1987 300SDL
Has anyone ever bypassed the CCU by hard wiring 12v (with a switch) directly to terminal connector going to the motor relay > blower motor?

Just want a reliable way to turn on the blower motor to get heat in the car.

Any advice if this is wise or not.

It seems like different parts go bad at different times and all of them are necessary to the proper function of the climate control.

Seems like the CCU is shot again. Why these things go bad so often?

After replacing the CCU ( a few times ) and the motor resister/regulator porcupine thingy, aux water pump motor, and that heater mono-valve, the blower motor not working again after several months. The motor works, its just the ccu is bad again I'm certain. I can return the Programma CCU as it;'s still under warranty. They might send me a new one and it might run for another six months or year before it goes bad again.

But it might just be easier to hard wire a blower motor switch and be done with it.

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1987 300SDL

You have probably already factored this in, simple hard wiring would eliminate speed control of the motor. It would work but I'd use a relay to feed the current under the hood to the motor. It would be a lot of current to continually run through a switch. Be very careful when routing the feed to the relay that it is fused and is routed in a fashion that protects it from cuts chafing etc.

IMHO you'd be better off to fix it as original.

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer

Don't know if you can completely by-pass the CCU, because that also controls the mono-valve, but there is a thread detailing a DIY fan speed controller for the 124 cars. Your SDL uses the same solid-state device to control the fan speed so I suspect it should work on the SDL the same way.

http://www.peachparts.com/shopforum/diesel-discussion/270584-w124-manual-blower-control-diy.html

Instead of Jereny's 124 solution you very well might be able to source continuous (key switched is probably safer, as well as adding an inline fuse)12 volts someplace to provide full voltage to the potentiometer and then attach its output directly to the wiring coming from the CCU going to the "porcupine" controller.

I've done the mod as Jeremy describes and it works excellent, it just depends on the CCU providing the 12 volt power source the way he has figured out.

Should be pretty easy to try, good luck.