300SD W116 Heater woes - PeachParts Mercedes-Benz Forum

pstyles · #1 09-22-2006, 07:35 PM

Ok, here's the latest to throw out to this trusty community.

My 1980 300SD heater blows cold, I put it to defrost and nothing but cold comes out, both in the inlet and outlet hose in the front of the engine appear warm. Does the control unit need to be replaced (I've heard these run $875+), I'm going on a long road trip and need heat.

Is there a way to bypass the system so I'm always getting heat? This would be preferable in the cold climate I'm in right now, rather than messing with the entire unit.

Any thoughts?

Thank you.

pstyles

1980 300SD

SirNik84 · #2 09-22-2006, 07:42 PM

MONO VALVE!

do a search for mono valve.

http://www.peachparts.com/shopforum/diesel-discussion/138252-monovalve-repair-completed-4-minutes.html#post1023864

I had the same problem last year.

read the threads, but if you simply unplug the valve it will blow hot. but it works so much better with a new valve. you can pick it up from fastlane.

they should make the mono valve thread stick during the winter because it seems like at least once a week someone asks about it.

pstyles · #3 09-22-2006, 09:00 PM

I don't believe my 1980 300SD has this mono valve. I see this aux-pump, and this large black box that I assume is the brains to the climate control system. I see two hoses on either side, can I by-pass this entire "black box" and connect the two hoses on opposite sides of this thing. It's starting to get cold, and I need heat!

Any thoughts?

pstyles
1980 300SD

#4 09-22-2006, 09:20 PM

I am not exactly sure here but I think what you call brain box is the mono valve. Try unplugging it, as stated above, but if that doesn't work I see no reason why the box can't be bypassed. In other words, attach a hose from the output of the heater core to the aux pump and don't use the mono valve. Likely you will need to supply a hose in absense of the space that the box takes up, some improvisation may be necessary. (90 degree copper elbow and two hoses?) It is impossible to catch all the coolant here, draining the radiator will still leave coolant in the heater hoses. If you have several drain pans try to keep them in general proximity to where the coolant will spill under the car.

pstyles · #5 09-22-2006, 09:23 PM

I've looked at some posts and by the looks of it, bypassing this "control unit" has other consequences as there are some vacuum lines that run to it. I may just have to buy an electric heater to plug into the cig. lighter. I can't think of any alternatives unless someone can suggest one.

thx.

Peter

1980 300SD

Ether · #6 09-22-2006, 09:30 PM

No monovalve.

You have the dreaded ACC Servo. It's the one notorious weakness of the otherwise glorious W116.

Look around, you can find quality rebuilds for much less than $875.

pstyles · #7 09-22-2006, 09:45 PM

Yes, looks like my 300SD 1980 has this ACC Servo. Is there anyway to bypass this without messing up the vacuum? I'm getting very cold!

thx.

Peter

2dieseljohn · #8 09-22-2006, 10:24 PM

You can bypass the Servo, but you will ony get heat on the defrost mode.
I got a kit from www.**************.com. I think it was about $50 or so ... The kit parts you could find on your own for a lot less at NAPA and the hardwear store. What makes the price worthwhile was the 3 page instruction sheet with 10 pictures. Took me less than 1hr to install. I then proped open the foot vents with a shim

. If that is out of your price range maybe I can help walk you thru the bypass.
Regards

pstyles · #9 09-22-2006, 11:11 PM

yeah, I'm hoping to go on a cross country trip in a few days so ordering the ACC bypass kit probably won't help me out.

I see two hoses on the fender side and two hoses on the engine side, then a yellow and a black vacuum hose going into this ACC servo unit.

The hoses look different sizes, but which am I to connect? And what should I do with the vacuum lines? I thought I saw this on an earlier thread, but you did seem quite familiar with this problem.

It feels like all the hoses are warm, is bypassing the ACC servo going to solve this problem?

thx. in advance.

Peter

2dieseljohn · #10 09-23-2006, 12:36 AM

Yes it will give you heat if you bypass the servo. think of this as a manual override. The line closest to the engine is the coolent flow to the heater core and the aux. water pump. The other line is the return line. If you disconect (drain the coolent

first) the lines and hook the hoses nearest to the engine to each other( if you use a stop cock or 'T' valve you can control your heat) and then conect the 2 hoeses nearest the fire wall to each other and now you have heat

The 2 vac. line are for keeping the fan off untill the water in the heater core reaches 100F so just hook them to each other and you can enjoy your heat without delay. If interested I'll post pics tomarrow of my bypass.
Regards

pstyles · #11 09-23-2006, 01:41 AM

Yeah, I may tackle the problem tomorrow.
Pics of your setup would be fantastic.

I may change the thermostat just to make sure that this isn't the root of this heat problem.

I appreciate your help with this matter, this board is great. As I become more familiar with these vehicles, I hope to share my findings and experiences!

pstyles

1980 300SD

Carson357 · #12 09-23-2006, 01:53 AM

my '79 had the same problem,i bought a rebuilt aluminum servo that fixed the problem. after i bought mine, i saw a new looking aluminum one in the junk yard last weekend. if you are interested i can go this weekend and see if it is still there

2dieseljohn · #13 09-23-2006, 07:59 PM

Hope you can now understand what I was talking about last night- I can control mine from the cockpit with a push pull knob, in is off, out is on, or half way is warm. You could just as easly install a 'T' and open the hood to change the Temp.
Regards

pstyles · #14 09-23-2006, 11:29 PM

Very creative of you! looks good.

Now my ACC servo unit has 4 hoses in total coming/going to it, two on the rhs, and two on the engine side. Looks like you just joined two, which ones were they?

Did you plug your two vacuum lines together?

pics are great! interesting fix! I need something temporary until I likely get a new unit. It's getting cold!

thx. again.

Peter

2dieseljohn · #15 09-23-2006, 11:54 PM

What you can't see is the 2 hoses nearest the firewall are hooked together also. The Vac. lines are hooked to eachother. I think in the picture you can see the vac lines at the top center a little to the left. So to re-cap Drain coolent undo the evil Servo then conect the 2 front hoses with a 'T' or on/off valve (engine side and fender front side too each other)then the back or firewall 2 too eachother. Then Vac lines to each other. Re-install evil Servo and enjoy heat

. I have many vac. issues so my heat only works on High Defrost.
Regards