'99 Mountaineer 5.0l A/C Electrical issue

[mhosbo]

1999 Mountaineer V8 5.0L 2WD

All fuses and relays have been tested. Dash switch has been replaced. High and low pressure switches have been tested and are working normally. Freon reads 40psi with a 85 degree ambient temp. If it makes a difference at all, the heater core is disconnected.

For some reason, the compressor clutch is not receiving power when the switch is turned on. I started the truck and with the switch to max AC, I hit the clutch wires with a voltmeter and they both read ground.

To temporarily solve this, I have run a power supply wire straight from the auxiliary fuse box, with a manual on/off switch. This works for now, but I have accidentally left the switch on and killed my battery a few times. So, I am finally annoyed enough solve the real issue. Lol.

I'm at wits end and totally clueless as to what direction I should go to fix this. A mechanic wants money up front just to diagnose the problem. If I had money to do that, I probably wouldn't be driving a 99 Merc.

Any help to the resolution of the problem will be greatly appreciated! If more information is needed, I can do more testing and reply with results.

 

[budwich]

well you need to take your "witts" a bit further... good testing but you basically need to "follow the voltage" to "save your money". Look for voltage "somewhere" at the low pressure switch (hopefully coming into it... ). IF it isn't there, you have an issue back thru your switch towards the fuse. IF there is something at the low pressure switch, then you have an issue towards the clutch.

 

[Turdle]

The PCM switches the AC compressor off during heavy throttle. JUst throwing this info out as it may have something to do with the PCM, or throttle position sensor? Somewhere is a wide open throttle relay --

 

[mhosbo]

Voltage was tested at the low pressure switch, and seemed to be working ok. I tested the 10AMP fuse in the battery junction and it is receiving power as well.

The WOT AC cut off relay is in the battery junction box under the hood, and it is working as it should be. I didn't think it would be the TPS, but I'll check it.

 

[budwich]

so you are saying that you "see" voltage coming in AND voltage leaving the low pressure switch. Continue moving your "wits" forward then, next the WAC switch for high pressure, check for voltage coming in (from low pressure switch) and leaving.

 

[mhosbo]

Power to both pressure switches is good.

What is the WAC switch and where is it located? I haven't heard or seen this acronym anywhere.