AC problem / Arizona = Bermuda Triangle? - Benzworld.org

in2dwww · 07-19-2008, 06:25 PM

Drove the 1994 S500 from South Carolina to California. It's one hell of a machine and is well-suited for the task. I actually can't wait for the drive back!

While in Chandler, AZ, we stopped for lunch.

But when we returned from the restaurant and cruised to Interstate 10, things got bad:
There was a sense that power was falling, then surging. We discovered it was in tempo with the AC compressor engaging/disengaging. (During the drive to our lunch stop, the car maintained 80c engine temp - but after the lunch break, the temp rose to 100c WHILE PARKED. The M119 runs hot and it was 110 degrees outside during lunch.)

All of a sudden, the oil gauge DROPPED to less than 1 bar and basically started hunting for 0. "Holy Shit," I exclaimed!

We were at an intersection with a gas station and pulled in. I checked the oil with the engine off and noticed that the level was fine, but the oil itself was running down the dipstick with a consistency of water. This, I know may be because the oil change I just had done in North Carolina before the trip was non-synthetic 10W30. It was obviously breaking down with the 110+ heat of AZ. We have already done 2000 miles to this point on 10W30.

A MB dealer was 4 blocks away and we pulled in, hunting oil pressure gauge, and a HOT engine. We all decided that the problem was just the oil viscosity, as AZ dealers use 15W40. "Change that oil NOW!"

So Chandler MB kindly took the car around back, swapped oil, gave it a nice wash to remove about 300 splattered bugs from the grill and windshield, and we were back on the road. Oil pressure gauge was accurate and reading fine after the oil change.

HOWEVER, this is where the fun begins - the AC compressor was acting up and not staying engaged. It would turn on, run for 1 second and shut off. It would continue to cycle in this fashion unless "EC" was activated. So we turn the whole HVAC off instead and drive through AZ\'s 105 degree day with the windows down.

So while the HVAC was off, the AUX fans were running. The car maintained about 85c engine temp from AZ to Los Angeles without the AC running.

The problem now: the AC compressor runs for 1 second and shuts down. I looked at the compressor clutch and it turns 360 deg in both rotations. I also pulled fuse 20 to reset HVAC and performed the HVAC diag to pull DTC codes - the are no codes.

AC pressure is 6 bar while the car is off. R134a charge is fine.

So I'm in CA and will take the car to a MB dealer on Monday. But what exactly is the problem? Compressor clutch is fine, there's a charge in the system, the engine can stay at normal temp without AC....

gold benz · 07-20-2008, 05:35 AM

You could try adding Cool Harness sensor. Less than $100. The relay keeps the aux fan running longer.

Exclusive Motors
8441 Miracrest Pl
San Diego, CA 92121
(858) 552-1112

Mr MB Motors
5557 'A' Reseda Blvd
Tarzana, CA 91356
(818) 708-8086

in2dwww · 07-20-2008, 08:37 AM

I've seen the cool harness, but my car only rose above 80c when we parked it. Presumably, the airflow during the travel aided in cooling, but when the car didn't have air flow for about 1 hour during lunch, the engine bay started to cook.

After that cook, the oil broke down considerably and then the AC compressor acted like it seized.

So I've scoured this board for clues on AC compressor problems and the only thing I came across is some sort of engine speed sensor. Could my AC compressor be cutting in and out in a second or two because the engine speed sensor is shot? I highly doubt there's a clutch problem, and when the compressor does kick in, you can hear the HVAC operating and almost feel slightly cooler air.

I've been racking my brain around this and I think it's electrical. Could the HVAC relay be causing the AC compressor to go wonky?

MikeJT · 07-20-2008, 10:30 AM

Quote:

Originally Posted by in2dwww

So I've scoured this board for clues on AC compressor problems and the only thing I came across is some sort of engine speed sensor. Could my AC compressor be cutting in and out in a second or two because the engine speed sensor is shot? I highly doubt there's a clutch problem, and when the compressor does kick in, you can hear the HVAC operating and almost feel slightly cooler air.

I've been racking my brain around this and I think it's electrical. Could the HVAC relay be causing the AC compressor to go wonky?

Your problem seems unusual because, as I understand it, the clutch is continually engaging and disengaging the compressor over a period of a second or so while the engine is running. On a properly functioning system, the clutch will stay energised until the evaporator temperature drops to about 3 degC (37/38 degF) and then de-energises until the evaporator temperature rises to 6 degC. This usually leads to cycling over a minute or more. If 3 degC can't be reached because of ambient conditions, the clutch stays energised.

You haven't mentioned whether the sensor values look alright. E.g., if your evaporator temperature sensor is reading very low or your system pressure reading very high, then perhaps this could cause it to cycle rapidly. I would try to see what is happening to these as the clutch goes in and out.

If the rpm signal from the back of the compressor doesn't match the engine speed signal, then the base unit will de-energise the clutch after a second or so. However, in my experience - and that of others - this often requires a key cycle before the clutch is allowed to have another go. Sometimes - but not always - it can be brought back in without a key cycle by switching the EC button on and off, but this usually requires a time lapse of several minutes.

Just in case yours works differently in this respect, I would examine the wiring coming from the back of the compressor (rpm sensor) to the connector on the front top, in case this has failed. I would also try squirting a degreasing agent into the clutch in case it has become oiled. I am in the process of checking my clutch in this respect and will post in due course.

Hope this may help,

Mike

in2dwww · 07-20-2008, 02:50 PM

Well, I'm dumbfounded right now.

Started the car this AM and ran the HVAC with the AC compressor on. It ran normally.

I let the car idle in P for about 10 minutes and then I decided to drive off. The moment I got the engine up to about 1500 RPM, the compressor started to cycle on/off every 3 seconds.

So I scoured the net some more and found that some ML's have a problem with too much pressure on the system's high side. The cause is from a "diaphram" or pressure switch on the high side that clogs from a combination of high pressure and shavings of metal bearings from the compressor.

I still think it's electrical - why would it work normally at idle with a cold engine? The aux fans come on and the air is ice cold. The HVAC diagnostic displays 6 bar pressure when the car is off.

Could the compressor clutch be on its way out? Is this a sign of early clutch failure? An increase in engine RPM or engine temp is the moment the compressor misbehaves.

in2dwww · 07-20-2008, 03:14 PM

So I found some info from R129 owners about their AC and followed a link to an E-class AC diagnostics page at:

Air Conditioning Diagnosis | EclassBenz.com

I used the HVAC diagnosis using instructions at:

K6JRF Auto Page

From my readings in the car, I entered the data into the E-class site's web page and it says that the AC is operating within the correct ranges.

So I think my problem is electrical. Where's the "evaporator temp sensor" on the W140? Could it be a blocked expansion valve? Or a faulty pressure switch? When the car is off, it registers 6 bar. When the engine is running and the compressor kicks in, it climbs to 30 bar and then the compressor shuts off and the pressure falls to 14.

in2dwww · 07-20-2008, 04:24 PM

Could it be a faulty AC pressure switch? Or blocked expansion valve?

MikeJT · 07-21-2008, 04:07 AM

The evaporator temperature sensor is located behind the centre console, on the driver's side, I believe. It's identified as component B10/6 and has a fairly short lead to a connector. I don't think that you can access it without removing the central wood panel, although I'm by no means certain. It may be accessible by simply removing the carpeting on the left side - I can't tell from the info I have.

There isn't a pressure switch, as such - it's a pressure transducer screwed into the receiver/drier under the hood. It's a 3-terminal device which should have between 4.75 and 5.25 volts between pin 3 (grey/yellow) and ground (pin 1, brown/yellow). The voltage to ground on pin 2 (blue/green/white) varies with system pressure: ~0.5 to 0.75V at 2 bar and around 2.5V or so at 18 bar. I guess you could check this but not sure how much help it might be. Of course, if the A/C control unit circuit monitoring this voltage is nbg, then this could also cause your problem.

6 bar sounds a bit high to me if the system has not been run overnight - mine is typically 4 bar - but don't know whether this is significant (over-filled system?). Others on this site have more experience.

Mike

fabbrisd1 · 07-21-2008, 06:45 AM

I had a problem that my aux fans were not coming on properly, and as a result the aircon was iffy at low rpm's at/under 1500.

My issue was traced to a faulty resistor pack, which is in-line with the circuit but placed up in front of the evaporator.

Long term failure of this specific part in these W140 of this period is common per my indie, who had the part in stock.

You got great service in Chandler, in Cali same-day-great-service can be hit or miss especially on Monday's.

For the drive back you may want to consider adding a quart of Lucas oil stabilizer which could help on the long dry deserts, helps retain viscosity in heavy load/heat conditions.

**Nimrodthewombat** · 07-21-2008, 08:23 AM

My 2 cents......There may be another problem that I haven't seen here yet. Unplug the wire from the coolant temperature sensor. I have seen some sensors go bad and tell the computer that the engine is overheating which will shut down the ACC compressor (if the engine recently overheated that may be a clue) or if your car has the cool harness already it may be possible that the PO accidently reversed the polarity of the harness sending a signal to the computer that the engine is overheated, again telling the ACC to shut down the compressor and turning on the aux fans while the car engine temp is at 80 degress centigrade.

Don't ask how I know about the second problem! :-)