Auto Electronics: The envoy- Again, passlock, proper signal

Question
QUESTION: Hi,
The light is on when weturn on the vehicle for the theft lock then turns on for breif as we try to start it but it does not stay on. It was when it ran though at times it was on as you drove down the road which I thought ment there was a problem but they told me No codes found. We replaced the spark control mod about a month ago but my husband wonders if maybe it is not giving proper signal? We are in the process of tracing wires now. Is there a way to test the main computer? Can that cause a no start problem?

BTW thanks for the info and help I really appreciate it. And you are right the dealership is not very helpful they just state to bring it back so thy can trace the wires at $65.-$75. hours. To steep for my budget as of this time.

Theresa R

ANSWER: Theresa, there is no test for the ECM that will verify it is good or bad. You have to make sure everything else is good before you condemn it. Check that the power feeds and grounds are good. Ensure that it is able to communicate on the network. No shorts on the 5v reference circuits and no shorted output devices. I thought you told me that you replaced the ECM? Without a doubt the ECM can cause a no start, it controls fuel and ignition. Don.

---------- FOLLOW-UP ----------

QUESTION: Sorry Don,
We did replace the ecm and it is showing that it is ok. All wires traced show no resistance. We are at the thought of the passlock again. Do you know how to get the passlock out of the vehicle completely? Can it be bypassed. there is no other reason we can see for it to  crank but not start.
theresa r

Answer
Sorry Theresa, I do not know of any way to bypass the system. The passlock module is part of the body control module. When the key is turned in the cylinder a signal is sent to the BCM. If it is the corrrect signal, the BCM will tell the ECM to enable the injectors for fueling the engine. The best and possibly the only way to diagnose these systems is with the factory scan tool. This device is called the tech 2 scanner, all dealerships will have one and they should be your best bet to figure this thing out. Don.