300SDL strange climate control problem
 

First of all, I am fairly sure that the climate control module in my console is flaky. The fan speed control buttons do not work consistently. I have remove it and cleaned the contacts, but it is still not quite right. I will replace it soon.

Now the problem, which I am not sure is related to the module: It is winter in CO and it is cold outside. During the week, coming and going from work (short distances), the heater works great. However, when I get on the HW, it does not heat hardly at all. As soon as I slow down or stop, it starts heating up again. I get back on the HW, and it starts to cool down. Also, the vents for the back seat never blow warm air at all.

The engine temperature gauge stays at 80 all the time. Never gets below or above 80 in the winter after the engine is warm. Can this be related to the climate control module or is it something else?

Thanks!!

Sounds like you need a new monovalve cartridge. Remove the old cartridge by removing the 4 screws holding the monovalve cover then lift out the old cartridge and the washers and whatnot. You won't lose much coolant. Check the mesh at the lower end. It'll likely be torn or out of shape. A new cartridge will restore heat at speed.

Sixto
87 300D

The monovalve diaphragm could be torn and give intermittent heat. But it would not explain why no heat on HW. I would do this check before I undo the 4 screws and lift the monovalve.

Unplug the connector to the monovalve. This will cause the plunger ( cartridge in Sixto's term ) to retract and call for heat ALL the time. Drive it on the Highway and see whether you have heat at all time.

Then put a constant 12v on the monovalve such that the plunger will be fully extended to block the coolant and there will be NO heat. This may be a bit complicated if you do not have an extra connector.

Good luck.

I agree with the previous posts, it is your monovalve causing the lack of heat. Also, the rear air ducts only blow outside air. In the winter it will be cold air, in the summer, outside warm air unless the Air Conditioning is on.
If the cold air is bothering you, slide the lever under the rear vent to the off position.

Thank you guys! I have something to look into. Now, where is the monovalve? :o I'll dig around in the FSM and the forum for pointers.

It's along the firewall inboard of the battery. Follow the heater hose from the back end of the water pump. It might be easier to get to the monovalve if you remove the ABS computer.

Sixto
87 300D

Monovalve test
 

300SDL

My SDl is showing the same symptom, (heat at idle and low speeds, no heat at highway speeds). This is with a newly-installed monovalve rebuild kit. I've confirmed that the coil-operated monovalve mechanism is operating properly when 12v is applied to it; when not installed, 12v makes the valve jump out of the coil about 1/2 inch or so. Installed in the system, with the monovalve two-prong electrical plug disconnected, the heat behaves the same as with the plug connected. That is, there is heat at idle and low speeds, heat diminishes to no heat as soon as I get on the highway.

Below, possibly under "too much info," are some voltage measurements I took.

I'm not sure the proper way to measure the voltage at the two prongs of the monovalve with it installed and the ignition on; I measured it from each prong to the metal monovalve cover(assuming it was a ground?), as well as across the two prongs.

Across the two prongs:
I get near 12v across the prongs when the system is off or when the temp dial is set to "min". I get 0 volts when the temp dial is moved to "max."

Purple wire to monovalve cover:
I get about nine volts from the purple-wire prong to the monovalve cover at all times with the ignition on, regardless of temp dial position.

Yellow wire to monovalve cover:
From the yellow-wire prong to the monovalve cover I get 0 volts with the system off or with the temp dial set to "min." I get about 9 volts when the temp dial is moved to "max."

Do I still need to test the monovalve on the road with 12 v applied in all conditions?

Does this help with ideas for a solution?

thanks
Bruce S

I have not had a chance to work on mine yet. It has not been an issue since I have mostly been driving around town. However, I got on a hw yesterday and was able to experiment a bit. What I noticed is that at 42 or 43 mph, the heat goes off. Lower speeds, it comes right back on. So, some electronic sinal is either not being interpreted or sent properly. That is pure guessing.

On my W126 I found it was easiest to get at the monovalve if I removed the diagnostic socket (2 screws). Changing it was super easy, and i didn't loose any coolant.

-J

That's a mechanical problem with the monovalve. Nothing electronic about it.

Cool. Sounds like an easy fix. Just need some time to come out for air.