When I used my '78 6.9 two weeks ago, it was blowing very hot air regardless of the setting of the temperature dial. I checked the resistance values of the rheostat there via the 10 pin test connector (pin 9 and 10) and saw a resistance of 550 Ohms at 65 F selected and 1700 Ohms at 85 F selected. I then took the car for a test drive.

Now, it only blows cold air all the time, regardless of temperature setting. So I wondered if the servo is stuck. The car has an aluminum bodied servo, and I thought those were more trouble free. Feeling the auxiliary pump I noticed that it seemed operational (slight hum), the lines to the servo were hot, but the lines from the servo were cold.

Any suggestions as to where I might look next? Thanks!

Robert

I think aluminum bodies are less prone to leak, but their likelihood to stick is probably the same.

Yours sounds like it is stuck.

There's a lot of information here about servos, but most will say that when they begin to fail, they don't suddenly get better.

I'm on my 3rd servo (this one is aluminum) in 10 years of 6.9 ownership. When this one breaks, I will be getting the electronic replacement from unwired tools.

The manual at the w116.org site has the test procedures and troubleshooting steps. Look at mercedesshop Vintage/helpful links/116.org/handbook

Thanks oxymoron - I do actually have the w116 chassis HVAC manual, and I just did a quick and dirty look to see if the rheostat was at fault. I discounted the servo because the thought the aluminum bodied ones were a final solution. It appears I may be mistaken. I will follow the procedure more closely and verify before I drop $500-700 in a new servo or the unwired replacement kit.

Besides the aluminum servo, my 6.9 has recently had its radiator rodded out, and new hoses all around to cure an overheating problem. I guess the crud that got the radiator also may have gotten the servo.

Does anyone besides Performance Products carry with unwired kit?

you can buy direct from unwired and I see them on e bay as well.
I sent somebody here on the board some internals from one of my old servos a couple of months ago. some of the innards look stout such as the gears and some are tiny springs.

my first servo bit it when the PO used some Stop Leak and the whole bottom of the mechanism clogged up. it doesn't take much foreign material to mess one up

It is only fair that people driving 6.9s (and 6.3s) don't get cooling/heating air!

__________________
82 300D....went to MB heaven
90 350 SDL....excercising con rods

Hi there,
If you go to the SL forum there is an advertisement for the unwired tools digital servo at the top of the page. It is worthwhile investing in, as it cures the servo problem forever, at least a lot longer than 3 years! My 450SL had the aluminum bodied servo in it, but the gears, etc above the thing were all corroded away. I installed the digital servo and am very happy with it.
The unwired tools people have some downloads available on their website that include vacuum diagrams, installation instructions, etc that you can look at before you buy.

__________________
Richard Wooldridge
'01 ML320
'82 300D 4.3L V6/T700R4 conversion
'82 380SL, '86 560SL engine/trans. installed
'79 450SL, digital servo update
'75 280C