I have a 79 300D 123 since it was new - just can't get rid of it! Have to turn blower motor on to engage air or heating. Turn defrost on - then turn on air button, then push high or low and ac engages and runs fine for about a min or two then shuts off - blower motor makes a very loud noise - motor 3 years old. Turn off ac - turn on defrost, move temp gage to high, push high or low button, heat engages runs for a min or two then switches off. Blower motor again runs loud and rough. One mechanic says Servo other says blower motor and resistor which gets hot and shuts off motor - anybody have some thoughts?
Thanks for your assistance
Ron

Got this response - thought it might be helpful to others....

Dear Ron:

You may want to post this repsonse so that others can see it.

Without a doubt it's the servo. One of the functions of the servo is to automatically control the fan speeds. The servo is bypassed (as a safety feature) when the "DEF" button is pressed. DEF turns on the fan and opens valves to direct vacuum to flaps inside the car. When this vacuum is supplied to the flaps vacuum is also applied to a switch which controls the fans when DEF is not pressed.

When the AC system is working there's a thermo-switch which prevents the system from operating until the engine coolant temp reaches 60 Degrees C. Here in the Southwest this is irritating because we need AC, not heat, and this feature keeps the AC from operating for awhile. You discovered the short-cut. If you press DEF first, this bypasses the thermo-switch (and other) controls and the system starts up right away.

Pressing DEF first should always start the fan and when you press any other button the system should stay on. Your car doesn't because your servo has failed. There's enough vacuum (300mm Hg minimum) to start but then the vacuum bleeds away because the servo is not doing its job. Vacuum control is another function of the servo. There also may be vacuum leaks unrelated to the servo but these are usually easy to repair or bypass. The fact that it takes a few minutes for the vacuum to bleed away is a good sign that your vacuum connections are fairly tight.

When the servo fails in this way the servo amplifier also fails and will have to be replaced. The ACCII Upgrade kit replaces both of these parts.

The noise in the blower motor means the motor has to be replaced. This is normal wear for a '79. Replacing the blower on this car is very easy. You can do this yourself. If you're willing to press off the fan from the blower motor shaft the replacement motor alone is inexpensive (~$100). You'll need a bench vise, hammer, and punch to do this. You can also buy the complete blower assembly ready to bolt-in.

The resistor box is not a factor in any of the symptoms you describe.

Sincerely,
Steve Schlanger
UnwiredTools Support Team

Where is that servo on an 84 190? Is it the unit behind the glove box with all the colored vacuum lines and a wiring harness from the climate control unit? Mine makes a rapid clicking sound ( like a relay) when calling for heat. Sometimes I don't get AC either.

I one new what the part was, but I've forgotten it and my links to the parts store where i saw it are lost. Can't find it in the parts lists.

__________________
Bob Tifft
84 190E (122K)
88 Volvo 240 DL (255K)
92 Honda Accord (184k)