Long and Covoluted intermittant A/C Problems with '88 300TE - Page 3 - PeachParts Mercedes-Benz Forum

larry perkins · #1 06-23-2007, 09:10 AM

i have been following this thread and trying to develop a convenient way to troubleshoot.
here is my question,has anyone tried to eliminate both the compressor speed signal and the engine speed signal for the purpose of determining if the compressor would run in the auto mode?
has anyone got access to the internal drawings of the kilma relay?
in most cases ac signals are rectified and converted to a pure dc value for the comparator circuits. it sure would be convenient to eliminate this possibility when troubleshooting the system.
by the way some one asked about the signal level at n22 pin 1 from s31/1 and it is high when compressor is off and pulled to ground when on.
also i am working on a 88 300te,86 300e and both have the same problem,at times wont cycle back on when in the auto mode but will run continously in the manual mode.
larry perkins lou ky

JFawcett · #2 06-23-2007, 04:31 PM

Seems like a whole lot of diagnosis work here....has anything been replaced? I would work under the assumption, since the AC does work - all be it sporadically, that it is probably a broken solder connection in the CCU in the dash (the push button unit). My 87 560SL was doing the same thing your car is doing. Compressor would come on and then shut down, the fan would come on and then stop for no reason. It was maddening!
Replaced the CCU with a rebuilt one from Benz Barn our of Georgia for $135.....everything has worked great since - even got the vents to work again (was so afraid it was a heater box thing!).
Even if that part does not solve your problem it will take a big known problem component out of the diagnosis and get you some new shiney buttons!

Good luck!

CTM VT 2K · #3 06-24-2007, 08:06 PM

I have not had any time in the last week or so to do any more diagnostics - though I did take some down time during my Rescue Shift to replace my aux-fan series resistor (which was quite well mangled. If I get a chance, I'll post pics.

The only change I've noticed in the last few days is that I do not even get 15 minutes of cooling now (down from the 20-25 I was getting a few weeks ago) - this tells me I'm probably chasing a freon leak. I've not been able to see any visible dye tracer, but I'll be hunting it with a black-light when I get a chance this week.

larry perkins · #4 06-24-2007, 08:18 PM

i tell u that fellow is good,got mine to my steam cleaner,washed that sucker real good around the a/c compressor,ran a 1/4 tank of fuel out yesterday and the ac cycled at any position i put it.
i really didnt want to post so fast as i will be driving this week in 94f temp but it sure looks like the signal from the ac speed sensor is getting clipped down to a level that wont operate the klima relay,with the crud becomming more conductive as the engine heats up(shorts across the output wires)
iin reference to my earlier subject on how to troubleshoot the system and bypass the compressor speed sensor,i wanted to be able to keep the relay in place for the elimination process. i know about the output contacts and all that.
ctm i bet a car wash will do the trick also if you dont have a steam cleaner. let us know.
larry perkins lou ky

CTM VT 2K · #5 06-25-2007, 09:06 AM

Let me see if I understand you correctly: You are saying that I should take my engine and get it steam-cleaned, with a specific focus on and around the a/c compressor, and that should fix the problem?

Richard Wooldridge · #6 06-25-2007, 11:00 AM

Quote:

The only change I've noticed in the last few days is that I do not even get 15 minutes of cooling now (down from the 20-25 I was getting a few weeks ago) - this tells me I'm probably chasing a freon leak.

Considering the information you have given earlier in this thread, what makes you think you have a freon leak? Has the weather been warmer in the last few days? If it has, it would indicate you are getting to a high pressure cutout sooner, which could be the result of an overcharge! The way to tell if you have a low freon charge is to connect a freon gage set and verify pressures. I'm beginning to think you should just take it to a qualified AC tech and get it fixed, as both your reasoning and diagnostic techniques don't seem to be getting you to a fix. Summer may be over before you arrive at the problem.

CTM VT 2K · #7 06-25-2007, 12:20 PM

It hasn't been warmer, it has infact been cooler, and yet I've been getting shorter and shorter running time.

I am, at this point, planning on taking it to an AC tech to get it fixed, but I'm waiting for cash-flow from my move and job-change to catch up (gotta love burocratic paperwork). Should be in a couple weeks, assuming nothing else strange comes up (Like having to replace major components of my wife's car because a previous mechanic couldn't do a simple job correctly!). My car is driveable, the A/C is (at the moment) a luxury item that I do not NEED to get back and forth to work with. Once everything that I NEED to have functioning taken care of, I'll start paying for the things I'd LIKE to have. That said, at my own pace, I expect summer will be over before I arrive at the source of the problem. So be it.

brewtoo · #8 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 · #9 07-14-2007, 12:51 PM

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

CTM VT 2K · #10 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 · #11 06-25-2007, 04:38 PM

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

CTM VT 2K · #12 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 · #13 07-03-2007, 10:00 AM

have you checked or bypassed the etr switch?

CTM VT 2K · #14 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).

gmercoleza · #15 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...

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