Thanks for the tip on the variable adjust. I installed it the other day and it works great.

the only trouble I'm having is my fan has a mind of its own. sometimes works and other times it doesn't. I'd like to take the fan logic away as I think one of the sensors is not allowing the fan to work.

I'm thinking about using this mod combined with a voltage divider to drive the fan with the potentiometer. if I use the 20kohm with the 20kohm potentiometer in series, I can drive the fan controller with 0 to 6 vdc from the 12v power.

any thoughts ?

thanks,

Wait do you have to put the potentiometer in the wire that plugs into the back of the control unit or do you have to take the unit apart and put the pot between the actual fan button and the circuit board?

You can run the blower completely manually. Only a few volts is needed on the control wire to get full speed from the blower. Use a fixed resistor and a potentiometer so that the maximum voltage is 3 or 4 volts. I have notes somewhere from when I did experiments; I'll try to find them.

Jeremy

__________________

"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

The blower controller (big transistor on heat sink located with the blower motor; sometimes called "porcupine") has three wires -- control, +12V, and ground. The wires go through a plug/socket (X64) in the firewall near the brake booster; at this point the wires change color, so be careful. The control wire is white/red where it goes to the climate control push-button unit (PBU), pin 12 of one of the big connectors on the PBU (the other pin 12 is a brown wire so they can't be confused).

You don't have to take the PBU apart, just cut that white/red wire and use a potentiometer to control the speed. You'll have to experiment but it shouldn't take more than 6 volts to get full speed out of the blower. About 1 volt gives you the equivalent of low speed. Anywhere between 1 volt and 6 volts should give you the speeds that the PBU gives in "automatic" mode. 6 volts and above should be full speed. About 10K Ohms should be good for the potentiometer with a 10K fixed resistor between the high side and +12V, the low side to ground, and the wiper to the white/red control wire.

Jeremy

__________________

"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

Guys, if you want to use the audio trim pot but feel 55k is too high, slap a 47k resistor in parallel with it. Now, it's 25k close enough. Also, it's nonlinear right, but that's probably ok because windspeed is not linear off a squirrelcage fan either. You might actually end up with more proportionally correct wind flow off a nonlinear pot. If you want to kill the 0 ohm deadspot in the middle of fade position then slap a series resistor online. This raises the whole range though, so you might want to use something bigger than 47k for the parallel resistor...

Just ideas..

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Cheers!
Scott McPhee

1987 300D

Hey guys. So, when I press "high" on my push button control unit the fan speed doesn't change. So, I was thinking about doing this, and hoping that it let's me "manually" adjust the speed of my fan, but I just wanted to clarify that the wire for pin 12 wire would go to "input" terminal, a ground wire would have to go to the "ground" terminal, but what about the positive voltage terminal? Thanks again!

Most likely your problem is the regulator, and the modification discussed here will NOT fix it. If you are not getting high speed fan when pressing the high speed button, it's 99% likely the regulator is toast.

Hope you are sitting down when you price new ones ($500 MSRP from the dealer). The aftermarket KAE regulator (about $200) is a bit more affordable, but avoid any other brands you may find under $100 as they are garbage. Another option is to get a used one from Pick+Pull or eBay, but those are a bit of a gamble, and there's no guarantee how long a used one will last.

Also remember the 86-93 (USA model year) W124's use a different blower and regulator than the 94-95. It would help if you updated your profile with location (USA?) and the year/model of your car...

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Dave
Boise, ID
Check out my website photos, documents, and movies!

This is a great idea, thanks for the tip! It works beautifully as advertised in both HIGH and AUTO modes. 20k seems to give it the complete range of adjustment. I installed it in the only blank spot I had left. Why the hell didn't the Germans do it like this in the first place?

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CENSORED due to not family friendly words

I have an early (1986-1993) blower and controller for sale in the parts section of this forum.

Jeremy

__________________

"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970