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  #1  
Old 08-21-2017, 10:56 AM
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AC wierdness

Hi I have a 1987 s124 diesel wagon. The previous owner had the AC converted to R134a, and it did not work when I got the vehicle. They'd replace the receiver drier and the compressor. I replaced the AC control unit, the klima relay, the expansion valve, and the black pressure switch. I did not like the pressures, so I replace the receiver drier. There are few specifications specifically for the 1987 so charts have showed me that it either holds 2.18 or 2.875 pounds of R12. Conversion chart show me that for R134 it should hold 1.7 or 2.3 lb. I went with 2.3 lbs. I'm an experienced mechanic and have access to a recharging machine. The car will cool beautifully and then cuts out the compressor. It will not come back on unless I turn off the ignition and turn it back on. Simply using the AC control won't do. I can do this at speed. The pressure will be approximately 20 pounds and 240 when it's running at 2200 RPM at 94 deg, cooling output under 45 deg.
It seems to be electrical but I am stumped.
Any ideas?
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Old 08-21-2017, 12:29 PM
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Have you validated the operating condition of your climate control unit? Programa 124-830-32-85 should be the unit unless you have Euro Manual.
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Old 08-21-2017, 01:31 PM
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Yes, and ordered it by application from our hosts.
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Old 08-21-2017, 06:02 PM
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If it helps, my W126s spec 2.9lbs and the W124 specs around 2.2lbs

2.3lbs of R134 seems significantly overcharged in that wagon.
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Old 08-21-2017, 07:17 PM
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The speed sensor on the rear of the compressor will disengage the clutch plate if it sees a variation in speed of the engine vs the compressor - such as a dragging compressor creating belt slippage or slippage in the clutch plate.
It will not allow re-engagement of the clutch until the ignition key is turned off/on. I had a similar experience just last week; compressor would engage and almost kill the engine, causing the speed sensor to shut off the compressor as the engine stumbled and hesitated. Thought it was a stalling compressor due to a clogged line or something raising the pressure, but then noticed the IAC valve was not working smoothy; cleaned the IAC and the problem has corrected itself. Hope that was all it was, keeping my fingers crossed. in your case, maybe it could be too high pressure from freon overcharge putting enough drag on the compressor to create a bit of slippage? Does the auxillary fan work?

DG
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Old 08-21-2017, 08:01 PM
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2.3 of r12 would equate to 1.82 Lbs of R134, according to http://www.tirereview.com/wp-content/uploads/Articles/04_01_2013/1123954jpg_00000062824.jpg
It cant hurt to try it but the pressure is low compared to this chart, AC Pressure Chart | R-134a Temperature Pressure Chart

I did the full electrical test and the only failure was the speed sensor, so it got replaced too.
New belt as well, maybe I will tighten it a bit and try again
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  #7  
Old 08-21-2017, 08:29 PM
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The compressor speed sensor is a common failure point. If you're at a point where it's reasonable to reaplce, do so. I struggled with this for my car, but since I would need to evacuate, flush, install a new dryer, etc, I opted to circumvent the Klima relay. You can find a lot of info on that here:

Compressor speed sensor.
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Old 08-21-2017, 09:21 PM
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Have you tried a new Klima relay? Maybe reflow the one you have.

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Old 08-22-2017, 01:42 AM
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Both are new. Thanks
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Old 08-22-2017, 01:52 AM
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Using Mxfrank's diagrams, check duty cycle or AC voltage on the engine input pin and compressor input pin when the AC is working and when the compressor cuts. Jump the compressor just to get a signal after the compressor cuts.

Hmm... maybe you can fit a momentary off switch to the Klima relay power pin to reset it without turning off the engine.

Sixto
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  #11  
Old 08-23-2017, 03:30 PM
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I hate to sound like a "did it all", but I did the Klima testing after the module didnt fix it, (resoldered the original before that). The speed sensor was the only failure so I replaced that, then relaced the Klima and it finally worked, when it wants to. I guess I will throw a belt tensioner shock at it next, as I cannot adjust the tension more. I have been tempted to bypass the relay, and the toggle switch sounds workable too, but they are both cheating, I want it to work as Mother Benz intended.Then, I will pull the dash and change out all the vacuum servos.
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  #12  
Old 08-24-2017, 12:41 PM
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You need to diagnose the system following the factory service manual, which has about 21 steps to test all the sensors and controls, in order to really fix it right. Note that the FSM calls for a special adapter / tester unit that plugs into the two harness connectors for the PBU; you can figure out all of the component connections by studying the wiring diagram.

Guessing and shot-gunning parts gets expensive and frustrating fast.

My guess is that either your KLIMA is bad, or one or more of the inputs to the KLIMA are bad, so it cuts out the power to the compressor clutch. However, the PBU may be shutting down due to a bad component somewhere drawing too much power through the PBU. When the AC stops working, can you push the defrost button and get full heat on full fan through the windshield vents? If yes, than the PBU is probably not killing the signal, the KLIMA is.
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  #13  
Old 08-24-2017, 03:25 PM
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Thanks. I did follow the 50 step testing procedures here,

A/c Relay - Mercedes-Benz Forum

before buying a new speed sensor, then a Klima relay from Peachparts. It all works right after that, except when it cuts off.
Thinking more about belt tension, no glazing is evident, but will oRder a new damper and spring payday, it does have 220,000 miles on it.

Last edited by ObsoElitist; 08-25-2017 at 12:04 PM.
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  #14  
Old 08-31-2017, 03:15 AM
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any updates ?
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Old 08-31-2017, 05:17 PM
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No. Thanks for asking. Today is payday, ordered new tensioner spring and damper, and just to be thorough, the only sensor not replaced, the red one. Should be here in a week
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