A little background... Two weeks ago I was driving the car with the AC on, everything working great. I turned the car off, then back on and the blower was only blowing out a tiny bit of air. I then turned the car off, then back on and there was zero air. I switched out the 30 A fuse just outside the fuse box with no change. Expecting that the original motor was just shot I replaced it with a brand new one (it worked just fine when I hooked it up to power). Plugged it in to the car's power and no joy. I tested the porcupine relay and there was resistance through it.. Next up the CCU. Just put in a rebuilt unit from Programma, still no motor. What's next...? Any help would be greatly appreciated. I've heard about the resistors under the brake booster failing but it is a low percentage. What do I need to check next?

Also, I just put in a new ignition switch prior to the motor failing.

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1987 300SDL 271,000
1999 E300 Turbodiesel 155,000ish. (Dad's)

This device is a typical culprit when the blower won't function properly. However, thankfully, I never had to deal with it.

I'd do some more research as to whether a resistance check is all that is needed to confirm proper operation.

The porcupine relay is a solid state device. It's really more like a power amplifier than a resistor. It takes the DC voltage from the car and switches it on and off rapidly, varying the percentage of time off and time on per switching cycle to regulate the speed.

It is called Pulse Width Modulation or PWM for short.

There is a connector near the brake booster that connects the vehicle harness to the porcupine and blower assembly down in the depths.

The wires change color at the connector, first color is the vehicle side and the second color is the porcupine side.

Black-Green (Red) wire = switched 12 Vdc (only in Pos II on the key) from 30 A strap fuse
White-Red (Yellow) = reference voltage from Pushbutton Unit (PBU), this is a dc voltage from 0.5 to 6 V dc that is proportional to the desired motor speed.
Brown (Black) = ground

If you put your meter on the power and ground you should get +12 V dc
If you put your meter on the reference signal and ground and High speed button pushed on the PBU you should get about 6 Vdc. With Low button pushed in about 0.5 V dc.

If you are getting the reference signal chances are the problem is in the porcupine or the motor. If no reference signal is being sent then look at the PBU.

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

At the red wire I'm getting .6v and 0v when I remove the connector an go off the pin. On the yellow wire getting 8v to 9v any idea when I'm problem could be? I've replaced the PBU an I can hear the A/c compressor kick on. Verified the fan is good by jumping it. I'm stumped.

Bump

I have a 1987 300D and I don,t think the turbo is kicking in, what test can I make?

Check the 30 A strap fuse on the drivers side shock absorber tower just in front of the fuse box. Chances are it is loose or has a hair line crack.

If you are not getting 12 V dc at all on the fuse then the next place to look upstream is the key switch, it has a dedicated set of contacts for the blower (made up only when the key is in the II position).

Electrical schematic available at w124-zone.com or startekinfo.com, the w124-zone one is clearer than the startek one.

__________________
The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

Mine are worn making it required to turn the key slightly counter clockwise to get power to the blower.

So start the car and then turn the key slightly counter clockwise. Not so far that you shut off the motor.

I'll be replacing my assembly in a few weeks.

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1986 300SDL 440,xxx