[heavyhaulerss]

my fan comes on when i hit a jarring bump in the road. then goes off 15--20 sec's later. also after my truck is at operating temp, when taking off from a stop my fan will kick on until about 4th-5th gear & then goes off. i do not use the a/c. this just started 6-7 day's ago. i replaced the selenoid fan switch. that didnt change a thing. i have a manual fan switch on dash, but i never heard of one going out in the on position. & if it did the fan would run constantly. if it's a short in a wire, then wouldnt the fan stay on also. bottom line is it going on by itself & going off by itself, without me running a/c or hitting switch. ????? :?

[Kranky]

You've got a loose or intermittent connection somewhere in the fan control circuit.

Whenever the circuit loses power due to this intermittent connection, the fan comes on. That's the default mode: no power= fan on, also no air pressure to the solenoid operated air valve that controls the fan clutch = fan on.

[heavyhaulerss]

could this problem be caused by a bad high/low pressure switch? the fact that it also comes on without hitting any bumps while taking off leads me to beleive it's more or something other than a intermittant connect/disconnect.

[Kranky]

Originally Posted by heavyhaulerss

could this problem be caused by a bad high/low pressure switch? the fact that it also comes on without hitting any bumps while taking off leads me to beleive it's more or something other than a intermittant connect/disconnect.

The pressure switch on the A/C receiver/dryer is also part of the fan control circuit, so that is a possibility.

Also check the wiring and connections to that switch.

[silvan]

Originally Posted by Splitshifter

The pressure switch on the A/C receiver/dryer is also part of the fan control circuit, so that is a possibility.

Big possibility, now that you mention it. One truck I used to drive, this thing would get horribly corroded after a winter's driving, to the point where the little connector lugs on the switch would just turn to green powder and disappear.

[heavyhaulerss]

I found out why fan was coming on. the wire at the high pressure switch on a/c compressor was broke & barely making contact by a few wire strands. new switch- $56.00.