sx spd c-5

I was wiping down the Vette after a short road trip, when I heard the HVAC fan come on and run for 10 or 15 seconds and shut off again.

The wife reports that it did it again today while she was making a phone call from the church parking lot after service, The car had been sitting for 1.5 hrs so it was not the engine cooling fan.

What is causing this??

sx spd c-5

The key was OFF (actually in my pocket) when I was detailing it. And the wife had the key in the ignition but not on...

The HVAC does not seem to heat as well as I think it should. Had to set the heat up to 73 degrees to feel warmth on a 55 degree day

dgrant3830

There are three fan relays in the engine compartment fuse box. One of those is or could be sticking. Here is the circuit for them, for a '99

dgrant3830

The fan relays are hot all the time and I've proved it at a time in the past. I manually closed the contacts of one of the relays and the fans came on. The schematic above shows that capability, therefore its possible he's got a relay issue or a PCM issue. I agree they should shut off but, I have had mine, one time last summer, I pulled into my garage and the fans continued to run for about a minute, then shut off. Its possible since the PCM is actually still running to whatever point that it is, its looking at the coolant temperture and keeping the fans on. Now if that happened in the winter, I would be thinking that the OP has an overheat or near overheat taking place.

toddk

The blower motor control module also gets constant power, and is susceptible to corrosion. When the relay on it shorts out, one of the failure modes is max fan (key in or out) which can kill the battery. There are also several resistors on the same module for the lower fan speeds. If they fail, the blower motor may not be provided adequate power or may not operate at all at lower speeds. "Full speed" is direct through the relay with no resistance.

This module is located in the passenger footwell wired directly to the blower motor, and can often get submerged when the udders on the firewall are clogged and water backs up (A/C condensate or carwashes). You should check and clean these udders if you haven't lately. Another symptom is a soggy passenger floor mat.

Edit: My bad, didn't notice you have an Automatic HVAC versus manual. My failure mode description above was for manual. However the control module in automatic HVAC is still getting constant power, and can fail this same way. It still is the likely culprit.

Todd

dgrant3830

Oh cr*p!!!!, You are so right. I messed up there thinking it was the radiator fans. Well, I'm not perfect. Sorry.

Bill Curlee

Here is the MODULE that is most likely causing your issue:







You can remove the module and see if its corroded or water logged. Here is one that I disassembled which is very easy:





This module failed and has a burnt circuit trace:





Heres the schematic of a duel zone blower control. As you can see,, the module can power the motor even with the key OFF!!



If you find nothing obvious, just order a new module. I thing AV or Advance have them.

If not contact Gene Culley or Patrick at www.gmpartshouse.com

sx spd c-5

That looks like a WINNER. I will pull the module out and check it out.
And clean the udders on the evaporator too!

I will keep you informed on the sucess