Yes, but the compressor isn't running all of the time. Maybe 20% of the time. When the compressor turns on, the engine fan kicks in.

No Allan, not even once! I even thought that something wrong with AC at first! :shock:

I've had this happen with one of our fire trucks. I took it to have a service done and also the ac charged. It was hot out that day and on the way back I had the ac on and after a while I noticed the fan was running non stop which it had never done before. I took it back and they said that they over charged it so when the system warmed up to normal it was showing high pressure in the lines which tells it that the system was overheating thus causing the fan to run all the time. They recharged the system with the proper pressure and it has been fine ever since. 8) 8)

replaced a 'micro-switch' in the dash, a little black bugger behind the ac control panel that has a 'clicker' and two prongs. now, 95% sure that fire is being sent from the panel to ... the breaker? all the breakers are old and have no labels. looking for a breaker diagram online. ac clutch still not engaging. sending fire from panel ... to wherever ... but no fire is coming out of the firewall into the ac dryer where the hi-pressure switch plugs in.

In your previous post you mentioned powering the clutch directly......if you do that for a few seconds does the small line get hotter & the big line get colder?.....if not you've lost the freon in the system.

The a/c only needs the fan on when there is low air flow over the condenser. At road speeds the fan doesn't need to run for the a/c to work. It operates independent of the compressor clutch.

i powered it directly right after my friend put some r134a into the system. he said there was some in there, and that it didn't need much. i haven't noticed any leaking, but i might have to color it or somethin to tell.

still having problems in the dash. behind the ac control panel is an air/electric module in which a 'micro-switch' is housed. the switch has two plugs and a lever. one plug is for the 'hot' coming in, at all times during which the ignition is on. when you move the lever on the dash panel to max ac, ac or defrost it depresses the lever on this little switch and the hot current moves through the switch to the other plug, going out ... wherever, i dont know. to and out the firewall i think. when i install a new switch i put it in and connect the hot coming in. turn on the ignition and depress the lever on the switch manually, check the other end of the switch with a test light to make sure the current is being sent through the switch. ok. then i plug in the wire that is supposed to receive this current from the switch and send the signal out the firewall. the switch breaks everytime i try to engage it properly. it's acting like a fuse, and not only is it not sending the signal, but it's 'blowing' everytime i try to engage it. if i try to bypass the switch, and just connect the two wires ... it blows a breaker below the dash.

before i had any kind of problem with my ac, i posted about my air horn. i have a 2000, frtlner classic. i removed the headliner and the overhead dash panel (with the speakers and storage panels) to access and replace the air horn valve. next day noticed that the voltage on my batteries was dropping more than usual when i turned the truck off, and then truck was 'dragging' to start again. when truck was off, one battery was warm like it was being used by somethin in the truck, and it was also slightly mishapen. but all lights were off, no plug-ins, no refrigerator, etc. next day, truck wont start, jumped it went to work. that evening replaced mishapen battery and one next to it, now all three batteries are less than month old. few days later, experiencing problem with ac, and clutch not engaging.

is there something that i could have disturbed, when messing with the horn in the cab or replacing the batteries, that would cause the current for the ac circuit to change? for it to blow a breaker, or keep blowing the micro-switch? maybe a hot wire, for an interior or sleeper light or somethin, is grounding or touching some metal in the cab? would this cause the batteries to drain, or problem with electricity the ac controls use?

i just dont understand, why, all of a sudden after four years of service, this switch in the ac panel blows like a fuse and continues to blow the replacement switchs. i dont understand what about the truck changed, to cause this reaction.

I'm not familiar with Freightliners wiring. my guess would be that the switch controls a relay.............till you get it figured out try using an inline fuse holder as a jumper.

It sounds to me like that is the switch that kicks in the A/C compressor clutch.

It also sounds to me that there is a dead short circuit somewhere in the wire from that switch to the relay that actuates the A/C clutch.

Be glad that the switch kicks out or the circuit breaker trips when you're trying to "hotwire" the circuit, otherwise you'd be seeing smoke and flames.

Find the short circuit and fix it.

Plugging in more switches and watching them fail isn't going to solve the problem.