Blower fan wont turn off after key out. - Mercedes-Benz Forum
 
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post #1 of 10 (permalink) Old 05-05-2019, 05:05 AM Thread Starter
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Blower fan wont turn off after key out.

Just bought a mint 1992 300se sedan, with only 100000kms on the clock. Paintwork, enterior excelleny, and EVERYTHING original, even an untouched first aid ki5 and Jack.
Absolutely love my german cars, and fell inlove with this at first sight. But, what sounds like my blower fan( Heater/ac fan) not turning off..(at all), even after ignition turned off. Have tried everything, now resorted to disconnecting the battery which is a pain in the proverbial...cause i loose all settings...seats and radio.
Any help would be greatly appreciated. TIA
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post #2 of 10 (permalink) Old 05-05-2019, 05:30 AM
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Here is some reading material. Blower speed regulator is most likely culprit but check it first.

https://www.benzworld.org/forums/w14...ower+regulator

https://www.benzworld.org/forums/w14...ower+regulator

https://www.benzworld.org/forums/w14...ower+regulator
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post #3 of 10 (permalink) Old 05-07-2019, 02:37 PM
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When I got my car the blower fan wasnt working so I popped off the blower cover to have a look at the controller. It was unplugged! I plugged it back in and it worked, fan blowing hard. Then it kept blowing when I turned the car off and removed the key. DUH! it dawned on me why it was unplugged. I swapped in the one from my parts car and it has worked great since. That is apparently a failure mode for that controller.

You treat that car like it's a Mercedes or something!.
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post #4 of 10 (permalink) Old 05-07-2019, 03:04 PM
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Garage
+1 for the regulator. Mine was doing the same thing, I replaced the regulator and it's working properly. FYI, disconnect the battery before the removal and install. I fried one regulator because I didn't take the time to disconnect the battery.
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post #5 of 10 (permalink) Old 05-08-2019, 04:10 PM
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Blower motor regulator transistor unit is regulating on the 'low side' of the motor, i.e. the grounded side.

The motor is directly connected to battery voltage 'B+' at all times regardless of ignition switch and this connection is un-fused.

The danger of doing regulator swap without disconnecting battery is that of touching that un-fused line to any ground, like the housing the motor is mounted in.

It won't blow the regulator, but it will make a juicy spark.

You need not ask how I know this.
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post #6 of 10 (permalink) Old 05-08-2019, 04:27 PM
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One thing to keep in mind before replacing blower motor regulator is that the classic syndrome of failure is that of increasing friction in motor drawing more & more current which eventually causes too much heating in the transistor regulator which kills it.

Given this, it is always prudent to measure the current draw of the motor to make sure it is not excessive. I was told that it should be no more than 10A. To do the test, you have to have an ammeter of multimeter that will allow >10A current measurement and these aren't the most common as most do only 10A. You don't need the regulator in the ckt to do the test, but you do need the ammeter in series to ground (since the motor already has B+ on at all times).

I meas mine at 23A. I tried to get OE Bosh motor and the last unit avail was built incorrectly and had to be sent back. I then got the ACM unit that comes with the regulator and that has worked fine. That's the easiest most straightforward path to success.

I did venture to disassemble the original Bosch motor and cleaned and rebuilt the unit just to better understand the reason for the excessive current draw, etc. After the rebuild the motor draws 5.5A. It's not the easiest job though. Must press off nylon cage, etc. and take apart motor hsg that was not designed to come apart ...
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post #7 of 10 (permalink) Old 05-08-2019, 07:50 PM
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Quote:
Originally Posted by daidnik View Post
One thing to keep in mind before replacing blower motor regulator is that the classic syndrome of failure is that of increasing friction in motor drawing more & more current which eventually causes too much heating in the transistor regulator which kills it.



Given this, it is always prudent to measure the current draw of the motor to make sure it is not excessive. I was told that it should be no more than 10A. To do the test, you have to have an ammeter of multimeter that will allow >10A current measurement and these aren't the most common as most do only 10A. You don't need the regulator in the ckt to do the test, but you do need the ammeter in series to ground (since the motor already has B+ on at all times).



I meas mine at 23A. I tried to get OE Bosh motor and the last unit avail was built incorrectly and had to be sent back. I then got the ACM unit that comes with the regulator and that has worked fine. That's the easiest most straightforward path to success.



I did venture to disassemble the original Bosch motor and cleaned and rebuilt the unit just to better understand the reason for the excessive current draw, etc. After the rebuild the motor draws 5.5A. It's not the easiest job though. Must press off nylon cage, etc. and take apart motor hsg that was not designed to come apart ...
What was causing it to draw so much current? Bad bearings? If so, are they ball bearings or sleeve?
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post #8 of 10 (permalink) Old 05-09-2019, 08:15 AM
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Jal,

Regarding the excessive friction, it was the following:

1. Dust and lint crud around the armature and in the housing. There was lots of that in there.
2. Dried out grease in the bushing bearings on the ends. This was also mixed with dust & lint.

I've learned from rebuilding a home fan that I've had for about 25 years that the best grease for the bushing bearings is stuff with lots of Moly. It lasts the longest.

Since I had the armature out of the housing, I also put it on a lathe and removed the ridges on the commutators at the ends of the brush track, but there was lots of brush length left on that 19yr old motor.
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post #9 of 10 (permalink) Old 05-09-2019, 10:11 AM
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Thanks!
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post #10 of 10 (permalink) Old 05-14-2019, 05:43 PM
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Me too

I also have a 92 300se. Same problem, but intermittent. First aid is to remove and replace the relevant fuse (18 on mine). That would either reset the system or mitigate the need to disconnect the battery. Long term fix was to replace the capacitor on the side of the blower motor. Easy job and now all sorted. Good luck.
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