1983 300td No heat at highway speed - PeachParts Mercedes-Benz Forum

ytmtnman · #1 10-03-2012, 08:46 AM

The heat works fine at idle and slow cruise speeds.
I have already tried: replace mono valve diaphragm, add coolant to spec, Park with the front of the car uphill, check auxiliary pump for operation.
If I pull the pigtail off of the monovalve I get full heat.
Any ideas on how to proceed?
Thanks!

vstech · #2 10-03-2012, 08:55 AM

Hmm still not workng? odd... there has to be a blockage in the heater line. the original monovalve was stuck, it could have been buildup, perhaps a good citric flush would clear it out.

vstech · #3 10-03-2012, 08:57 AM

the car has not been flushed since I got it.
also, remember there is that heater hose heater in there, it may be blocking the flow at highway speeds. try removing that from the equation first...

vstech · #4 10-03-2012, 08:58 AM

wait.
with the monovalve unplugged, you get full heat at highway speeds?

ytmtnman · #5 10-03-2012, 08:58 AM

It was a while ago that I checked disconnecting the monovalve. I'll check again today.

Should I go for one of those flush kits with the garden hose adapter or is there a better way?

vstech · #6 10-03-2012, 09:02 AM

gimme a call.
if you search citric flush you'll find PLENTY of info on the right way to do it.

ytmtnman · #7 10-03-2012, 09:03 AM

.... And I prefer to keep the block heater inline. If that was the problem then wouldn't I see other symptoms associated with that?

vstech · #8 10-03-2012, 09:04 AM

I only mentioned it because of it being in the heater line and it may be causing a blockage, and removing it would be easy.

vstech · #9 10-03-2012, 09:05 AM

did you look at the block to see if there is an MB heater in the block itself? it may just need a cord... I don't remember.

ytmtnman · #10 10-03-2012, 11:45 AM

I can now confirm that with the monovalve unplugged I get blazing heat at highway speed. Does that mean that the acc is suspect?

vstech · #11 10-03-2012, 12:41 PM

yup. either you have a short to power/ground on a wire, or you have an ACC issue...

ytmtnman · #12 10-03-2012, 12:44 PM

Does the ACC send variable voltage to the monovalve, or is it 12v/0v on or off?

cho · #13 10-03-2012, 01:05 PM

Quote:

Originally Posted by ytmtnman

Does the ACC send variable voltage to the monovalve, or is it 12v/0v on or off?

it should be variable... as voltage sway,so the monovalve rod sways up down...

.

ytmtnman · #14 10-03-2012, 01:36 PM

I think I have found the problem......the ACC unit that came in my car is from a w126. Part # 126 830 02 85.

Would anyone with an ACC unit compatible with a 1983 300td like to swap mine for yours?

qwerty · #15 10-03-2012, 01:37 PM

Quote:

Originally Posted by cho

it should be variable... as voltage sway,so the monovalve rod sways up down...

.

Not true. The positive terminal to the monovalve is hot with system voltage whenever the key is on. The monovalve is controlled by switching the ground side of the circuit, either by the temp control module or the push button unit, depending on mode/temp selection.