In two previous articles I outlined the problems and solutions on a 2005 Buick Lacrosse. Symptom and Solution. Now for the root cause of this failure. In every case that I found, similar to the vehicle I was working on was that the condition was preceded by a battery event. Most notably loose and corroded battery terminals. The positive battery cable terminal had been recently replaced, the negative battery post was undersized and the negative battery cable terminal was loose as a result. The thinking is that these ECMs are very susceptible to battery voltage spikes.
I found one reference to a poor body ground located just behind the battery tray as shown in the following picture. The article only referenced a problem with two of the gauges not working. This one had already been cleaned and tightened before arriving at my shop. I did reseal the connection by spraying it with paint. I did this while I already had the battery out of the car.
I crimped soldered and sealed new ring terminals onto the existing cables.
I then attached them to a marine style battery terminal for a positive connection. No pun intended but it fits.
To replace the battery, the engine compartment cross brace had to be removed. It is installed back into position in the next picture.
I also find it easier to remove the hold down nuts for the under hood fuse box and position the box slightly out of the way.
When completely reassembled the harness attaches to the under side of the brace.
One key note about the negative battery terminal. The nut that tightens the terminal clamp onto the battery post is beveled on the underside. I fully removed the nut. Cleaned both surfaces and added a little bit of grease to allow the beveled lip to work properly as the lever it was designed to be. When I have more time I intend to do an article on just how these terminals work. I also have to look for a supplier of replacement nuts as they do seem to wear out.