No heat, winters coming - PeachParts Mercedes-Benz Forum

DrewGerhan · #1 09-29-2006, 10:48 PM

so winters on its way and my 300SD has no heat. i did some searching and checked the aux. water pump. she works fine and the monovalve seems to be good too. i drained the coolant and replaced with 50/50 mix of zerex Z-05, in between there i also added a new thermostat and o-ring. before i did all this i put the defrost on and it would only get warm when i revved to about 4k and i dont want to hear a diesel go that high in the r's ever again. anybody got any ideas on this?

TheDon · #2 09-29-2006, 10:53 PM

how long did you let the car warm up? hows the CCU looking? mines a lil fuzzy but its still kickn

SD Blue · #3 09-30-2006, 12:44 AM

Have you checked the board-board solder connections in the ACC controller? It controls everything you have mentioned so far.

Jeremy5848 · #4 09-30-2006, 02:21 AM

Quote:

Originally Posted by YoungBenz

....before i did all this i put the defrost on and it would only get warm when i revved to about 4k ...

Seems to me the only thing that revving the engine to 4000 RPM will do for the heat is to increase flow and pressure in the cooling system. That said, maybe you have a partially clogged heater core. If so, very little water will be flowing and there will be little heat -- as you have described.

What does the temp gauge say? If it is at a normal 80C or so, you should have lots of heat. If the monovalve and aux water pump are both working (how do you know?) and the engine is not overheating, then you are not getting water flow through the heater core. Another possibility is a clog somewhere else in the water flow to the heater. You'd better hope it's the latter, 'cause I think I read here that getting at the heater core is a real *****.

dmorrison · #5 09-30-2006, 11:32 AM

Start the car and get it up to normal operating temperature, about 80C.
Remove the wires from the mono block. You should not get full hot in the heater core. It takes 12 volts to close the mono valve. So if you remove the wires your getting full heat. If it does not get hot then flush the heater core. the 3 tubes going in and out of the firewall that have the black rubber hoses going to them. A garden hose will do the job, just not to much pressure, the system operates at 15-20 PSI. If the heater core allows a good amount of water to flush through it then it is probably the mono valve, they can stick. If it DOES get fully hot then we have to look at the control panel and amp.

Dave

adrenelin500 · #6 09-30-2006, 12:16 PM

I agree with blue on this one, last winter I started to have problems with the heat in my 82 300sd. Seemed pulling over after driving for five minutes and shutting the car off and restarting again would help at first but that stopped working eventually.I pulled the ACC and examined it and found nothing to my untrained eye.So I just ran a wire from the fuse box right to the blower motor with a switch and this gave me heat to the winshield, but unfortunatly nothing else.Anyway after reading a post on how that melting the sauder on the board inside the ACC could be a fix i ventured on.Upon inspection of the board i quickly saw that the trail of sauder that leads from point to point on the board was missing a section.I layed down a strip of sauder where it was missing and it corrested the problem.Apparently the sauder on these boards are susecptable to breaking apart from vibration in the car.Hope this helps.

DrewGerhan · #7 09-30-2006, 05:35 PM

Quote:

Originally Posted by dmorrison

Start the car and get it up to normal operating temperature, about 80C.
Remove the wires from the mono block. You should not get full hot in the heater core. It takes 12 volts to close the mono valve. So if you remove the wires your getting full heat. If it does not get hot then flush the heater core. the 3 tubes going in and out of the firewall that have the black rubber hoses going to them. A garden hose will do the job, just not to much pressure, the system operates at 15-20 PSI. If the heater core allows a good amount of water to flush through it then it is probably the mono valve, they can stick. If it DOES get fully hot then we have to look at the control panel and amp.

Dave

i removed the wires from the monovalve and tada she works great!

! im so glad it wasnt the heater core. since the monovalve is not plugged in now does that mean that it was at fault(broken)?

SD Blue · #8 10-02-2006, 10:58 AM

Not completely, but that is where I would start. The monovalve is controlled by the ACC controller.

To repair the:
Monovalve-$30-15 minutes
ACC controller-$0 (if you have the tools)-4 hours+

dmorrison · #9 10-02-2006, 11:58 AM

Now use a volt meter and connect the leads to the mono valve wire harness. Turn on the heater and vary the temp from full hot- the voltage should drop to near zero, to full cold-- the voltage should increase to near 12 volts. Allow 2-3 minutes between readings to allow the CC amp to work. If you get these normal readings then it is your mono valve internals ( replaceable ) or the mono valve case could be bad ( replaceable)
If the voltage does not vary then the problem is upstream of the mono valve and we will have to approach that.

Dave