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1994 S600 Coupe, 1995 S600 Coupe
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Discussion Starter #1
A/C has worked fine all summer, until now. Start the car, and A/C works very well. Runs, cools the car, goes to low fan mode, and compressor cycles off eventually to defrost evaporator. Air from vents gets warmer and warmer, but compressor doesn't come back on. When this first started happening, the compressor would eventually cycle on, but only after the air got pretty warm. Now it doesn't come back on. I've verified that the clutch isn't engaged on the compressor. I first thought the expansion valve might be sticking, so I wanted to verify that the compressor wasn't running.

If I shut the car off, and immediately restart, the compressor cycles on, just like nothing's wrong, and the car cools. Just turning the HVAC off and back on while the car is running doesn't restart the compressor.

Yesterday, when the car was cool, I checked the sight glass and saw bubbles, so I added some R134a till they disappeared. I thought low R134a might be the problem, but it happened again today. I looked at the sight glass again, and saw significant bubbles. I'll check again tomorrow when it's cool, and see if there are bubbles then, as I know you can get them when the system is quite hot. Today it was near 98°F and quite humid, so the system was working hard. I'd think if it was low R134a, the compressor wouldn't come on even after cycling the engine.

I haven't done any STAR diagnostics yet, so I don't know if I have codes or not. I'll do that tomorrow. I'll look at live data to see if any sensors look weird.

Anybody have any ideas where to start? If the system detects a fault, does it not allow the compressor to engage until the car is turned off and back on?

Does anyone know if any particular sensor can cause symptoms like this? I've done a fair amount of car A/C work in the past, so I'm familiar with how the systems work, in general.

Thanks.

Jon




Jon
 

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Registered
1994 S600 Coupe, 1995 S600 Coupe
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1,700 Posts
Discussion Starter #2 (Edited)
Checked sight glass for bubbles this morning, found a few, and added maybe an ounce of R134a. High and low pressures with gauges appear normal. Ran a STAR diagnostics scan on the HVAC system, one code for the recirc pump which has been working ok, and it cleared. Nothing related to the sensors or the a/c shutting off.

Checked live data of the sensors, and all appear normal. Will drive the car later and if the problem recurs, I'll check live data then.

Just checked Startekinfo and Starfinder, and the a/c clutch signals go thru the Base Module, so I need to pull codes there also. Will report back. Basically the HVAC control in the dash sends a signal to the BM to engage/disengage the compressor clutch, and the BM reports back to the HVAC control. The compressor also sends an RPM reading to the BM, which apparently allows it to detect a slipping or missing drive belt.
EDIT: Just scanned the BM. Two codes, one for a blocked a/c compressor clutch, and one for a slipping drive belt. The belt was new about 15,000 miles ago. Not sure what the "blocked clutch" means. Hope it's not the clutch or compressor going bad or locking up. Cleared both codes, and they didn't immediately recur.

Jon
 

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Registered
1994 S600 Coupe, 1995 S600 Coupe
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1,700 Posts
Discussion Starter #3
Went driving, had the problem again. Shutting off the car at stoplights and restarting did not solve the issue temporarily, until the third time I did it. Came home, and got the compressor clutch blocked code again from the BM. Either I've got a compressor or clutch that are hard to get moving, or I've got a intermittent connection between the BM and compressor. After the car cools down, I'll pull the BM and check for bad connections on the affected pins. Will also clean the BM fuses.

Jon
 

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Registered
1994 S600 Coupe, 1995 S600 Coupe
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1,700 Posts
Discussion Starter #4
Pulled of the air cleaner over the compressor, and pulled the plug connection on the compressor. Opened the plug, and all connections to wires looked good. Pins on compressor side looked questionable, so I sprayed every thing with Dexoxit, and reassembled

Pulled the fuse blocks to access the a/c harness connector underneath. Separated it, but everything appeared pristine. Reassembled and moved on.

Pulled the Basic Module from the coffin box and opened it up. Found 4 open relays inside. Burnished the contacts on all 4 with a burnisher. Also found some questionable solder joints at the pins that interface with the compressor, 2 for the clutch, and 2 for the RPM sensor. Reflowed the solder on all 4. Also cleaned the contacts on the module and on the connector in the coffin box with Deoxit.

Drove it in stop and go traffic in 98° heat for over 1/2 hour with no issues. Too early to declare success, but so far, so good. If problems recur, I'll swap the BM out of my other coupe to rule that out, and go from there.

Jon
 
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