This case study took an unexpected turn that could only happen in certain areas of the country. The vehicle in this case study is an 1992 Cadillac Eldorado. When this car was dropped off, I was told that it was a no start caused by the anti-theft system. The first step I took on this one was to confirm the no start. I turned the key to the run position and nothing happened besides power being cut to unnecessary devices. Next step on my list was to check for codes, none present. So I decided to print out the starting system electrical diagrams. The first image is the diagram I used.
Power goes from the battery, to fuse, to ignition switch, to the starter enable relay that is controlled by the pass-key decoder module. Then the power goes from the starter enable relay, thru the transmission position switch, to the the starter. I diagnose electrical problems 3 ways; #1: by finding the easiest testing points, #2: by testing at points in which multiple tests can be performed, and #3: by going to common failure points.
After looking at the wire diagram on the starting system, I choose to use #2. I saw the starter enable relay as a great place to check for power after the ignition switch, check to see if the anti-theft system was enabling the starter, and I was able to jump power to see if the starter would engage. Testing from here let me divide the circuit into 3 smaller circuits and to also check for the three things this circuit used. Power, ground through the starter and the anti-theft enable. I figured I would also be using my 1st way to diagnose but when I found out that the starter enable relay was under the driver side carpet I knew it wasn’t my 1st way but it was my 3rd way. How many corroded wires and bad grounds have you found under carpet? I sure have found my share.
As I started pulling trim off and started pulling back the carpet I ran in to something not everyone gets to see. This car was just pulled in out 0*F weather and the carpet was completely frozen rock hard. I had to pry back the carpet far enough to get to the relay and connector. I then check the relay for power at terminal 1 and 2. Then I checked to see if the pass-key decoder module was actually trying to ground out the relay, which it was. I finally jumped power from terminal 1 to terminal 4 which engaged the starter. Now I know that the power supply circuit, anti-theft system, and the rest of the starter circuit are all good besides the relay itself all at one location. The problem with this car was that the relay filled with water from the sunroof that was missing and then it froze solid. You think they see that in Arizona! The relay still worked after I thawed it but it still got a new one and off it went.
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