The mighty 300SDL climate blower motor has stopped blowing.

From the many posts, I see that I should test the blower motor at the plug where it connects to the harness. This plug is a three-pole on my 300SDL, leading to red, black and yellow wires. These 3 wires go to the PWM "porcupine" assembly. The red wire continues from the PWM to one pole of the motor, while a fourth (blue) wire comes out of the PWM goes to the other pole of the motor. I applied 12v and a ground to all the combinations of terminals at the plug, which is what I think I'm supposed to do to test the motor. No result at all from the motor.

However, when I spin the blower by hand, the voltmeter shows voltage across the blower's two poles. Is this telling me that the motor is OK and that the porcupine thing is not sending any voltage to the motor?

Also, the blower does not easily "free wheel" when I tap it; it moves easily, but has a bit of damping to it that stops it as soon as the force of my fingers is gone. Is this how the blower motor should feel?

Finally, I've never found the 30-amp strip fuse that I keep hearing about.

Anyone think I'm on the right track?

thanks!

Sounds like your W126 wasn't upgraded to the strip fuse. Mine wasn't either.

My fan does exactly what you are talking about. It spins easily, but doesn't freewheel. Until I get a new blower motor, I've rigged some 10 gauge and a 30amp switch. Even with the massive wire and the 30 amp switch and 30 amp fuse, its still getting too hot to touch at the switch and the fuse. Tells me there is too much resistance.

It may be possible to refresh the fan with fine sandpaper and new brushes, but new motors are less than $100. So I'll just put a new motor in. I just hope the squirrel cage doesn't snap while I'm swapping the motors.

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77' 300D, "Cartman" SOLD @ 150K (didn't know what I had)
83' 300SD, "The Superdon" 325k+ @ 28mpg
95 E320 wagon, "Millennium Falcon" 231k+ @ 24 Mpg
95 E300D, "Sherley" 308k @ 33.69 Mpg, currently anticipating a head
99 Suzuki Intruder "Trudy" @ 45 mpg

Swapping the motor is difficult because the squirrel cage likes to break. Try new brushes first. I'm still trying to find a source for them.

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1991 F250 super-cab 7.3 IDI. (rebuilt by me) Banks Sidewinder turbo, hydroboost brakes, new IP and injectors.
2003 S430 - 107K
1983 300SD - Tanoshii - mostly restored ~400K+.
1983 300SD - Good interior. Engine finally tamed ~250K.
Monark Nozzle Install Video - http://tinyurl.com/ptd2tge

Further to this thread, got the blower motor out, tested it with 12v to the two motor leads, no nothing. I had tested it in place as well, but hoped I was wrong.

This motor does not want to be taken apart; the caps that the armature goes through are crimped on to the case. If I didn't destroy them taking them off, I'll clean it up & put brushes and report back, with some pics if I'm lucky.

thanks

Bonifide brush failure there. You do have to bend some metal but that should not be an issue. You may also need to solder the new brushes in.

__________________
1991 F250 super-cab 7.3 IDI. (rebuilt by me) Banks Sidewinder turbo, hydroboost brakes, new IP and injectors.
2003 S430 - 107K
1983 300SD - Tanoshii - mostly restored ~400K+.
1983 300SD - Good interior. Engine finally tamed ~250K.
Monark Nozzle Install Video - http://tinyurl.com/ptd2tge