Cooling fan only works when ac is on

HI. I have a very thorough, step by step procedure that will help troubleshoot this issue. follow carefully, to isolate the problem, Use extreme caution when preforming this inspection procedure.


Step1
Check for broken wires or loose connectors around the fan circuit. Inspect connectors at the fan motor, relay, sensor or heat sensitive switch, and the Electronic Control Module (ECM)--your car’s computer control system. Also, make sure to check for a possible blown fan fuse. These are common and overlooked troublesome spots that may cause a fan to fail.

Step2
Run and bring the engine to warm temperature. With the engine running, use a voltage test light to check for power to the motor fan. Be extra careful and make sure to keep your hands and tools away from the belt, fan or any other engine moving parts. If voltage is reaching the fan motor, the test light should glow.

Step3
Turn off the engine after you see the light glow. Apply direct voltage to the fan motor from your car battery using a pair of spare wires. If the fan fails to operate, replace the fan motor. If the motor operates, your problem is in the motor connector.

Step4
Locate the heat-sensitive switch or heating sensor if the fan motor operates with direct voltage and the test light did not glow. You should find the sensor in the radiator, engine block, or thermostat housing.

Step5
Measure the resistance across the heating sensor with the multimeter. With the engine at cool temperature (engine off), it should register infinite resistance; with the engine at warm temperature (engine off), you should read low resistance. If both readings state infinite resistance install a new heating sensor, that’s the cause of your failing fan.

Step6
Check the action of the fan relay if the heating sensor is registering variable resistance. Your service manual should specify the power and ground wires according to color codes and the proper way to test it. If the fan relay fails the test replace it with a new one.

Step7
Check the connections going to the ECM after you determine the fan relay is working properly. If you find broken wires or loose connectors, make the necessary repairs. If you suspect a defective ECM take your vehicle to a service shop for a computer analysis. In most cases, the ECM is rare to be the cause of a failing fan. The above steps should take you to the root cause of your problem and help you fix the cooler fan.

You will need to check the fan relay, fuse and temp sending until The fan is turned on automatically when you turn to ac and defrost. The rest of the time it is controlled by pressure. I doubt it is the fuse because it is working on ac. Good news is that you can still get heat just by leaving it on ac but putting your temp all the way up until you find the problem. (assuming you are in an area like I am where it is getting coooolddd out there.

Hoe this helps.