Mono Valve in 1983 SL
 

Hi,

I need some help with my 83 SL. The heat is on at all times and doesn't respond to the AC control unit (other than just turning everything off). Based on everything that I have come across this seems to indicate that the mono valve is bad. Is that the right conclusion? Could the AC control unit be bad as well? Could it be that the electric conneciton is bad?? Sorry for all the questions....just need some help.

The monovalve is a good place to start. The valve gets a constant 12V and the control unit switches ground to close the valve. So if the valve does not have power and ground, it will always be open which allows hot water to flow to the heater core.

So the test is to remove the connection at the valve and remove the connector's plastic shell. With the engine running and the system operating, test the red/black wire for a constant 12V. If you have that, connect only the black/red wire and manually ground the other pin on the valve. You should hear a soft clunk. This means that the valve is operating, and the problem is with the wiring (unusual) or somewhere else.

Be very careful not to short the red/black wire to ground while you are testing. Also, you did check fuses 6 and 7?

The Somewhere Else could be the dash temp sensor or its foam tube, the temperature wheel, or the internals of the control unit.

My car had the same problem. NAPA has the mono valve repair kit for cheap.

Dan

monovalve
 

Usual failure mode is a ruptured monovalve diaphragm. Simple fatigue from heat and motion cycling. If that is the problem, the kit previosly mentioned will fix it.

I would look there first. It takes about 5 minutes. Unplug the input, remove 4 screws and pull the valve solenoid with attached diaphragm out to inspect.

The foam tube connecting the interior temperature sensor to the controller may also be deteriorated but that manifests as a poorly controlled interior temperature with a lot of lag before change. Most of us replaced that foam tube with a simple piece of plastic tubing.

classic symptoms, all hot or all cold. As suggested just test it, easy fix. However you have the dreaded ACC servo on that car correct ?

Monovalve
 

Hi--

I have an '81 380SL with no heat. All I get from the vents is outside air temperature when I turn the heat up high.

Could this be the monovalve or the hated ACC servo?

How does one check the servo?

Thanks in advance for your assistance.

Joe

If you have the servo (I think it is) it's on the right side just in front of the firewall, has sort of a flat top on it and a bunch of vacuum lines going to it.
If you can get it off and figure out which 2 sockets run the small electric motor and are good enough to jump battery power and ground to it, that's probably the best test to make on it at home. Typically the complex guts of the thing have seized and you need a new one. I think there is a linkage on it you can see go back and forth if it isn't seized. "Real" diagnosis is of course always the best, but get's pretty involved and requires a special jumper harness. Don't know if anyone has simpler test programs online to help diagnose or not.
Gilly

Monovalve
 

Just got the monovalve and will try to make the repair. I have no idea about the ACC servo. How expensive is it to get that replaced. Is this something I can replace? Thanks so much for your help!

ACC is about 400-500$ if you get the aluminum body one. Not a bad DIY, the clamps are usually rusted and a pain to remove, have spares as well as hose rplacement in case you have to resort to cutting:)