ghack

I have a 99 grand Cherokee with a fan that doesn't come on. I read a troubleshooting tutorial that made sense, but the symptoms don't. I measured fan resistance at 1.6 ohms. The fan works if I power it directly. I have four wires to the relay, two big and two small. As I understand it, the gray(big)wire is the 12 volt power source which I confirmed w/ a voltmeter. The black(small)is ground for the switching circuit. The blue/purple(other small) is the power for the switching circuit. The green(other big) is the output for the switched circuit. I checked and got proper grounding on the black wire. To test, I put 12 volts to the blue/purple wire, but only got 1.5 volts on the big green wire. I suspected a bad relay, so I bought a new one, and got the same results. Any thoughts as to what the problem could be? Could the new one be bad also. I bought the better Borg-Warner relay with the lifetime warranty.

ghack

I was reading the test wrong. I was measuring the voltage between the blue/purple wire and ground. The test is supposed to be between the blue/purple wire and the positive terminal(full 12volts). The voltage should be 0 when cool, and 12volts when hot(or when the A/C compressor starts). I still have an issue, but hopefully, this will help pinpoint it better.
So I'm guessing, as it heats up, the pcm starts to reduce resistance to ground, so when it allows the full 12v, the relay closes and sends 12v out to the fan.

ghack

I found the old relay didn't work, the new relay works, but doesn't get signal from the PCM. I tested the relay by grounding the 3rd wire(blue/purple) and the fan comes on. If the PCM isn't sending a signal,do I need to replace it, or is there a reset? When I hooked it back up with the 3rd wire going to the pcm and not direct to ground,the voltage to the fan started at 12v, but kept dropping off. The fan never came on. Not sure of the significance of that.