1999 kw t800 a/c fan problem

Replace the resistor next to the fan motor, watch fan motor temp. after the res. is replaced to be sure its not over/heating and pop breaker?

 

Check the fan switch, I had a similar problem in one of my Freightliners it would also only run on high.

 

I had the same problem (blower motor working intermittently) in a late 90's T800 I drove. Although the resistor is a good item to check (sorry I don't have any ohms spec on it), I found a bad resistor usually doesn't give an "intermittent" problem. In my experiences, it either failed completely (open circuit) or not.
The fan switch is a possibility for being faulty, since it won't run on speeds other than high. In my case, it turned out to be severely corroded terminals & brushes in the blower motor itself. But my blower would stop, intermittently, regardless of the speed setting.

Let us know what it turns out to be.

 

On a 2001 t600 had similar problem every time ac clutch kicked in the power draw would exceed resettable breaker and fan would shut off ... After 2 dash switches and 2 different fan blowers and a few other things which I can't remember the mechanic eventually set fan up on a relay which solved problem .. He had a tester which he put in place of a fuse which read draw thru the breaker after relay was installed draw never got close to breakers capacity . Probably not the correct fix but after a few thousand dollars least it was working and wiring under dash wasn't pushed to max limits. The relay was on the high fan setting low and medium were thru switch like normal

 

There is a resistor on passengers side open door look where door hinges there's one in the dash there two small screws I think 10mm

 

My 03 w9 is dooing this also help!

 

Check the resister that Alberta mechanic is talking about. I'm not exactly sure but if the fan will only blow on high it should read out of limits on the resistor. Reason for only blowing on high is that' the highest fan speed doesn't run through the resistor.

 

It blows but will shut off on its own then start up again I just put the new blower motor two weeks ago it's driving me nuts