1993 RR LWB 4.2. My a/c compressor cycles on and off every 10 seconds. It blows ice cold when on and warm when off. FREON IS OK!!! I have tested high and low pressure several times and it is within LR parameters. Jumping the pressure switch makes no difference.

I have traced the problem back to the air conditioner thermostat control (i was told this is only in the 93 and 94 models) which is located under the dash all the way to the right (LHD model) on the bracket that bolts the dash to the firewall. It is a little black box marked Eaton and has about 6-8 black wires coming from it. Two wires seem to go to something behind the evaporator. I assume it is a thermostat probe that tells the black box when to kill the compressor or turn it back on by changing it's resistance when the tempurature changes. I am assuming this because the black box clicks just before the compressor clutch relay clicks. That is when the compressor turns on or off. If I jump the two black wires from the thermostat control box that go to the probe, the compressor just stays on indefinately and the AC blows ice cold...39-40F. I did this to make sure there was not a signal coming from the ECU trying to shut down the compressor. But this is overriding something.

I have a workshop manual but it only covers 1987-1991 so I don't have a good wiring diagram for the AC (feel free to email me one if you have it)

So....what is the two wire thing behind the evaporator? Is my thermostat control bad? Or is it this probe? One more thing that might be important: The compressor clutch relay sparks a little when it clicks open. I have put other relays in its place and they all do that too. Someone suggested a bad diode on the compressor (I would assume the diode pack) or is this just normal and the problem is elsewhere. I'm close to fixing this so, someone please help me get it figured out!!! Thanks

 

Before you over pressure your a/c and blow it, go have someone with a real set of guages, check your operating pressure's. You could be overcharged causing the real cold blow temp. at the dash and the 10 second shut downs.
Do this before you try anything else, may save you alot of time and money.
Mike

 

Like I said in my original post, I tested high and low pressure--with my manifold gauges--and they are fine. But I found the problem. The evaporator temperature sensor drops out when the AC blows. I tested resistance across the two terminals. It starts at 2100ohms and goes up for 5-10 seconds until the resistance goes open which it should not do. So, off to the dealer tomorrow. I do appreciate the "use a real set of gauges" though because a lot of people test with just the crappy gauge from the recharge kit on the low side and as you know, that tells you nothing.

 

Hey guys I'm having a problem with my 94 RRC LWB A/C also.
I took it out for a spin down the interstate on Saturday when it was real hot here 102+. The air was blowing cold for about half-hour, then it got warmer and then started blowing warm. No leaks, good pressure. compressor clutch does not engage. Checked fuses and will check relays.

Alpinab7 the "the two wire thing behind the evaporator" is the Dual Pressure Switch.
The switch, located on the high pressure line between the receiver/drier and the expansion valve checks refrigerant pressure and by means of the relay module controls the following system functions:
IF refrigerant pressure drops below 29 lbf/in clutch is dis-engaged. When pressure rises above 29 lbf/in clutch is re-engaged.
IF refrigerant pressure rises above 455 lbf/in clutch is dis-engaged. When the pressure drops below 375lbf/in clutch is re-engaged.

As mike said; if the charge in the system is not just right the clutch will dis-engage to protect the system.

 

Actually, what I am describing is the Evaporator Thermistor. Basically a temp probe to tell the t-stat when to kill the compressor. Got a new one and i'm all good. Oh, except now I have a no start.....not related though.