beltrunner

Here's the deal on my '93 LT1...the primary cooling fan works as it should. #1 fan comes on when A/C is turned on... or with A/C off, the fan comes on when coolant temp is about 227 degrees...cycles off when temp drops to 221 degrees.But the secondary fan will not come on either when A/C is turned on, or when coolant temp gets over 236 degrees.My actron scanner lets me cycle both fans on & off, so I know the secondary fan motor & relay are good.I also switched relays just to be sure, to no effect.With motor running and A/C on, the primary fan is on...but when I unplug it (it stops,of course) & then immediately plug it back in, the secondary fan then comes on and runs until the A/C is turned off...then the secondary fan will not come on again as it should.Any ideas on this? Thanks in advance.
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Injuneer

I assume you are reading the ECT sensor with the scanner, and you are seeing a temp in excess of 235*F, which is the programmed "on" point for fan #2. When you unplug fan #1, it sets a code. When the code sets, the ECM turns on fan #2. Minimum on time is 30 seconds. The establishes that the ECM can control fan #2, as does turning it on with the scanner.

Do you have a stock chip in the ECM?

The factory manual indicates that A/C head pressure above 248 psi will turn on fan #1 and fan #2. How does that figure compare to what you are seeing on the scanner?

What codes are set?

The factory manual also indicates the fans are controlled based on inputs from the VSS, but does not identify the criteria that activate or turns off the fans. Is the VSS working correctly?

If you do not have access to a factory manual, I can give you a link to a free download, or I could send you a .pdf of the diagnostic section for the cooling fans.

beltrunner

Thanks for the quick reply, Injuneer. My daughter drives this car now, so I kinda try to fix my kid's car as quickly as possible (whereas I sometimes put up with broken stuff on my own car for awhile because...well, you know the thousand reasons). As far as I know the VSS system is working okay. And my little Actron scanner does show coolant temps, and will activate both fans with the key-on-engine-off only. The car was fine until yesterday, when the temp gauge went all the way into the red, and the "check gauges" light came on...and then a frantic cell phone call from my kid, "Dad,what should I do?"...the A/C was on at the time, and the primary fan was on...she couldn't tell if the secondary fan was on. I told her to shut the A/C off, and the gauge slowly came out of the red. When she got home, the primary fan was on, and secondary fan was off...temp gauge still almost at red (not usual for this car)...there was no voltage going to secondary fan...when I unplugged primary fan, the secondary fan came on (crap, for a second I thought I had fixed it). I swapped fan relays & both seemed okay...#2 fan still would not activate. I did not check A/C head pressure, but cold air was blowing. I only have a Haynes manual for this car.Any info you could give me would be much appreciated.

beltrunner

And the the ECM is stock...

beltrunner

So now it seems to be "fixed"...but I really didn't fix anything (I don't think)...the coolant temp sensor passed all listed tests...then, with the A/C on, the evap temp was 48 degrees(duct temp 55 degrees...ambient temp 85 with 60% humidity)...high side pressure at 245lbs...coolant temp at 230 degrees...#1 fan on...#2 fan off...I added 6 oz. refrigerant to system...high side went up to 255 degrees...evap temp down to 37 degrees (duct temp down to 44 degrees)...coolant temp at 230 degrees...and #2 fan turns on! I turned off A/C and #2 fan turns off...I blocked air flow to radiator to raise coolant to 240 degrees, and #2 fan turns on...when coolant temp dropped to 230 degrees, #2 fan turned off...I turned on A/C and when high side pressure reached 253lbs, #2 fan turned on...the system seems to be working okay for now...before, #2 fan would not come on regardless of coolant temp or A/C being on or off...I see that my A/C pressure might not have been high enough originally, but #2 fan still should have come on when the coolant temp got high enough...go figure...I just hope that it's not some intermittent ECM problem...thanks for all the help...my kid's back on the road!1