I've got an intermittent voltage supply to my AC compressor. I'm thinking there must be a sensor somewhere in the circuit, or maybe a relay? If so, where? When I hot wired it for a few minutes the AC worked great. TD

 

the signal comes from the dashboard AC controller to the low pressure switch on the AC reciever/dryer near the firewall to a high temp switch on the radiator to the compressor clutch.

more or less. what year and model car do you have? the details change year by year.

 

It's a '93 940 wagon. I'll trace that circuit tomorrow. As it's intermittent, I know everything is capable of working. Just have to find the weak link. TD

 

I'm assuming you have the MCC manual temperature control? ECC (electronic climate control) is more complex.

green red wire comes out of pin 10 of the dashboard controller, and goes to the LOW pressure sensor thats on the reciever, this has two pins. unplug it, start the car, turn on the AC, and check if one of the pins has about 12V relative to chassis ground (the other pin will have nothing, don't worry about it). switch the A/C off, and that 12V should go away. if don't you see that, something is wrong at the dashboard control module.

now, short those two pins together on the harness connector, start the car, and turn on the AC, does it run? if so, then either the pressure is too low, or the low pressure switch has failed (it will kill the compressor to run it without freon pressure).

from the low pressure switch, the other green red wire goes through a connector or two and ends up at the HIGH pressure sensor which is the violet one on the condensor manifold at the bottom of the radiator. this opens if the pressure is too high.

from that high pressure sensor it goes to a connector or two and is plugged into the compressor clutch, powering it directly.

 

The saga continues. I have no voltage at the sensor on the receiver so it's under the dash I go. Is the dashboard control module right behind the the push button switch under the heat-level-dial, center of the dash?

I'm doubtful that the pressure is to low as I charged it the other day as the compressor was cycling frequently as it does when you need a charge? It took 12 ozs and compressor stopped cycling and all was normal until the supply voltage was gone the next day.

 

its the whole module thing with the control panel.

is the ventilation fan working at all speeds? barely audible at 1, whoosh at 5 ? the control panel won't turn on the compressor if it doesn't see power going to the fan.. the way its wired, if the low speeds are fried (typically due to the fan resistor pack being burned out), then it doesn't see any juice even if the fan does work on '5'.

 

The fan motor works on all speeds. When you say "pin 10 of dashboard controller", it that on the back of the fan speed switch? Is that a plug I can just pull off? I've got pretty good access as the instrument cluster is out and I've altered the console under the heater controls and installed an 850 radio, CD, plus cassette unit. I'm not sure how one would get the entire dashboard control mudule out? There seems like there is some bracketry under the module but I'm not sure if I can get it out of the way.

Just a point of clarification, the 12vs should be present in the wire plugging into the sensor on the receiver, not the sensor itself? Either way, I'm getting nothing there. TD

 

I bleieve pin 10 is on the OTHER connector, behind the main part of the temp/ac control, and not the fan switch which has its own 2 connectors with 6-7 wires total for the 5 speeds, a power input wire, and a power sense power that goes to the rest of the control panel.

and yeah, the 12V should be on one of the wires, not the sensor itself. the sensor itself is just a switch. open circuit for too-low pressure, closed circuit for adequate pressure.

as far as removing the control unit... I think the bezel around it unsnaps and comes off, exposing some screws that hold it to the dash console frame ?

ahhh, from the 700/900 FAQ...
some potentially useful diagnostic and repair tips here?
https://www.volvoclub.org.uk/faq/Hea...olACNotWorking

 

be careful with those vacuum lines on the back of that control module, btw. if you mess those up, all your air flaps will be fubar.

 

Success getting the entire module out. It was quite easy since my 850 radio mod removed some of the obstructions. I've got continuity between pin #10 on the heat control plug & the receiver plug under the hood. I'll start to check for voltage which I assume arrives first at the fan plug and then goes to the heat control plug. Any tips as to which pins should produce continuity with others before I apply current? I have enough wire slack that I can reconnect it all as it's hanging out of the dash. TD

 

fan speed control pin B6 should be a yellow-red wire to main controller pin A7 (the fan speed control has two connectors, a smaller one C and a larger one B, while the main controller has a 14(?) pin connector A). Accessory switched battery power via fuse 16 should go via a black wire to main controller pin A12. main controller pin A1 is a white-black wire to resistor module pin ? which is jumpered to resistor module pin ? which is white-black to ground 31/10 (near right door hinges). The resistor pack pin numbers are too fuzzy to read, but it looks like both are pin 1, one on the middle connector and the other on the lower connector. this resistor pack is on the outside of the fan air-box behind/below the glove box.


that 7XX faq page I linked above suggests the common module failure mode is cold soldering, another failure is one or another relays inside the control box.

 

All checks out as far as continuity & voltage at the connectors. I jumpered pin #10 and #12 on connector A1 and the compressor sprung to life. I did not run long but it seems to indicates the wiring is all good. I took the metal cover off the module and there was no indication of anything getting hot. Think I should turn my attention toward "the resistor pack"? and the "ground near right front door hingers"? I haven't found the resistor pack or that ground yet. As an "OldVolvoGuy" I know that bad grounds run rampant!

 

the resistor pack is all about the fan speed, if the fan is working at all speeds, the r-pack is good. its on the outside of the plastic casing that the fan is in, this is behind the glovebox and passenger kick panel, but if its not bad, no worries.

its quite likely that the circuit board inside your heater/ac control panel is bad, and needs some resoldering. again, see https://www.volvoclub.org.uk/faq/Hea...olACNotWorking

 

Lots of great info in that link. Thanks for that. I've got the module out and partially disassembled but want to run my logic by you before I consider resoldering the circuit board as that might be flirting with disaster considering my skills set!

Here's how I understand the circuit;
#1. Plug A pin #10 only receives current when the fan is on even if the AC button is in the on position.
#2. Therefore there must be a pin on the fan switch which provides that current when turned on and bench checking the continuity of that switch should indicate which pin that would be.

Here is what that check revealed;
#1. Current comes into switch via plug C #1 and there is no continuity with any other pins when fan is off.
--In fan speed #5, continuity only with C #2 (like direct drive?)
--In fan speed #4, continuity is only with B #1
--In fan speed #3, continuity is only with B #3
--In fan speed #2, continuity is only with B #5
--BUT ... In fan speed #1, continuity is with B #4 and #6

I would think that there should be a pin in plug B that has continuity with C #1 all the time that the fan is on and my simple mind says perhaps that would be B #6 as that would be more or less sequential. Therefore the fan switch could be the problem. I will test that by next plugging it all back together and see if I get voltage to the receiver sensor at fan speed #1 only.

What say ye? TD

 

what I already said, fan speed control pin B6 should be a yellow-red wire to main controller pin A7

the fan speed switch goes to 3 resistor pack, which has 4 resistors in series. speed 1 goes through all 4 resistors to get to the motor, for the slowest speed. speed 2 goes through 3 of them, speed 3 goes through 2, speed 4 goes through 1, and speed 5 bypasses all the resistors and goes directly to the motor. the fan speed switch is connecting power to one of those 'taps' on that resistor pack...

said yellow-red control wire is connected to the speed 1 position and goes into a high impedance input in the control unit box. regardless of which 'tap' on the R's the power is connected to, that speed 1 tap will output 12 volts as long as its going into a high resistance at the control unit side (eg, the fact that speed 5 goes directly to the motor, means the yellow-red wire has all 4 resistors in series with 'power' but that doesn't matter if the AC control circuit is just checking for voltage).

does that make sense?

 

p.s. as I said twice, if your fan is working at all speeds, the fan speed switch AND the resistor pack is 100% AOK (OK, if the resistor pack looks charred on a visual inspection, even if its still working, its somewhere en route to its eventual death... but it usually doesn't die until a dying fan motor requires too much current, and cooks the speed 3 or 4 positions. said dying fan motor will usually be quite noisy. a 740/940 fan should be almost silent at speed 1 and very quiet at speed 2. and smooth at the higher speeds albeit there's plenty of wind noise at speeds 4 or 5.

 

Thanks again for the analysis. Just thought I might have been onto something by the way the continuity checked out on the fan switch. I previous got the connectivity picture but I still don't understand why pin B6 is only in the circuit, suppling voltage, in speed #1?

I just plugged it all back together this AM and ... everything is working perfectly, at the moment. I suppose it will continue to work until I'm in the middle of Nebraska in a few weeks.

I'm guessing that if it happens again, I will try replacing the resistor pack. In the meantime I have "IAC" (idiopathic-air-conditioning), the medical term for "we don't know the cause for the situation!"

If I learn more, I'll let you know. Again, thanks for your wise counsel. TD

 

A new anomaly has occurred in this mystery. The voltage going from pin #10 to the Receiver is still intermittent but seems to be affected by moving the temperature dial on the HVAC controls. I know that the AC on button only sends voltage out pin #10 when the the fan is operating but does that circuit also run thru the temp dial? TD