Another strange heater problem - PeachParts Mercedes-Benz Forum

ThePhoenix · #1 03-12-2009, 07:27 PM

I have an '83 300SD (dubbed "The Phoenix" by the wife, see attached pic) which when I first got it had a problem with heat always coming out of the vents on the freeway regardless of how the controls were set. I was told when I got the car a few weeks ago that the A/C drier unit needed to be replaced, so I installed a new one. This did not fix the problem.

Yesterday I installed a rebuilt climate control switch assembly. While I was in there I saw a vacuum hose that had been disconnected from a round white plastic module with a single rearward facing nipple and then capped off. I reconnected the hose and hooked everything back up to the new control switch.

This seems to have helped a bit, but I am still getting slightly warmer than ambient temperature air out of the dash vents even with the fan off and the temperature control set to minimum. Turning up the temp in slow increments seems to deliver reasonable incremental increases in temperature. Curiously enough, the rear seat vent at the back end of the console puts out ambient temperature air no matter how warm the air is coming out of the dash vents. What can be causing this?

The A/C pump does not engage at all regardless of the temp setting. Am I correct to assume that I could test the control system by setting the controls to minimum temp with the fan engaged and the engine running, checking for voltage at the two wires going to the pump, and if there is no voltage I could test the pump itself by applying 12v to these wires? If so, which one is positive?

Last of all, I think I have seen here that a non functional tachometer can make the A/C not work? If this is the case I am in the process of hunting down a working tach amp and maybe this will cure my problem. Any helpful tips are greatly appreciated.

ThePhoenix · #2 03-13-2009, 10:57 PM

Today i set the climate control to maximum cooling and checked for voltage at the wires going to the the A/C pump. There was no voltage showing, and the air coming out of the vents was noticeably warmer than ambient outside air.

I then jumpered 12v+ to the green wire to engage the pump. This dropped temperature of the air coming out of the vents to about ambient temperature. This seemed to be progress, especially considering that I don't know if the A/C has been charged recently.

So, I need to figure out what controls the voltage going to the A/C pump. I assume it is controlled by a thermostat somewhere inside the car?

(On a side note, my instrument cluster has the LED unit for showing outside air temp, but there is no wiring going to it and no sensor that I can see in front of the radiator. Not sure if this is related to the inside climate control system, but it would be nice to know the temp outside. Was this standard equipment or an option?)

I also took apart the heater valve next to the battery. The internals looked to be in pretty fair condition, though there were some small cracks in the rubber seals. I suppose I will get a rebuild kit just in case. I checked the wiring coming to it and there was voltage being delivered with the car idling.

Anyhow, if anyone can offer me any advice I'm all ears. Otherwise at least I have an online journal I can refer to as try to figure this out...

::matthew · #3 03-13-2009, 11:02 PM

A non working tach could mean a fuse is blown, the tach amp doesn't control climate control.

the heater valve by the battery, your monovalve, needs to be working properly before you go too much further...

ThePhoenix · #4 03-13-2009, 11:31 PM

Quote:

Originally Posted by ::matthew

A non working tach could mean a fuse is blown, the tach amp doesn't control climate control.

the heater valve by the battery, your monovalve, needs to be working properly before you go too much further...

Thanks for clearing up the tach issue, though it isn't a fuse. I think I've narrowed it down to a grounding issue in the cluster. I've got my hands on a couple of spare tach amps, so hopefully I will get to the bottom of that soon enough. At least now I know that it has nothing to do with the climate control, which I read somewhere here and I thought sounded far fetched.

How do I know if the heater valve is working properly? Is it as simple as voltage in, valve closed; no voltage, valve open? I assume valve open means the top of the plunger is sticking further out of the top of the unit? How far does the top of the plunger move? Should I be able to see it move up and down as I manually apply voltage?

These kinds of things keep me up at night.

::matthew · #5 03-13-2009, 11:46 PM

let me cloud things up again...I'm not as familiar with 126s, but in my 85 123 the A/C and tach function share some sort of electrical source. As it was my first week of ownership, I'm not sure what the indie did, but conveyed to me that a fuse (perhaps a relay(klima?, OVP?)) was bad and once replaced the A/C and tach worked.

search "monovalve" and you'll find tons of info, but if it was torn, it's toast and you need a rebuild kit.

ThePhoenix · #6 03-15-2009, 07:49 PM

How far should the monovalve move? I put voltage to it manually and it just made a slight click with no discernible movement. Should I be able to grab the exposed top of the plunger and move it up and down? If so, how far?

John Schroader · #7 03-16-2009, 04:05 PM

If your car is set up like my 300D, then the monovalve is basically a solenoid that is opened and closed incrementally by a variable resistor -- the temp wheel in the CCU cluster. Should open partially or totally, depending on the temp setting. As for power to the compressor, I think there are a couple of switches in series with the relay that controls it. I'm not sharp enough on AC work to know just what they are and have had a difficult time finding exactly the correct wiring diagram. On my car, one is beside the glove box. Can't recall where the other is. Probably not a good idea, but I wired around them and my AC worked.

ah-kay · #8 03-16-2009, 04:56 PM

Quote:

Originally Posted by ThePhoenix

How far should the monovalve move? I put voltage to it manually and it just made a slight click with no discernible movement. Should I be able to grab the exposed top of the plunger and move it up and down? If so, how far?

The valve moves about 1/2" down. You should hear a click as the solenoid force the plunger to go down. The plunger blocks off the flow of hot coolant. It is 0V ( open ) when calling for heat and 12V ( close ) when calling for ambient or A/C. If you get hot air when it is not called for, try measuring the voltage across the connector. It should be 12V.

Good luck.