How Do I Know When My Ecm Is Bad?

threearmsinjune · November 24, 2007

Hello,

I have a 1989 GMC K2500 5.7L. I have just replaced the motor and I am having trouble getting it to start. Within just a few cranks I seem to flood the cylinders with fuel. I have tested the TPS and MAP sensors and replaced the CTS. The plugs for the TPS and MAP both have a good input power and ground, but when back probed they are nonresponsive. The TPS when backprobed doesn't vary in voltage when the throttle is rotated. I replaced it and still it does not register a signal back to the ECM. The MAP also does not register a change when hooked up to a vacuum pump. I find it hard to believe that I have had multiple sensors go out at once and a brand new defective. How do I know that the ECM end of the signal wires are completing their circuits? The injectors are spitting out tons of fuel. The plugs are firing just fine at an acceptable timing point. I have 160 psi cranking compression wich is certainly acceptable. I have compression, spark and fuel. I have to question the ECM but how do I know it is time to replace it?

I have a brand new fully charged battery and I show no trouble codes, but I may have lost them with the battery replacement.

Thanks for your help.

Chris

neonhomer · November 24, 2007

I am not a GM tech, but a good hand at automotive electronics. Give all your grounds a good check. You might have a bad ground, causing voltage problems to the ECM.

If you can get your hands on another ECM, try swapping that in and see what happens. Try to find an ECM from the exact vehicle as you have. I don't remember if the late 80's ECM's used the EPROM or not, but you want to avoid having to swap that EPROM if you have to. Eliminate the ECM as a whole.

You might also want to gain access to a diagnostic scanner, like a Snap-On MT2500 or the like, so you can monitor what the ECM is seeing.

threearmsinjune · November 25, 2007

I am not a GM tech, but a good hand at automotive electronics. Give all your grounds a good check. You might have a bad ground, causing voltage problems to the ECM.

If you can get your hands on another ECM, try swapping that in and see what happens. Try to find an ECM from the exact vehicle as you have. I don't remember if the late 80's ECM's used the EPROM or not, but you want to avoid having to swap that EPROM if you have to. Eliminate the ECM as a whole.

You might also want to gain access to a diagnostic scanner, like a Snap-On MT2500 or the like, so you can monitor what the ECM is seeing.

I found a possible open condition in my lighting circuit that somehow was sending voltage into the block. With the battery ground disconnected and a test light hooked to the neg pole of the battery, there was power coming through the block. I pulled fuses until the light went out and came to the courtesy lamp and also the stop lamp fused circuits. With these circuit faults eliminated I am going to go back out tomorrow and see what the sensors are saying........

I can get a reman ECM for about $75. I would have to keep my chip and swap into the reman core. I really would like to keep from buying parts on the premise of "maybe that's it". For some reason I tend to save more money this way...........

Thanks for the input.

Chris