AC stops at speed

[joel price]

my 87 300D has a cold AC that works very well but lately it sporadically stops working. Yesterday travelling down interstate at 70 mph ac blowing cold and suddenly it stopped. This morning cranked up to come to work and AC working fine. Several days ago AC worked great all day several short trips in car and then sat for afternoon came out to go home from work and no AC---I don't really know what to search for in the archives.
What's going on?
thanks in advance
Joel

[Kestas]

It'll do this if the belt slips. This is to prevent the belt from snapping should the compressor seize. It'll reset itself after a restart. Check and see that the belt tension is working properly.

There has been much written about this. If you were to search a/c stopped, or climate control you probably would get a ton on it, but I will answer the sanme question again.

It sounds like the belt is slipping or the compressor is trying to lock up. With the engine stopped turn the front of the compressor by hand and make sure it turns smoothly with no metallic feel. If so, then examine the belt and tensioner.

Good luck,

[jcyuhn]

Tons of discussion on this in the archives. This car has a function which compares the RPM of the a/c compressor to the RPM of the engine. If they don't match, the a/c is shut down (until the engine is restarted) on the assumption that something is wrong.

Two common causes are compressor clutch gap and grease/oil on the compressor clutch. The former can be measured with a set of feeler guages, such as are used to check valve clearances on an engine. Clutch gap spec. is 0.50mm. They seem to work OK up to 0.80mm or so, then clutch engagement gets flaky. As for grease/oil, this is an easy DIY. Pickup a spray can of degreaser or brake cleaner and spray it in the compressor clutch gap. Rinse and repeat. That alone has fixed this problem on many an older MB.

- JimY