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Discussion Starter #1
Hi there

I'm a new member and have been viewing this forum for some time now and have saved hundreds of dollars fixing things like hard shifting trans (turned out that the vacuum line to the back of the inlet manifold came off after a recent engine steam clean) and some other general repairs. This is in my opinion, the best Mercedes site going around ! Well informed and excellent response time, unlike some other forums I have noticed.

Anyway I have a problem with my a/con, it doesn't operate properly. What I do know is that the unit appears to be fully charged, the fuse to the compressor is ok, the lights on the console switches come on when the unit is switched on, but the clutch is not operating/engaging.

I noticed in one of the forum reply's regarding the thermo fans in front of the radiator that one member suggested that these work only when the a/con is operating. Is this right ? The reason I ask is that the fans came on and then off rather quickly today when I was running the car doing some exhaust leak checks. It was about 36 deg celcius in Adelaide, Australia today - around 100 Farenheit. Will they only work if the a/con clutch is operating ? If so then perhaps the clutch was engaging intermittently today ?

I'm fairly mechanically minded & more than willing to have a go at fixing this myself - cheaper also !

Is there any way to 'hot wire' the clutch to see if there is a power feed issue ? I took a quick look at the wiring but couldn't see any broken wire to the compressor.

Any help would be very very welcome indeed as it is very hot here at present.

Cheers
AussieAndy
1989 500SEL
 

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87 420SEL, 89 560SL, 92 300TE
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Hello and welcome!

I had a similar problem with my A/C recently, and it turned out to be a faulty "hi-temp sensor" on the intake manifold. There is a small sensor at thr front-center of the intake (about 1-inch in diameter) with two connectors on it - one two-conductor and one single-conductor. The single-conductor connector sends a signal when the coolant temperature exceeds a certain value, so the A/C doesn't overheat the engine. If it is faulty, and signals over-temp when there is none, the compressor will not start.

There is also another temp sending unit nearby, but it has only one connector on it.

If you start the car, engage the A/C and pull the single-conductor connector, the compressor will start if this is the issue. I left mine disconnected until I got and installed the new sensor (about $40 from the stealership, IIRC) a couple of days later. It worked fine- you just need to watch the temp.

I hope this helps, as this is a quick (about five minutes to install), easy and cheap fix.

Good Luck!

-Robert
 

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Discussion Starter #3
Fantastic !

Thanks for the speedy reply Robert ! I waited four weeks for a reply on the hard shifting problems I had with my trans, with not one single reply. I also have a 1988 BMW 730i and have used the Roadfly forum (questions and answers) for that car and the response is thick and fast. Not the case with Mercedes though, funny isn't it !

I'll your suggestion out first thing in the morning (it's about 1.00am Adelaide time at the moment) and I'll advise how I get on. This sounds like an easy and inexpensive fix.

Thanks again Robert.

Cheers
Andrew - AussieAndy

1989 500SEL
1989 420SEL
1988 BMW 730i
 

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Discussion Starter #4
Robert

I tried your suggested diagnosis and unfortunately had no joy. I disconnected the single connector and unfortunately the clutch did not start spinning.

The clutch turns ok with your hand (although not really easy to spin) it just will not start up on it's own.

If you or any of the other members have any other ideas I would 'really' appreciate it.

Cheers
Aussie Andy

1989 500SEL
1989 420SEL
1988 BMW 730i
 

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87 420SEL, 89 560SL, 92 300TE
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62 Posts
Well, heck, then. I guess the next thing I would do would be to check the A/C clutch by running jumpers from the battery directly to the compressor (the "hot-wiring" you mentioned)- unplug the existing connections first, then touch the "hot wire" from the battery to the "start" terminal and see if the compressor starts up.

If that's a no-go, you'll have to check all the standard A/C problems. These vehicles have a low refigerant (freon) pressure cut-off also, so if your freon pressure is too low "at rest", the compressor will not start. I can't recall all the other conditions that must be met for the compressor to start - all I can remember is the high-coolant-temp cutoff and the low-refrigerant-pressure cutoff. The diagram MBL posted should help - it was the reference I was using when I found the high-coolant-temp cutoff problem on mine. If you know a good A/C shop, I'd have the freon pressure checked. If that's the problem, a quick recharge will correct it. That sensor could also be bad, but I don't know where it is located or how difficult it would be to replace. You'd probably have to "break-into" the freon system to replace it, though, so you'll need a recharge afterwards even if you don't need it now.

If the system has not been previously converted to R-134a, and you plan to do so, the conversion process should correct this problem, since a new compressor is needed.

On the "aux fan" question-- IIRC, the fan starts up when coolant temps exceed a certain value. I don't think the A/C compressor must be on for the aux fan to start.

Good luck, and let us know what happens!

-Robert
 

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If you think your system is charged, your refridgerant OIL level may be low. R12 is very caustic, and needs a certin type of oil to keep it lubed. Tho I think a change to R134a would be the best way to deal with the problem, since R12 is REALY REALY EXPENSIVE. The swap out will hit the wallet hard at first, but the recharge should be much cheaper in the future.
 
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