Long and Covoluted intermittant A/C Problems with '88 300TE - Page 4 - PeachParts Mercedes-Benz Forum

brewtoo · #46 06-25-2007, 02:41 PM

I did not read thru all this so forgive my basic question if this has been answered.

Do you have bubbles in the sight glass?

CTM VT 2K · #47 06-25-2007, 03:56 PM

I don't think so. The sight-glass is quite dirty, and difficult to clean enough to see anything.

brewtoo · #48 06-25-2007, 04:38 PM

Clean it with the eraser on the end of a pencil.

CTM VT 2K · #49 07-03-2007, 09:36 AM

Ok. I cleaned the sight-glass, replaced the Aux Fan Series Resistor, cleaned the electrical connections, and did a spray-cleaning of my engine (with extra attention to the AC compressor).

The AC now works indefiniately at MAX cool and MAX fan in HOT weather.

Any other condition causes it to fail at between 10 and 20 minutes.

In 95°F ambient temps, with the fan on MAX and the temp on MAX COOL the vent temps never get much below 50°F, but it runs for as long as I need it (or even longer than I can stand - I don't need the cabin to be in the 50's when I'm dressed for a nearly 100°F day). IF I change any setting, the compressor shuts off very quickly. If I start with the fan on the minimum blow setting, the vent temps get down into the 40's, and the compressor shuts off after about 15-20 minutes.

In 75°F ambient temps, with the fan on MAX and the temp on MAX COOL the vent temps get down into the low 40s, and the compressor shuts off not long after getting the vent temps below 43°F (typically about 15-20 minutes).

In temps below 70°F, with the fan on MAX and the temp on MAX COOL the vent temps very quickly get into the low 40's, and the compressor shuts off not long thereafter - sometimes in as little as 5-8 minutes.

My assesment:
I have corrected several problems that were contributing to the problem, and I may be down to the last problem, or handful of problems.
The key issue I am seeing now is one that I believe to be thermodynamic - if I keep the system running at full blast in hot weather it never chills below a critical point somewhere within the system - thus it never shuts off the compressor. If I do not keep enough fresh HOT air blowing over the system (and thus keeping the system above this critical temp) the system will chill below the threshhold, and the compressor will shut off.

I take this to mean that I may have moisture inside my system (freezing and blocking a line, thus driving pressures to trigger a safety cutoff), or something similar.

Or am I really out in left field? I'm taking care of my forward Flex-Coupling this month, so professional AC repairs are not in the budget until next month.

david s poole · #50 07-03-2007, 10:00 AM

have you checked or bypassed the etr switch?

CTM VT 2K · #51 07-03-2007, 10:13 AM

Quote:

Originally Posted by david s poole

have you checked or bypassed the etr switch?

I have not. Basically, I have not done any new checking inside the cabin. I will work on checking this first thing - probably tomorrow (have the day off, so I can spend it tinkering).

jcyuhn · #52 07-03-2007, 11:14 AM

Sounds like the compressor runs until the first time it cycles off, then won't restart.

I don't know if the ETR has a failure mode where it lets the compressor run, then shuts it off. I would guess the answer is no, because the compressor always runs when the car is cold started. I can't see the difference between letting the ETR sit and warm up in a parked car vs. driving down the road with the climate fan running.

To me this sounds like the compressor clutch gap is out of spec. (How many times have I written that statement?) Failure to re-engage when hot is a known fault on Nippondenso compressors when the gap is out of spec. DTDT myself.

- JimY

david s poole · #53 07-03-2007, 11:56 AM

the failure mode on etr switch is to shut down at 36degf [supposed to come back at 39-41] and take it's own sweet time to close contact again.[just like an independent teenager].

CTM VT 2K · #54 07-03-2007, 12:29 PM

Quote:

Originally Posted by jcyuhn

Sounds like the compressor runs until the first time it cycles off, then won't restart.

I agree.

Quote:

To me this sounds like the compressor clutch gap is out of spec. (How many times have I written that statement?) Failure to re-engage when hot is a known fault on Nippondenso compressors when the gap is out of spec. DTDT myself.

- JimY

I lack the equipment to properly check that gap, or I would have had that addressed long ago. I am trying to isolate out everything I can, so that I don't have to have pay my mechanic to go on an open-ended hunting expedition. If I can nail down two or three specifics to check (the compressor clutch as a whole is already at the top of the list) that saves my mechanic time and me $. Either way, next month (Assuming nothing else blows up before then) I'll get the AC system properly fixed one way or another.

CTM VT 2K · #55 07-03-2007, 12:30 PM

Quote:

Originally Posted by david s poole

the failure mode on etr switch is to shut down at 36degf [supposed to come back at 39-41] and take it's own sweet time to close contact again.[just like an independent teenager].

Would such a failure then prevent the AC from re-starting until the engine has powered off and cooled down?

david s poole · #56 07-03-2007, 12:36 PM

i have seen that occur yes.

jcyuhn · #57 07-03-2007, 05:58 PM

Quote:

Originally Posted by CTM VT 2K

I lack the equipment to properly check that gap, or I would have had that addressed long ago.

All you need to check the gap is a set feeler guages, as are used to set valve lash or check a spark plug gap. I haven't purchased a set since I last owned an OM617 several ages ago, so I don't recall what they cost. But 5 bucks at your local parts emporium oughta do it. We're not talking rocket science here.

- JimY

gmercoleza · #58 07-03-2007, 06:24 PM

Feeler gauges are about $7 at AutoZoo. Just be sure to keep 'em oiled - I just looked at mine, and they are pitted with rust from sitting too long, so now they're useless...

CTM VT 2K · #59 07-14-2007, 12:42 PM

Update:

My indepedant mechanic verified that the AC pressures are NOT going out of spec between running and failure.
When running in HOT weather, in bumper-to-bumper traffic, occasionally my engine temp climbs above the A/C compressor turn-off temp - turning off the compressor. When the engine temp cools back down, the compressor turns back on. He and I agree that it's most likely my ETR.

While I had the dash open to pull my instrument cluster (to repair my odometer as discussed in another thread) I tried to locate the ETR so I could bypass it. Unfortunately, the .PDF doesn't seem to reconcile with my actual system. Can anyone tell me how to properly locate the ETR to bypass it?

In this picture (looking from the driver's well towards the center column, as specified in the .PDF) the circled area is what I think MAY be the ETR.

CTM VT 2K · #60 07-14-2007, 12:51 PM

Here is the graphic portion of the .pdf on my ETR