87 300SDL --- weird, erractic AC issues - PeachParts Mercedes-Benz Forum

daw_two · #1 05-01-2012, 12:18 PM

Okay, folks, here are the 2 symptoms:

Symptom #1 described:
About 2-3 weeks ago, I'm driving on a warm day with the AC cold air blowing out the center vents.....and the air starts getting warmer....then hotish. I'm running errands, so I stop the car. Came back to the car in a few minutes, start the car, and viola, I have cold air starting to come back thru the center vents.

I'm thinking this is a possible KLIMA erractic malfunction.

It happen 3 times yesterday. AC ice cold air starts getting warmer....turn off key; restart the car, and I have ice cold air again.

Symptom #2 described:
I was playing with the AC a lot this morning. If the AC is on MAX cold (feel the click in the thermostat wheel on the ACC), the AC is icy cold and blowing thru the proper vents including the center vents. I adjust the thermostat just a bit, with the fan speed on any setting, and the center vent seems to switch.......hardly any air flow thru the center vent.....can hear the roar of air moving....buts it's not coming out of the defrost vents.....I turn the thermostat back to MAX cold, and the air flow returns to the center vent.

Weird, huh?

I replace the Mono Valve on this car (87 Champagne) back when I bought it, so, I'm thinking it's not related to it.

eatont9999 · #2 05-01-2012, 01:16 PM

A similar thing happened to my 300SD yesterday after I topped it off with R134a. At idle the AC was cool, not near as cold as I would like, but noticeably cooler than the outside air. I decided to drive the car around the parking lot for a few minutes to see if the AC would get cooler past idle. Well, it stayed the same until it stopped putting out cool air. Suddenly, I was getting ambient temp air. It wasn't hot like the heater was on but about the same as the outside temp.

I have yet to get cool air from the car again.

ah-kay · #3 05-01-2012, 01:22 PM

Problem for symptom #1 is Kilma relay. It happened to one of my SDL, A/C cut off after a while then return after turning off car. I hacked the relay to only monitor the freon switch and everything is OK. I ran the hacked relay for about 1 year without issues until I found a good one in a JY.

Cannot offer anything on #2. Good luck.

daw_two · #4 05-01-2012, 02:54 PM

Since we all know about the engine speed compared to the compressor speed in the KLIMA....and since the 87 Champagne car had a small power steering leak (over the edge --- replaced the seal/gasket on the lid), I'm thinking I will take a shot at cleaning the speed sensor on the AC compressor and see if that fixes the issue.

In addition, I guess I will increase the priority of the vacuum pump replacement, serpentine belt replacement, and tensioner damper. The dancing tensioner is probably impacting the AC compressor speed:
Bought some 1987 Champagne

Anyone want to chime in here about the KLIMA circuit?

Thanks, BC for this post ---- AC challenge: 300SDL compressor cut- out

I've been doing alot of reading....

eatont9999 · #5 05-01-2012, 05:02 PM

I noticed my compressor clutch is not engaging when I have the AC on. What are the correct low and high side pressures for a MB AC system? What should the ambient pressure with AC off be? How can I bypass the Klima temporarily?

brandlj · #6 05-01-2012, 06:14 PM

Hey David,
Symptom #1: Probably Klima doing it's job correctly if your serpentine belt is slipping which is allowing the speed sensor on the compressor to sense this and shut off compressor. If the belt is moving around a lot because of the tensioner, speed of the compressor may not be the speed of the engine.
BUT also, the kickdown switch could be flaky. Kickdown switch shuts off compressor. Also full throttle microswitch in the engine compartment will shut off compressor. Check those things.

Symtom#2: Probably not a Climate control pushbutton unit failure. Default mode for 87 sdl is all of the air will blow out of the defrost only. BUT turning off the car and turning it back on resets the PB unit until it fails again.
But you say you hear roaring, but no air coming out. I am leaning towards the vacuum tree on the passenger side of the footwell. Perhaps an erroneous signal is closing the vacuum to the center vent pod. Of course it could also be the center vent pod. When mine went bad it closed off the center dash vent, but then all the air blew out the foot and defrost.

What you really need is the Climate control Paper book manual so you can run the step by step diagnostics. I have one and will loan it to you if you would like. Just drop me a line.

Summer is almost here, Memphis summer is NE Louisiana summer. It sucks!!!

daw_two · #7 05-01-2012, 08:14 PM

Yeah, John. I think Summer is here today. I wish I would have had an extra shirt with me today, so I could change after coming back from lunch.

Anyway, yeah, I know about the microswitch, but had to find this picture of it to know where it is located:
http://www.w124performance.com/images/OM603_misc/microswitch.jpg

For now, I just unplugged it.

Hmmmm....studying the KLIMA relay circuitry here:
AC challenge: 300SDL compressor cut- out
and
it looks like I can leave the full throttle microswitch unplugged. EDIT --- I seriously doubt it can be left unplugged if the KLIMA is plugged in the socket.

I measure the resistance of the speed sensor using sockets 9 and 11 ---- ouch.....measured 686 ohms. Looks like my speed sensor is out of range.

daw_two · #8 05-01-2012, 11:11 PM

So, I attempted to build a simple ice cube relay to control the AC compressor with just the ACC thermostat wheel. After that failed (I guess there's a reason why I'm not an Electrical Engineer), I kept staring at the KLIMA relay that I have from that 1991 350SD parts car. I turned it over and saw the labels on the pins --- they are the same as the 300SDL. Plugged it in and cycled the key and ACC unit and presto, I have working AC.

I did test to see if jumping "hole" 5 & 7 and sure enough that powers up the compressor without any protection.

Oh.....btw....after I turned the car off, I remeasured the resistance between "hole" 9 & 11 (speed sensor) and measured 707 ohms. I'm guessing the range of 530 to 650 ohms is a cold measurement.

So, it looks like my existing KLIMA relay was the culprit.

Anyone want to give me a lesson in reading/interpreting the KLIMA relay diagram?

brandlj · #9 05-02-2012, 07:29 AM

Awesome news! Yeah the speed sensor resistance appears to be to high, but then as things get older...
Anyway, I am glad you had another KLIMA relay. Try and stay cool. We are looking at record highs here for the next 7 days with no prospects of rain.

ah-kay · #10 05-02-2012, 11:29 AM

Quote:

Originally Posted by daw_two

So, I attempted to build a simple ice cube relay to control the AC compressor with just the ACC thermostat wheel. After that failed (I guess there's a reason why I'm not an Electrical Engineer), I kept staring at the KLIMA relay that I have from that 1991 350SD parts car. I turned it over and saw the labels on the pins --- they are the same as the 300SDL. Plugged it in and cycled the key and ACC unit and presto, I have working AC.

I did test to see if jumping "hole" 5 & 7 and sure enough that powers up the compressor without any protection.

Oh.....btw....after I turned the car off, I remeasured the resistance between "hole" 9 & 11 (speed sensor) and measured 707 ohms. I'm guessing the range of 530 to 650 ohms is a cold measurement.

So, it looks like my existing KLIMA relay was the culprit.

Anyone want to give me a lesson in reading/interpreting the KLIMA relay diagram?

Good job. I do not realize that you have a 19i91 SD parts car. The Kilma should be compatible. So the time I spent last night to hack the Kilma circuit and PM'ed you was all in vain.

The problem is the Kilma, not the speed sensor in the compressor. Speed sensor rarely goes wrong unless it is open circuit. The speed sensor measurement is done by a IC differential circuit. One cannot fix it unless one knows the part number.

ah-kay · #11 05-05-2012, 01:22 AM

The switch at the rec/dryer is a pressure switch for the SDL. There are 2 switches in the r/d. One low pressure to indicate freon is present and up to pressure. The other one is high pressure switch ( 15 bar? I cannot remember off hand ) to activate the aux fan at low speed.

daw_two · #12 05-07-2012, 09:28 AM

Quote:

Originally Posted by ah-kay

The switch at the rec/dryer is a pressure switch for the SDL. There are 2 switches in the r/d. One low pressure to indicate freon is present and up to pressure. The other one is high pressure switch ( 15 bar? I cannot remember off hand ) to activate the aux fan at low speed.

So, is the high pressure switch in contact with the R134A ? At what pressure does it come on? I thought the FSM indicated the switch on the drier was a temperature switch.

Thanks for the clarification!

ah-kay · #13 05-07-2012, 12:18 PM

Quote:

Originally Posted by daw_two

So, is the high pressure switch in contact with the R134A ? At what pressure does it come on? I thought the FSM indicated the switch on the drier was a temperature switch.

Thanks for the clarification!

I looked at the FSM manual for 300SDL pressure switches at rec/dryer:

Freon presence switch - Off 2.0 bar On 2.6 bar
Aux fan switch - Off 15.0 bar, On 20.0 bar

Hope this help.