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99 Merc S420 (80k Mi) 07 Lexus LS460 (21k Miles)
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Discussion Starter #1
I just had my A/C system overhauled after finding trace amounts of R-134a in the R12 system. I replaced the drier, refrigerant temperature sensor (controls both aux fans and threads into top of drier), and refrigerant pressure sensor along with all new R12 and oil (entire system also evacuated). The car only took 3 12oz cans of R12. The bubble reader on the top of the drier indicates that it has a correct charge of R12. The problem is that the 2 front aux condenser cooling fans are short cycling. They come on for a few moments, then go off, then come on, then go off. This is at idle. I don't know what they are doing when I'm driving. They didn't do this before. They would come on as needed, then go off for a long while, then come back on for a long while.

Is the new temp sensor the problem? Or is the drier giving me a false reading and the system just needs more R12? If I unscrew the temp sensor off the drier, will I start releasing R12?

Any ideas?
 

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defective switch perhaps?? What are you high/low side pressures? Could be a relay on the way out as well (one of the ice cubes in the fuse box).



Jonathan
 

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99 Merc S420 (80k Mi) 07 Lexus LS460 (21k Miles)
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Discussion Starter #3
I'm going to get the pressure readings next week when it is warmer out. It's hardly hot enough to turn on the A/C right now.
 

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Quite...! I'd also try swapping around those relays..

Jonathan
 

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99 Merc S420 (80k Mi) 07 Lexus LS460 (21k Miles)
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Discussion Starter #5
If I unscrew the temp sensor, will the R12 start gushing out?
 

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99 Merc S420 (80k Mi) 07 Lexus LS460 (21k Miles)
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Discussion Starter #7
So, if it does come down to the temp sensor then I'm going to have to pull it off, loose all my new R12, kill the drier and be back to square one again!! Hopefully the charge is low, but not showing in the drier window (why ???).
 

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Instead of that I'd wire up a switch under your dash... I had just this very problem on my 87 560SEL.. So instead of losing all my R12 I invested about 10$ in wire and a switch. I rather liked that set up as I could switch the fan on before I hit traffic, or I could leave it on for a minute after I turned the car off..

Either way it's a very handy mod and can Easily be placed out of site yet readily reachable.

If your window is clear I suspect a faulty switch or relay.

Jonathan
 

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1991 300SE
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If you know some repair places, you could have them remove the R12. Then, you take your car home and swap out the switch. (or do it in the parking lot!) Having the switch out for a little while should NOT ruin the dryer. Then go back, evacuate the system and have them recharge it. Even if they charge you a few bucks, it should be worth it.

I also like the aux fan switch idea from Jonathan. I'd go to the wrecker and acquire an extra switch that goes into your center console or above the climate system, and hook that switch up to a relay to run your fan. That way. it looks stock. :)
 

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99 Merc S420 (80k Mi) 07 Lexus LS460 (21k Miles)
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Discussion Starter #10
If you know some repair places, you could have them remove the R12. Then, you take your car home and swap out the switch. (or do it in the parking lot!) Having the switch out for a little while should NOT ruin the dryer. Then go back, evacuate the system and have them recharge it. Even if they charge you a few bucks, it should be worth it.

I also like the aux fan switch idea from Jonathan. I'd go to the wrecker and acquire an extra switch that goes into your center console or above the climate system, and hook that switch up to a relay to run your fan. That way. it looks stock. :)
Good idea, but I'm not putting in a manual switch. These were designed to be automatic and that's way its going to stay.

Could the drier be giving me a false reading? There is somewhere around 34-36oz charge in the system right now. MB specifies 46oz. Drier says its fine and the compressor isn't short cycling a current charge.

What should the high and low pressure size reading be? I'm going to hook it up to a set of gauges next week and see if it just needs more R12.
 

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Automatic is nice, but I Really came to prefer the manual.. And you being an M103 which already likes to run warm I think a manual switch could have multiple benefits. If the system calls for 46 and you have around 35oz's you are indeed short.

The sight glass is handy, but gauges are really where it's at. I haven't looked @ a set since last summer, but you want to see around 30-40 degrees F on your gauge..I forget that that translates to in pressure. Your High side shouldn't be in the 150-250 range depending on exterior temps.

Jonathan
 

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1991 300SE
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The pressure switch yes, the temp sensor no. Both go on the dryer.
Actually, I think that's correct. You would think that would be an intelligent design in these cars. Just barely unbolt the existing temp sensor. If it doesn't leak out refrigerant, then you're golden! :)

(I still think your R12 level is low.)
 

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99 Merc S420 (80k Mi) 07 Lexus LS460 (21k Miles)
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Discussion Starter #14
I had my mechanic hook up his set of gauges and we watched the pressure on the high and low side. They seemed OK, but a little low. So, I added another can or so (some didn't make it due to a bad can tap), so it should be up around the 46oz charge range now. I let the car sit and idle with the A/C on auto, set around 66-68, in the shade for about 30 minutes and just watched it. The outside are temp is about 76. The fans didn't seem to short cycle, except for once. The air also seemed a few degrees cooler. I think my problem has been solved. It seems to be operating correctly now.
 

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1982 280 se aut. ecc. - sold
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Good for you!
Thanks for reporting back. This might help people in the future.

gr J
 
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