Car is 1989 260E.
A/C is normally as cold as it can be. However, when engine vaccum drops to near 0 (ex: while accelerating hard), very hot air would come out of all vents. When vaccum returns to normal, cold air is back.
This certainly has to do with some malfunction of some traps in the system. (vaccum actuated)
What should I be looking at and where. Thanks
JackD

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There are some cars that have a wide open throttle switch that kicks off the compressor while accelerating hard. I don't know if MB ever did this, but you might research this. If they did, then this is normal operation.

Good luck,

Thanks for your suggestion but I don't think this is the problem. If the compressor would stop under acceleration, it would take a few minutes for the a/c to get hot. In my case, it is almost instantanious. 4 or 5 seconds after a hard acceleration, the A/C turns into a heater, and back to full A/C 4-5 seconds after returning to narmal acceleration.
JackD

this is normal. my car does it, too. a/c compressor
shuts off to help engine achieve demand for power.
the air that comes out in the process is outside air,
whatever the temperature is.
if you say your air is hot, it might be engine heat air.
check vent hoses!!

Thanks for your reply, but this situation is not normal.
I have been driving this car since 1994 and never had this problem before. it only started
last week. What is coming out of the vent is certainly not outside air. It is heated air (very hot).
Any other idea? Thanks
JackD

It could be the vent doors that distribute the ventilation are moving when they shouldn't. Some cars have a check valve in the vacuum system to maintain vacuum during acceleration, in order to keep the doors in position. If you notice a real effect during acceleration that wasn't there before, I'd suspect a vacuum check valve went bad.

__________________
95 E320 Cabriolet, 159K

I agree with Kestas. Given all the vacuum damper controls and such under the dash, there is probably a leaky diaphragm or something that is not staying position with low vac.

Good luck,

I'm pretty sure both Kesta and Larry comments are making a lots of sense.
The problem is definately vac. related.
Now , where are those check valves??
JackD

No compressor cut-off during full throttle accel except on diesel MBs.

Unfortunately your problem is pretty strange for an 124 car. It does not have a vacuum controlled heater valve as in the 201 cars. If you are loosing vacuum the only way this can affect heat is for the recirc door to go full open (something not normally possible in A/C position). If this happens the center ducts should close and the air should go through the defrost ducts. And this is only going to give heat if you are already mixing heat at the temp setting you are running. In other words if ypou are in the MIN position the heater valve is dead closed and mixing hot outside air will only make the air warmer but not heated. If on the other hand the heater is working because you are set at 78 and there is mixing to achieve a temp greater than A/C (45 deg), then when trhe doors move heat actually can occur.

Another way this happens is while asking for mixed air the controller can't achieve a satisfactory heater core temp while idling do to the aux water pump not working. As a result while sitting at a light the control duty cycle of the heater valve is increased and increased (because at idle no water is flowing). Now start of from the light and the core is flooded with hot water due to the open state of the valve. This usually is only a winter condition in Florida (my experience), but could easily occur in the evening with large mixing. To check run in the MIN position and this won't happen. Also check by pinching off the heater hose.

The doors will move if vacuum is removed but they dont directly affect heating and are easily identifiable as the door position will quickly change.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

Thanks Steve for the explanation.
My A/C control are normally set at 72 deg., year round.
This setting is kept all year round, either with an outside temperature at -30 or 95 deg. it has always kept a very constant temperature. At times, I increase or decrease it by 2-3 deg. which result in a slight variation in temperature.
I do not have to apply full throttle to repeat the problem I have.
A normal to brisk acceleration from a traffic light will cause the problem.
Hot air come out all dash vents (including center). And it is not outside air, it is definately heated air. As soon as the vac. returns to normal,(cruising), A/C works perfectly.
Any other idea? Thanks
jackD