Is w124 heater blower motor supposed to run all the time?
 

I have a 1987 300TD w124 wagon and it has had a chirping sound from the dash for the last few weeks. It definitely sounds like a motor bearing/bushing on its way out. I assumed it was the air sampler motor (which previously had made some noises for me), but I removed that today, and the noise persists. You can hear it very clearly when outside the car, by the fresh air intake and monowiper.

Basically, if I have the car on, regardless of whether the climate control is on or off, I can hear this chirping/squeaking motor. If I turn on or off the heater fan, you can hear the chirping change speed as well, so basically I am virtually certain it is the heater blower motor. I think I have two distinct problems 1) the squeaking and 2) the motor running all the time. So a couple of questions:

1) Is it correct for the heater blower motor to run at low speed even if the entire climate control system is off? If not, does this suggest a particular failure for me to address? Maybe the fan motor brushes or a switch somewhere has failed?

2) Is there a consensus regarding lubing the motor shaft vs. buying a new motor vs. buying a new motor with squirrel fans attached? I am on a tight budget unfortunately.

I don't know about their relevance, but I wanted to give two more details about my car. My A/C is non-functional, so I have only been doing these tests with the heater on or off. And on rare occasion, the entire climate control does not respond to my pressing of buttons, until I raise the fan speed to high. This seems to remind the entire system to start working, and then the other buttons start working again. This last observation is not a big deal to me because it happens so rarely, but I wanted to share it on the off-chance it relates to my questions above.

Thanks for any help you can provide.

I do not believe that the fan runs with the ACC turned off. To be sure, there is a 3-wire connection just ahead of the windshield where you can disconnect the power to the fan, and see if your noise stops, or pull the fuse.

The blower fan should not run when the climate control is switched off. You could have a problem with the pushbutton unit or less likely the motor speed control module located under the blower motor on a heat sink.

Mercedes used 2 manufacturers for the pushbutton unit in the W124. If it is a Bosch there is a 5 volt regulator in it that gets so hot that the solder eventually cracks. Also cracks develop on the switches and temperature control potentiometer connections.
The other more common unit is made by MK. It's failures are usually solder cracks on the switches and temperature potentiometer. Sometimes electrolytic capacitors.

To test the blower control module you could disconnect the yellow wire that is in cavity 3 of a 3 terminal connector behind the brake booster. That connector has red, black and yellow wires in it which go to the speed control module. The yellow wire is the control wire from the pushbutton unit. If it is removed from the connector and the fan still runs, then the module is bad.

For lubricating the blower bushings I have always used transmission fluid applied with a syringe. If the brushes and commutator are still in good shape it has always worked well for me. Avoid getting fluid on the brushes and commutator.

Thanks so much for the help so far. You guys are awesome.

By removing the strip fuse for the blower motor located near the normal fuse box, I was able to confirm that it is the climate control blower motor making the ugly noise, and that the blower motor is running anytime the car is on, regardless of the setting on the climate control unit.

Using Rick76's method, I removed that yellow wire from the circuit. The blower motor continued to run, even though the climate control unit was now not involved. So if I understand the advice, this means that I have a failing motor speed control module.

Did I get that right?

Sounds like a failed blower regulator to me. It's on the back side of the motor.

Fairly easy to access, pull the trims off the base of the windshield, take out the wiper motor, and just below that is the blower.

I would:
- hit a wrecking yard for a motor and regulator. If you find a 90 or newer 124, buy its wiper assembly too, they improved it from the 1st generation and it fits.
- the climate controller changed from 87 to all newer 124s. The aux fan control was redesigned. 87 the temperature switch in the head directly drives the relay for the fan and a second part of the switch cuts off the compressor on overheat. 88 and up the head sensor is a thermistor and the aux fan relay drive and over temperature compressor cutoff are managed by the in dash controller. Long story short they are different part numbers. If you are handy with wiring you can probably upgrade to the newer controller. You will need the water neck sensor from the later model donor.

- while you have the blower out, get a can of evaporator cleaner (or scrubbing bubbles bathroom cleaner) and clean the junk off the evaporator. Check your drain hose first!

If getting a used blower speed control module it might be a good idea to remove the module from the heat sink, clean the mating surfaces and apply a fresh coat of the white heat transfer paste. It tends to dry out over time and become less effective. You can buy small quantities of it or go to anywhere that repairs electronics and get a thin smear applied.

Thanks guys. I don't have the time/patience to repeat jobs, so I have decided to go with a new KAE-brand speed control module, but simply clean and lube the blower motor. The OE part was too rich for my blood, and the $40 no-name versions on ebay scared me. Perhaps I am just paying more for the KAE branded version of the same junk, but we shall see.

Based on my reading, it appears some of the w124 blower motors have accessible brushes, while others are hidden. While I have it all open, I may try to renew the brushes if possible, but I don't want to overcomplicate things.

I will post an update once I have it all together again. Thanks for the great advice. It is so hard to find out the details you guys provided in my searches, so the collective brain of PP is definitely better than Google.