I have a 1987 300 TD wagon. The air conditioning was upgraded a few years ago from R12 to one of the supposedly-more-environmentally friendly refrigerants (134a I guess). The air conditioning is acting up, and I would really appreciate some thoughts as to possible causes and remedies.

Here are the symptoms:

1. When the AC works, it works well. Blows nice and cold. However, it does not always work.

2. If I set the temperature control to "min" and push the low fan button, the AC seems to work fine on low.

3. If I set the temperature control to "min" (or any other cold setting) and push the auto button, the AC works fine for a little while; but, it stops cooling after 5 or 10 mins. If I turn the ignition off and back on, it resets and works fine for 5 or 10 min again, then stops cooling. I can keep resetting the ignition every 5 or 10 min, but that gets old pretty quick and may be dangerous to do while driving. Sometimes on longer trips, after resetting the ignition several times, the AC will continue working for the duration of the trip -- but that is rare.

The fact that resetting the ignition makes the AC work for a while leads me to believe that a relay or some other electrical component is bad. Any ideas?

Does the fan continue to blow, but the air is no longer being cooled? Or does the fan slow down or stop? It's not exactly clear which problem you are experiencing.

It sounds as if you have the classic 124-chassis-ac-cutout-problem. It's been discussed here a million times. Briefly, your car has a circuit which compares the speed at which the a/c compressor is turning to the speed of the engine. If there is too large a difference between the two, the system assumes there is a problem with the a/c compressor and shuts it off to preserve the single fan belt. The compressor remains shutdown until the ignition is recycled. About 99% of the time there isn't a problem with the compressor.

Try searching this site using phrases such as "compressor cutout" and "klima relay" - you'll receive quite an education.

Here's my short list of possible causes:
1) Worn/dirty/stretched serpentine fan belt
2) Defective serpentine belt tensioner
3) Compressor clutch dirty/oily
4) Compressor clutch gap outside specification
5) Defective klima relay
6) Intermittent connection between compressor RPM sensor and klima relay
6) Intermittent connection between engine RPM sensor and klima relay

The good news is that none of these problems are too expensive to cure. The bad news is the challenge in determining exactly which one(s) your car suffers from. I too own an '87 300TD and, as you can probably guess, suffered from intermittent air conditioning. My specific problems were #3, 4, and 5 from the above list.

Oh, if you mean the fan stops blowing any air into the interior, cooled or not, then ignore the above. One possible cause of this problem is a defective ignition switch. Try jiggling the key to see if the fan comes back to life.

As you seem to suspect, the problem is not with the fan but with the cooling. Thanks very much for pointing me in the right direction. I will pull prior discussions on this topic and will use your checklist as a road map. Thanks again!

flbiggs,

jc gave you some good things to check, but there is one thing to check that is more likely to be related and is a somewhat common part to fail.

There is a miniature fan that draws a sample of the interior air through a tube and blows it across a sensor for interior temperature measurement. If this fan is not turning there are many weird problems that will happen with your climate control. There are two different locations for this fan in the 124 series. Mine is an '88 and the fan is located to the right of the glove box. If you pull out the glove box it will be to the right. The other location is somewhere at the base of the console or "hump" on the right side.

Another way you may be able to check for proper fan operation is to put some smoke at the little intake grille to the right of the inside rear view mirror. If the smoke is drawn into the grille, the fan is working. You can light a match for a second, shake out the flame and immediately put it near the little grille to see if it draws smoke. The fan is powered on whenever the ignition is on.

Good luck,

Holy cow!

My 1987 300TD does the exact same thing as yours, FLBiggs. The car will cool for a few minutes and then the fan blows warm. I am wondering if you have cured your problem yet. I am eager to fix this problem.

Please respond if you can. Take care.

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1987 300TDT 165K MILES

There is a high probability that the problem with these two cars is one of two things, a loose, greasy or improperly tensioned drive belt, or a bad cabin temperature sensor fan has gone bad.

Good luck,

Heh. That's really cool (at least for a few mins. hehe). I haven't fixed it yet because of life's many other demands, including newborn son who is hard to keep hands off of.

I did do a lot of reading on this site about the problem. Based on many extensive posts on the site, I further observed: (1) that I dont have the problem if I run AC on low and (2) when not on low, the cutout seems to be triggered when I come to a stoplight or stopsign. These things, particularly item 2, leave me pretty dang sure that the AC compressor clutch plate is dirty or spacing is slightly wider than spec (0.5 mm). As I gather from the other posts, (please forgive me if i butcher this) when the AC is on low, there is low pressure on the clutch plate, and hence no slippage. When the AC is on high, there is only slippage when I slow down, causing the voltage to drop and the clutch solenoid to loosen up a bit. When slippage occurs, of course the cutout circuit gets triggered.

I have been running the AC on low without incident. When I get around to it, i will probably just take the car into the local MB garage (Westover Hills Mercedes, Richmond VA) and have them clean the compressor clutch plate and adjust spacing. If i figure out how to do it myself, I may try. Can anyone advise as to this adjustment??

I'll keep you informed. Please let me know how you come along with this.

Other than the little AC snafu, isn't that a great car? Mine is so great, it makes me laugh whenever i see it or drive it.

No need to go to the dealer to clean the clutch plate. Just grab your favorite degreaser and spray it into the air gap between the clutch plate and the driven plate. Let soak, and hose off. I used Simple Green - I always have a big spray bottle handy in the garage 'cause I'm always getting something dirty.

Adjusting the air gap is quite easy, but significantly more involved. It requires a simple, but specialized tool. My local Mcparts place lends them for free. Simply remove the 12mm nut which retains the clutch plate and thread in the body of the remover tool. Then screw in the big bolt part of the remover and it pushes the plate right off. In theory it's simple - the hard part is that there is insufficient clearance between the compressor and radiator for the tool, so the radiator has to be pulled to make room!

The actual air gap is adjusted via spacer washers on the compressor drive shaft. It becomes obvious once you remove the clutch plate. There are usually two washers, so you can just remove one of them to close the gap up to specification. Of course, measure the gap with your feeler guages before taking it all apart...

I like Larry Bible's suggestion about the cabin air temp. sensor. I had a perplexing AC problem a couple summers ago. The AC would come on strong, and after a minute or two the fan speed would inexplicably go to low. Another minute goes by, then the fan speed kicked up appropriately to high again (the cabin was quite warm so the fan SHOULD have been running fast the whole time). This cycle would continue indefinitely.

Scroll down for the answer (in case you want to cipher it out logically first):

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Turns out I had just detailed the interior of the car and while cleaning the center AC registers, had left them both pointed straight up, i.e., pointed at the temperature sensor Larry was referring to next to the courtesy light. Therefore, the AC would blow nice cold air at high speed right at the sensor. The sensor would then decide the cabin was nice and cold and would tell the climate control to reduce the fan speed. The sensor would warm up and kick the fan speed up, etc. etc.

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Steve
'93 400E

Hi all,

I found this thread through a search which exactly describes my car's symtoms, only I have a 1986 300SDL, not a 300TD. Where would I find the cabin temperature sensor fan on the SDL series?

I'll start the diagnosing there and with the belt and clutch issues.

Thanks

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Later.........Gregg

(86) 300 SDL
(00) GMC Yukon XL 4X4
(97) C5 Corvette
(02) BMW R1150GS Adventure
(00) KAW Nomad 1500 FI

My symptoms were different, but the root cause was a seized auxiliary pump drawing too much current.

The pump is under the washer fluid bottle in the '87 300D, plug is accessible with the bottle in place, unplug and see if it changes.

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Gone to the dark side

- Jeff