No heat in my 300D - PeachParts Mercedes-Benz Forum

TulsaBenz · #1 11-17-2005, 09:40 AM

My 1979 300D heater is not functioning. AC works fine, blows ice cold. Set temp for heat and it still blows cold. Every once in a while the heat will kick in, but it is rare. What do I do?

Any help is appreciated... Its cold here in Tulsa!

boneheaddoctor · #2 11-17-2005, 10:29 AM

Lay odds its the evil vacuum servo....but hopefully its something else...

is the A/C actually shutting off and do you hear the servo doing its thing going from cold to full heat?

TulsaBenz · #3 11-17-2005, 10:43 AM

I don't think I hear the Servo operating, but I will have to try it and see to know for sure.

boneheaddoctor · #4 11-17-2005, 10:58 AM

Quote:

Originally Posted by TulsaBenz

I don't think I hear the Servo operating, but I will have to try it and see to know for sure.

Have someone listen to it....you should also hear it park a few seconds after shutting the car off. And when you turn the key full on without actually starting it. Have someone listen or turn the key for you to listen if your hearing is not very acute.

Hearing nothing at all is not a good sign....Servo and servo amp should be replaced in pairs always. One can smoke the other.

mdawson · #5 11-17-2005, 03:24 PM

First, check the mono valve. If that's not it...

There is a flapper valve in the HVAC system behind the glovebox that seperates hot air from the heater core from the rest of the system. This valve is operated by the vacuum servo referred to earlier.

Warm the car up
Pull the glovebox out so that you can see rod and arm on the valve.
Start the car up and place HVAC in defrost.
Did the arm on the valve move
No? try twisting it by hand in the direction of the servo (it pulls open).
Did you get heat?
Yes? check if the servo is getting vacuum.
No? You've gotta dig deeper into the console to check the controller and the vacuum solenoid valve that delivers vacuum to that servo.

boneheaddoctor · #6 11-17-2005, 04:18 PM

Quote:

Originally Posted by mdawson

First, check the mono valve. If that's not it...

There is a flapper valve in the HVAC system behind the glovebox that seperates hot air from the heater core from the rest of the system. This valve is operated by the vacuum servo referred to earlier.

Warm the car up
Pull the glovebox out so that you can see rod and arm on the valve.
Start the car up and place HVAC in defrost.
Did the arm on the valve move
No? try twisting it by hand in the direction of the servo (it pulls open).
Did you get heat?
Yes? check if the servo is getting vacuum.
No? You've gotta dig deeper into the console to check the controller and the vacuum solenoid valve that delivers vacuum to that servo.

THe 79 doesn't have a monovalve....It has the Chrysler designed Vacuum servo assy if it is ACC equiped.

boneheaddoctor · #7 11-17-2005, 08:50 PM

Just for grins and in case it was overlooked.....Have you checked the fuses yet? And have you checked the Aux water pump for function?

gdemoss01 · #8 11-17-2005, 10:01 PM

Another thing to check is the auxilliary pump next to the servo. It should come on when the key is turned on. Or check to see if it runs by jumping it direst to 12 volts. It rund hot water thru heater core.

Rev. Dr. G.

Scott G. · #9 11-17-2005, 10:07 PM

I had the same problem with my 280ce. Ice cold air but no heat... try turning the heat on and run it through defrost, it work for me

meangrean · #10 11-18-2005, 02:16 AM

hey bonehead is right,90 percent of the time it is the vaccum servo.they do have a problem all the time ,to bad they are quite expensive.if you cant afford it you can bypass the valve for the winter and replace or get it checked out for the winter. Harald

TulsaBenz · #11 11-19-2005, 03:14 PM

It looks like I have a vacuum issue of some type. The car will not shut off when I turn off the key. Also, my brakes and door locks are not functioning correctly. No lines seem to be disconnected. It sounds like all this could be related to one issue? Fuses look good. I'm going to start looking at the heater servo as suggested.

TulsaBenz · #12 11-19-2005, 03:16 PM

Where is the vacuum servo? If it is out, could that be the entire problem?

meangrean · #13 11-19-2005, 05:41 PM

Ok now you are saying that your brakes are not working proper,are you saying like hard pedal preasure.If so than it is your vaccum pump that is gone.the pump is rebuildable or just replace it

TulsaBenz · #14 11-21-2005, 09:40 AM

Yep... thats just how they feel. Very hard preasure.

meangrean · #15 11-21-2005, 03:32 PM

ok than it is either your brake booster has a major leak or you vacuum pump packed it in.uncrew the big line to the booster and start the ca.if you dont feel much vacuum come out of the line it's the pump,possible the check valve in the but i have not seen that go bad.Harald