutes) the engine dies and the fuel pump does NOT turn on with the key in the "ON" position, and the check engine light does NOT come on when you first turn the key on.

Any help confirming an issue with the ECM ? What's a good way to test / troubleshoot that thing ?

Thank you !
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Old 07-11-2018, 01:50 PM
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Kevova
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The check engine light part sounds like ecm. Verify power and grounds ideally when it's not working. Also check if there happens to be any codes. ECM is 92-93 LT1 only can be hard to find. Remans normally send one to get one. The a companies who will repair yours.
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Old 07-11-2018, 02:17 PM
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RacerX10
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Originally Posted by Kevova View Post
The check engine light part sounds like ecm. Verify power and grounds ideally when it's not working. Also check if there happens to be any codes. ECM is 92-93 LT1 only can be hard to find. Remans normally send one to get one. The a companies who will repair yours.
Thanks for your reply .. do you know where I can find a pinout on this thing to verify ground and power ?

The ECM has a part number of 7-9278 on it and has the 4 connections on the top (grey / red / green / brown)

Last edited by RacerX10; 07-11-2018 at 02:19 PM.
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Old 07-11-2018, 02:31 PM
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Might take a look at the IGN switch..
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Old 07-11-2018, 03:09 PM
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Originally Posted by confab View Post
Might take a look at the IGN switch..
Hadn't thought of anything that simple Does the IGN switch power the ECM ?

Even when it's shut itself down (after warming up) and I don't get the check engine light, I *do* still get OIL light on dash since the engine is "on" but not running. Would that eliminate the IGN switch as a culprit ?

Thanks !

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Old 07-11-2018, 03:38 PM
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I appears that the wire harness for the ECM draws power from the starter lug (where the main line from the battery positive terminal comes up) so I guess that's where I start looking. I guess the ECM stays powered up all the time.
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Old 07-11-2018, 03:44 PM
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ECM has both constant 12v and ignition 12v feeds. Checking during failure should confirm ECM or car is at fault. You could try to Google ECM wiring diagram or pin out. I don't have a way to share it.
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Old 07-11-2018, 05:14 PM
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I found all the diagrams I need to chase this down tonight at the shop. Thanks a bunch for your help.
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Old 07-12-2018, 10:34 PM
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Thought I would update this thread with the solution. Pulled the ECM out, took it out of it's case and did a little poking around while it was still hooked up. Found out that I could reproduce the problem with slight tapping or very slight PCB flexing of the ECM PCB. Re soldered all the connections on the IC's attached to the heat sink (those are usually the most common points to find cold solder joints) and the car seems to be running fine now. Ran it about 30 minutes in the shop and never missed a beat.

Thanks again for all the advice given.
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