ajs37

The cabin fan in my 1991-C2 continues to run with the ignition turned off. The fan speed is not affected by the fan switch unless the ignition is on. I had to remove fuse #1 to get it to stop. This is not the tiny fan that circulates air around the A/C controls which is on a delay switch.

Randall G.

Hello ajs37,

Well, the bad news is there's no easy-to-replace relay powering the interior fan. The interior fan power supply--as you've learned--is fused directly from "always hot" Term. 30.

Fan speed is controlled by the "blower final stage," which I understand to be nothing but a big rheostat. The final stage is "downstream" of the fused, 12V power supply and the interior fan motors. When the ignition is off, the fan power supply is isolated from ground. With the ignition on, the final stage controls the motor current by varying resistance between the motors and ground.

So ... it looks like either the CCU is "telling" the final stage to run the motor(s) with the ignition off, or the final stage has failed in a current passing condition.

I'm looking at the wiring diagram for the final stage. Here's how I think it operates:

-Pin #1 from left hand blower
-Pin #6 from right hand blower
-Pin #5 to G31 of the CCU = RH blower control
-Pin #8 to G2 of the CCU = diagnostic?
-Pin #3 to G34 of the CCU = LH blower control
-Pin #4 to G3 of the CCU = diagnostic
-Pin #2 to ground
-Pin #7 to ground

--12V power from always hot Term. 30 passes through fuse #1;
--to the motors;
--goes out the motors to pins #1/#6 of the final stage;
--input from CCU pins #31/34, at pins #3/5, determines how much resistance to place between pins #1/6 & #2/7;
--current flows through the final stage to ground (pins #2/7), its value being determined by the signals from the CCU.

So, if you measure resistance between pins #1/#6 & #2/7, and find a low value, this would indicate that the final stage has failed in a always-on position for at least one motor. (I'm assuming the final stage is disconnected from the CCU, and the default/high resistance control voltage is 0 volts.)

I've never put my paws on it (that I can recall), but the shop manual says the blower final stage is located about even with the base of the windshield, just forward of the windshield, just right of center in the luggage compartment.

Alternatively, I suspect (but don't know for certain) that the CCU tells the final stage what resistance it should create through a voltage signal. So .... you could measure voltages being supplied to pins #3/5 (from G31 & G34 of the CCU) with the ignition on, then measure again with the ignition off to see if it changes.

Best of luck!

Randall G.

One more thought. I see in your signature that you live in Orange. You're welcome to stop by, and we can install my spare CCU in your car long enough to see if your interior fan quits running with the ignition off. If not, then your problem has to be with the final stage.

John Boggiano

Just a thought -

You're not talking about the tiny fan on the back of the HVAC control unit that draws air over the cabin-temperature sensor, are you?

That fan runs for about 20 minutes after you turn off the ignition.

Randall G.

Hey John,

You'll find the answer to your question on the last line of ajs37's initial post. <img border="0" title="" alt="[Wink]" src="wink.gif" />

John Boggiano

</font><blockquote><font size="1" face="Verdana,Tahoma,Helvetica">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Helvetica">Originally posted by Randall G.:
<strong>Hey John,

You'll find the answer to your question on the last line of ajs37's initial post. <img border="0" title="" alt="[Wink]" src="wink.gif" /> </strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">LOL - I swear that guy must have edited that in after I replied!