Heat output problem with W123 300D - PeachParts Mercedes-Benz Forum

240diowa · #1 11-30-2005, 10:41 AM

Can someone help me diagnose my heat output problem? I only get heat when I'm cruising down the road and even then it isn't much, I basically get no heat at Idle. Don't know if it is the mono-valve or the auxilary water pump. Some threads I read say that if you get heat at idle and none at operating then it's the monovalve, I kinda have the opposite. My kids are freezing in the morning...

boneheaddoctor · #2 11-30-2005, 10:45 AM

Can you clue us in to what year this is? Aux water pump not working gives these symptoms..

sailor15015 · #3 11-30-2005, 10:49 AM

Could be a combination of both. Is your car getting up to operating temps like it should, somewhere around 80-95 degrees Celcius?

boneheaddoctor · #4 11-30-2005, 11:27 AM

THere are three different coolant control systems possible on the W123...

only one of them is a monovalve...we need to know what year it is and if its ACC equiped or not.

240diowa · #5 11-30-2005, 12:31 PM

It's a '78 300D with AC

boneheaddoctor · #6 11-30-2005, 01:01 PM

Quote:

Originally Posted by 240diowa

It's a '78 300D with AC

You have no monovalve....you have the Evil Vacuum Servo monstrosity....assuming its not a Euro with the two knobs and slide levers....If you have the vertical row of push buttons and one rotory wheel with the red, white and blue tape on it, then you have ACC and the evil vacuum servo.

apply power directly to the Aux Water pump and see if it works...I bet it doesn't. it pushes the coolant through the core at slower speeds....which is what your symptoms indicat is not happening.

#7 11-30-2005, 01:28 PM

Quote:

Originally Posted by 240diowa

It's a '78 300D with AC

Ignore my previous monovalve post.

How long does it take your car to get up to temperature (80C) in winter conditions? Does the temperature fall back below 80C after it's initially warmed up? I'm wondering about your thermostat.

#8 11-30-2005, 10:48 AM

I would start with the monovalve, it's the most likely problem. Take it apart and have a look, it only takes about 5 minutes. If required the rebuild kits are cheap. My experience with a non-funcioning aux pump was that I had normal heat while driving and a reduction of heat at idle, nothing like what you're describing.

240diowa · #9 11-30-2005, 10:54 AM

My pride and joy is a '78 300D (with only 82K miles and no rust). Anyhow it does get up to operating temp but it takes awhile (it was around 12 deg this morning). I basically get little or no heat while idling but when cruisiong at 70 on the interstate I start to get some heat, If I drop below 30 mph my heat basically stops. Where is the monovalve located?

240diowa · #10 11-30-2005, 02:11 PM

My heater control does have the buttons vertical and the red/blue white wheel on the left. Over lunch hour I opened the hood and found this round thing (1.5" diam X 4" long) with two wires connected to it sitting next to the servo monster. It looked like a solenoid type valve. The hose coming in was warm as well as the hose coming out. The servo monster (that thing must have at least 20 vacuum lines going to it) appears to be leaking coolant. I tried tightenin the hose clamps but it didn't help. I'll check power to this thing and see what happens. Does the aux water pump/servo run at different speeds or is it on/off application?

boneheaddoctor · #11 11-30-2005, 02:15 PM

Quote:

Originally Posted by 240diowa

My heater control does have the buttons vertical and the red/blue white wheel on the left. Over lunch hour I opened the hood and found this round thing (1.5" diam X 4" long) with two wires connected to it sitting next to the servo monster. It looked like a solenoid type valve. The hose coming in was warm as well as the hose coming out. The servo monster (that thing must have at least 20 vacuum lines going to it) appears to be leaking coolant. I tried tightenin the hose clamps but it didn't help. I'll check power to this thing and see what happens. Does the aux water pump/servo run at different speeds or is it on/off application?

that leaking water thing is not a good sign.....usually that sounds the death knoll for them.....they are about $350 + and core for the servo monster...you need a new servo amp too when you change it...one will smoke the other if you have a bad one.

Try to unplug the aux water pump and jumper 12V and a ground to it...and listen...you should hear it run if it does...do this with engine off. Hope that is your problem...they cost far less.

I went through this 2 years ago on my W116...I had a siezed up Aux water pumpt too, a toasted Vacuum Servo, and a burned out Servo amp.....the parts are identical between your 300D and my W116 as far as these are concerned.

240diowa · #12 11-30-2005, 02:31 PM

So is the round the (1.5" X 4") my aux water pump? Is there a way to determine what part of the vacuum servo is bad? Is it worth buying a used one, Should I install a different system that would be compatible with the heater controls? Does the vacuum servo also control the AC? If not is it possible to bypass the servo and have heat on all the time and regulate by opening a window??

240diowa · #13 11-30-2005, 02:33 PM

The mercedes shop shows a monovalve kit for my car
PART NUMBER SEARCH RESULTS WITH PRICES

Vehicle 1978 Mercedes Benz 300D
Part: Mono Valve Repair Kit

Note: Heater Valve
Electromagnetic heater valve.
Chassis: 211432-

Product Brand Availability Retail Our Price

R3023-15174 Mono Valve Repair Kit Bosch IN STOCK $80.00 $49.45

#14 11-30-2005, 02:42 PM

Strange, do you have something that looks like this in the heater line?

http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1CQ0J3JZ41O90VH5S3&year=1978&make=MB&model=300-D-002&category=All&part=ACC+Mono+Valve

boneheaddoctor · #15 11-30-2005, 02:50 PM

Quote:

Originally Posted by 240diowa

So is the round the (1.5" X 4") my aux water pump? Is there a way to determine what part of the vacuum servo is bad? Is it worth buying a used one, Should I install a different system that would be compatible with the heater controls? Does the vacuum servo also control the AC? If not is it possible to bypass the servo and have heat on all the time and regulate by opening a window??

the vacuum servo is an assembly...you don't fix it....few venture to even try.

Used ones may be bad or fail at any time....or not..its a gamble.

THere is a difital repacement availible that works far better...but its a $750 system....Performance producs sells it and so does fastlane....

but bypassed or not the Aux water pump still needs to work...