83 300SD Weird Heat Problem - PeachParts Mercedes-Benz Forum

cabazon · #1 02-14-2008, 11:08 AM

I hav an 83 300SD . When driving the car often times there is no heat, The valve to the heater core is closed. On the climate control you can push buttons all you like with no change just lots of cold air. The only way to get instant heat is to shut off the ignition ie: all the way off and on again and Voila heat every time. Then if you push the off button anytime later. The only way to get heat again is to shut off the ignition then on again. Any suggestions here?

blackestate · #2 02-14-2008, 01:16 PM

I would do a search on the monovalve. Sounds like it is having issues. This allows the water to flow to the heater from the engine.

cabazon · #3 02-14-2008, 04:35 PM

This valve seems to work fine Id doesn't seem to get power to turn it on unless you shut off the ignition and turn it on again . Must be something electrical like a relay or something .

Johnt49 · #4 02-14-2008, 07:26 PM

Could be the relays on the CCU board. Sounds like the contacts are dirty causing intermittent contact. If you remove the CCU and disassemble it you'll see some relays soldered to the board. You can clean them with a small picce of 400 wet/dry paper folded over to clean both sides of the contacts at once. It happened to mine. Been over a year now and still working well.

tangofox007 · #5 02-14-2008, 09:16 PM

Quote:

Originally Posted by cabazon

This valve seems to work fine Id doesn't seem to get power to turn it on unless you shut off the ignition and turn it on again .

Without power, a normally operating monovalve will open and provide more heat than you can stand.

tangofox007 · #6 02-14-2008, 09:20 PM

Quote:

Originally Posted by Johnt49

Could be the relays on the CCU board. Sounds like the contacts are dirty causing intermittent contact. If you remove the CCU and disassemble it you'll see some relays soldered to the board. You can clean them with a small picce of 400 wet/dry paper folded over to clean both sides of the contacts at once. It happened to mine. Been over a year now and still working well.

According to my schematic, the relays in the CCU control the blower,vent flaps and aux pump, but not the monovalve. Am I overlooking something?

jap · #7 02-15-2008, 08:49 AM

I get heat only if i push the economy button, the low fan speed button, then turn on the car. It's just barely enough heat to keep warm. And I never .... NEVER use the climate control, because it becomes very difficult to get heat to come back. (Sometimes if i stop at a stoplight after the heat has stopped working, the heat will start again).

Also the engine hardly ever gets over 50-60 C in sub-freezing weather. Could this be causing the lack of heat in the cabin?

tangofox007 · #8 02-15-2008, 09:22 AM

Quote:

Originally Posted by jap

Also the engine hardly ever gets over 50-60 C in sub-freezing weather. Could this be causing the lack of heat in the cabin?

You bet. Sounds like it's time for a new thermostat.

FLYWULF · #9 02-20-2008, 08:21 AM

I have the exact same problem with my 87 300SDL.
I get a short blast of very hot air if I turn off the car and restart it with the heater or defroster on. The heat then slowly becomes colder until it is gone. If I need more, got to shut it down one more time. It seems that the monovalve is working just fine. It is the vent flap that switches from the evaporator in the AC system to the heater core that is the problem. I will try the fix on thr relay contacts as mentioned above and report back with any results.

Eddie E
87 300 SDL 316k

FLYWULF · #10 03-03-2008, 01:37 PM

Well I took out my CCU opened it up and found no relays to fiddle with. This heat problem is driving me berserk! Mine is a 87 300sdl could it be a different printed circuit board (PCB) than earlier models?. I have used two different CCU's so far. One was re soldered the other the original and still the same problem. Are there any other suggestions, I really appreciate the help.

Graplr · #11 03-03-2008, 02:20 PM

Quote:

Originally Posted by FLYWULF

Well I took out my CCU opened it up and found no relays to fiddle with. This heat problem is driving me berserk! Mine is a 87 300sdl could it be a different printed circuit board (PCB) than earlier models?. I have used two different CCU's so far. One was re soldered the other the original and still the same problem. Are there any other suggestions, I really appreciate the help.

Have you used a mulitmeter to diagnose that it is sending the correct voltage to the monovalve?

Put the meter on volts. Put the meter on the wires coming into the monovalve. Change the heat slowly (cool to hot or hot to cool). You should notice a change. This could eliminate the CCU as the problem if you get a full 12 volts when on cool and 0 volts when on heat. If you do get this then the monovalve, or possibly the vent flap pods are the problem. If you do not, then it is electrical (most likely the CCU itself)

Does your engine temp drop at all when you get no heat?

rrgrassi · #12 03-03-2008, 02:44 PM

Monovalves have been known to get stuck in the "closed" position. Unplug it's power and see if you get heat. Also, a stuck open thermostat and low coolant level can cause no/low heat issues.

Also, check to see if your car uses the servo or not. I do not remember what year the servo was dropped.

carnut · #13 03-05-2008, 06:53 PM

Remove the four screws holding the top on the mono valve with engine cold, radiator cap off!! Gently lift the insert out of the mono valve (pay attention to how the wave washers and spacers stack on the insert). You will most centainly find a torn rubber diaphram! Replace with a monovalve repair kit from an advertiser on this site. Classic symptom of ruptured mono valve diaphram. Also do "search" for mono valve to locate it and what it looks like. Running 50 to 60 degrees is a symptom of a missing or stuck open thermostat.