Help with climate control servo - PeachParts Mercedes-Benz Forum

bigbob2 · #1 07-23-2007, 10:57 AM

77 300D

I bought a 77 300 D. that was mechanically a basket case, but had a nice body and interior. I have fixed everything and I am down to the last thing on the list, that being the heater control servo. The PO has cut and plugged the heater hoses effectively disabling the heater. While a lot of heat is not necessary in Florida, with our high humidity, some would be nice to defrost the windows. I have looked at the servo made by Unwired ($700). It may be that it is the way to go, even though it is expensive, unless some of you knowlegable people know of a way to rig up a valve with some sort of a control mechanism to by pass that servo. But, I still am at a lost as to how you would get the vacuun to the vent pods. Any help would be greatly appreciated.

LUVMBDiesels · #2 07-23-2007, 11:43 AM

I was planning on going to my local boneyard this week. If you are interested I will see if I can find a used servo. I will see how much they want for it. I will let you know how much they want and if you want it I can pull it and send it to you.

-- Can't be sure if is any better than the one you have so I hope it will be cheap.

Joe

bigbob2 · #3 07-23-2007, 01:25 PM

LUVMBDIESELS: If you can find a heater servo in a boneyard that works, it would be worth its weight in gold to me. However, as I understand it, these things had a 100% failure rate. If they can guarentee it is in working condition, grab it. If not, then pass on it. As I understand it, they are not repairable. Perhaps someone who knows more that I do, can comment on its repairability.

michael cole · #4 07-23-2007, 01:36 PM

these servo units are definately repairable success will depend on your skill level and patience.there are no repair kits currently available but many times a little dismantling and TLC will do the trick.my 1980 300sd has the original servo unit and its still functioning well despite 27 years and 400 kkm.ive had this unit apart several times.if you need any assistance just ask

bigbob2 · #5 07-23-2007, 01:47 PM

Michael Cole: I have never been into this servo and and not sure what to look for , or what to do about it if I find something. Any advice and guidance you can offer will be greatly appreciated.

michael cole · #6 07-23-2007, 02:21 PM

the first thing to is get a used one and carefully try to disassemble keeping track of where everything goes.this thing comes apart in stages and will go back together the sameway.most common reasons for failure are a seized drive motor,worn wiper contacts or a cracked case

KAdams4458 · #7 07-23-2007, 08:00 PM

I'm in the same boat with my 77 300D. Mine has a large crack in it, from which a little coolant seeped, until I did a cooling system flush. Flushing it must have removed the deposits that were sealing the crack, because now it gushes out half a gallon of coolant after fifteen minutes of running.

I've been looking at used ones for several months, and I have yet to find a single used one that doesn't have a crack in it. Repairing the cracked case is out of the question. None of the uber-epoxies I have at my disposal will bond effectively with whatever that plastic is. at this point, I'm close to giving up on it and may just bypass it with hoses.

Lycoming-8 · #8 07-23-2007, 11:18 PM

I understand that the part on the servo that cracks is plastic, but what part of the servo is this? The upper or lower half? I currently have an 80 300D with one that, at first glance, does not seem to have any plastic. What gives?

hardyboy1704 · #9 07-24-2007, 01:27 AM

Everyone: If you want to get rid of the servo box, you can easily just replace it with two hoses---as I have. But, if you want to do it the correct way, you need a four (4) port valve that will allow you to switch (manually) back and forth from one line to another. I have searched around and, at least according to most auto stores, you can retrofit your car with a heater valve. It's a rare part so good luck in finding it.

But don't the 240D cars have such a valve? I thought I saw someone writing about them on this forum.

michael cole · #10 07-24-2007, 09:17 AM

some cracks are ok.the critical area is where the bottom of the plastic case attaches to the metal base thru which hot coolant will flow.if it leaks here coolant will destroy the drive mechanism located directly above.there may be some used units out there with the all metal case which came along later.that would be a find!this system is a thing of beauty when it works

KAdams4458 · #11 07-24-2007, 10:08 AM

Mine is cracked nearly in half, and coolant even comes out the top of the motor housing.

It's done for. doubt that if even if I could disassemble it and managed to repair the cracked housing that I could do more than stop the leaking. The internal parts are probably toast at this point, but maybe not. I just need to start collecting broken units, if I can manage it. I might then be able to cobble together one working unit.

For the time being, I'd love to know more about the switch over valve. On many cars I've owned/worked on, there is a simple ball valve which is operated by a sort of teleflex cable. These block coolant from flowing through a hose to the heater core when closed, or allow coolant through when open, and are generally somewhat variable in the amount of coolant they pass in intermediate positions. I remember this exact valve from my '75 Chevy truck years ago.

The only reason I haven't run out and bought one is that pesky auxiliary coolant pump that the Mercs use. I don't think the motor would take kindly to running while the valve was closed off, and coolant flow to the heater core is apparently insufficient without that motor in place and operating. Ideally, we need a valve system where as one valve closes off coolant flow to the heater core, another opens and redirects coolant from behind the auxiliary pump, back to the return side of the heater core, so that the fluid the pump motor is pushing always has a place to go.

Where does one get such a system that also uses only teleflex cables to control it? I have no idea.

kerry · #12 07-24-2007, 10:08 AM

Quote:

Originally Posted by Lycoming-8

I understand that the part on the servo that cracks is plastic, but what part of the servo is this? The upper or lower half? I currently have an 80 300D with one that, at first glance, does not seem to have any plastic. What gives?

Someone developed a replacement with an aluminum bottom portion to replace the original plastic. You probably have one of those.

bigbob2 · #13 07-24-2007, 03:45 PM

I think I understand the part about by passing the servo and putting a heater valve in place to control the amount of heat. But... what about the vacuun that servo controls. How would the vent pods get the vacuun they need to open? I am assuming the the vacuun is what holds them open???

Lycoming-8 · #14 07-25-2007, 12:25 AM

Originally posted by Kerry Edwards:

"Someone developed a replacement with an aluminum bottom portion to replace the original plastic. You probably have one of those."

Thanks for the info Kerry,

So maybe this one might have some utility left, since it does not have the cracky plastic parts!!

KAdams4458 · #15 07-25-2007, 05:53 AM

Quote:

Originally Posted by bigbob2

I think I understand the part about by passing the servo and putting a heater valve in place to control the amount of heat. But... what about the vacuun that servo controls. How would the vent pods get the vacuun they need to open? I am assuming the the vacuun is what holds them open???

Oh... You know, I hadn't given that any thought, yet. I guess I'll only have defrost mode? I don't care at this point, as long as it's not blowing hot air all of the time and my car isn't dumping expensive coolant on the ground.