Three years ago my AC stopped working. I hot wired the compressor clutch and found appropriate suction-side pressure (periodically adding R-134) and function, so was driving it that way until now – with a switch that would connect 12 V from the cigarette lighter through a wire glued to the exterior of the car to the AC clutch.

This fall the AC clutch failed (bad pulley bearing). Found compressor operational, but replaced it with a reconditioned unit anyway because I had already purchased it. Found only a couple of ounces of oil left in the system and it was all in the evaporator - guess one should add oil with freon if you have a leaky system. With new O-rings everywhere, I think my leaks are gone.

After learning more about AC while doing this, I’ve come to appreciate that it is not a great idea to continue driving with a hot-wired AC clutch. The pressure switch on the suction side was working. Found a switch connected to the high pressure line that was open at zero pressure (I presume this is a high pressure cut-out switch - and that it should be closed at less than ~300 psi -correct?). The local Volvo dealer told me after some fumbling that it was a pressostat and wanted an unreasonable amount of time and money to provide a replacement. I replaced it with a Porch part with appropriate specs. Also found the AC fan relay had failed. Would activate, but high current side remained open. Found corrosion (but continuity) in the wire bringing high current into this relay. Replaced the relay with a GMC part with appropriate specs, and fastened a lug above the corrosion on the wire.

What happened after thus restoring the original configuration is this: the AC fan ran always when the ignition was on (even with the AC switched off). The AC clutch didn’t engage, but I saw 0.3 volts appear on the clutch after the AC was switched on – guess I have bad soldier joints in the CCU.

Here is something very strange. I then disconnect the wire that carries about 0.1 amps to activate the AC fan relay. Now the AC clutch activates as expected following ~10 second delay after AC switched on! The clutch also works if I hot-wire the low-current side of the AC fan relay. I’ve been poking around the postings here, hoping to find a definitive description of the AC wiring for my 740 – but apparently it is not to be found – and I’m not anxious to pay Volvo to see it.

Now finding that turning up the temperature setting on the console with the AC on only runs hot water through the heater while running the compressor continuously. I was hoping that a thermostat would be used to cycle the compressor to maintain a specific temperature. Is there no AC thermostat in the 740?

Am I over looking anything important here?

John Mattox, North Carolina, 1991 740 with 168k miles.

I had rebuilt an '87 740Ti A/C system from the ground up and also worked on a '91 740Ti I once had. Cursed the system as it is!
I am happy to have the totally manual '89 240 A/C system that has the simple manual thermostat and clutch circuit compared to the 740. Take heed from the advice below, and make the mods which avoid the issues you ran into.
One observation I will make on your use of GMC A/C relay is that it may be the sole reason why you have the intermittent problem with clutch and fan behavior, where the relay is not rated for the current the complete circuit is consuming, thus dropping the voltage to both and disabling one or the other, depending on which is on first. If you really wnated to fix that, put another control relay in the fan circuit (off the battery) that trips off of the A/C relay and it won't overburden the A/C control circuit.

The delay for clutch start is built into the system at the MCC, it's designed to keep the engine as unloaded as possible during start up. Why it's doing that when the fan is disconnected is another story. Most of the volvo 740 condenser fans I've seen recently have completely rotten fan connections. The wires as they go into the fan housing seem to have lost all the insulation. If you have a short there that could be sinking away all your current. Considering how wimpy those original 740 condenser fans are and the cost of a new one I would replace it with an after market hayden condenser fan. A 14” is 60 bucks, will fit and move much more air than the original. If you use the relay wiring kit available you can just tie back the volvo fan circuit and and use the clutch power as the signal for the fan relay. Source the fan current from the battery. This gives an a/c on = fan on configuration. If you want I can post an image of the bracket I made to mount this fan using the original fan mount holes.

For the 740 CCOT w/134a system you should see 40 -> 45 low side 290 -> 330 high side on a 90 -> 100 degree day with the condenser fan working, engine at 1200 rpm, the center vent temp should be 53 -> 65. If you are getting colder, good for you. If the fan is not working or the condenser is dirty your high side will be above 300 . Either way if the original r12 prossostat cuts out at 300 you are not going to get much out of the A/C system on a hot day using 134. The new pressostat for 134 I bought must be at 350 because I'd didn't have any clutch cycling and the gage was a little above 320 when I charged the last 740.

That pressostat on the high side, below the orifice valve, is available from NAPA for about 20 bucks, under the make and model. All of these pressure switches are also made by a company called santech (not scantech) and your local AC re builder can order the exact one you need. Also if you have switched to one of the newer zexel compressors w/o connections on the compressor you will need to tee off a high side 134a connection from that same high side pressostat connection if you want to have a hope of charging the system correctly. Note there is also another pressostat on the accumulator, that is the low pressure switch.

first things first; did you check to see if your condenser coil is dirty/blocking air-flow?

Turns out that my car has manual climate control, MCC, so there apparently is
no thermostatic control of heat nor AC. I hope to eventually substitute the automatic climate control, ACC module - would appreciate comments regarding the
feasibility of this.

Turns out the high pressure switch is used to activate the AC fan - don't know details - this function is mediated by the MCC module - but I think the switch should be open at low pressure. Too bad I didn't keep the box my "replacement switch" came in so I could return it.

Now plan to manually switch 12V to suction side pressure switch to activate AC - at least until I can swap out the MCC module for ACC.

Now that I’ve learned a little about electronic climate control (ECC not ACC), I expect that I will be content to stay with MCC. ECC incorporates sensors for cabin temp, ambient temp, AC coolant temp, and vehicle speed. Would it be worth the pain I expect it would entail?

VVPete may be right – the GMC A/C relay may have prevented the activation of the compressor clutch by drawing more current (at 0.1 amps) than the MCC was designed to supply (when I had the wrong high-side pressure switch – closed at low pressure).

Now that I’ve put the original high-side pressure switch back in, open at low pressure, I haven’t heard my AC fan go on yet. Anyone know what the turn-on pressure for this switch is? Should it be a different pressure for R-134 than for R-12? For MCC control, it appears that the AC fan relay is activated directly by this pressure switch.

I also appreciate the comments of festus3001 about replacing the AC fan and other parts, and the offer to post an image of the bracket he made to mount a replacement fan using the original fan mount holes, as well as nominal high-side low-side pressures and air temp.

I don’t plan to replace my AC fan in the near term. It works fine – if I hotwire my GMC relay to activate it. That is a 30 amp relay – the Volvo AC fan draws about 15 amps. I don’t think I want to run it whenever my compressor clutch is activated. If I drove a lot in city traffic (which I don’t), I would reconsider this – or perhaps install a manual switch in the cabin to activate it. At 20 MPH or better, my AC works fine without it. Also not seeing any cycling of the compressor due to the low-pressure switch (which is wired in series with the clutch and opens when the pressure drops below about 30 PSI).

And finally, an interesting thought from Tony Turner to pass along:
They tell me that once the evaporator gets down to temperature the compressor cycle is actually less drag because the working pressure remains lower. Normal systems that cycle the compressor off for longer periods cause the compressor to work harder ---- or at least that's what GM says.

Mike, Yes, I checked to see if the condenser had blockage of air-flow - it was OK - in fact I replaced the condensor because of minor damage from road stones (was not leaking) because I had a better unit on hand. There was minor blockage due to an accumulation of leaves behind the AC fan which I don't think would have been possible to clear without removal of either the AC fan or condenser.


As long as I'm posting let me share here useful advice on this project that has been e-mailed to me by Tony Turner:

I'd stay with the MCC [rather than retrofitting with ACC] there is ALOT less to go wrong.

As far as the diagrams go ... AllData subscriptions
are $25 for the first car per year and $15 for each additional car. I
have all my cars but one covered with a subscription ----- they have
tons of stuff that you won't find in a service manual.