Blower regulator test
 

I spend about 2 hours this morning replacing the blower motor on a 87 300d,just to find out that nothing is wrong with the old one...ACC seems to work fine,pushing the buttons I can hear the valves working,also I tested the blower regulator behind the brake booster? and got power on the red wire.My questions is,how can I make shure that is the problem,can I bypass that regulator,jump the wires?

Power at the red wire simply means you have 12v feed from the fuse box.
The regulator is in the ground line for the blower [ switched ground] and it is triggered by a variable voltage from the yellow wire coming from the PB unit.
Look for 2-8 volts at the yellow wire with the PB unit ON. If you have that, then the reg is not tripping.
In that case , the blue wire coming from the neg side of the blower motor
to the regulator can be jumpered to ground. If the motor then works , you have a bad regulator.
Just make sure you have trigger volts at the yellow before diagnosing the reg. as bad....

Arthur,

I did my test on our W124 500E since I have NO blower movement, and here are my results:

On the 3-pole connector, I have +12VDC on red; good continuity to ground; and 7+ VDC on the yellow control wire on HIGH & Defrost, about +2VDC on "low" speed.

How can I bypass the regulator and access the blower motor directly to see if the blower still spins/works ? In other words, is there an accessible blower power connector that I can access to add 12VDC ?

TIA,
:-) neil

The blower pos. is hot all the time . It is the ground side that the reg switches..
So, as I stated in the test post, you ground the blu wire that goes from the motor to the reg. If the blower then works, the reg is not working
[ assuming the reg is getting trigger voltage at the yellow wire.]
That is all the regulator does .. it switches the ground side of the motor.
[ called a Switched Ground]

Ugh! Which means I have to PULL apart the unit first!

:-( neil

There was also some discussion about the motor having a bad ground thru the screws. Check the resistance to ground from the motor frame!

Yup......

That's why we have you check the firewall plug for power first.

Motor gets grounded through the regulator.. the frame does not get grounded.
I think the post you are referring to is the ones that the fuse block mounting screws get tightened too much and ground out... different proble/diagnosis.