engine fan RPM drops to nearly zero

[Dustin Gebhardt]

I've been trying to diagnose and fix poorly performing AC while at idle and stationary. Thinking that there should be an audible difference in the viscous-clutch fan with the AC on or off, and not hearing one (even though ForScan shows an increase in fan RPM and command percentage), I decided to replace the fan clutch. I figured that it was likely the original and at 300k+ miles, perhaps it just wasn't functioning like a new one. I bought an SDK clutch from RockAuto and installed it. I also replaced a defective evaporator temperature sensor (which I had jumpered several years ago to bypass it) and a blower motor resistor (the blower fan speed was stuck at 100%, but this just started happening this week). I doubt those last two pieces are relevant, but who knows. After the clutch was replaced, a 10-minute driveway test went fine. I took it for an extended test drive through town and roughly 20 minutes into the test, my temperature gauge shot up and I got an engine overheat warning on the dash. I pulled over and checked under the hood. The fan was barely spinning. The AC had also kicked off. I took it home and hooked up ForScan, but by then the fan was back up and running again. So I let it idle in the driveway. Again, roughly 20-30 minutes later, I heard the AC compressor kick off. I checked ForScan and it showed that the fan command percent was still >20%, but the fan RPMs were almost zero (they normally were in the 700RPM range at idle). Cylinder head temperature was also climbing quickly. AC high pressure line was still high and the compressor was trying to kick in every minute or two, but only for a second. This is the ForScan o-scope page after shutting the Expy down and restarting it, thinking that this might restart the fan, but it didn't work.



I'm assuming that the new clutch fan is bad. Any other reasonable conclusions?

 

[twodollars]

Does the fan drive you replaced have an electrical connector goimg to it, or just the thermostatic coil on the front? If there's no connector to it it only operates in reaction to being heated by hot air flowing over it from the cool pack. Is there an electric fan on the cool pack somewhere? If so, that's likely what the pcm controls. What you describe in your test drive makes me think you have air in your cooling system, a bad t stat, or some type of restriction to air flow through the cool pack. Have you had the cooling system opened up lately?