Sufficient evidence ECM is bad

joncheryl · May 22, 2020

2000 Suburban 5.3L. I'm having problems with the injector circuit for cylinder 3 in my car. I removed the connector on the injector and verified that it's getting power (constant 12V when key in ON position and 14V when engine is running). When a test light was put on the ground terminal it illuminated but didn't pulse. When I looked at the voltage though, it was bouncing all over between 0 and 14V. I turned the car off and noticed the ground terminal had 1.7 kOhms resistance to ground when the engine is off. Thought that was funny as it shouldn't be connected to ground when the car is off and if there was a short, the resistance should be 0. I traced the wiring and didn't see anything wrong. I then found a diagram for the ECM connectors, located the terminal for the cylinder 3 injector (ironically terminal #3) and found the same 1.7 kOhm resistance to ground at this terminal. This was the terminal on the actual ECM not the wiring harness.

1. Is this sufficient evidence that the ECM is bad?

2. Can I just go to a junkyard and pull an ECM out of another 2000 Suburban with the 5.3L and put it into my car with no problems?

3. If not, does anybody have experience with flashmastersecm.com? Judging from ebay, they look reputable.

davester · May 22, 2020

The injector isn't constantly powered (as in, as both power and ground connections all the time), and then gas would just be coming out of it all the time. Instead, it is powered intermittently, depending on a whole bunch of factors, by the ecm. I think it received constant 12V, and the ecm varies when/how long it is grounded to control how much fuel is injected, but I'm not sure.

To really test it and know what's going on, you would need an oscilloscope or similar testing equipment.

sefiroxx · May 22, 2020

A typical 12v test light may not switch fast enough to allow the ecm control. You'll need a noid light to confirm.

Also, a mechanics stethoscope can be used to verify operation. You'll here clicking of operating.

I believe, (will need to find the FSM), the ground is controlled by the ecm.

carnau · May 22, 2020

28 minutes ago, davester said:

The injector isn't constantly powered (as in, as both power and ground connections all the time), and then gas would just be coming out of it all the time. Instead, it is powered intermittently, depending on a whole bunch of factors, by the ecm. I think it received constant 12V, and the ecm varies when/how long it is grounded to control how much fuel is injected, but I'm not sure.

To really test it and know what's going on, you would need an oscilloscope or similar testing equipment.

This is incorrect and correct all in the same sentence which is crazy. lol

Your injectors have power to them at all times which it should have. The ECM controls ground pulse which makes the injectors spray fuel.

If you removed the connector and you have a constant ground on the control side your injector would pulse one time and be stuck open and then drip fuel.

These ECM's are known for corroding up along the lower part of the connector. So disconnect both connectors to the ECM and see if it is corroded. If it is repair.

If it is not, Check the signal circuit for ground with the ECM disconnected. If it still lights up, This would be an indication that the circuit is shorted to ground.

If it doesn't light up with the ECM disconnected and when you reconnect it, It lights your test light. You have a BAD ECM.

Now, One last thing. Your test light, Is it LED? The reason I ask is because an LED test light doesn't require a lot of Amps to illuminate and could be giving you a faulty reading.

I recommend an inexpensive/cheap test light with a bulb because it requires more amperage to illuminate it. ( https://amzn.to/3gi3hHd )

That or a Noid light which is your best bet. ( https://amzn.to/3ensZsn )

Now, about the ECM. No. You can't take one out of a junk truck, install it an everything work. It has to be programmed. You can however buy an ECM from a parts yard or ebay/amazon , drive to the dealership with your old ECM. install the new ECM while there and ask them to program the ECM and the Theft system. They will probably charge you 1.0hr Labor.

Flashmastersecm or whatever is the same as getting a dealer to reprogram it. I personally would rather have someone program it local just in case there are any other concerns you run into.

Here is the Schematic for your system in case you didn't have 1.

joncheryl · June 23, 2020

Thank you carnau. Helpful reply. I was using a plain non-LED test light and there was no corrosion at any connector.

Just to conclude this for the potential future reader, my findings were sufficient evidence that the ECM was bad. I ended up buying a remanned ECM for $90 from flashmastersecm.com and no more misfires. I did have to go through a security sequence thing that was simple (you can youtube it) and I didn't have to do a "crankshaft variation" reprogramming. No core charge either.