Good day. I have a 92 Volvo 740 Base Wagon with 396,200 miles on it. The motor runs beautifully. But I'm having some issues with the AC. I have no AC and live in Texas. So, that's a bummer. I replaced one of the hoses (brand new), I refilled it with Freon (It has the R134 conversion). I replaced the AC Control Unit inside the cabin, and the only thing is that it blows air from the 1 thru 4 on the ****, but when I turned it to 5 (speed of air from the unit), it turns off. And no cold air. I'm no mechanic so, some of the terminology I'm using might be a bit confusing, so I apologize in advance. Any ideas?
On the computer, it's giving me a 2-2-1 code, but that's because I replaced the knock sensor, the temp sensor, the oil trap, the PCV valve and cleaned the throttle body, so the hoses, which I'll be replacing soon, might have moved.

 

Well, the fan speed is unrelated to the lack of cold air. Is there gas in the system? Is the compressor turning on? Probably not. Measure the pressure in the system first.

 

I took it to the shop and put the right amount of refrigerant, but yes, it seems that the compressor is not engaging. I've seen posts about bypassing some things and connect it to the battery, but I'm unsure to 'where' in the compressor should I connect it to. Also, I read someplace that the compressor might have some part (inside, it seems) that it is easily fixed. Also, don't know which one. A new or even a rebuilt compressor is a bit expensive, so I just want to exhaust all possibilities.

 

On the drier/receiver there is a pressure sensor. Unplug it and jumper the two terminals on the plug side, not the sensor-you can use a paper clip. With the A/C turned ON, and the plug jumpered, the compressor should then turn if all is well and this will indicate a bad sensor: common problem. If the compressor does not engage and there is enough pressure in the system, the problem is most likely the compressor providing the control unit is OK... Does the control seem to work OK besides the fan speed issue?

 

I've attached a birdseye view of the drier/receiver. I don't see any pressure sensor. Only the valves. Under the "6 Nm" one is another valve. Am I missing a sensor?

 

Mind if I butt in? What should the low pressure number be on an 80 degree day at idle and at 2000 rpm??

 

As your picture sits now, at the 11 o'clock position, there is a nipple thing--there is where the pressure valve normally goes. An there should be a wire with a place for two prongs where they connect to the switch. Is it anywhere around there?

 

whats that wire at 8 o'clock going to?

 

I'm sending 3 pictures of the surrounding area. There's a barrel connector with no cable. The other connector around is hooked to another cable that goes to a bigger connector that is under a support that I can barely move, hence the unclear picture. Should I buy and install the missing sensor?
And btw, Mr. Pierce, your Volvo looks gorgeous.

 

hah, my volvo's clear coat is peeling, silver just hides it well.

um, ok, those two connectors in your first pic look like the oxygen sensor (2-wire connector is for the heater, 1 wire is the signal).

according to my 1992 wiring diagram, the low pressure switch on the side of teh reciever/drier has 2 green-red wires. one comes from the HVAC dashboard control panel pin 10, the other goes through a couple connectors and ends up at the high pressure switch which is on the condensor manifold at the bottom side of the radiator (there's 3 color coded sensors on that manifold, the high pressure switch is the violet one). after the high pressure sensor/switch, it goes through a couple more harness connectors and drives the black wire to the compressor clutch.

its quite possible someone disconnected the low pressure sensor and jumpered the two wires together. this is bad because it allows the compressor to run even when there's no freon, this will burn out the compressor.

 

On the other picture, there's a barrel connector with a cable cut out which is green/red. Should I look for another of the sorts and try to connect them to the non-existent sensor? Should i check if the compressor is working? Is there a way to check if the compressor is working without dismounting it?
I did found (accidentally) the 3 color coded sensors at the bottom of the radiator. The one in the middle was disconnected. And had been like that for a while. I removed the rust with some cleaner and reconnected it; again, unsure of what was supposed to do.

 

Looking closely to the picture, you can see both red/green wires. They've been cut off. The only thing to do is to put the non-existent sensor and plug back the wires, right?

 

the missing sensor is two pins... its a switch that connects both pins if the pressure is OK, so there should be two red/green wires that went to it, one wire from the dash and one that goes to the other sensor. my drawing shows the violet sensor on the lower manifold as the leftmost one, I believe the other two sensors are for the electric fan speed.

my 92 740T, which has a Volvo R134a conversion kit...

 

Once I get the sensor back on, and connect the wires correctly, but still doesn't work. What would be the next logical step?

 

determine if there's voltage at one of those wires when the AC switch is on. if not, then the AC control unit is suspect. if yes, determine if there's voltage at the violet sensor on the condensor manifold, and if there's voltage to the black wire at the compressor. if there's voltage at the compressor clutch and its not engaging, then the compressor clutch is bad. if the clutch is engaging, and its not chilling, then we're back to freon.

 

Extremely stupid question.
On the AC Climate Control, the "AC" button, when on, should be pressed "IN", right? The light in the panel should be green, right? And orange when off, right?

 

depends on the year/model. on my 1992 740, pushed in is AC off and its orange, popped out is AC on and its green. my button is labeled AC Off, too.

 

May the inter net be your friend. R-134a System Pressure Chart | AC Pro

 

Thanks. Good info.

 

I realize this is an old thread, but where is the "condensor manifold", and does this apply to a 1990 as well?
I've been having intermittent issues with my AC since I had it serviced last year. Literally from the moment I drove away. I almost turned around and took it back to the mechanic, but I went over a bump or something and it started working. It was blowing air fine, but no cold air until I went over the bump. I've checked everything I know of in the engine bay except the "violet sensor" you mention. I also just started looking at the control module because my fan is now running intermittently, but I think that's just a bad connection. But I think I read somewhere there's something to be checked behind the glove box as well? Thanks!