AC Cutting off

[KermitF]

Hi guys,

New problem on my '90 300E. When I accelerate w/AC on, AC ceases cooling. Air still blows, but it is not cool air. Eventually cool air will return, but it seems as if the harder I accelerate, the longer the wait - after having reached a steady speed - before cool air returns. I normally leave temp wheel set to 75, w/AC button on - all the time.

This is my third summer w/car and have never experienced this problem before. Recent work on car includes new head gasket - last week. AC was worked on last fall, so I have not really put it through a real workout until last two weeks (I am in Atlanta).

Anyone have any ideas as to why this is occurring?

There is a rotation sensor on the compressor that will cut out the system whenever the clutch or belt slips. See if the belt is tensioned properly and in good shape without any oil on it. Also see that the clutch is not slipping.

Good luck,

[jcyuhn]

The symptom doesn't quite map to that cause, though. The rotation sensor setup (which is definately problematic) cuts out the a/c until the engine is shut off and restarted. It's very much an all-or-nothing deal. Weak a/c during acceleration has me scratching what's left of my head.

Here's an idea - leaky monovalve. Acceleration runs the engine at higher RPM, which runs the water pump faster, which equals more output from the water pump. Some hot water is sneaking past the monovalve into the heater core. Hot water in the heater core equals less cool air out the vents. When he stops accelerating and engine RPMs drop, the flow of hot water ceases. But it takes some time for the heater core to give up all its heat tothe air stream. Hence there's a time during which the air temperature gradually drifts back down to nice and cool.

I'd disassemble the monovalve and see what it looks like. That's my theory and I'm sticking with it.

- JimY

[gmercoleza]

If I read your description correctly, mine does the same thing and it appears to be engine vacuum related. I think I will need to eventually do a podectomy. It only happens when outside temp is close to the temperature that I set the wheel for (usually occurst between 65 and 75 degrees). In this scenario, if I accelerate briskly, I can feel the air coming from the outer vents turn from cold to neutral to warm (possibly even heated). I think the manifold vacuum that holds the plenum in position to direct "a/c cooled" air out of these vents is not holding and succumbs to the lower vacuum which naturally occurs during acceleration. I'm pretty sure this is the cause, because if I accelerate very gradually the problem isn't nearly as evident. Somehow, the a/c pushbutton unit must have something to do with this, since the problem disappears when the outside temp is above 75 or so. And the problem only affects the outer vents; the center vents continue to blow cold a/c even as the outer vents appear to blow heated air.

Next time you drive, see if your problem is similar to mine. See if varying rates of acceleration have differing effects on the temperature of the air coming from the vents.

[speace]

While I have experienced what Jim said about the A/C cutting out util the car is restarted I have also encountered times when it came back on its own in a few minutes. A new tensioner took care of my problem. Don't rule out belt slippage just yet...

[KermitF]

Guys,

Thanks for good feedback. A few additional comments:

1. I keep temp wheel at 75, but temp here in Atlanta has been in high 80s, low 90s lately, so my problem does not appear to be same as GMERCOLEZA'S.

2. Interestingly enough, when the car had head gasket done, my mechanic left me with an unsolicited quote for a "Drive Belt Tensioner Shock/Strut"
This was done in the last two weeks. (I wanted to try head gasket replacement myself, but engine started leaking, smelling of burnt oil pretty badly in past month, and since my son and I are driving up to Indy tomorrow to see McLaren-Mercedes win F1 race, decided to take car to mechanic.)

Does his recommedation to replace the "Drive Belt Tensioner Shock/Strut" sound like it would cure slipping belt/tensioner problems?

[speace]

I would say he is recommending the belt tensioner be replaced as well as the front shocks. I'm not sure what model you have, but on most Mercedes the front shocks are integrated into the struts.

These belt tensioners seem to only last a few years. To be honest, the way I know to replace the tensioner is when I accellerate abruptly to pass someone and hear the belt shreek immediately after I let off the accellerator. The A/C usually cuts out and then comes back in a couple of minutes if I don't restart the car first...

I just realized that you are in Atlanta. I highly recomend MBS Motors on Pleasant Hill at Buford Highway.

[zafarhayatkhan]

Turn the temp control dial to max. cold position and see if the problem persists.

[KermitF]

Hello gents,

Back from Indianapolis and the F1 "race".

AC saga continues.

Discussed my problem w/my mechanic in AM of last Thursday and turned out it would require more time to assess situation than I had before I had to leave for Indy. We discussed the symptoms I had and his description of those for bad tensioner/slipping belt did not exactly match my problem. (I had a subset of the problems, but not all of them). I drove back home on I-85 and had only mild overheating (@98 est.), but no other symptoms, so decided to take car to Indy. Car ran fine on interstate, but wnet to @ 98 when in slow traffic. Got to Nashville and decided to ride around town for a bit to see city. Temp repeatedly went up to 110, so beat it out of town pretty quickly. Car ran fine on interstate again. When we stopped for evening in Indiana, popped hood and did a check on engine. When squeezing upper radiator hose, noticed water glistening in a joint on plastic mechanism located under coolant tank ('90 300E). So as not to risk blowing this open, did not use AC for rest of trip (weather in Indy was great anyway) and never had problem recur. Now that I am back home. I am using AC, but I drive short distances in light traffic, so no problem. Here is my question ... would that mechanism on the fender liner, under the coolant tank be the monovalve?

No, if it is what I think you are describing it is the auxilliary water pump. If you have a coolant leak YOU NEED TO FIX IT ASAP! Any coolant leak during hot weather can ultimately result in engine disaster.

Since you are actually using the a/c on short trips, I don't think the monovalve is your problem, but it could be leaking some coolant through it I suppose, although I haven't seen one fail that way.

I think you need to go back to basics and check hi/lo pressures and vent temp. Give us these readings along with ambient temp at the time and we can start from there.

Good luck,

[KermitF]

Aux. water pump replaced and situation mildly better, but not corrected (overheating does not occur quite so rapidly, but does consistently recur).

Coolant system now rid of leaks, so I believe.

I am getting a bit of what I think is belt screech from time to time - on startup and possibly upon gear shifts (auto trans).

BTW speace, I think my mechanic's comments related to a shock associated w/belt tensioning, not my struts (or shocks) - as I replaced all of those last year and mechanic aware of this.

Car definitely will overheat in traffic, so I use heater and/or hi fan speed and/or pop hood (if traffic crawling - which it often does in Atlanta). This controls problem (temp has never gone above 115), but is obviously not a solution. I do not face much traffic day to day, but do on weekends. Car also will overheat on interstate if I am going up long hill.

I have purged air from system by removing temp sensor and filling 'til coolant flowed out (did not take much water, so I do not think air pocket a candidate - if this a valid way of removing air pocket)

Thermostat seems to be working OK and coolant is not yet due for flush.

Have not yet checked AC pressures, Larry, as I have confirmed car will overheat w/out ac on - which I think means my problem is not AC issue.

I am thinking belt tensioner. Need to check PO's records to see when last replaced; and do plan to check radiator for hot/cold spots to rule out radiator clogging.

To be continued ...

You need to check your belt tensioner.

Good luck,

[aqua190]

overheats in traffic but fine when moving briskly.......maybe the fan clutch is going bad?

[speace]

KermitF

I'm getting confused. Is the problem that the A/C cuts out intermittently, or that the car overheats, or both?

For an intermittent A/C I would focus on the slipping belt. For overheating in traffic I would make certain that the AUX FAN in front of the radiator is working properly. Then check for a bad fan clutch, weak tensioner, and if all else fails, replace the radiator. I've changed the radiators in both my 92s and it improved their abilities to cool quite a bit. Radiators seem to lose their efficiency as they age.

What part of Atlanta are you in? I'm up the NE I-85 corridor.

[KermitF]

speace,

Forgot to thank you for also pointing me in direction of belt tensioner - which was my problem. I am in NE corridor of I-85 also ... Duluth.