Then it's not engaging. That circuit might havd been affected by touching the monovalve wires. Back to the wiring diagram...

Sixto
95 S420
87 300SDL

I took the PBU out and looked at the boards to see if anything was fried, and everything looks ok on it. I figured that maybe it wasn't telling the compressor to turn on. From the wiring diagram on page 307, it says that the electronic unit for temperature control also tells the compressor to turn on... so I guess that's the next thing I should check for burns.

__________________
Gabriel
'91 350SD 162k
'83 300SD 210k - rear-ended and scrapped
'84 300SD 210k - totalled

That's how you read p307? I read it as the mode selector determines if the compressor should be on. The refrigerant pressure sensor and evaporator temp sensor are interrupts.

Temperature is not controlled by cycling the compressor but rather by modulating the heater core.

Sixto
95 S420
87 300SDL

Yeah, I agree with you that the compressor low pressure cut-off switch and the evaporator temperature regulator are interrupts. But The electronic unit for temperature control (on the bottom right) has an input to the compressor clutch. The only other devices that could fail (from that diagram) are the PBU and the relay.

Out of those 3, the reason I think it's the EUTC, is because in Climate\83-603.pdf, it says "During electric test of monovalve (with voltmeter, test lamp or the like) be sure to avoid a short, since in such a case, the electronic switching unit for temperature control may be damaged". Now, that document is about testing the system with the adaptor, but I think that what it's referring to is the same thing (electrically) to touching those two wires.

__________________
Gabriel
'91 350SD 162k
'83 300SD 210k - rear-ended and scrapped
'84 300SD 210k - totalled

I interpret that to be the other way. When the compressor is on, there's a 12V signal to the temp controller so the temp controller knows the compressor is on. If my interpretation is right, you should be able to disconnect the temp controller and still have the compressor function. In fact, that might be the way to survive in Florida.

Sixto
95 S420
87 300SDL

If you're right, then the relay or the PBU would be the problem. I wish there was a way to check if the PBU is sending the signal to turn the compressor on.

__________________
Gabriel
'91 350SD 162k
'83 300SD 210k - rear-ended and scrapped
'84 300SD 210k - totalled

There is. Open the file ETM/126td83-85.pdf and go to file p3/doc p103 to locate the compressor relay. Hopefully it's a 4-pin relay as ETM/ACC83-85.pdf suggests. With the ACC off and the engine off, find the ground wire. With the ACC off and the engine running, find the live wire. With the ACC in DEF and the engine running, see if one of the remaining wires has 12V. If so, you know the PBU works. Jump the constant 12V wire and the last pin to check the compressor. If it works, you have a bum relay.

Or the relay socket in the fuse box is numbered like the wiring diagram in which case there's no guessing.

Sixto
95 S420
87 300SDL

The relay had 5 pins, but one of the pin sockets didn't seem to have any metal in it, which I'm guessing means it's not connected to anything.

I tied wires to each other pins on the relay, then put the relay back in the socket and measured the voltages between a random wire I picked and every other wire. I figured eventually I'd find either the ground or the 12V wire and there'd be a voltage difference. I was wrong... the biggest voltage difference I got was 1V. Both with the ACC on and off.

I checked for continuity and 2 of the pins had continuity, all the rest didn't. I'm not sure what that means.

Since I didn't get a voltage difference between the wires, I replaced fuse 11, thinking it might not be providing power to the relay (even though the blower was working), but no difference.

__________________
Gabriel
'91 350SD 162k
'83 300SD 210k - rear-ended and scrapped
'84 300SD 210k - totalled

In general, an electrical relay consists of a coil and a switch. Energize the coil and it pulls the switch closed. So you'd expect one pair to have continuity. If you're daring, you can try very briefly sending 12V into one then the other switch contacts with the ACC off and engine off. One contact goes to the compressor and you should hear the clutch engage with a heavy click, the other goes back to the PBU and should be safe seeing 12V if the PBU is off... I think.

Sixto
95 S420
87 300SDL

sixto, it was the low pressure cut-off switch. I bypassed it, and the clutch engaged. It looks like freon leaked out at some point in the past couple of weeks.

__________________
Gabriel
'91 350SD 162k
'83 300SD 210k - rear-ended and scrapped
'84 300SD 210k - totalled