I've spent the day attempting to diagnose the following symptom:

The a/c compressor will engage for about four minutes and shut down. It won't come back unless the vehicle is shut down and restarted.

I have done the following:

1) Checked the low pressure switch. Found a ground on both terminals when connected to 12V.

2) Isolated the system from the Klima. Removed the Klima and installed an ice cube relay in it's place.

No change in symptoms. Four minutes of compressor engagement and then it drops off line.

I'm at a loss as to what other device would control the compressor engagement. AFAIK, there is no high pressure switch that shuts it off???

Any additional thoughts would be greatly appreciated.

What's an 'ice cube' relay?

The pressure switch on the receiver/dryer is a dual switch. One switch will open if the pressure is too low, and the other switch opens if the pressure goes too high. The connections on this pressure switch come from the push-button control, and the ACC (klima?) controller. When the proper button is pushed, this line is grounded. It is the ground signal that tells the ACC to turn the compressor on. The ACC will turn the compressor off if the engine temperature gets too high, or if the compressor speed does not agree with the engine rpm. From you symptom, (compressor stops and will not restart until the key is cycled) I'd first want to look into the speed signals going to the ACC controller. Jumper power to the compressor clutch and measure the signal coming from the speed sensor. Mine was under specification at about .17 volts at idle, but works fine. If it changes after 4 minutes, find out why. A slipping belt or incorrect compressor clutch gap are possibilities, as is a failing compressor. If the signal is steady and the compressor continues to run with direct power to the clutch, then you'll need to look at the engine speed signal, the coolant temperature sensor, and the compressor request signal. If all those signals are correct, then it's the ACC controller at fault.

__________________
1989 300 SEL that mostly works, but needs TLC

An "ice cube" relay is one of those cube relays that look like an ice cube. There are about six of them in the fuse box.

Please correct me if I am mistaken:

I have eliminated the following items because the Klima (ACC controller) is not in the system. I have removed the Klima and replaced it with an ice cube relay. The relay switches power from to the compressor based upon the ground signal from the CCU off pin #10.

---engine temperature
---compressor speed
---engine speed


So, the only control to the compressor is via the CCU. If pin #10 gets a ground signal, the compressor engages.

The question is what could possibly interrupt the ground signal from the CCU?

The only thing that I see is the pressure switch. I believe that this pressure switch has only a single low pressure function (1986 wiring). However, to rule out the pressure switch, I removed the two leads from the switch and jumped them. Same results.

Just to confirm, the switch is the lower of the two switches on the dryer?? In fact, the switch is actually in one of the lines from the dryer. There is a second pressure switch (high pressure for aux. fan??) on the very top of the dryer.

Pressure switch: Looks like the pressure cutoff switch is 'binary', will open on either hi or lo pressure .
http://catalog.peachparts.com/item.wws?sku=W0133-1628267

Temp switch:
http://catalog.peachparts.com/item.wws?sku=W0133-1626582

Sounds like jumping should make the compressor 'ignore' hi or lo pressure...

Best Regards,
Jim

Thanks. I've definitely got the right switch.

Is it proper to jump this switch to eliminate it?

I tried this and found no difference in the results, so, I'm assuming that it is not the problem.

Any other ideas where the CCU would get a signal to drop the compressor??

I do not have a schematic for this chassis , but you may want to consider power feed interuption instead of the switched ground fault.
It is possible that you have a poor connection at the fuse feed or relay connection .
Hook a test light to ice cube coil side and load side and see if power is dropping at the terminals of each.
If suspect , bring a bat+ jumper over to ice cube pos side and run the car for a test .. see if it drops then.
Same goes for ground side .. Run ice cube with its own ground wire and see if you get a drop after the time limit you are getting.