In order for you to test the engine cooling fans using the AC- the AC must be working. That's all the AC, not just the blower inside, but the compressor must be turning also. If you have low refrigerant in the AC system, then the compressor won't turn, and the engine cooling fans won't come on.
The engine cooling fans won't come on due to engine temps until your engine temps are much higher than you would think. That's why we suggest the AC method to test the fans. You can use the electronic odometer trick
to monitor the temps that the temp sensor is reporting to the ECU, and the fans should come on at 115C. All temps reported in there are in C. High speed fans come on at like 118C. There's not much of a spread.
Likewise, if the engine cooling fan doesn't come on, and the blower fan in the cabin doesn't come on, then the AC won't come on- if that AC light isn't lit up, then that control circuit isn't complete. You can still test the engine cooling fans with the AC on normal and the blower on 2 or whatever fan speed works. As Sailor wrote, if the cabin air blower doesn't work on any speed except 4
, then the fix is typically the blower resistor. If the blower doesn't work on any speed, then you should test the fan motor using a simple continuity test, or test for resistance between the pins. If you get resistance, the motor probably works, so replace the switch. I'm not going to get into how to test the switch. 99% of the time, in the situation I mentioned above with no blower speeds working, the culprit is the blower motor itself. If the blower works on some speeds but not others, like in your case it works on 1, 2, but not 3, or 4, then the culprit is the switch itself.
Now, back to your engine cooling fan problem- right? Isn't that what we're supposed to be fixing? You have another thread about pressure testing your cooling system for a leak. You also mentioned a clicking relay under the dash. This relay has been identified before in low voltage situations. Can you use that electronic odometer trick to tell us what your voltage is with the engine off, and then with the engine running, and then with the engine running and the lights on high beam. That might help with some diagnosis. Your whole problem might be a weak battery connection, bad battery, or bad alternator.
Now, once that is done, I can tell you how to force the fans to come on another way, and somewhere in here there is a method of bridging the low speed and high speed circuits so that your fan runs on high speed when it should be on low speed. All that does it make the engine cool off quicker- maybe lose a tiny bit of fuel economy- that's it. Here we get into "I'm not there" like was mentioned by others before. I have to go look up the thread because I don't know the exact wire colors to tell you. If I was there, I could figure it out by looking. I also don't know the exact relays to tell you to go to- if I was there, the information is in your owner's manual.
Anyway, try the stuff I mentioned. Make sure that AC light is on before you check the engine cooling fans. It is entirely possible that you need new fans, however, the problem is typically in the plug-in connector, engine cooling fan resistor, wiring, or something else. You'll want to be thorough because cooling fans cost like $200, but a diode would cost like $5 or less.