I have a 1997 old body style diesel F250 and the A/C is broken. So I have not used the blower all summer.

Recently I went to turn it on and found that if the air selector is in any position except off the fan motor is running at full speed and the control **** has no effect.

I looked at the schematic and the variable speed appears to be done by a variable resistance in the ground. So it seems to me like a short to ground somewhere down line. I looked under the hood and moved all wiring with no effect.

I am wondering if anybody knows of a likely place for this presumed ground to be or if I should look elsewhere.

Thanks

 

I'd check the resistor.

 

X2
Classic symptoms of a bad blower motor resistor....hi speed only.

 

The difference is it is high speed in all positions. If it was the resistor according to the drawing it would be high speed in high position only and no blower otherwise....

 

The classic symptom is when only the high position on the selector does anything. OP's problem is that all the positions cause the blower to run on high speed.

OP, if you have a factory schematic, the grounds are labeled with a Gxxx number. Elsewhere in your manual you should see an overview of where each of those ground points are.

I would start by checking at the selector switch. See that you have one wire that has a clean low resistance path to ground, then check the others to see if each one has a different resistance to ground. If so, then check the switch to make sure it's switching. If not, then go to the resistor and see what's going on there.

 

The circuit is very basic:






My guess would be a shorted blower motor resistor (rare) or a short between the Ac/Heater control switch/blower motor and ground. Check the Orange/Black wire.

 

Found it--grounded sheet metal screw into wiring harness.

It was much easier to type the above than to actually find it.

I am glad it is over now is all I can say.

Thanks for the help.

Birken