1995 E320 Climate Control

[300CE]

Earlier this year I purchased a very nice well care for low 80k mileage - 1995 C124 from southern California. During my PPI I noticed there were some erratic heating issues and I’m now at a point where I can tackle the last item on my to do list, the unreliable heat issues.

The A/C works very well as it gets very cold. When turned to max cold the blower fan cycles to very fast and then the blower speed steps down when the temp wheel is adjusted. This is the same in AC and EC modes. It seems not to be calling for / or missing hot water.

My test of the heat, sometimes when the temp wheel is near max heat I don’t get any heat. If I turn the control unit off and then on in the defrost mode I’ll get plenty of hot air from the defrost and out of the side vents. When I place the temp in the middle, 72 degrees, I get warm air out the side vents for a while and cooler out the center.

This is what I’ve tested thus far:

I’ve checked for trouble flash codes, none indicated.

Aux heat pump, it turns off and on with ignition switch, it measures 3 amps and even hot temps on hose connections from the right hand feel test.

Mono valve; I get heat when I unplug it, measures 13.2 ohms ( in range 11-19) Replaced Heater Mono valve and aux heat pump.

Interior Temp Sensor; revealed expected changes in ohms from 3 to 24 when changing temps from 100 to 32 degrees in range.

Sampler Blower, no suction at inlet will not hold tissue. Replaced sampler pump, No leaks on suction tube to blower, pump runs very little suction at inlet to blower but will hold tissue. No dust / dirt blockage!

I need help and guidance of what to do next.

[HGV]

I went through a series of trouble shooting on my w124 wagon. I had erratic heat. First thing i did was to check the vacuum solenoids that control the dampners. Check the vacuum at the y connector in the engine compartment on the driver side below the wiper. It is a check valve y fitting. Check the rubber to make sure it is not cracked. One line goes to a vacuum chamber in the front passenger side wheel well, the other goes to the vacuum valve behind the passenger side air bag. If all is good there. Check the vacuum solenoids in the car. Easy to get once the passenger airbag is removed. There is a 10mm bolt that is accessed from below the dash. You will need a long extension. I did not unplug the air bag, just put it on a box to keep it elevated. If you changed the sample air blower, you already know how to do this. Remove the brackets that hold the airbag in place so you can get access to the vacuum solenoids. They are all color coded, but take a picture just in case. Pull them off and use a mity vac and pull vacuum and see that they all hold. Check the rubber connectors to make sure they are not cracked. If all is good. Check the heater core temp sensor located behind the push button controller. Mine was pulled out of the heater box and was reading the temp in the dash instead of the heater core. Hope this helps. That was my problem.

[300CE]

Thank you I'll have a look.