Heater Blower Issue '78 300D

[bscrecords]

Hello! Let me say that I have learned more from these boards that I could ever imagine.

My daily driver, 78 300D just developed a fairly weird issue: The Lo and Hi fan speeds stopped working. Note: the defrost mode works well, fan blows hard and hot. I am very versed in this car, I have had few electrical problems and few vacuum problems, but obviously, I have a problem now!

I'm going to tear into it this evening and start looking for the issue and I'm sure I'll track it down, but anything in particular that I should be looking for first?

Thanks in advance! Erik

[bricktron]

no schematic at hand, but i think there is a resistor pack in that circuit which could potentially blow out. i would find it, disconnect one side, and test with an ohmmeter.

[bscrecords]

Thanks. I'll find it.

[Bobbywheels1212]

My 300d has the exact same problem. Please let me know if you find a fix!

[bscrecords]

I will update. Got the control unit out, but then a friend stopped by and reminded me how cold it was outside.

[uberwasser]

Often the "only defrost" problem is related to issues with the climate control servo. Can't say for sure. However, the defrost only is basically the fail safe literally for safety's sake when the servo fails to work in any other mode.

Do some searching on here and over on peachparts.com and you'll find lots of info on doing diagnostics.

[bscrecords]

Well, it was not the dreaded servo. Alas, I had corroded contacts. That climate control unit is a bear to break down.