1990 560SEL blower motor inoperable - PeachParts Mercedes-Benz Forum

biobenz617 · #1 07-22-2007, 02:37 PM

I have a 1990 560SEL with the HVAC blower motor not working. The climate control unit seems to be operating everything else because I can feel cold air through various vents at different button positions while driving down the highway, but no blower even when in defrost mode.

I dug into the dash this morning and got the motor running inconsistently after unplugging and replugging to test voltages. This system is different from what is shown in the shop manual (CD) in that there is a huge heat sink on top of a small electrical connector on the inside of the blower motor cover. Instead of two wires directly to the motor, there are three wires (red, black, white/blue) going to the heat sink, then two (red & blue) to the motor. Also, I couldn't find any preresistors under the hood. I am getting 14V between red and black before the heat sink, and a variable voltage between the white/blue and black wires depending on manual speed setting, so I don't think it is the CCU. Motor resistance is 0.4Ohm. I am getting 14VDC or 26VAC (tried both settings on multimeter) out of the heat sink device (I think blower regulator), which does not change with manual speed settings.

Is there a method of testing the blower regulator? That looks like a pretty pricey part

Is this climate control system better shown on the W124 shop manual CDs?

Thanks

Arthur Dalton · #2 07-22-2007, 02:42 PM

Run a jumper from the blu wire at motor to ground
That should give you full blower. if it does , the reg is bad...if it doesn't , the motor is bad.
[That is assuming all the voltage reading you posted are correct]

david s poole · #3 07-23-2007, 10:25 AM

yes easy deal,common problem,if you were able to get it going intermittently,prob the motor cause when the reg goes byebye it stays that way.you can buy just the motor and take everything apart and rebuild.motor only=$100 approx.

biobenz617 · #4 07-23-2007, 01:15 PM

Thanks for the help!

Yeah, I was wondering if it was the motor. The only way I could get it working was running the engine with climate in auto mode with regulator & motor disconnected for testing CCU, then connecting the regulator and finally the motor. I'll check again to see if there is something loose on the motor.

Should I be getting different readings out of the regulator with different speeds? As I posted earlier I had a constant voltage no matter what speed setting.

Arthur Dalton · #5 07-23-2007, 04:25 PM

< >>

You don't have variable reading out of the reg, you have a var. voltage ONTO the reg ..that is TRIGGER voltage from the CC
The 12v wires at the reg [ red/blk] are simply supply voltage for the reg internal circuit. They do not change. EVER...
What the reg does is it controls the GROUND side of the blower .That is why I asked you to GROUND the blu wire. The blower circuit works on a SWITCHED GROUND circuit controlled by the reg. ..and that is controlled by the variable trigger voltage from the CC. When the reg gets trigger voltage, it GROUNDS the blu wire..that is it's function.
So , as I said, you go ground the blu wire at the motor with a jumper ..that takes all circuits out of the blower b/c the red for blower is always HOT. [ no switching is done on the HOT side of the circuit].. you now have a DIRECT 12V bat source feeding the blower motor for testing.. [ red for 12v pos and blu grounded for 12v neg....just like you took the blower out of the car and hooked it to a Battery]...all other controls are OUT of The Circuit..this is just a blower motor test. The blower will then spin at High/full speed b/c there are no controls in the circuit. If you wabt to get fancy, you put an Amp meter across the fuse link when you do this ground test and that will tell you if your motor is drawing too much of a load..if it is in spec, you now know you do not have a blower problem.
If you do that test, then I will tell you the next sequenced one ..which depends on the YES/NO of that one...............................

biobenz617 · #6 07-23-2007, 08:20 PM

Sounds good. That explains why my multimeter got an AC signal out of the regulator. I'll check it out again.

BTW, is the W124 or W201 climate control the same or similar as the late model W126s? I was trying to find info on my MB repair library CDs, but only mentioned the obsolete pre-resistors.

Thanks,

biobenz617 · #7 07-31-2007, 11:02 AM

Replaced the motor and it's all working again

Another sign it was the motor was that it would almost never spin by itself when connected and powered up, but would come alive when I rotated the blower blades a few degrees, connecting the next motor winding. Either bad contacts inside or a couple of bad windings.