Expert: Rob Painter - 8/27/2010

Question
QUESTION: What will cause an ECM in a 1991 Cadillac Sedan DeVille with a 4.9 liter engine to keep going bad after its replaced with a new one?

ANSWER: Hello,

I would assume that you had a qualified tech looking at this.
I cannot with any certainty diagnose this problem through the Internet.

What it sounds like though is you are getting a surge in voltage going to the ecm to keep burning these things up.

By design, there is a constant 10 volts on the computer power supply circuit.  Any surge to over 10 volts will fry the ECM.

You will need full wiring schematics for this car (available at www.alldata.com) and work backwards from the ecm, find out what else is on that circuit causing the spike.

This is not going to be an obvious wiring problem. This is going to be either a momentary positive to ground like the 10v wire hitting steel when going over a bump or more likely the sensor or part that keeps this power feed to the ECM at a constant 10v.

Remember, every thing in the car is 12 v except for the ecm. Resistors are a way to bring down voltage. There may be some kind of resistor control part in the ecm power circuit that is failing.

I personally after working on thousands of cars have never seen where the ecm kept burning up and the answer I am giving you is  somewhat limited because I don't have a schematic for your car to study what feeds the ecm circuit.

I hope I have assisted you in some way and if I did please rate my answer.

Thanks

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

QUESTION: I perfomed a diagnostic test on my 4.9 liter 1991 Cadillac Sedan DeVille, and the E71 PCM diagnostic code came up. What will cause an intermittent map sensor reading?

Answer
Hello,

As I had stated before, you are going to need more information specific to this 1991 Cadillac. The car is 20 years old and I disposed of the schematics, code trouble shooting, electronic information for this type of vehicle 15 years ago.
I just didn't have the room or the need to hold on to information to one vehicle, when in fact, we service hundreds of different years, makes and models.

You need exact answers to your problem before it gets more costly.


You have three options:

1) hope you can find a Cadillac dealer that is still sitting on the manuals for this car. You will need the body manual, emissions manual, electrical manual, component location manual if you are dealing with books like Mitchell. Chances are pretty slim you  will find a dealer with all of this in that vintage.

The systemyou have was made obsolete in 1994 when obd II became the standard that it is now.

2) hope you can find all the manuals either through Helm or on Ebay.

3) this is the option I personally use when I need to search out a specific problem and solution on a specific vehicle. Has never failed me yet. Http://www.alldata.com.

I get nothing to plug this site. A while back a guy complained that he had to pay money to diagnose his problem with his car. Well, I was not about jto pay for it and how would I know if the diagnoses worked or if you had to go farther-all this without physically being at the car!

His complaint was that he had to pay about $25 for a one year membership for access to every piece of unedited factory information about the specific year make and model.

You use thier chart to scroll to your car. Then dowmload. You havr access for a year. If during the year you get another car, the informatiom for a second car is $15.

There are flow charts that may direct you past the map sensor here.

I try to give my readers the most accurate information, otherwise my credibility is blown and I send the reader on a goose chase and I realize thier time is valuable too.

That is why for your problem with ECMs, i hope the information from alldata will assist you in getting the correct answer in the first place.

Please do me a favor and let me know how this works for you.

Good luck!