Electrical issues can be hard to diagnose, especially when standard testing doesn’t reveal the problem. Sometimes you have to go inch by inch through the wiring to find the cause. SureTrack community member Mlopez2008 had a Mercedes-Benz CLK320 in their shop with a message on the dash and the cooling fan running any time the key was on. They posed the question to the community, and with the help of fellow community member Richard_C and others, they were able to resolve the issue quickly.
The following Community Fix summarizes the interaction between members of the community that led to the solution.
2005 Mercedes-Benz CLK320 3.2L
Cooling fan remains on, repaired wiring harness.
Customer states that “Coolant Service” message is displayed on instrument panel.
Diagnostic Steps Performed/Parts Replaced
Confirmed message on instrument panel. Found that the cooling fan is on full high any time the key is on. No blown fuses found. Power and ground are present. Don’t have probe to test for pulse from ECM. Got an aftermarket fan from Import Direct (Continental). Hooked it up, and the fan didn’t come on with the key, which at first I thought meant it was fixed. I attempted to activate the fan with scan tool. When doing so the fan never came on. Mercedes doesn’t give much for description and operation. I’m starting to believe there may be an ECM issue. Has anyone seen this concern?
Looking through the wiring diagram I checked the wires for continuity. Probably something I should have done earlier. In any case, connector 4, Pin 39 Green/Blue at ECM to fan connector pin 4 Green/Blue had no continuity. I followed the harness back from the fan and just after the plastic insulator for the harness it tees off. The Green/blue wire was chaffed and corroded about a foot and a half back from connector. Just inside the wheel well.
Repaired the wiring harness by replacing corroded and chafed section. Fan now works as it should.
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