I had the same issue with only one radiator fan working full speed for a couple of months until eventually they both went and I had to replace the whole assembly. I have since installed 3 separate fan assemblies with the following results:
1). Ordered the Dorman from Amazon and installed using the procedure in this thread (I have the factory tow package). Worked great in the garage and then the next day I noticed the temperature gauge rising again and the fans weren't on. Took it to a mechanic and was told there was a loose connection in the module. They repaired it and it worked for another half day before burning out again. I decided to return it and buy another assembly locally so I could return it easier if needed.
2). Bought the VDO (Murray) FA70600 from O'Reilly and the fans were operating with the AC compressor and/or when the engine came up to temperature. The only problem was that when they did come on, they'd wind up to full speed and were extremely noisy and caused a lot of vibration. I ordered another unit (same brand) and waited for it to arrive so I could take it in and do a warranty exchange. I figured the speed problem was something else that I'd have to figure out but I wanted to exchange it for the vibration issues anyway since the fans didn't seem to be properly balanced.
3). Installed 3rd unit last night (Murray FA70600) and the fans immediately came on with the AC compressor at a much more reasonable speed. However I was concerned that they seemed to be cycling too often (about 10 seconds on and 20 off). The AC compressor was cycling at a normal rate it seemed...at least not as often as the fans were. I had a 12 minute drive to work this morning with the AC off and when I arrived I made sure I could hear the fans running before shutting off the car. During lunch I went out and tested it again with the AC; cycled 10 on and 20 off which wasn't enough to keep the AC compressor cool so it was blowing warm air the whole time. I still need to test the cycle times without the AC to see if it does the same thing. The engine didn't overheat on the drive to work but I didn't hit that many lights so I'll have to test it out later tonight.
What would cause #2 and #3 to behave like this? My initial thought is that I've just gotten unlucky every time in receiving fans with bad modules, but I don't know if I should be looking at or replacing any other sensors that would cause this. I'm just thinking if it were something else causing it, then the most recent fan assembly would have behaved the same way and ran full blast. Any thoughts?
