Intermittent 4WD Module Problem – 1996 Chevrolet Blazer
This case study will be pretty short and is just a nice example how a picture is worth a thousand words. This 1996 Chevrolet S-10 Blazer would not stay in 4wd and the check engine light was on and there was a code set for the 4wd system. I do not remember which one because I diagnosed this Blazer long before I ever thought about having a web site, let alone doing case studies. I am lacking a lot of information a lot of you would like to hear, so I will make this more about the power of a picture.
I believe the code was a 4wd module error code, so I went there and checked all powers and grounds. Everything look good on the multimeter but I felt I needed to dig in deeper. This 4wd issue was intermittent so I decided to hook up my scope to the 4wd module ground, power in, and power out to PCM (I believe). While on the test drive I could see if I was losing a ground or one of my powers. As you can see on my scope pattern I was losing the 4WD 12 volt signal out to the PCM. The image shows the worse case of my power issue and most of the time it would only be one spike to ground very intermittently. You might have been able to catch the problem on your multimeter when it was the worst, unless your multimeter has a record feature, but it would have been hard and inconclusive. I don’t think a test light would even flicker over this problem. The scope showed me the problem clearly and a new 4wd module took care of this problem for the customer.
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