A/C and heater problem - PeachParts Mercedes-Benz Forum

#1 10-19-2002, 02:28 PM

I have posted this problem a few weeks ago but did not have too much respons to it. Here it goes: Car= 1989 260E with only 246K.
Summer: A/C was cold and operating perfectly but at times, it would turn to full heat. A gentle tap on the gas pedal and it would turn back to cold.
Now the heater season is here (was 37deg. this morning). The heater works perfectly but for no reason will switch to cold A/C .
The switches (from hot to cold or cold to hot) seems to be vacuum related. It always switch at high vaccum (idle) and switch back to what it should be at low vacuum (acceleration).
Can anyone help? Thanks
JackD

#2 10-19-2002, 07:36 PM

guess I have just discovered as new problem for Mercedes as knowbody seem to have and answer to my previous post.

#3 10-20-2002, 09:26 PM

Solution anyone?
Thanks

#4 10-21-2002, 09:31 PM

I've got the same problems with the same car ... very strange!

#5 10-21-2002, 10:17 PM

Salut Pef.
It must be something related to the type air we breathe.
There must be an expert on this forum who have seen this problem before.
it just can not only happen to us.
jackD

Gilly · #6 10-23-2002, 11:52 PM

Yeah Jack, it is strange sounding alright. A check of the water valve insert (rubber bellows) may be a good starting point. The airflow doesn't change, correct? Just the temp of the air changes?
What would happen if you had hot air, then it switches to cold air, then you hit the defrost button without revving up the engine? You could also try this:
Locate the watervalve. If it is heating normally, then switches to cold air, park the car but leave it running (in the failed state), then go under the hood and pull the electrical connector off the watervalve. The default setting (no control to the valve) is full heat, so it should go to full hot air. If it does not do this, I would have to say either a bad valve (more likely) or a strange heater core or heater hose problem, maybe a collapsed hose, but nevr seen it happen before.
But I'd pull the insert out of the water valve first and look for a tear or small hole in the bellows. The insert part has a repair kit available fairly cheap if that's the problem.
If none of this pans out I'd say either a temp sensor problem or a bad push-button panel. (Should be tested)

Gilly

Piotr · #7 10-24-2002, 12:21 AM

Similar (but not the same) problem with 190D (1985).
When driving with the heater button ON and set for heating (about 22-24C) the car seems to cycle from heat to cold to heat. So, for instance I get the heat until the car (interior) is warm, and then the climate control clicks and the AC comes on, until the heater kicks ON again. and so forth... This does not happen with the EC (Economy) button pressed ON. Because my AC was kaput last winter, I don't know if it's normal.
On top of that, when in the EC mode, the heater seems to get stuck and I have to "restart"it by moving the temp control to max (only happened twice). Any thoughts??????

#8 10-24-2002, 10:54 AM

Thanks Dan. Disconnecting the electrical contact at the water valves reverts to full heat as is should.
All water hoses (upper/lower rad, to/from heater were replaced only a few months ago, along with the water pump, so I think this is unlikely the cause.
The temperature switching always happens when the car idles for a few minutes or when crawling in heavy traffic. Air flow changes as well. When it switches from cold A/C to hot, I get hot air comming out of the center vents, which I don't think is normal.
Could it be related to some vacuum operated flaps under the dash?? Thanks for your help

joe p · #9 10-24-2002, 10:58 AM

If this car is eqipped with a evap temp senso and heater core temp sensor, sounds to me like 1 of them is failing. I have had a few 124's that would go full heat w/acc for no reason and the evap temp sensors were bad.

Gilly brings up a good point as well, check that monovalve.

Joe

#10 10-24-2002, 01:23 PM

The mono valve was checked and seems to work as it should. How do I determine if my car is equiped with those evap/core temp. sensors. As it switches from cold to heat (when A/C is required) and from heat to cold, (when the heater is required) woud it not be surprising those two sensors failed at the same time?
I'm still thinking it is somewhat related to a vacuum flap problem, but which one? Thanks for yout input

joe p · #11 10-24-2002, 03:47 PM

1st off is this a US car with ACC? (thumbwheel for temps) If so the sensor for the evap is stuck int the l/h side of the case behind the dash over the gas pedal.

Joe

Gilly · #12 10-25-2002, 12:08 AM

Joe could be right, we all see different types of problems. My experience on this is that it usually comes down to a new push button control panel, assuming that this is the same control panel as a US model. Joe has already asked if it has the auto climate control or not. The solder joints can crack and cause problems liek you describe. You are right though, usually you don't get hot air out the center vent, on that car I think it's only supposed to be fresh or A/C cold air. But this push button panel can screw it up that way. That's why I'm leaning more towards the push button panel than a sensor. In a perfect world the system would be tested using a break-out box to check the sensor readings. You can also test for a bad solder joint by running the system with the push button panel removed with the connectors hooked up of course, and kind of twisting the controller slightly to see if the system goes into a failed state. I recommend only a new push button panel, I see the rebuilt ones failing quite often within a year or so (if this proves to be the problem).

Gilly

ps, Problems with the vacuum actuators and the vacuum control block are fairly rare on this, plus you also have the air temp problem going on at the same time. Gut reaction is that it's a controller fault, but the actuators and control block can be tested as well, not what most DIY'ers can normally do without borrowing special tools (breakout boxes and adapters, etc)

#13 10-25-2002, 02:24 PM

The car has thumbwheel temp control like all other US cars.
I sepnt a few hours this morning trying to replicated the problems I have.
1) Swithching from heater to A/C: This morning, outside temperature was close to the freezing point. Started the car and a few minutes later, the heaters starts blowing hot air. Normal. Inside temperature gets to the setting temp (about 68deg). No hot air is comming out of the center vents (also normal). All of this was done at fast idle (1,500-1600RPM). I let the car return to normal idle (700RPM) and about 30 seconds later, the heater turns to cold (A/C) with cold air coming out of all air outlets (dash/floor). A little tap on the gas pedal and the heater turns back on. (nothing's blowing out of the center air vents).
Then I put an air dryer directly in the temp. sensor (near the rear view mirror). A/C turns on nice and cool as long as the car is running at fast idle. If I let it run at idle speed, the heater's on. Again, a little tap on the gas pedal and I'm back at full A/C (normal). I'm totally lost
JackD

joe p · #14 10-25-2002, 05:37 PM

check the aux coolant pump.

Joe

Gilly · #15 10-25-2002, 08:46 PM

That controller can really mess with your mind Jack, i'm still leaning towards that. Try running the same type of tests with the water valve disconnected and see what results you get.

Gilly

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