marginal heat 85 300D - PeachParts Mercedes-Benz Forum

tobybul · #1 12-04-2006, 12:49 PM

First winter for me on my 85. Heat is very marginal. I have checked most everything and considering the low miles and "newer" age of the car, I believe the CCU is fine. I know the aux pump is working and so is the mono valve. Just for kicks I disconnected the monovalve and it did not improve the heat. In fact its better when its connected.

I am wondering if the thermostat may be reversed. The temp gage show good operating temp consistently (90c). Thanks for any input.

rrgrassi · #2 12-04-2006, 01:01 PM

If your Tstat was reversed, your car would overheat. Have you felt the water lines runnng to the heater core?

After I installed a t-stat, max heat, even at 25f ambient, will roast you. My car was missing the t-stat. The clue for me was operating temp between 60 and 80c on days where the temps were in the 40's and 50's

carnut · #3 12-04-2006, 01:02 PM

Pull the cover off the MONO valve and verify the rubber diaphram is not torn. If the valve fails electrically you will have full heat, if the diaphram is torn it will produce luke warm air. 4 screws and the top comes off. Do it cold engine, radiator cap off, less spillage that way.

tobybul · #4 12-04-2006, 01:19 PM

Quote:

Originally Posted by rrgrassi

If your Tstat was reversed, your car would overheat. Have you felt the water lines runnng to the heater core?

After I installed a t-stat, max heat, even at 25f ambient, will roast you. My car was missing the t-stat. The clue for me was operating temp between 60 and 80c on days where the temps were in the 40's and 50's

my temp is consistent at 90c no matter the outside temp. I realize its suppose to overheat if the stat is reversed but I wondered if the tsat housing bypass is keeping just enough coolant to go thru thats why it does not everheat.

As to the monovalve, I mentioned I tried disconnecting it since it supposedly fails open but has no effect. In fact, it seems the air gets colder when I do this.

I don't know what a torn diaphram will do?

goodpoint on the hoses. my theory was that the aux pump is trying to draw coolant but since it has limited flow due to the bypass that its not giving me the heat i want.

rrgrassi · #5 12-04-2006, 01:29 PM

Do you have a way to do a temp bypass of the monovalve? Monovalve issue sounds plausible as well.

I do not think the built in bypass in the t-stat housing allows enough flow to prevent overheating. If it did, then it would be like operating with out a t-stat.

mplafleur · #6 12-04-2006, 02:25 PM

If the monovalve is torn, it may allow coolant into the solenoid body and push the valve closed.

tobybul · #7 12-04-2006, 02:46 PM

Another thing i notice is at idle after it has reached operating temp that the heat warms up a bit more.

Its in the 20s here today.

is a torn diaprahm obvious once you take the mv cover off? can the diaprahm be removed temporarily to let coolant go thru more freely? if i remove the diaprahm, should i disconnect power to the mv?

Brian Carlton · #8 12-04-2006, 02:52 PM

Quote:

Originally Posted by tobybul

is a torn diaprahm obvious once you take the mv cover off? can the diaprahm be removed temporarily to let coolant go thru more freely? if i remove the diaprahm, should i disconnect power to the mv?

When the cover comes off, the central piston and diaphragm comes with it. Pay attention to the assembly with the washers and spring. It's in a specific sequence.

Then, look at the rubber diaphragm (the larger section........not the very tip) and see if there are any tears in the rubber.

If so, that's your problem and you can get one on e-bay for $20.00.

No, you cannot remove the diaphragm and run the engine. You'll have to reassemble it with the torn diaphragm until you get the new one.

tobybul · #9 12-04-2006, 03:10 PM

thanks again brian!! I was at this other post where there were good pix of the diaprahm.

do u think this is another possibilty in case the diaprahm is fine:

on the aux pump.. not sure yet if this is even the case but what if the aux pump does not turn on at all? I know my aux pump works b/c i've tested it with direct 12v pwr. what tells the aux pump to turn on? the CCU?

i be chkg the mv and the aux pump tonight. am assuming that i will hear the aux pump running by simply turning the key on and the dial on heat. I guess I could put a voltmeter on the aux pump terminal and see if there is pwr.

I know there is pwr to the mv. seems like its on all the time b/c i had the dial up and dn and it did not cutoff.

Brian Carlton · #10 12-04-2006, 03:24 PM

Quote:

Originally Posted by tobybul

thanks again brian!! I was at this other post where there were good pix of the diaprahm.

do u think this is another possibilty in case the diaprahm is fine:

on the aux pump.. not sure yet if this is even the case but what if the aux pump does not turn on at all? I know my aux pump works b/c i've tested it with direct 12v pwr. what tells the aux pump to turn on? the CCU?

I've disconnected the pump on the SD and the heat is fine........so, I'm doubtful that it's the culprit.

Check that diaphragm........it's usually the problem.

tobybul · #11 12-04-2006, 04:18 PM

okidoki, just got home and I'll wait for the engine to cool dn a bit. Boy, it is nasty out there right now. Snow coming dn hard, roads slippery, but the D got thru it.... except I could see my breath inside the car.

i'll post what i find.