Quote:
Originally Posted by Diseasel300
Even if it does, the speed control shouldn't have any effect with the PBU switched to "off". The OP states that he can change the fan speeds when the blower is acting up and the PBU is "off".
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Let me explain some more detail. The transistors are usually arranged so that the power source goes straight to the motor through the transistors. The PBU control only affects the control signal to the electronics in the blower regulator.
The way it’s done in the 124 and later models is the PBU puts out a 0-6 V dc reference signal to the regulator. The regulator takes this 0-6 V dc signal and creates a PWM drive signal for the transistors. The on-off ratio of the PWM is increased as the control voltage increases. Thus increasing the effective “on” time and the amount of energy passed to the motor.
There is one more consideration. MB design philosophy on climate control is that in the event of a control failure, the system should failsafe default to max heat, max blower, and air flow to defrost. So if the reference voltage drops all the way to 0, then the regulator drives the output to 100% on.
The motor voltage in this era came through the key switch, terminal 15X. This terminal is live only in position 2 so that the blower is stopped during cranking. The PBU has nothing to do with actually supplying power to the motor.
So if a transistor shorts, it would be 100% on constantly without ability to control. So chances are that is not the problem.
What may be more likely is an intermittent loss of power to the PBU or the control line between the PBU and the regulator, both of which would force intermittent 100% blower output due to the failsafe. I had this happen on the 87 300TD. Fuse 7 had a weak spot and crumbled when wiggled.
I just went back and read the original post again. If you can change the speed even if the PBU is off, then you have a bad PBU that is throwing a reference signal when it shouldn’t. The way it turns the blower “off” is that it really puts out a very low reference signal, low enough that you can’t tell the blower is running, but not enough to throw it into emergency mode. Even in “off” the blower still runs at very low speed to ensure there is always fresh air coming in.
The W124 style control has 3 blower buttons, low, auto, and high. There is no “blower off” even if you push the 0 button as I described. The “low” and “high” modes send a fixed reference voltage to the regulator to create a low and high speed. Auto puts the PBU logic in control and the blower speed is modulated according to the temperature sensors.
Maybe what is happening is you are commanding “off” but the reference voltage is dropping low enough to throw the blower into emergency mode. If you select the “low” or “high” blower speed, the reference voltage climbs enough that it falls back into the regulation region and obeys your speed request.