1978 A/C Servo - PeachParts Mercedes-Benz Forum

mobenz · #1 07-09-2008, 09:37 AM

I just replaced the A/C servo on my 1978 300CD with a rebuilt unit. Still no blower on Hi or Lo or Bi-level Auto, just on Defroster. Is there another component in the system that could be causing this?

Thanks in advance for any help.

Lexxani · #2 07-11-2008, 10:57 PM

you have an "ac amplifier" in terms of other components as well as various vacuum switches. My guess is your problem is vacuum related.

You might begin my checking the switches on the dash panel and working toward the servo and vac pods (that operate the recirculation flaps and blower.)

79Mercy · #3 07-11-2008, 11:04 PM

You probably have a vacuum leak. There is main vacuum switch that turns the blower off if there isn't enough vacuum in the system. Start checking lines and actuators with a mityvac.

I have this same problem only it is not as bad as yours.

Mark DiSilvestro · #4 07-12-2008, 02:30 PM

All my Benzes have manual heat-AC, but I seem to recall hearing/reading about some electronic ACC component in the dash that gets fried when the ACC servo fails on these cars.

Happy Motoring, Mark

Lexxani · #5 07-12-2008, 02:47 PM

Quote:

Originally Posted by Mark DiSilvestro

All my Benzes have manual heat-AC, but I seem to recall hearing/reading about some electronic ACC component in the dash that gets fried when the ACC servo fails on these cars.

Happy Motoring, Mark

mark, I think that occourance you reference is the servo goes if the aux waterpump fires (or vice versa). I think the amplifier ('that component in the dash') is insulated from the madness.

Mark DiSilvestro · #6 07-12-2008, 04:05 PM

Having dissected one of these Chrysler servos from a friend's '79 300D, what happens is the heater valve in the servo leaks, subjecting the servo's gear-train and motor to coolant vapors until everything corrodes, the motor seizes/shorts out, then takes out whatever in-dash component (amplifier or relay) operated the servo motor.

Happy Motoring, Mark

79Mercy · #7 07-12-2008, 04:10 PM

I haven't ever had a servo to fry, all mine just start leaking.

Mark DiSilvestro · #8 07-12-2008, 04:21 PM

The 300D was a rustbucket, and my friend didn't want to invest in restoring the ACC, as the car was mainly for his teenager to drive to school. Since the ACC system's failure had defaulted to the high-defrost mode, I simply bypassed the servo's heater valve with a firewall-mounted, cable-operated, heater-valve, using a lawnmower control under the dash to regulate defrost temperature.

Happy Motoring, Mark

UT_Tech · #9 07-20-2008, 02:28 AM

This indeed vacuum leak. These are time consuming but inexpensive to fix. PM me if you need details. If you want to get the system functioning right but the full digital upgrade is too pricey there's another alternative:

http://unwiredtools.com/accii-diy.asp

bigmaq · #10 07-20-2008, 10:11 AM

UT-TECH, how, exactly, is the leg and center vent "manually" changed on the ACCII DIY version. I assume the vacuum pods no longer control these vents, so what does the driver do to change from "cool on center vents" to "heat on leg vents."

This is interesting. I wonder if you could just tap into the existing vacuum lines to these pods and attach a T to an existing vacuum source, then add a "vacuum switch" that you manually turn on/off.

Thanks,
bigmaq