1985 Chevrolet Cavalier • 102,000 miles

Fan will not come on. Every componet in circuit has been replaced including ECM. All wires to fan, censors, relays, checked back to ECM.
January 9, 2013.

There's a couple of ways to approach this kind of problem. One way is to bypass the relay. The relay is two circuits; the low-current control circuit run by the Engine Computer and the high-current fan circuit. If you pop the cover off the relay and squeeze the contact, the fan should run. If it does, you can forget about the entire high-current circuit and its fuse or fuse wire, the fan motor, and the motor's ground wire. The problem is in the control circuit.

You can also unplug the relay and use a piece of wire to jump the high-current terminals. Those will be the two with the fatter wires. If the fan does not run, test those two terminals for voltage. One must have 12 volts all the time. If one does, leave the jumper wire in place, then test for that voltage at the fan motor's connector. If it's there but the fan still doesn't run, you have a bad motor, a break in the ground wire, or one of those terminals is stretched.

When you've thrown all kinds of parts at it and logic says everything is okay, but the circuit still doesn't work, there are two common things to look for. One is a stretched female terminal in a connector. If it's not making a good connection, you won't see that when you unplug it to take resistance or voltage readings. The second thing only pertains to high-current circuits protected by fuse link wires. If you find 12 volts feeding one terminal at the relay socket, test it again after you connect the jumper wire. If the 12 volts disappeared, the fuse link is burned open and you're getting a misleading false 12 volts through the carbon track left behind inside the insulation after the wire burned open.

The second way to approach this is to use a scanner to activate the fan circuit. Make selections on the menus and sub-menus until you see "Radiator Fan Relay". The scanner will communicate with the Engine Computer and ask it to cycle the relay on and off about once per second. If the relay clicks on and off, you know that part of he circuit is good and the computer has control of the relay. If the fan doesn't run, you can make voltage tests to find the break in the circuit. If the fan does turn on and off, the high-current circuit is also working, and that just leaves the sensor circuit.

Jan 9, 2013.