Alright, I'm kinda stumped. I'm awful with electrical and I hate it very much.

Here's the situation.

01 Ram, 5.9 gas

New AC Compressor (which I just installed today) is not engaging and the low side will not take any more R134a. Low side pressure is around 90, high side around 50.

Jumped the low side switch...nothing

Swapped relays around...nothing

Checked the horn fuse (some people have reported the horn fuse making their ac compressor not run)...nothing.

I have power to the relay in the junction box, but no power to the compressor connector. If I jump the compressor off the battery it will engage, so I didn't get a dud.

Now I can't remember, when you're sitting in the cab and you turn the switch from OFF to MAX AC, should you hear some sort of click? I don't get one, but I can't remember if it used to click or not.

I honestly don't know what to do next. It's been in the 90's and humid as **** for the last week and without ac I'm one grouchy mother ****er...the fact that I dropped $150 on a new denso compressor(following service manual troubleshooting) and that didn't fix the problem is making me much more grouchy.

 

Need to have the controls set to some variety of A/C, and the fan on.

HVAC controls send a voltage to the low pressure switch, then to the high pressure switch, and from there to the PCM. If the PCM actually sees voltage from the switches (C3-22), AND the a/c request circuit (C3-23) it will ground circuit C13, which is the ground side of the a/c relay control circuit.

So, make sure you are getting at least 5 volts thru the pressure switches, and that you are getting power TO the a/c relay. (fuse 11 in the fuse block on the side of the dash is control circuit supply, Fuse J in the PDC supplies power via the relay to the compressor.)

 

AC has been set to max, fan speed to max, and temp to max cold the whole time (other than when I was fiddling with the switch to listen for clicks).

I have 12 volts at the high pressure switch.

I took my multimeter and poked around in a couple spots in the ac relay socket and got 12v, so that theoretically means the switches are good then since it gets power after them? Like I said, I don't understand electrical...

Fuse 11 in my dash fuse box says ignition. I don't see anything that says control circuit supply in either box.

 

It's a multipurpose fuse, so, if it's good, it's good.

Relay gets power from two places, fuse 11 is for the control circuit, Fuse J in the PDC (under the hood) is power for the compressor. Should have power in two places in the relay socket.....

The circuit for the pressure switches comes from the HVAC controls in the dash, goes thru both switches, and then to the PCM. No relay involved. Might wanna check and see if you are getting 12 volts at the PCM. (connector 3, pin 22, Brown wire.) and also pin 22, (same connector, light green with white trace)

If you have 12 volts at both of those pins, then something on the relay side is giving you grief.

 

Pulled it and it looks good. Didn't test it.

I'll check for power in multiple relay spots tomorrow, it just started storming.

I don't see a fuse J, but there is a fuse labeled AC something or other (10amp) and it's fine. I even swapped it with another and that didn't change anything.

I'll also check the PCM tomorrow.

 

Was driving home from the gym today pondering this issue and though...why don't I just bypass the high pressure switch as a test.

Bypassed it, and compressor fired right up. Didn't run it like for any longer than it took to do the test, but it verified for me that my high pressure switch is the issue. Got a new one and I'll put it in tonight after work.

 

Dear lord...I ****ing hate this thing.

New switch...nothing. Jump the high side switch again...nothing.

Check voltage, have 13.5 volts at the compressor but won't engage.

Pulled all the PCM connectors out and inspected (looked fine). Plugged them back in.....fire it up....and it works.

So I fill it with 134a and now it's roughly at the factory capacity of 32oz and the pressures are right....but the compressor runs all the time. Hopefully it's just because it's hot out and it's idling. We shall see. Probably not. **** I hate electrical gremlins.

 

If the pressures are right, and outlet temps are good, doesn't matter if the compressor is running all the time or not.

 

Really? So it's normal for the compressor to be running the entire time when AC is selected on the dash? In the years I've owned the truck, it always cycled a little at idle. A couple times a minute or so. Maybe it's just been out of spec all these years....

Pressures are within spec and don't really move around at all because the compressor is always on, capacity is roughly correct (32oz give or take), and the temps are quite cold (haven't measured but colder than I ever remember it being). I just assumed that it running all the time was abnormal because it's always cycled before.

 

It probably was low on refrigerant before.

Sounds to me like it is workin' great. Don't over think it.