300D 1980 123 heater - PeachParts Mercedes-Benz Forum

Fisherman · #1 01-05-2003, 08:57 PM

Hi Guys,
I think my monovalve needs rebuilding but where is it on this model? Does it reside under the controller up front int he engine compartment or is it mounted on a firewall or what?
I looked all over the thing today and tested vacuum at teh controller etc but did not see anything that looked like it would take the fastlane monovalve repair kit.

In other words I'm lost here!!! Help!!!

kerry · #2 01-05-2003, 09:06 PM

I may be wrong, but I believe the monovalve was used beginning in 1981. If you have the dreaded ACC servo, (the large item mounted on the fender well behind the front of the right tire with a ton of wires and vacuum hoses connected to it in addition to the heater hoses) then you don't have a monovalve. What problems are you experiencing that is making you think the monovalve needs to be rebuilt?

Fisherman · #3 01-05-2003, 09:22 PM

I do have the dreaded box... It was repalced according to the paperworlk I got with the car about 10 years ago..
I have hot water getting to the pump, and the pump is working correctly. The small diaphram mounted on the three way on teh firewall is working correctly. \
There is no hot water in the lines past the acc box. The pump is trying to p[ump but it appears that a valve is not opening in the bottom of the acc box to let the water flow through.

I get no heat from the vents in any setting.

Is there any diagnostic procedures for this box? Also the acc controls seem to work properly but again I have no clue what should be doing what when I turn the temp wheel or push the buttons.

One item that I am pretty sure about though is that the outside air solenoid in the dash is gone since I have air blowing out the middle vents in all settings.

I am also in need of a vacuume schematic for this car as I am having a dickens of a time trying to tame this tranny into shifting without huge flares and early upshifts. It will not downshift either using the kiskdown switch so I have a lot of work to do here too...

I love new cars... Luckily I paid very little for it.

kerry · #4 01-05-2003, 09:49 PM

There are diagnostic procedures for the ACC. They're on the CD. You can pick up a copy on ebay for around $15 usually. Your problem is above my level of expertise. I agree that the ACC servo is not opening the valve in the bottom that permits hot water to flow. It could be a vacuum or electrical problem in either the servo itself or the push button unit. I'd try the unit in the full defrost position because that is the position where a lot of functions are overridden.
I believe the diagnostic procedures are quite complex. If there is a junkyard near you, I'd be inclined to pick up a cheap used pushbutton unit and try that first since the servos are a lot more expensive. (I'd do this after a quick check to make sure that the vacuum system is reasonably intact. If there are other vacuum problems in the car, I wouldn't mess with the heating system until those problems were chased down)
I just remembered that there is an amplifier in the system (I think behind or near the glovebox) that fails. Check out the symptoms for that failure before acting on my prior comments.

Fisherman · #5 01-05-2003, 09:59 PM

Thanks Kerry!
Oh boy, now I really like the simplicity of my 115.

Can you bypass the unit to get heat into the car. It is like 25 degrees here now and not real pleasant without heat...

Mommy's not gonna like this "investment" I made..

kerry · #6 01-05-2003, 10:09 PM

Do a search on heater or ACC servo and you'll find threads describing a bypass. People take a piece of pipe (copper) and join the heater hoses under the servo. I was going to do that on mine when I was having problems with the servo but decided by the time I did that, it would be better to spend my time replacing the servo.
If you end up replacing the servo, there is an metal bodied version that is superior to the original plastic body. You also might find a used one on an old Chrysler Imperial since they used the same servo.

Fisherman · #7 01-05-2003, 10:23 PM

Good info ont he Imerial.
Thank you again...

I did some searching ealrier but was not looking for the "servo" side of my problem.
Now at least I have a better idea of what to look for.

driad_98 · #8 02-25-2004, 10:48 PM

I'm having the same trouble as Fisherman. I found this thread while looking for one that describes the ACC servo bypass, but I'm still having a hard time finding a thread that explains lucidly the proceedure and the results. I have some things to do before I bypass it, but it IS leaking. (Is there a way to fix it's leaking, btw?)

I have another push button unit, so I'll be trying that, but I don't think my aux water pump is working... if the aux water pump isn't working, and you bypass the ACC servo, will water make it to the heater core? Thanks for the help, gents.

Ash

kerry · #9 02-25-2004, 10:56 PM

There are two hoses coming into the servo from the front and two leading out at the back. These hoses are on the bottom. Join the front ones with the back ones with short pieces of pipe. Water will then run to thru the heater core all the time.

driad_98 · #10 02-25-2004, 10:59 PM

Cheers, man. Should I leave the servo in place? Will it still try to operate the vacuum elements?

kerry · #11 02-25-2004, 11:09 PM

Leave servo in place. I don't think the electronic and vacuum controls will know the water valve is bypassed.

driad_98 · #12 02-25-2004, 11:14 PM

Of course! Thanks.

Fisherman · #13 02-25-2004, 11:32 PM

I made some plugs that I could use temporarily during chageove so that I did not need to replace the lost coolant.
The only hose that needs to bypass the servo is the block side hose. The other has a permanent bypass built into the servo. Why it even goes through it I have no idea other than maybe for the thermo sensor on the bottom of the servo. This part is tied to the yellow vacuum line (I think) or black that controls when the system turns on. If the water temp is too low then the system does not operate except in defrost mode which bypasses this circuit.
Anyway, you can just bypass the servo on the block side of the unit and you wil have constant full on heat. The other way to go is to buy a standard heater control valve (you can probably get one at a wrecking yard real cheap but it may not work too well) and then put a bowden cable connection under your dash or somewhere else to control the flow of water through the heater core. You know, pull on the cable and it gets hotter push it in and it turns off.
I bypassed mine for a short time and I hate to say it but I don't remmeber if all the controls worked like normal or not. You definatley need the water flowing by the on/off vavle on the fender side as this is the main vacuum valve fro the whole system. Wihtout this sensor allowing vacuum through the vents and things won't work I think.
Now you could also bypass this sensor by just connecting the two vacuum lines that run "under' or the bottom fo the servo. This is where the switch/sensor is located and it opens the vacuum from the source line to the rest fo the system.

Sorry if this is a littel incoherent I'm really beat but had to respnd to one of my original 300 threads...

I forgot how much I didn't know...

driad_98 · #14 02-26-2004, 12:24 PM

I'll likely bypass both hoses, as the servo block's leaking. Do you think there's any reason not to just remove the vac operated valve downstream of that temp senson? Or maybe lock it open somehow... I guess I could just hook up both vac lines, as you suggested... just seems like it's asking for more vacuum trouble.

Quote:

You definatley need the water flowing by the on/off vavle on the fender side as this is the main vacuum valve fro the whole system. Wihtout this sensor allowing vacuum through the vents and things won't work I think.

Which valve is this? The vac valve on the firewall? Part of the servo? (Thanks a ton for your guidance, guys, you're great!) Upon re-reading, I think you're talking about the one on the firewall... why not remove it?

Fisherman · #15 02-27-2004, 09:47 AM

I think they are black and yellow. Just need to connect these two together.
The switch is located on the bottom of the servo and just keeps the system from turning on until the water temp is up a bit. No sense in blowing cold air.
Defrost setting bypasses all this as it is the default/failsafe setting.

Just take a rubber connector and connect the two lines together. I don't think any water actually goes into the servo via this passage though on the fender side. It is only there to allow water access to this switch which is mounted on the bottom of the servo. I could be wrong but you can bypass thsi one too if you feel you need to.
I just figured on mine that the hose to tube connections were asking for trouble so I only did the one side.
Obviously you can't remove the servo entirely since all the vacuum stuff passes through it anyway. Someone is really laughing about this setup somewhere...

My '76 BMW 2002 had one of the adjustable ball valves that controlled the heat on it. I seem to remember them costing about 10 bucks but that was quite a while ago. You'd have to figure out a mounting point for it and for the cable end but shouldn't be abig deal to rig a temp control using a six foot cable and the ball valve.
Good luck.