I'm looking for someone who actually KNOWS what they are talking about.
Please do not bother answering if you want to tell me I need a new Heater/Air Condition Controller - the module that mounts on the center console. I now own 3 of them.


The PROBLEM:


The driver's side temperature goes full hot only on the driver's side seemingly at random.
This happens regardless of the set temperature and regardless of whether or not the air conditioner is being used. It will happen when I want heat, when I want A/C, or when all I want is ventilation air.
If I turn off the A/C and the fan and wait 20 minutes to an hour, I can then restart everything and it all seems to work normally again, sometimes for days. Then the failure repeats.


I am now up to 4 garages that swear it is the controller but I no longer believe them. Three controllers and you're OUT! Does anyone have any other suggestions? I have not noticed any pattern as to when this problem occurs. It seems to be completely random. I have been "tolerating" it for 5 months now.

 

The hot water flow is controlled by the double solenoid valve thingey you see just in front of the battery.
The valves are controlled by the module you have already replaced and supply a ground to either/both valves to actuate.
The valves are supplied 12v via fuse 15.

I think the valves work bass-ackwards of what you'd think ( read that somewhere ), IE energizing the solenoid stops the flow, but I could be wrong, never verified this, too messy.
( try pulling fuse 15 and see if it goes all hot/cold )

There is also a thermister in each left/right heater core that tells the control module what's happening there.

NOTE: would you be willing to part with one of the control modules at a reasonable price ?

 

The two valves under the hood control the heat that passes through the heater core, if one jams then no control will move the valve. These valves are side by side and near the battery.
With the heater control in the OFF position check the temperature of the hoses at the front of the valves, they should be cool or cooling down. The valves stop or hinder the coolant flow out of the heater cores. There are two cores, one for the driver and one for the passenger. These valves are normally open, so will supply hot air, one could be stuck in the open position and not moving to the closed position when signaled to do so.
I am a bit vague on the total system but I think .....
Air passes over both the AC and the heater cores, the heater core should cool down when the valve shuts and slowly allow the AC to do its work. That's if the heater was on prior to switching on the AC.
There is a diverter flap and I have forgotten what it does, I think it supplies air that has not passed over the AC and heater cores.
I should look at the Service Manual, I assume you have (?).

 

Great minds think alike, or so I'm told.

 

Golly Gee WOW, a compliment from Dave, can you visualize my tail wagging ?

 

Just don't start dry-humping his leg, OK? LMAO...




Frankenmerc, The heat valves (two of them right in front of the large flat fuse box in front of the battery) need that 12v to close, if your not getting the 12v when A/C is on and ***** are to full cold, you found part of the problem. If the 12v IS at both valves then the valve is either stuck, or the coil that moves that valve (electromagnet) is 'open'. Check those two valve voltages (with A/C on full) and let us know the findings. Just be sure to check both wires at each coil, not one and a ground. Do this twice (once each coil) at the terminals that are plugging into the valve solenoids. Good luck!




BTW, how would you like to donate one of those control heads, so I may troubleshoot it (for the multiple failures that keep happening to them). They consistently fail, and I have a few ideas I would like to try to correct them?




.

 

Greyghost and ala_xfire,


I may be convinced to sell or trade the extra control units once I'm convinced I have FIXED the problem. Does anyone have an extra shop manual lying around? If they are not the cause of the heater problem they are both still good BTW.

 

Most download the .pdf service manuals from the site at the links provided. I don't know where the links are because I have a hard copy of the 08 manuals and use them to reference my inquiries. There are a number of posts that are 'stickies' and are there on the top of each category of posts. Jump through the titles of the posts, or do a search for 'service manual'. That should get you close. Anyone else have any added help for this guy?




.

 

The Service Manuals can be found here.
Down load and save what you want.

 

I read that as "Heather goes full hot" and thought I could maybe help you out but I see there is another issue.

 

Heather going full hot wouldn't be considered an issue, more like an opportunity

 

This is the post I have been searching for, it describes my "moist ventilated air" and intermittent heat issues almost exactly. Now, I'll go troulbeshoot the issue better informed on where to look. Thanks for the post and the informed replies. Can't wait to meet Heather when she goes full hot. wink.

 

I had a similar problem.

What I found was that where the heater control valves operate, there is a small copper wiring around them that engages/disengages the magnet to move the valves up/down. This will effectively control hot/cold air.

I completely disassembled the units and found that coolant had leaked out of the small rubber gasket and into the copper wiring. One of my valves was constantly shorting out and was jammed ON. The result of this was hot air all the time on one side, regardless of if aircon on/circulation on/off / whatever.

I tried to clean the entire unit and get a good circuit running, but because the circuits on both sides need to be complete for the valves to function, I just threw it away and bought a replacement for £20. Works now.

If you want to diagnose, dismantle the valves to the point where you can see the copper wiring and use a volt meter to check the AC going through; it should adjust in accordance with the heat setting in the cabin.

 

I'm posting to this old thread rather than starting a new one. I'm getting tired of having to replace the heater control valves! I don't need heat. Is there a way to disable the valves from being in the (default) heat position so they can't override the A/C? I was thinking that the valves being in the down position would be the position that shuts off the flow of warm coolant through the system. So was wondering if I put a double spring on the valves to hold them in the down position, might that work? I have plenty of spare parts after changing out the heater control valves FOUR times already. Looking for some advice from the experts! Thanks . . .

 

If the milder AC temperatures do not use warm air mixed with the cold air you could just cut of the water to the valves, but I think the cold air is mixed with warm air to get the milder temperatures.
The heater core is behind the evaporator so the AC in that case would rely on fan control, the EVAP coil may freeze up in humid areas with no hot or warm air.
That is my thoughts but I am no expert on it.

 

Big thanks to all who contributed to the potential solutions to this problem.
I read all the posts and then used an infrared laser thermometer. to read the temperature on both sides of the pipes at the control valves, sure enough one side was the same as the feed pipe but the drivers side valve significantly lower, bought a replacement fifteen minutes to sap problem solved

Many thanks

 

I don't own a crossfire but the engine and frame are the same as a Mercedes SLK series 170. The "duo valve" failure is a common problem. My failed after 23 years of service and you can find the methods to restore it to full operational use. Go over to the Mercedes SLK 170 forums and you will find numerous posts that explain and show how to overhaul this valve assembly.

 

I don't know if the OP is still here. The post is 10 years old.

 

Tank Man. I have rebuilt and reinstalled my duo-valve. I noticed you rebuilt your's. Since you got your controller back have you any issues with crazy heat? Mine still jumps around.

 

If it is not the duovalve, it is one of these things:
1) The climate control module's PWM output to the drivers side of the duovalve is failing - probably a bad pass transistor on the main board. Replacing the module will fix that.
2) Corrosion on the pin at the duovalve for the driver's side coil. (I have not seen corrosion do this, but it sure can do it. The fact you rebuilt the valve tells me you have already disconnected and reconnected, which often cleans enough corrosion to clear such an issue for some time.)
3) Corrosion or bad crimp on a pin in one of the connectors between the duovalve and the climate module. (I found this ONE time, back in about 2017 in an SRT Roadster.)
4) Failing battery power to the duovalve (but this would cause BOTH sides to go hot).