Blowing hot AND cold

[bpat]

Sometimes my climate control system will suddenly start blowing really hot air when I want it cold. This usually seems to happen after I've been running for a while. Weirdly, it will often simultaneously blow HOT air out of the two outside vents nearest to the doors and COLD air out of the two center vents. I swear it is heating and cooling at the same time! It does this even if I turn the climate control dial all the way to cold. I can sometimes get it to stop this if I turn the whole AC system off for a few minutes. Does anyone know what might cause it to do this? Someone mentioned a valve that lets hot water into somewhere getting stuck open. Does that make sense and where would that be? It's a really annoying problem. I like it cold.

Bruce

 

[Rudy]

duo valves of the [Euro-only manual climate control] heating system (under the windshield - passenger side)

 

[Jelmer]

Either that (well, mono-valve in the case of auto-climate systems) or the temperature sensor near the roof controls is wonky.

 

[vexed]

My A/C would blast hot air, it was the monovalve. I don't think there is a way to test it.

 

[maw1124]

Sometimes my climate control system will suddenly start blowing really hot air when I want it cold. This usually seems to happen after I've been running for a while. Weirdly, it will often simultaneously blow HOT air out of the two outside vents nearest to the doors and COLD air out of the two center vents. I swear it is heating and cooling at the same time! It does this even if I turn the climate control dial all the way to cold. I can sometimes get it to stop this if I turn the whole AC system off for a few minutes. Does anyone know what might cause it to do this? Someone mentioned a valve that lets hot water into somewhere getting stuck open. Does that make sense and where would that be? It's a really annoying problem. I like it cold.

Bruce

Another suspect would be the "evap temperature sensor"... I was starting to have that problem on my car. I didn't fully understand all that my mechanics told me about the car's system, but since many of the other components were new, that was where we ended up. I wouldn't throw a part at it, because there are too many in that system, but it's worth a look.

Good luck.

 

[samiam44]

Test the separator fan first. The mono valve caged changed in 94. There is three or four pipe model. Epc is wrong on the split.

Did you pull codes on the system? Don't think evap sensor would cause this. You can chk it's resistance per fsm.
M

 

[Phoenix127]

Sometimes my climate control system will suddenly start blowing really hot air when I want it cold. This usually seems to happen after I've been running for a while. Weirdly, it will often simultaneously blow HOT air out of the two outside vents nearest to the doors and COLD air out of the two center vents. I swear it is heating and cooling at the same time! It does this even if I turn the climate control dial all the way to cold. I can sometimes get it to stop this if I turn the whole AC system off for a few minutes. Does anyone know what might cause it to do this? Someone mentioned a valve that lets hot water into somewhere getting stuck open. Does that make sense and where would that be? It's a really annoying problem. I like it cold.

Bruce

I am having the same issue. What did you find the problem to be? Seems a monovalve would either work or not but failing occasionally sounds more like an HVAC control...

 

[Gatorblue92]

I am having the same issue. What did you find the problem to be? Seems a monovalve would either work or not but failing occasionally sounds more like an HVAC control...

I recently went through this with my 93 400E. I was getting hot air from the side vents but cold air from the center vents. Replaced the CCU and nothing changed. Replaced the mono valve and it’s ice cold now.

 

[Kyiv]

My A/C works just fine, as far as blowing cold air. Recently, I've had to use the heater on the few colder mornings. I start off in a foggy weather in my neighborhood and halfway towards Cleveland the car's interior is well warmed, it becomes more sunny and I shut the heater off. Last time I did it belatedly and actually switched A/C on to cool off the interior a little. While the center vents blew cold air, side vents blew hot for 30 or so seconds, maybe even a minute before they air turned to cool... Longer than normal it seemed... Its that in the diaposon of normal?

Secondly, as soon as the heater is turned on, there is a definitive, pronounced sound of water flowing, burbling behind the dash. I remember that last winter the same sounds was also present, but it was much further in the background.

My monovalve is a new MB unit, installed about 2 years ago.

 

[gsxr]

All of the above sounds normal, kiev. The easiest solution for the heater core gurgle is to turn up the radio.

 

[Kyiv]

All of the above sounds normal, kiev. The easiest solution for the heater core gurgle is to turn up the radio.

Really? Dave, all of your cars do that too?

 

[gsxr]

The side dash vents always take a while to catch up with the temp changes - design limitation.

The gurgling varies from car to car but it's usually present when first adjusting temp from cold to hot, particularly if it's a large temp change, not a small one. If the coolant level is correct/full and you aren't losing coolant, what you hear is most likely normal and not 'fixable'. I vaguely recall MB adding an aeration line below the wiper mechanism that might have been intended to help with this, but I don't remember when it was phased into production, or if the V8 models all had the 'upgrade'?

 

[gerryvz]

What about the factory "anti-gurgle" kits they made?

 

[Kyiv]

What about the factory "anti-gurgle" kits they made?

I've done some googling prior. It was for 1988-1990 cars, or something like that