Hey all, ive recently been working on a 91 745 to put my wife in, she currently drives my 780 and with another kiddo on the way a coupe aint gonna work.

Ive resoldered the MCC unit, all solder joints that looked bad and still cannot get 12 volts to the low pressure switch. going to be heading to the pick yard tomorrow to grab a unit, So since ive got a 91 745 which MCC can i use in this car, from what years and can i steal from a 940 if i cant find a 740.

Ive read through the FAQ on the MCC issues and have done everything in my power to get 12 volts to the Low Pressure switch and just cant seem to do so, i do not know if the heat works and with it being 90 in the day time i really dont want to find out. the Blower Motor is functioning as well as the vent change function, the only time i see lights on the Recir and A/C button is with the headlight switch on, is this normal? yea they do change color with the headlights on when i push them in and out.

Okay i went to the pick yard today, ( the expensive yard ) anyways about a week ago i took the compressor off this 91 740 and the system was still charged, so i took the MCC out of it today, slightly different then the one that was in my 745, strange. anyways im going to install it today and see what happend. Ill update my progress/success.

When you get this figured out, let me know for the FAQ.
stephenringlee at hotmail dot com
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See the 700/900 "FAQ" at the menu bar top screen left side.

I have finally figured out the issue of my MCC not working and it was not in the FAQ so this should be added.

After re soldering and replacing the relay i still had no luck in getting the A/C to function ( kick the compressor on ). I checked wiring and everything in between and all was working as it should. i have 2 MCC units, one with a Snow Flake logo and another said A/C Off. both were identical otherwise. ive spent about 15 hours total trying to solve this mess, I took a good hard look at how this A/C button actually functioned, i noticed it relied on the Turn Knob in the middle which controls the vent's and defrost, after turning one of the units to defrost i heard the compressor kick, it still would not kick with the A/C button, i took the complete knob assembly from the other unit which i determined to have a bad electronics board and put it in the one which kicked on when in defrost mode, this required un soldering the 2 black or Red wires ( depending on Snow Flake or A/C Off button ) and re soldering them to the other board. i installed the unit once more and it worked! So please add to the FAQ that when all else fails it could simply be the Vent Control Assembly that is preventing the A/C button to function.

See photo here
http://www.brickboard.com/IMAGELIB/volvopic.htm?js_link=1&id=10134

Got it: many thanks for tracking this down.
Steve
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See the 700/900 "FAQ" at the menu bar top screen left side.

Steve
I have two different units, one with A/C Off switch and one with a snow flake button. both hook up the same on the rear and appaer to have the same PCB board, did not compare P/N's. They both work as specified. sadly in my case both units have bad relays, i verified this by taking my dremel tool and cutting the top off the relays. they were not making contact, the only time i could get them to contact was with a screw driver, pushing them together didnt do anything unless i pushed really hard. Worn contact's, they are not really serviceable due to the lack of room within the relay.
Ive ordered the relays from digikey ( as specified in the FAQ ) and once i install them i will search out this thread and update it.

I have worked a lot with these. Resoldering IME is at best a temp fix unless it's truly Resoldering, removal of old solder and put new, not just reflowing.
I have one unit that is this way and works, and another unit that I got the replacement relay and that works also.
There are two similar units, one has a snowflake on the button and one has AC. I don't know if the different buttons sig real differences, I was advised by someone fairly knowledgeable that they *may* work the opposite of each other, not sure though. But if those buttons match the unit matches, so 91-2-3 will have compatible ones. I have all those years myself and they have the same unit, 7 and 9.

I finally figured it out, check my reply to Steve. 2 units with 2 different problems, was a big mess, but i was able to salvage one.

Very involved, congrats on your fix!
Looks like the two models are the same, just the button logo is changed, is that right? It'd be good to have that established I think.

yes they are the same, Different button and different color wiring from the panel to the PCB board. position of the wire's are the same as well as all other aspect's, they are Interchangeable with no modifications.