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ACC Servo replaced, but no blower motor function

Started by ceast3141, 26 August 2023, 12:45 PM


I have a 1980 300sd. I am troubleshooting the AC right now.

I started by checking my fuses, and I have replaced all f the original fuses with ceramic ones.

I have no blower motor function with DEF on, so it is hard to say if I have a vacuum issue. My next step was to check the amplifier in the glovebox.

I noticed my AC servo has been replaced with something like unwired tools' AC servo replacement.

Initially, I saw the green vacuum line out from behind the driver side firewall was disconnected, and the fairway vacuum connector was t'ed closed where it would go. Is this part of the UT ACC Servo installation?


'68 250S
'77 6.9 Euro
'91 300SE,
'98 SL500
'14 CLS550,
'21 E450 Cabrio


Might be best to source a good used Chrysler ACC unit.  These aftermarket concoctions tend to work for a while and then they get temperamental.
1977 450 SEL 6.9 - Astralsilber


I'd check the blower motor first. See if it's getting 12V in def, or apply 12V and see if it runs. I don't think servo issues would cause no blower function on def.

There is a relay near the blower motor behind the glove box that allows for the backup defrost function. That might also be the issue if of the blower motor works but is not getting 12V in def.

Have you tried to find a diagnosis procedure for this issue in the service manual library?
1980 300SD


I was able to change out some old fuses, and now the blower motor works in DEF. No movement for any other setting.


You need to triple check for vacuum leaks.  Check all vacuum lines and rubber connectors as well as one way check valves.  Any vacuum leak will make the system misbehave.
1977 450 SEL 6.9 - Astralsilber