Stumped! Cooling Fan Won't Turn On!



I have a 2000 Dodge Grand Caravan that has 195,000 miles. My radiator fan won’t turn on and I’m stumped!

The issue presented itself with a check engine light and the radiator cooling fan not turning on. The trouble code indicated the cooling fan relay was faulty. In all cases below, I tested the radiator fan by turning on the air conditioning (thus forcing the cooling fans to run) and also for it to come up to a high temperature coming close to the danger zone.

1. Fuse - first tested the fuse and it was OK. Had to start with simple things first.

2. Cooling fan relay - I replaced the relay which has a reputation for failing on the Dodge. Still the fan did not kick in.

3. Cooling fans - tested the cooling fans and the motor was seized. Replaced the cooling fans and tested them by jumping them. The cooling fans still do not turn on. There is no voltage leaving the cooling fan relay and coming into the cooling fan harness.

4. Wiring - tested the wiring between the relay and the fans. They checked out OK (good continuity and resistance check). There is no voltage leaving the cooling fan relay. Now it’s getting more complicated… I unplugged the wiring harness from the ECM and tested the wiring between the ECM and the cooling fan relay. A-OK.

5. ECM - it is unclear how to check the ECM for functionality of the cooling fan. There are 3 wires (supply, ground, and switch wire?) The supply voltage and ground is good, but I don’t know what voltage the switch wire should be. I’ve read that it is pulse width modulated so I can’t measure this with a simple voltmeter. I’ve also read that the switch wire goes to ground thus triggering the relay. I don’t know which is correct.

Several mechanics I talked to were skeptical that the issue is with the ECM, they knew of very few instances of this being the culprit. I even replaced the new cooling fan relay with another new cooling fan relay on the off chance that the replacement was faulty. Also because I was running out of ideas!

Did I miss something in my logic and diagnostics? I’ve run out of ideas and have resorted to wiring a switch directly to the cooling fans and toggling them on and off. Any help in diagnosing this problem is greatly appreciated! Thanks!


What about the temp sender that tells the ECM what the coolant temp is ?


Looking at a diagram for the fan circuit it is a pretty simple design and your testing proceedures look good to me. The relay switches power to the fans and the coil is controlled by the ECM like you said. I doubt very much the control voltage to the relay coil is pulsed. It either will go to ground and turn on the fans or supply power to the coil and turn them on. If so then try placing a ground connection on the Lt grn/dk blu wire that goes to the ECU and verify that the fans turn on. If they work ok then the ECU isn’t turning the fans on for some reason. It may be defective. If it is you can purchase a rebuilt one for a lot less cost over a new one.

D-C brings up a good point about the coolant sensor. If that isn’t working correctly that could be the real trouble. If is working then the ECU would appear to be the problem.


My 2002 had in addition to the relay, a fan control module that was mounted on top of the steel front bumper,behind the bumper cover and next to the radiator on the drivers side. I don’t know if the 2000 has this but you can look. I could see mine by removing the black plastic cover between the grill and radiator.


Having an older car my first thought was “thermostat.”