Like the title says, I have no voltage to the ac clutch relay. Using the key on key off method, I get code 33, clutch relay. Everything I read points to the PCM??? Anyone know who rebuilds them at a reasonable price, or where to get a used one (I keep looking on ebay, but $250 and up with no guarantee that it works). Any other suggestions?. I currently have the clutch wired to a switch on the dash through a relay (also through the low and high pressure switches), and it works, just want to fix it correctly.

 

UPDATE, I reset the computer (batteries disconnected, turned ignition switch to start for a few seconds). I now have 13.2 volts to the 30 terminal on the relay, but no voltage to the 86 pin with the AC on. I believe I should be getting voltage there, when I turn the AC switch on, which then closes the relay and sends current out of the 87 pin (to the clutch via the pressure switches. I do have 13.2 volts at the low pressure switch, but nothing at the high pressure switch near the compressor clutch. I'm still thinking the PCM?, but that's an expensive guess?.

 

I had one a while back with a spread terminal at the high pressure switch that caused a open. Remember that the pressure switches are in series and the conection is broken or one of the switches is open you wont have ac function. I used an oscilloscope to watch each of the four wires. When I found that the channel that was connected to the high pressure outgoing wire lost voltage, I followed that to my lose wire. Sounds like the low pressure switch is open or a wire is broken between the two switches. Do NOT replace the pcm its not bad.

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I would suggest Pressure switches , and run more testing on wiring .... I work on 45' buses ... its almost always a switch or a realy .... Ive done 3 clutches in 2 yrs ... never a controll unit ..... not that it couldnt happen ....

also check your orfice tube and see if its plugged .... if it is your pump will kick on and the right off ... this is some times how I know where to go .... do not under any circumstances jusm a pressure switch unless you have guage's on the system ... just helped another guy who blew his high pressure line up ...LOL ... good thig he was outta the way

 

Thanks for the reply. Makes sense on the low pressure switch. I'm thinking I should skin the wire back on the low pressure switch, and see if there's voltage leaving the switch with the system running? I do have voltage to the low pressure switch, so maybe I should just replace it as a start, and go from there?

 

Dont skin the wire, use a paperclip *without the plastic coating* to backprope the connector. Unplug the connector first and check for continuity or a "open" in the switch. If the switch is open you'll need to put gauges on the truck and verify refrigerant pressures are within range. If they are and the switch is open you replace the switch. If not you recharge and try again. If this dosent work then test the low pressure switch by back probing the light green and white wire at the high pressure switch (koer ac on) if voltage is present wiggle the wires at the low pressure switch. If voltage is lost you likely have a damaged terminal. This method prevents misdiagnosis caused by the paperclip causing engagement that would be there under typical conditions.

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Got it. I'll do that when I get home this evening. The only thing that puzzles me is that I currently have my compressor clutch rigged via a toggle switch mounted on my dash (been like that for 8 months). To do this, I pulled the clutch relay, ran a jumper from the battery to the 30 pin on the relay, ran another hot wire from the battery to my toggle switch, then from the toggle to the 86 pin on the relay. Then ran a jumper from the 85 pin on the relay to the battery ground, and finally I ran a jumper from the 87 pin on the relay, to the 87 postion on the power distribution panel where the clutch relay plugs in, and I've have cold AC using the toggle switch for 8 months. Being that I ran a jumper from the 87 pin on the relay, to the 87 position on the distribution panel. I'm assuming when I switch the toggle on, power is running through the relay, then through the trucks harness via the 87 pin receptacle in the panel, through both pressure switches to the clutch which does engage when the toggle is on. This leads me to believe that the low pressure switch must be working because the clutch does engage. Sorry to be long winded, just trying to communicate what my pea brain is thinking!!!

 

Why did you eliminate the relay instead of repairing the fault?

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Because I have no idea where the fault is, or how to locate it, and it's 100 degrees here every afternoon on my 1 hour drive home everyday. I'm not very good with schematics. I have power to the terminal that the 30 pin plugs into, so I know the relay is hot when it's plugged in. What I don't have is power to the terminal where the 86 pin plugs in when the AC is turned on (which would trigger the relay and supply voltage to the 87 pin, out to the clutch). I even replaced the switch/controller on the dash, with a known working one from another vehicle. So my mind tells me something is wrong between the PCM, the AC switch in the dash, and the distribution panel where the relay plugs in. I was hoping someone had experienced this same issue before. Sounds like I need to get my hands on a schematic, and start studying?

 

Lets look at it from a factory wiring stand point.
When the a/c switch is selected ground flows from the black and orange switch feed through the light green wire to the A/C High Pressure Switch through to the low pressure switch via the light green and white wire which then pulls a bias voltage to ground via the brown wire which therefore indicates to the pcm all conditions are met. From here the pcm energizes the coil side of the relay if the pcm is working correctly.
The coil side of the relay is fed 12V via Fuse 9 which is a 10 amp fuse.
The coil side of the relay is energized when the pcm senses all required conditions are met (pressure switches meet criteria and A/C Switch on) via a dark blue and orange wire (c13) which is connected directly to the pcm. When the pcm commands the a/c clutch to engage this wire will switch to a ground circuit. When this occurs the coil closes and transfers 12v from the 20Amp feed fuse to the a/c clutch.
Now the method I explained is using basic testing for a pull up style circuit. The testing is basically the same, Except for the fact that presence of voltage is an indication of a open (or the lack of a ground, or a disabled system), and less than 100mv indicates that the system is engaged and working correctly.

The diagrams have pin numbers and all required information to allow you to test the system correctly. With the relay removed and on a toggle switch you've seriously jeopardized your A/C system. It's likely that the pressure could have been low or high, and without the proper measures to disable the system damage could have been done to the compressor (cant remember if you're using a PCM signal to engage the clutch) . At this point I'd highly recommend finding a qualified shop to diagnose the system. There's going to be some working out to get this situated. My recommendation to them would be start by returning the truck to a completely factory wiring design on this system. Diagnose the circuit and get the electrical system situated first. At that point the pressures and function of the mechanical system can be addressed. If you did use a pcm signal for the relay, kudos to you. Continue to diagnose the high pressure and low pressure switch circuit.



All that said, I don't mean to be insulting but we see implementations come in like this all the time. Often wiring like this causes significant damage to related systems and in some cases even fires or fried modules. If you question you're knowledge in circumstances like this it's best to find someone to help you at-least get the problem diagnosed and recommend a path for repair. While it may "work" it may not work within the designed parameters.
In this case you're a/c compressor may have been damaged by low pressure which would require a at minimum a compressor, a condenser and a dryer. The system in it's factory form would have prevented this type of failure.
Hope the post helps.