Originally Posted by CINNEMAN

When is the top cooling fan on the passenger side supposed to engage? I thought when the AC is on both fans should be running. i have yet to see that fan turn on at all. when i turn the ac on, only the lower high speed fan turns on. Any input? Car is a 98 coup a4.

Originally Posted by 8VETTE7

One of us is mistaken I thought the OP meant the fans that cool coolant for the engine cooling and you thought he meant the interior fans????

Originally Posted by 8VETTE7

BOTH fans are supposed to runint low speed mode and high speed mode.

The fans are wired in series for low speed mode so that about 1/2 of battery voltage is dropped across each motor hence a lower speed of operation. When high speed mode is commanded the relays in the circuit (below) change the wiring configuration to parallel mode so that battery voltage is across both fans and they run in high speed.

Factory settings for the low speed and high speed modes are as follows:

When the coolant temp reaches 226 degrees both fans come on in low speed mode. When the coolant temp drops to 219 degrees the fans are commanded off. If the coolant temp rises to 235 degrees then the fans are commanded on in high speed mode. When the coolant temp drops to 226 degrees the fans revert to low speed mode. If the AC is turned on then the fans are commanded on in low speed mode at 185 degrees. They remain on in low speed mode if coolant temp rises until the coolant reaches 235 degrees. At 235 they are commanded into high speed mode. When the coolant drops to 226 the fans revert to low speed mode for AC on. ( ie the fans remain on until coolant temp reaches 178 degrees)

In all cases if the vehicle speed exceeds 35 MPH the fans are commanded off irrespective of coolant temp. GM apparently felt that air through the fans at speeds above 35 mph with fans free wheeling was superior to fans operating.

Here is the wiring diagram:




You can start by checking the 3 fuses indicated but if your passenger fan is not operating in low speed mode or high speed mode, you really need to inspect the fan. Blade may be missing or jammed or the motor electrically bad.

As an FYI it is possible to reprogram the turn on temps for both low and high speed modes. The fans were not designed for continuous operation so it is prudent to select temps that prevent continuous operation at speed under 35.....