W124 a/c woes!
 

Hi all,

So I got my W124 ('94 E420) out of storage recently, and the a/c has been acting up. Most of the time it works great--with the temperature dial turned to maximum the car freezes me out most of the time. Sometimes, however (usually after the car and the a/c have been on a while) the a/c just cuts off--the blower blows, but the air temperature is the same as if I'd hit the EC switch. If I leave the car off for a few hours though, it works as normal. Does anyone have any idea what could be wrong?

Thanks!

I would start with the poly-v belt. If it slips even slightly, it may cause the a/c clutch to lock out. I don't know if it's supposed to reset automatically after this, but it has been my experience that the a/c will usually not work until you turn the engine off and restart it. If you have a code reader, you may see a DTC code 7 (poly-v belt slips) on the base module (socket 8). If you haven't changed the belt in a while, it may be a cheap fix.

Also, if you are in a really hot climate (like here in Arizona), the a/c pressure may exceed the limit and prevent the clutch from engaging. This has happened to me a few times in the hot summer where I have parked the car with ambient temperatures of 113 F and the under-hood temperature must have been incredibly high.

Also, I was told by a tech once that under really high temperatures, the pressure in the system can be so high--even with the engine off--that when you go to start the engine and engage the compressor it takes a lot of force just to initially get the compressor turning and it could cause the belt to slip and lock out.

In Switzerland:D?

lol

Slipping belt makes sense and it is easy to inspect the belt.

I am going to go with a more basic guess...
Your throttle suctioning mechanism... sometimes the Txvalve...but in more complicated systems ( than my type One ) it may have other components...
is not cutting off the cooling of the evaporator at a high enough temperature... thus below freezing fins on the evaporator are freezing the moisture in the air on the fins.... and cutting off the cold air... until you leave it to melt off and start over....

Hmm.. the v-belt was replaced last year and seems to have sufficient tension. Also, if I leave the car off for just a few minutes (eg getting gas) the compressor doesn't kick back on (but it will if the car has been off for a few hours). It's been pretty warm here in Columbus, OH (not Geneva! oops, updated that).

It definitely behaves like it's freezing over and then working again when it's thawing out. Where could I find the throttle sucking mechanism/txvalve?

Thanks!

Can you provide us with a temperature reading out of the vents on the dash when it is blowing cold?

Haven't checked recently but last time I did it was 42 F.

Sounds about right.

I only have knowledge of type one AC's.... the simplest MB AC...notice my signature...

Someone with your kind of car..
or a FSM of your type of car will have to speak up....

You would have had more help if you had posted this to the Diesel forum..... More AC guys with later models over there...

If you are driving the car and it quits then you turn it off and crank it back up does it come on? With the engine off try spinning the "inside" portion of the compressor pulley to see how easy it is to turn.

Nope, the car has to be off for at least a couple of hours for it to kick back on. I'll try spinning the compressor pulley tonight (car's at home in the garage).

I'm wondering if the clutch is dropping out after the compressor gets hot? I have seen this before. Are the components original? Feel the tension on the belt while you are under there to verify that it is pretty snug. What kinda shape is the tensioner in?

You are suffering the same symptoms I am.

I normally get 40F out the vents on a 95F degree day with 90% humidity (excellent) running on the highway. 50-55F at idle.

I am getting codes 5 and 6 in pin 8 of the diagnostic module.

You probably have the same.

In my troubleshooting, I changed the tensioner as well as the belt with no success.

I was observing the compressor cycle at idle, it was cycling fine. At about the tenth cycle, I noticed the clutch would start slipping and then stop (not engage, and A/C stops working) AHA !!

I think the clutch material is either wearing out, or the gap should be adjusted (in my case). I have not done it, but now I know why the A/C stops, sporadic clutch slippage.

You may want to check that.

Pedro

Hmm interesting.. the belt and tensioner are fine; I just checked. I'm planning on getting the system pressure tested Monday morning. Thanks for the tips!