1980 300sd - servo probs, manual monovalve question - PeachParts Mercedes-Benz Forum

topanga · #1 11-22-2006, 04:21 PM

Hi, friends,

As you may recall, I had a ton of trouble at the start of this searing hot summer with my 300sd and its a/c troubles (thread here a/c blows nothing but hot air, 1980 300sd)

So it's time to get the heat going... and I have a bunch of questions. I do hope you'll forgive me for my ignorance, but I really am trying to understand this car in addition to just turning it over to someone to fix.

So here's the deal:

I couldn't get a/c to work in summer. All a/c components worked, but I could not get hot air to stop coming into the cabin. You all helped me diagnose and fix this. We diagnosed it as a sick servo and rerouted the air.

The way I got it fixed was to join the heat tubes together, bypassing their journey thru the servo. In addition, something was done under the dash. I could not see this part of the fix and I am sorry I didn't pay more attention. Voila - cold air!

Soooo... it's time to get heat going in the car, and I took it in and we reversed the heat bypass. That is to say, we unjoined the heat tubes so that now they DO pass thru the servo. So far so good? Right. Now I can't get HOT air. The air to the cabin and thru the defrost is running cold. The hot tubes are nice and hot, but the air thru the vents is chilly. (A/c switch in "off" position.)

My mechanic tells me it's time to put in two monovalves: one on the a/c lines up by the servo, one on the heat lines up by the servo. This is fine with me, and is more than likely what I need to do, but here's my problem: the air isn't just cool, it's pretty cold. Thus I feel, based on what you guys tell me, that it's going through the evaporator. And that if we do install the manual monovalve, it will be fine and hot right up until it gets routed over the evaporator, at which point it will cool off, and will then come into the cabin chilly and cold. I feel that I don't JUST need to do the monovalves, I also need to change the flap positions. I think it IS hot air venting in, but it's being routed over the evaporator and cooled.

My mechanic disagrees. He says "it's your servo, and it probably broke more over the summer." His english is not great, so I am double-checking with you guys to make sure that if I go ahead and get the dual monovalves, I will be able to, manually, get hot air in winter and cold air in summer and not vice-versa.

To reiterate, my question is this: if I bypass the servo, with manual valves on the hot and cold lines, will I get hot air when I want it and cold air when I want it with no further monkeying? Are the two bypasses, when used together, going to do the job?

I will point out that one thing he said was, "of course, you will probably always need to run it on defrost." This tells me that he's not planning on me having working floor vents for heat. This sucks. If it's what I have to do, it's what I have to do. But is that really true? I would REALLY like to have floor heat this winter, or as much as I can get below windshield level.

I believe that what was done under the dash to get a/c flowing was some flap moving. I also believe that this consisted of moving a grey dial next to the evaporator in a clockwise and counter-clockwise motion. I've fiddled with that to little result. I'd be happy to play with all those cables if I had any idea what I was doing if it would give me floor heat. I'd be more happy if I didn't have to, because I have zero clue what I am doing with all that.

Again, excuse my ignorance. I'm trying to save $, trying to be smart about how to keep the car up and trying to LEARN!!!! happy thanksgiving...

dave1980_300sd · #2 11-30-2006, 11:54 PM

I've got a 1980 300sd, too, and I just rigged it for the winter (my servo is broken as well).

I read through the other thread, and I'm not sure how much your system has changed from original, but maybe this will help...

You don't need to put a valve in the AC lines. Just the heater hoses on the front side of the servo (as it's seen when you're standing next to the passenger fender). Looking at the servo from that side, the hoses on the left can be connected directly to one another (this is the coolant return circuit from the heater core), and the manual valve should be put in between the two hoses on the right side of the servo (the supply circuit for the heater core). also, look under the servo and you'll see two vacuum lines entering into it. remove these from the servo and connect them to each other (if the servo were working, there's a vacuum switch in there that only opens once the coolant has reached a certain temperature, preventing the fan from blowing out cold air. with the lines connected directly to on another, the fan should work whenever you press one of the buttons).

So, if you're set up this way, here's what you do:

winter: open the manual valve to let hot coolant into the heater core. disable the AC compresser somehow (just cut power to it, maybe wire a switch into its circuit, or find the vacuum swith that controls it and make sure it stays off) so it doesn't come on (mine would even with the dashboard switch on "off). Now you'll have hot air all the time and can rig the flaps however you want, or just use the buttons.

summer: close the manual valve to preven hot coolant from reaching the heater core. enable the ac compressor and rig the flaps the way you like them. that should leave you with zero heat and AC when you want it (as long as the rest of the AC stuff is working, of course.

hope this helps...

- dave

rgnprof · #3 12-01-2006, 05:10 PM

Dave, (or anyone else that can specify):

I am looking to follow your bypass instructions:
Can you clarify the connections for me from the picture below?

The front of the car is to the right (picture was taken from the passenger fender) - one of those hoses is a short hose that runs to auxiliary water pump.

My car is a '78 300D with the ACCII system and my servo is leaking around the base and now I have a vacuum leak - nothing vacuum works, car won't shut-off, etc. I am assuming that my vacuum is leaking at the servo...

If I make this change - and connect the 2 vacuum hoses from the underside of the servo together - will my AutoLo and AutoHi buttons work? Right now, all that works is the defrost button and it does blow out heat.

Thanks, Ryan

kerry · #4 12-01-2006, 06:43 PM

Try to find a used servo, it's much simpler.

dave1980_300sd · #5 12-01-2006, 06:58 PM

I'm not sure, but I'd guess that maybe if your case cracked really badly it could cause a vacuum leak between those two lines... it certainly won't hurt to connect them together, and yeah, I believe it should return function of auto hi and auto low. not sure if that'll solve all your vacuum problems, but it's worth a look. to rule out the servo as the source of your vacuum leak, I'd connect those lines to each other, and then unplug the big vacuum connector from the top and plug all of the ports. if you still have a vacuum leak at that point, it's most likely a coincidence it occured at the same time as the servo breaking.

you could always plug the main vacuum line to you climate control (on my car it's the green and black line under the hood) and see if that cures the leak. if it does, then you know that the leak is in fact somewhere in the climate control system. this would be a quick way to prove or disprove the theory that the leak is caused by the servo failure.

hope this helps,

- dave

topanga · #6 12-01-2006, 09:26 PM

My hot air comes on now, and it does delay before coming into the cabin. So the fake monovalve thing did work, is reversible, and my servo seems to be somewhat functioning. Ran errands all day in 22 degree weather and enjoyed heat the whole time!

I wonder about the above advice to get a used servo -- everyone keeps remarking that they're only made to last five years. I suppose it's worth it to get a little more out of your ccu, but it's a pretty short-term proposition, I think.