1979 300SD servo quesiton - PeachParts Mercedes-Benz Forum

turbozeke418 · #1 04-01-2019, 02:40 AM

What powers the servo or should I say, what tells the servo to power up and do whatever it does? If I connect a jumper to pins 3 and 4 and reverse them, the servo works fine. Only my defroster button works. Hi Lo Bi do nothing. A couple weeks ago, the buttons all worked and when I rotated the knob up and down I would get warm or cool air until the fuse melted. (air compressor is not hooked up at all). I also do not hear the servo kick on when I turn the car on or push any buttons or when the car turns off. The fuses are all OK and so are the 2 secondary fuses within the fuse box. This is in addition to my melting blower fuse #3 when I turn the defroster on it gets too hot to touch and over 160*. I'm not sure if these are linked somehow or not.

I need the dumbed down description of what makes what happen (chain of events) if possible so I know where to check for power. Everything inside the dash looks great. The amp (whatever that's for) the hoses and plastic, etc. It all looks practically new still. I removed the fan and jumped it to a 12v source and it runs perfectly and also is nice and free. I have the electrical diagram, but looking at, it might as well be hieroglyphics because it makes no sense to me. Correct me if I'm wrong but in this diagram at item #11- Fuse #3 (my melting fuse) goes to 3 areas-
1. item #1- the the adjustable spin knob for hot or cold temp.
2. item #10- the pre-resistor for the blower
3. item #18- the double contact relay
Therefore, it must be one of these 3 causing it to melt? If that's not the case, I give up on reading electrical diagrams.

Alec300SD · #2 04-01-2019, 04:28 AM

If you have vacuum leaks (like my cars do) the servo won't operate as intended.
Defrost mode will still work though...designed as a failsafe.
I know I need to replace a few door lock and pod diaphragms in both cars in order to regain 'climate control'.

The 'evil servo' explained:
1971 Chrysler Imperial Auto-Temp 2 Service Reference Guide - Session 281

turbozeke418 · #3 04-01-2019, 04:36 AM

Quote:

Originally Posted by Alec300SD

If you have vacuum leaks (like my cars do) the servo won't operate as intended.
Defrost mode will still work though...designed as a failsafe.
I know I need to replace a few door lock and pod diaphragms in both cars in order to regain 'climate control'.

Now that you mention that. Before I installed my heated power leather w126 seats into it, all of the buttons worked but would blow the fuse. After wiring everything in and putting the radio and drivers side knee panel back in I noticed the car wouldn't shut off and coincidentally the buttons on the hvac stopped working. I guess I disconnected something by accident. I didn't know that effects the servo kicking on. THANK YOU!!! I guess I'll take all of that apart today and check it out. Up to my interior mod....locks vents all worked and car shut off instantly.

turbozeke418 · #4 04-02-2019, 02:24 PM

FIXED! It was in fact the blower motor. I took one out of another car and wired it up and no melting fuse and it doesn't even get hot. I also fixed the Hi Lo Bi controller. The vacuum connection on the back of the climate control unit was completely disconnected. Hooked it back up and everything is perfect.