Help! My friend's 87 GT auto engine cooling fan will only cycle on for a few seconds at a time with the A/C on. However, the fan will turn on when the engine temp raises to 235 or so and remains on until the engine temp drops to normal.

What is going on?! It used to run continuously with the A/C on.

The cooling fan can activate in three ways - through the coolant temperature sensor, the a/c control head when it's set on NORM, MAX, or Bi-LEVEL, or when the high side pressure the A/C high pressure line exceeds 280 PSI. You've confirmed that the coolant temperature sensor is working properly. Since the fan is cycling, I'm guessing that the a/c control head is not activating the fan correctly, and instead the fan is only being activated when the high side pressure in the A/C line exceeds the set amount.

Here is the wiring diagram to help explain, courtesy of Bloozberry: https://www.fiero.nl/forum/Forum2/HTML/120108.html

If you notice in that wiring diagram (and I've confirmed this on my Fiero as well) that defrost will activate the compressor, but not turn on the cooling fan like the NORM, MAX, or BI-LEVEL.

[This message has been edited by masospaghetti (edited 07-16-2012).]

I think you're right...it's probably the head unit. We discovered two issues..

1, the fan switch had failed so we would overheat climbing a grade.
2, no cooling fan with the a/c on.

We replaced the fan switch (by the thermostat housing) which fixed the general overheating issue but still could not get the fan to turn on with the a/c. I didn't have my service manual with me so we had to poke around with a test light and jumper to see what was getting power and at what time.

We had ruled out the relay, fan switch and fan. Now we had to figure out if the ECM sends the ground to the fan during a/c on, if the switch at the back of the compressor was working properly or if it was the head unit. I learned here on the forum that the cooling fan does not turn on with the defroster so I assumed that the ECM does not control the fan when it sees the a/c on.

Since the cooling fan is cycling on for 5-10 seconds at a time with the a/c on, we're assuming the pressure switch on the compressor is doing its job. Therefore, we must have a defective head unit. Thankfully I now have my factory service manual to follow step by step. We really appreciate your help, taking the time to point us in a direction. I'll post to let people know what we find.

Bypass the pressure switch on the drier to make sure it is not causing the problem due to intermittent function as it responds to the pressure changes in the drier. Disregard, if that pressure switch doesn't work the compressor should not be energized.

[This message has been edited by Joseph Upson (edited 07-16-2012).]