I'm new to this site and just bought a 1980 300D with 212K on it. I have been trying to figure out how to fix my heater problems (the only activity I get out of the unit is defrost) and I can see that this is a fairly common problem. Review of the many threads discussing similar problem leads me to conclude that there sure is a lot of complications and high dollar parts in this car in order to get heat and defrost, which seems like a pretty simple concept.

Upon further reading of a very good, detailed post on how to bypass the servo, I am left wondering why just bypass it, why not completely remove the servo and the quagmire of potentially leaking vacuum lines along with it and simply have a conventional cable operated heater control valve on the firewall. Has anybody tried this? It looks like somehow, a person could leave the vaccum system in place in the dash for opening and closing ducts, then regulate hot water into the heater core with the manual heater control valve and direct the air it with the push button controls which still utilize vacuum.

Any ideas would be appreciated.

I have a 1979 300d and would be interested as well. I would like to also figure out a way to get my AC to work w/o the servo... any ideas?

I bought a used servo on ebay for $186 and it works great. It is worth every penny because I can just set the dial and the fan speed and temperature are controlled automatically. The servo doesn't let the fan turn on until the coolant is hot. Very cool.

You can manually change or control the flow of coolant, but the Evil Servo also operates the ducts openings and closings I believe. That is where it would get sticky......
Much info under "Evil Servo"

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Jimmy L.
'05 Acura TL 6MT
2001 ML430 My Spare

Gone:
'95 E300 188K "Batmobile" Texas Unfriendly Black
'85 300TD 235K "The Wagon" Texas Friendly White
'80 240D 154K "China" Scar engine installed
'81 300TD 240K "Smash"
'80 240D 230K "The Squash"
'81 240D 293K"Scar" Rear ended harder than Elton John

when i attempted to unhook my servo at one time there were a lot of electrical demons jumping up. the car wouldnt turn over with it disconnected. i had a huge draw from it at one point as well. when i plugged the electrical back in, all was fine.

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1980 300D

yep, on the top rear side of the servo there is a gang connector of vacuum lines, they are linked to vents and stuff in the cabin. It's best to leave those connected to the servo, even it is not controlling coolant.

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Sonny


86 190D N/A 2.5L Auto 265k "Ruby" -Sold-
79 300D 242k "Condi" my first -sold- RIP
2013 chevy sonic hatchback - had to for work

Performance Products has a Digital AC Servo Upgrade Kit for $600.
http://www.performanceproducts4benz.com/productpage.aspx?pid=109850&cid=3&sid=383

[IMG]http://content.************************/main/109850/NO.jpg[/IMG]

I put the digital replacement listed above in my 77 300D last year, have been totally pleased. A little pricey, but we bought the car new and will always have it, so well worth the money.

Ron

Yes, you really can't operate the vacuum pods under the dash without the servo in place. The bypass system is good to solve a cracked servo with leaking coolant but functional otherwise. Without the electric/vac portion of the servo working, you're going to be stuck with the defrost default. You could manually wedge open the footwell vents and get heat to your feet, that would seem to be the simplest solution without replacing your servo if it is not leaking and you are actually getting heat. But I don't think you'll get much temperature regulation, just full heat in the defrost mode plus heat on your feet with the vents wedged open.
Cheapest solution is to look for a used one at the pick and pull. Best solution is either a rebuilt aluminum bodied one or the computerized replacement. Or, move to Florida.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

I did this modification a few years ago after chasing gremlins constatly on my 1980 300SD. Also, I couldn't see $600 or so for an electronic servo that may ormay not solve the heating.

I took out the servo entirely and replaced it with a bypass valve activated by a push/pull knob that I ran to underneath the steering wheel through the firewall. The fan control I think I ran to the vacuum connection in back of the dash and the fan speed is controled by the dash buttons. The only issue is because of the vacuum control the fan will always run on at least low.

The A/C i connected to an additional fuse block controlled by an on/off switch installed again below the steering wheel in the dash. The normal dash switches control the fan speed.

When you want A/C turn on the switch and set the fan setting to the level you want and you got cold air. When you want heat you use the push/pull know to control the amount of heat and also set the fan to the level you want.

It is not as elegant as the original but for about $50 - $75 it works and I'm not concerned with any vacuum or electrical issues popping up.

I also did the same thing with the glow plugs: push a button to turn them on, hold the button in for as long as you want them on then start the engine.

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Rev. Dr. G. Church of the SubGenius
It doesn't take a genius

Thanks for all of the input and ideas. I am still thinking it all over...I must admit that I still think that it is feasible to end (cap or allow to remain open) all of the vacuum hoses going to and coming from the servo and get some sort of valve that does the selection manually...it seems like I have had cars in the past that you could hear vacuum opening and closing doors in the dash when you selected the various settings (when the key was off) until the brake booster was out of vacuum. Maybe I could find such a valve by going to Autozone and lying to them by saying "I have a ....1975 chevy pickup.... yea... thats it and I need the heater valve and the heater/defrost selection switch that goes in the dash"
Anyway, the other remaining "bait" for eliminating the servo and all of the associated vacuum lines is that I would hit the loto on extra/virgin Mercedes vacuum fittings and hoses (couplers, Y's, T's, 3 ways, 4 ways, etc) that I could use to replace my swollen ones under the hood...all of the complicated vacuum lines and fittings associated with climate control and servo (in the dash) look brand new...

I will keep you all posted.

Hey Spleen.

Any update on your attempts to get around using the servo? I have had mine out for a few months now, completely bypassed with a copper pipe, but I have NO control over anything. It's constantly blowing heat, but that doesn't work too well in the Texas heat.

I'm just looking for anything I can do to get any sort of cool air flowing through there.

Thanks!

http://www.performanceproducts4benz.com/productpage.aspx?pid=109850&cid=3&sid=383

Or, you could buy a used working servo on eBay for around $100, it should last a few years, when it breaks sell the core on eBay, buy another used one... Or buy the rebuilt one with the aluminum body for $440 from GermanStar.

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1980 240d
1999 SL500

I think a lot of the time those servos leak and become plain worthless.

I wrote up a guide here a few years ago on how to bypass on someones thread after I figured it out; that is what I did and I have never looked back

Cost me a couple bucks, and I get mad heat and defrost!
the regulation is my fingertips, though...

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Think Alternative Energy!
300CD '80 (now gone but not forgotten...)