I'm having trouble finding some information about the AC system on this '93 960. It began by working great for the first 10 minutes or so, then quit working for about 5 minutes, then worked again. After a few days now it starts by working great then quits altogether. I had the compressor replaced last year (Volvo compressor) and I have no reason to think that is a problem. I thought it might just be low on freon so I hooked up a gauge to check the pressure. With a gauge connected to the low pressure side, the pressure readings when the car is started from cold look to be in the correct range, and the clutch kicks in when the pressure goes below a certain point. What happens after awhile is that at a certain point the clutch does not engage and pressure begins to build, moving eventually close to 100 pounds. So I'm thinking maybe there is a high pressure switch just like there is a low pressure switch, and it is failing, causing the pressure to build so high. But I can't find a diagram or any direct reference to such a switch online, or rather, I see items claiming to be high pressure switches for sale but no reference in any troubleshooting guide as to where it is located. Does anybody here know: first of all is my thinking correct about the possible cause? And then, whether there is such a switch on this system, and if so, where it is located?
Let me add that I do have good airflow through the condenser, and power to the low pressure switch when the clutch is engaged, and none when it is not (which seems the way it should be). Also, nothing appears to be freezing up.

 

"and the clutch kicks in when the pressure goes below a certain point" - sorry don't understand that - (the clutch should disengage/stop spinning the compressor if the pressure is too low) but I do understand that the clutch stops working - so yes static ac pressure could be about 100 psi on the low side -

Here's a diagram from a 1994 - sorry I don't have a 1993. If you have the three pressure sensors on the condenser - it should be the same as a 1994.
Relay 2/10 is energized by the ecc module 4/6, the Engine control unit 4/12 lets that signal pass (if the car is not overheating) to close the contact in relay 2/10 and send compressor power to the low side switch, then the high pressure switch and finally to the compressor.

 

Thank you for the diagram and the info.

"If you have the three pressure sensors on the condenser - it should be the same as a 1994."

I do see three "somethings" side by side, all different colors, at the bottom of the condenser.
I guess I remembered the switching on/off of the low pressure switch backwards.
I have to confess my brain struggles with understanding electrical processes.
In other words, I'm still confused as to what might be happening; which components are likely to be suspect,
and how to test them. So at first, the clutch is engaged and there is power to the low pressure switch connector (checked with a voltmeter).
After it warms up to operating temp, the clutch disengages, and there is no power to the low pressure switch connector even though the pressure begins to build toward 100 lbs.
Does this point to any specific component? Or is this not enough information to say?
I assumed a low side pressure in the 40-50 range indicates sufficient refrigerant? Give or take a few pounds on either end of this is the range the gauge moves between when the clutch is cycling off and on.

Thinking more about it, am I correct in suspecting the relay itself?

 

The low pressure switch would turn off power to the compressor is the pressure got too LOW (below about 20 psi) Anything above that and that signal goes to the high pressure switch on the condenser 7/41 that shuts off power of the pressure is loo high then finally to the compressor.

But you have no power at the low switch, Find the fan/compressor 2/10 relay - The pink with white wire coming from the ECU (with engine running/ac on) gets a ground signal from the ECC passing thru 4/12 the ECU . ( as I mentioned if the ecu think the engine is overheating it will turn the compressor off) you could ground that terminal (w pink/white wire) on the relay and see if power is then sent thru the green w orange wire to manually send power to the compressor via the two switches. Or use a jumper wire to send 12v thru the green/orange wire to power the compressor.

 

Looking at the diagram, it appears the 2/10 relay operates the fan as well. So can I assume that if the fan is working properly, the culprit is more likely to be the high pressure switch?

 

It looks like that box 2/10 includes the electronic resister for fan speeds as well as the compressor relay. So no- if the fan works that has nothing to do with the compressor relay.

If you want to test the high pressure switch - it should have 0 resistance - until it senses maybe 300 psi - then it will be an open circuit shutting off power to the compressor. In other words it allows current until the pressure is too high.

Diagnose first - then solve the problems -it's cheaper and less frustrating that way. Or if you have another car the air conditioner works normally in to swap parts from for free - do that and not have to figure it out - either way the problem gets solved..

 

"( as I mentioned if the ecu think the engine is overheating it will turn the compressor off)"

Which component would cause the ECU to think the engine is overheating? That sounds like maybe a temp sensor somewhere.

​​​​​​​Also, there is no pink/white wire coming from the ECU

 

This is the fan relay. The plug in the center has wires with colors that match the wires on two of the sensors mounted at the bottom of the condenser. I believe these are the low and high speed (fan) pressure sensors. I'm not clear on how these relate to the low pressure switch on the accumulator, which I associate with the ac compressor clutch circuit.
One of the things I thnk I'm seeing is that once the clutch cuts out for good (when the car warms up to operating temp), the fan kicks into high and stays there.
Surely that's a clue...?

 

Yes that is the RADIATOR FAN relay. And yes 2 of those pressure sensors on the condenser are part of the radiator fan circuit and will turn the RADIATOR FAN on when there is a certain pressure in the ac system - maybe low speed at x pressure and high speed at x+ pressure. You have a two speed radiator fan. The 3rd sensor (one on the far right as mounted in the car, 7/41 in the picture above) turns off the compressor if the pressure gets too high - for instance if the RADIATOR fan is broken, then pressure will get too high but the engine may not be overheating (yet). The HEATER FAN resister and compressor relay 2/10 looks like what's in a previous post and is mounted inside the car - maybe behind the glove box near the heater fan motor as shown. The low pressure switch on the accumulator under the hood simply turns the compressor off if the pressure is below about 20 psi.

Your clue - certainly if the engine is overheating or the engine temp sensor is telling the ECU the engine is too hot - that will turn the compressor off and possibly run the RADIATOR FAN on high. Some of those 6 cylinder engines had 2 engine temp sensors - one on the thermostat housing and the other on the back of the head. Don't know if your 93 has one or two. A 1994 has two and only one of them reports to the Temp gauge on the dash. Pictured below is where the temp sensor 7/16 is on the back of the head and where the COMPRESSOR relay 2/10 is located inside the car. .

You have not mentioned if you have checked for self diagnostic codes - or are aware that you can. The ECU and the Climate system, as well as many other control units can store self diagnostic codes - maybe that will give a clue. Instructions can be found here. https://www.volvoclub.org.uk/faq/EngineOBDCodes.html


Heater fan resister, compressor relay

 

"...The 3rd sensor (one on the far right as mounted in the car, 7/41 in the picture above) turns off the compressor if the pressure gets too high - for instance if the RADIATOR fan is broken, then pressure will get too high but the engine may not be overheating (yet)." ---so a faulty high pressure sensor could be at fault? Can I assume there should be constant voltage here, and if not the problem is elsewhere? also, is it correct that disconnecting the connector and checking across the two contacts would be the way to test for voltage (same way as is done at the low pressure switch)

"The HEATER FAN resister and compressor relay 2/10 looks like what's in a previous post and is mounted inside the car - maybe behind the glove box near the heater fan motor as shown. The low pressure switch on the accumulator under the hood simply turns the compressor off if the pressure is below about 20 psi." ---- in other Volvos I drive, power to the low pressure switch comes through a relay behind the dash; however, I have never experienced a situation where there was power at the low pressure switch switch at first and then not, indicating that the relay fails during operation (I realize this is anecdotal, but the consistency of the symptom I'm experiencing in relation to engine temperature has grabbed my attention for the moment and I'm thinking this relay (if I can find it-I haven't been able to put my eyes or hands on it yet) is probably not suspect- at least not yet)

"Your clue - certainly if the engine is overheating or the engine temp sensor is telling the ECU the engine is too hot - that will turn the compressor off and possibly run the RADIATOR FAN on high. " ---how does one test this beast?

"Some of those 6 cylinder engines had 2 engine temp sensors - one on the thermostat housing and the other on the back of the head. Don't know if your 93 has one or two. A 1994 has two and only one of them reports to the Temp gauge on the dash." ---the temperature gauge is showing the engine temp in the proper range, so I'm hoping it has two and one of them is the culprit.

Pictured below is where the temp sensor 7/16 is on the back of the head and where the COMPRESSOR relay 2/10 is located inside the car. .

"You have not mentioned if you have checked for self diagnostic codes - or are aware that you can..." ---I followed those directions this morning to no avail. However, there is one part of the instructions that are not clear to me. So you start with the AC switch in the "off" (out) position. According to the directions: “Retrieve the DTCs by pressing the A/C button in and releasing it within 5 seconds.” I can't tell from that wording if I'm engaging the AC switch and then disengaging it or just engaging it. When you press the button and release it (take your finger off it) once, you are turning the AC on. Is that what this instruction indicates?

Thanks so much for the added images. I'm going to go now and see if I can locate those temp sensors and have another look for the relay near the glove box. Unless I can figure this out this evening, I'll be driving my daughter's '93 940 which is currently in my driveway so I can try to diagnose an intermittent cold no-start situation. Oh joy!

 

"....Find the fan/compressor 2/10 relay - The pink with white wire coming from the ECU (with engine running/ac on) gets a ground signal from the ECC passing thru 4/12 the ECU . ( as I mentioned if the ecu think the engine is overheating it will turn the compressor off) you could ground that terminal (w pink/white wire) on the relay and see if power is then sent thru the green w orange wire to manually send power to the compressor via the two switches."

I think I found the 2/10 relay. It's almost impossible to access. But the wires are easy to see and I do see a pink/white wire. After reading this post about 10 times, my brain is finally (I think) beginning to understand. Tell me if this is what this post means: I think you're indicating that if, by grounding that terminal, power is then sent to the low switch, that indicates power is coming to the relay (from the ECU) but not making it through the relay indicating the relay is at fault. If grounding this terminal does not result in power going to the low switch, the next place to check is at the ECU.

 

If power goes away from the low pressure switch - either the relay is erratic, the ECU is turning it off because if think the car is overheating (kinda makes sense because the radiator fan is on) or the signal is getting lost inside the CCM,

I would be reluctant to ground the wire you mentioned (actually ground the terminal on the relay that closes the relay to send power to the low pressure switch) unless I it was unplugged from the ecu (that will be hard to do and keep the others plugged in).

How does one test the Temp sensor (ECT)? Volvo gives the info below - not helpful. It's going to be a simple thermister - resistance changes with temp - but don't have the specs for that. Maybe the internet does somewhere. The temp sensor that is used for the Fuel injection system (ECU) is the one on the back of the head. The one in the thermostat housing appears to be only for the gauge. Included is a pic/part number of the sensor.

Retrieve DTCs. - From the directions on the that linked earlier obd website " If the 1992+ car is so equipped with connector B, socket 1 is for the climate control, socket 2 for cruise control, socket 5 for the SRS and socket 6 for the memory seats." - The A connector is black, B connector is gray and right next to it if I remember correctly. All codes for all control units are read from the blinking light rectangular box under the hood. Check all the systems you can, record and erase all codes in the eng, trans, ccm, wherever else. If the ECM thinks the car is overheating - you may have a code in the ECM

The ccm code retrieval process you describe sounds like what was used on early 850 CCMs on cars that only had one box with 6 terminals and the codes flashed out on the CCM

 

Wow! Thank you so much. I think I'll try to find the specs for that temp sensor online, or ask my mechanic. I'm guessing it will be a certain resistance reading?'

I do have the OBD2 feature, so I can follow up on that and report back.

Just out of curiosity I googled "can a bad temp sensor cause the ac to not work?" Most of what I found seemed completely backwards to me; language to the effect that if the sensor went bad it would send a signal that the temperature was 0 or something like that, so the radiator fan would stop working, which seems like just the opposite of what you'd want.

 

"The temp sensor that is used for the Fuel injection system (ECU) is the one on the back of the head." --if this sensor is related to the Fuel Injection system, if it was faulty in this case and crapping out when reaching operating temperature, wouldn't that cause the car to run differently once it reached that point?

 

Yes I would imagine so. But the ECU is designed to keep the engine running as long as possible. No matter what. Too lean, too hot, too rich, too cold, no voltage in the battery - (on newer cars) let's shut everything else off (steering, radio, ventilation, etc, on newer cars) and keep the engine running. Do you want the engine to stop because it's a little hot and you are driving at 3 AM lost in a very dangerous area? No - So the Ecu will keep the engine running as long as possible.

Can you imagine the lawsuits? "I was driving and my car shut off for no reason and someone wanted to clean my windshield - I had to give them $3! " Or worse.

 

I'm wondering if at this point you know enough that, if it were your car, you would be ready to buy a part, or is there something I missed that I haven't checked yet? (after I jump the pink/white wire to the orange/green wire)

 

Out of curiosity, I unplugged the electrical connector to that rear sensor to see if the ac would work with it unplugged. I let the car warm up first, then got on the highway to see what would happen. I hadn't gone very far (1/2 mile) when the temperature gauge began to rise. I had to turn the car off and park on the side of the road so it wouldn't overheat. I let it sit for about 10 minutes, saw that thew gauge was in a reasonable temperature range, so I tried to crank it. It won't start. I'm wondering if by unplugging that sensor I've triggered the computer to do something wacky.

 

While the car was cooling I thought to check the upper radiator hose. Sure enough, it was hard. I began to suspect a bad thermostat. Once the car cooled, I plugged the sensor back in and it cranked right up. The upper hose had settled a bit, but after running the car awhile in the driveway it got hard again. I replaced the thermostat, which was physically broken (I've never seen that before). After driving down the highway a couple of miles, the ac is still kicking on and replacing the thermostat appears to have been the issue. I think I got misdirected by the fact that the temperature gauge never moved out of an acceptable range. I guess that's a testimony to the system's design to get you home no matter what. ? Anyway, thanks for the suggestions and great information & diagrams!

 

So after replacing the broken thermostat, plugging in the ECT sensor (behind the head), and clearing the codes, the car runs fine but now neither the AC nor the cooling fan are coming on. I'm trying to understand what has changed that results in this behavior as opposed to what was happening before, with the AC not working but the fan kicking into high. The fuses look fine.