Hi, please confirm that basic checks have been done and the next step...(93 940NA Wagon, 155K)

AC compressor does not click on or engage. The on/off button does nothing(AFAICT). Found voltage going to the compressor - 12V with engine running. Found good ground at the back of the compressor at the ground wire terminal. Checked the resistance from the power wire through the compressor to ground, at 3.4 ohms, this agrees with the faq for good. An AC shop verified today the system is properly pressurized and no leaks. (was just evacuated and refilled 3 months ago) Drove it around, no change.
Shorted the pressure switch at the accumulator, no change. Found voltage at that switch jumping around at idle, from 5V to 10.5V between the hot terminal and ground. It was inconsistent.
What's the next step, is it time to check the solder joints on the dash control board? Thanks for your thoughts.

I think Spook has it right, as usual. My '93 945 AC compressor would not come on but I measured nearly 12 v from compressor lead to ground. I was puzzled that the compressor was not comong on. Eventually I pulled the control unit out of the dash and inspected the printed circuit. It was just as Spook described, the solder connections at the small,black relat box were cracked. I resoldered the relay connections and the AC now works fine.

What was happening was this: the cracked solder joint made a high resistance connection. This allowed enough current to flow to let the voltmeter read nearly full battery voltage. When the compressor tried to run, the high resistance, cracked, solder joint would not let enough current through to energize the compressor clutch. If you know Ohm's Law, think of a series circuit with the 3.4 ohm clutch in series with something like a resistance at the crack of 100Kohm. A 10 megohm input meter would read nearly the full supply voltage but not nearly enough current would flow to operate the clutch.

There is a way to test this without pulling the control unit. If you connect your voltmeter to the compressor connection from the rear of the connector without disconnecting the compressor, you can measure the voltage when the compressor tries to engage. If the solder connection is cracked, the voltage will drop to nearly zero. With the connector open, the voltage will be nearly 12v.

All of your responses have found me well, thank you. I will try measuring voltage at the connector through the compressor to ground to see if it has dropped, as well as giving a clean jumped battery voltage to the compressor to see if it engages. All of these are pointing at the control board (manual AC Controls) so I expect we're headed in that direction. Thanks for all the detail.
- Landon
93 940NA wagon

I had an odd condition at the pressure switch: I had somehow leaned on it when doing work on the engine, breaking it and causing the terminals to fail intermittently. The a/c would engage, then as current passed over the terminals they would apparently heat up and fail. On, off, on, off: typical low charge condition, but it was caused by the pressure switch. So check that you have regular voltage at the switch and its terminals.

That being said, if 12V applied directly to the clutch does not move it, then the clutch is bad. Try a jump from the battery to test it.
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See the 700/900 FAQ under 'Select Link' button on the top right.

My reading of your post would indicate that there's 12 volts across the clutch, yes?
It seems to me that if you have 12 volts across the clutch and it doesn't engage, then the clutch must be bad.

Tell me more, as I'm working on the same problem. Do you have a 960 or 940? Do you have ECC or ACC control. I have the wiring diag and should be able to help.

Dear Landon T.

Good a.m. and hope this finds you well. Yes, although the pressostat may have failed. Usually, if the control unit has failed, jumpering the pressostat should get the compressor to cycle on.

To be sure, you can re-solder the joints, on the the printed circuit board (PCB) , inside the air conditioning control unit.

Two items will need attention:
(a) The relay - a black plastic box about 3/4" (20mm) long, 1/2" (13mm) wide, and 3/8" (10mm) tall - is affixed to the PCB by four pins.
(b) the main connector, which has 12 or 14 pins.

The solder may be cracked, but microscopically so. Thus, re-flow the solder at each point. Use a max 25-watt iron. Use rosin core solder (not acid core, which is for non-electrical jobs).

Put the tip of the iron on the pin for 1-2 seconds. The solder should turn from gray to shiny silver. At once remove the iron. The solder should cool and revert to a gray.

If a joint appears to have too little solder, apply a very small amount. Be stingy with the solder. You do not want to create solder bridges - connections between two things that are not supposed to be joined.

Hope this helps.

Post back with your results.

Yours faithfully,

spook