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post #1 of 8 (permalink) Old 07-16-2010 Thread Starter
kbj
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1991 740 A/c issues

Allright I have searched the forums and come up empty. I have 91 -740 Turbo. Where is the mysterious "relay" mentioned in the Haynes manual that turns the darn A/C on?

I have jumpered the power wire to the compressor and it turns on and runs just fine! Bad news is, it doesn't get cold. ( Ok so I must have a leak too (:-( ) Jumping the low pressure switch does not turn it on.

I have pulled out the control panel and found that the A/C button activates a micro switch in the vacumn line area. According to the wiring diagram there is a delay relay somewhere, but it is not in the relay bay.
Can anyone please tell me where it is?

What else could cause the power to the compressor to fail to get there?

The Volvo dealer thought the compressor was frozen, not the case!

Vacumn leak? Separate vacumn pump for the A/C?

Same problem with the cruise control. Does not activate. The wiring diagram again shows a mysterious brain box and vacumn pump for it.

Where are they!!!???

I'd like to help my 82 year old father out, but am getting nowhere with the repair manuals or the dealer.

Any advice/expertise with these issues would sure be appreciated!

thanks.
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post #2 of 8 (permalink) Old 07-24-2010
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Have you tried changing out the switch with another one to see if that is the issue? Here is a link on Volvo relays that may also help you out.

Volvo Relays
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post #3 of 8 (permalink) Old 07-26-2010 Thread Starter
kbj
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Thanks for the info! The relay locations and fault diagnosis should be very helpful.
The switch is part of the control panel and does not appear to be seperately replaceable. It actually is just a lever that seems to operate a micro switch or vacumn gate in the vacumn line area input to the main control box. There are no wires going into this area, only vacumn hoses!
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post #4 of 8 (permalink) Old 08-01-2010
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FWIW, I feel your pain.. I'm experiencing the exact same problem with my '91 740 Turbo. Because it is *really* hot here in TX, I've been running with a long jumper wire clipped to one of the wires going to the low pressure switch on the accumulator. Yes, it does make a difference which one, because you do not want to bypass the switch and just have the compressor run constantly. I first ran a jumper wire from the battery to the wire that goes to the compressor, just like you did, except I had the key off/car not running. I unplugged the connector on the low pressure switch on the accumulator and used my test light to see which prong was getting current. I then stripped a small bare spot on the wire going to *the other* prong (the one that was notgetting current) and connected my long jumper wire with alligator clips to the battery. If you do this, you can run the AC and stay cool while you try to solve the original problem. Of course, it's a PITA to have to clip and unclip the jumper wire every time you shut the car off for more than a few minutes, but it does beat sweating.

Right now, I'm thinking the easy/quick fix is going to involve replacing the control panel, which includes the on/off switch, fan switch, etc. I pulled one out of a wreck today and hopefully will get a chance to install it sometime in the next few days. I'll let you know how that turns out...

P.S. After re-reading your original post, I realized that you mentioned that the AC did not cool when you jumpered the compressor directly from the battery, although the compressor did come on. This would almost certainly indicate low or no refrigerant, and in this case you most definitely do not want to run the compressor much or you risk seizing it from lack of oil, which circulates with the refrigerant. If you hook a jumper wire as I described above, the compressor will probably not come on because the low pressure switch will not allow it. In my case, about 3 days after running the AC with the jumper it started blowing hot air again. I thought my jumper had come loose, but all connections were good. Long story short, I had developed a leak which caused the low pressure switch to shut down the compressor, saving it from frying. I had to find/fix the leak and recharge the system to get cool again and resume my search for the on/off switch problem. Sounds like you may be in the same spot.

Last edited by Joseph/TX; 08-01-2010 at 02:46 AM. Reason: addition
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post #5 of 8 (permalink) Old 08-01-2010 Thread Starter
kbj
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740 A/C issues

Hi, thanks for your reply! So are you only supplying power to the one prong on the low pressure switch? Not to the compressor directly?

I hope your control panel swap helps, but I don't see that micro / vacumn switch being as likely to go bad as I do the relays that apparantly are located behind the glove box. I am going to try to find some of those at salvage and change them out. Let's hope one of these possibilities works!
let me know what results you get and if you could just exactly what jumper/jumpers you are running. If you are only using one on the low press switch, does this mean that the compressor will cycle on and off?

Thanks

kbj
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post #6 of 8 (permalink) Old 08-01-2010 Thread Starter
kbj
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740 A/C issues

Hi again! Upon reading your full post ( the whole thing didn't show on the link) I see what you mean. We have had some success with older cars (ie: 93 and older R12 setups) just putting in R134 and they have worked fine.
I'm not really interested in spending the $ to upgrade the system to r 134 and may just see what happens with a shot of R134.
Anyone else tried this?

But first, I have to get the darn thing to turn on! Seeing as there is no power coming to the compressor, I have to think that the compressor delay relay is at fault. Jumping the the low pressure switch does not turn it on.
I will have to check salvage for some relays.

Meanwhile, if anyone knows why my 97 Ford Ranger all of a sudden has nothing but static on the AM radio when the ignition is on - please chime in! (Yuk, Yuk!)
Relay issue?, faulty plug - doubtful, because happens also when key is on, not running, when running, but not when on Accessory key setting.???

What else in an ignition system can cause radio static?

thanks!

kbj
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post #7 of 8 (permalink) Old 08-01-2010
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The plug that connects to the low pressure switch has two wires, each of which corresponds to a prong on the switch. You need to jump to the wire that carries current into the switch, not out of it, so the switch will work. The current flows through one wire into the switch and back out the other wire to the compressor if the switch is closed; which it will be if the accumulator is over 25 psi or so. If the system is empty, there will not be enough pressure to close the switch. If the system has a low charge, the compressor will pull the pressure down below the cutoff point and the switch will open, cutting power to the compressor. You can run a jumper wire to either of the wires on the plug and the compressor will come on as long as there is sufficient refrigerant charge (assuming the switch itself is good) but only one wire will run the current through the switch, which is what you want. If you have zero charge because of a leak, the compressor will come on only if you bypass the switch, in which case you are running it without oil because the oil circulates with the refrigerant. Very bad. It sounds to me like you have already lost all your refrigerant, since you said that the accumulator didn't get cold even when you ran the jumper directly to the compressor and forced it to come on. My friend, it sounds like you are going to have to spend some money to get cool. You need to leak test the system and the best way to do that is to take it to an AC shop that uses a mixture of nitrogen and R22 to pressurize the system up to about 200 lbs at which point the leak can be located with an electronic detector. The shop I use charges around 50 bucks to do this. Alternately, you could charge it yourself and use whatever method you prefer to find the leak. The problem with dyes and soapsuds is that you can't see every possible leak spot. What if the evaporator is leaking? Are you going to take the dash apart to check for dye spots? Electronic is the only way to go, in my book. Once you have found and fixed the leak you can resume searching for the reason it won't turn on with the switch. Hopefully by then I will have found the reason for the same problem in my car, which I will surely pass along. Your radio problem sounds like a bad antenna connection to me. FM is a lot less sensitive to this than AM; I've had cars that would get FM with no antenna at all, not so with the AM.
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post #8 of 8 (permalink) Old 08-01-2010
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By the way, please don't just try adding "a little bit of R134A" to see what happens. A little bit won't be enough for cooling or leaktesting and if the system is not already converted from R12 you will just make a mess inside. The mineral oil that is used in an R12 system wil not mix with 134A and therefore your compressor will get no lubrication. IF you just add ester oil on top of the mineral oil that's already in there without flushing the old stuff out you will have way to much oil to get any kind of good cooling.
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