I have a 99 F350 7.3 diesel and the A/C clutch wont disengage.I am looking for advice on how to find the problem I know nothing about the ac system.Thanks.

 

The compressor is engaged when ever the engine is on?
It would be electrical. Could be a relay or the mode switch. You'll have to trace it.
Get a Service manual, or at least a HAYNES repair manual for your truck, Haynes have electrical diagrams in them.

 

I have the same problem with mine now. At least, I think it's a problem. This is the first truck with A/C I have ever owned, and I've only been driving it for a month. But it doesn't make sense that the A/C compressor is engaged when the temp control **** is set too hot!

I see you also have the 6637 filter. On mine, the air intake sensor is just tucked in between two relay boxes on the fender well. Not sure if that is a related issue, or not. I don't think so. I don't know if the A/C sees this sensor or not. The owner's manual states that the A/C will only function if the outside temp is 43* or higher. But I don't think this is the sensor it gets data from??? Either way, that's not our problem... A/C blows cold air just fine.

The owner's manual states that the air conditioner can operate in all modes except panel (shows drawing of arrow blowing in face) and floor (arrow blowing on feet). My compressor clutch engages on all settings exept these two (and "off"). No matter that the mode selector switch is not set to the "A/C" or "MAX A/C" setting- the A/C compressor will still engage- even when the temp control is on hot. The air gap at the compressor wheel/clutch face in the "off, panel and feet" selector position on mine is at 0.020, which should be plenty. In otherwords, the compressor is not binding up and is does disengage on these settings it is not supposed to work on. See if your's will disengage in any of those settings. If so, at least you can stop the compressor from running all the time.

My question: is the compressor supposed to be engaged in all settings except panel, off and feet???? If yes, then this seems like an incredibly poor design, robbing power and wearing the compressor unneccesarily.

If I want outside air just to vent in on say the defroster/feet function and the outside air temp is a cool dry 68*, then the compressor will still be turning/engaged, even though it's not needed to cool or dry the air??? Makes no sense to me!

 

That's how Fords have been made for many years. You could always add a manual clutch control switch.

There is no sensor that directly measures ambient temperature and then controls the compressor clutch. In general, R134a contracts enough at a point somewhere between 40 and 45° st that the pressure inside the system drops below the cycling switch set-point (around 45 PSI). When the set-point is reached, the switch opens and disables the compressor clutch until the system temperature rises enough to increase the pressure, re-enabling the system.

 

sho89- thanks for the reply. A/C operation is new to me, never had one before. The compressor always engaged (except the settings mentioned above) just seemed strange to me. Thought it might damage the system, as Springerpop mentioned in the air gap clearance post, but that was a different circumstance.

I have an owner's manual, but of course, it only tells 1/10th the story. I need a big boy service manual, so I guess I should pony up. thanks-