1986 300SDL blower motor/regulator question

So here I am trying to troubleshoot my 1986 300SDL's blower motor not working. I've got that heat-sink-looking thing on the floor and see there's a red and blue wire each going to the blower motor. I put my multimeter's + probe on the red wire and the - probe on the blue and I get 13.6V (with very slight fluctuations). Turning on the fan swich on high/auto/low does nothing to the voltage. It stays 13.6V. Assuming the blower motor provides the ground I put the + probe on the red wire and grounded the - probe and get 13.6V. Then I put the + probe on the blue wire and keep the - probe grounded and I get 0.55V. These values DO NOT CHANGE whether on the low, auto, or high-speed fan buttons. What's bad then....the blower motor or the regulator?TIA.

[Arthur Dalton]

The reg is a switched ground
so.... ground the blue wire and see if the fan runs.. if so, reg is not completing the groumd [ which can also mean the trigger voltage to the reg is absent]
Try that and post

Thanks for the reply, Arthur.

If you meant take the blue wire off the blower motor leaving the red one attached, then ground the blue wire I did that. The blower doesn't move, make a sound, nothing.

[Arthur Dalton]

NO
I mean for you to run a ground wire from the blue wire terminal on the motor to ground . This means you have power on the red side and ground on the other end of the motor,[ thus, eliminating the reg ]
The motor needs 12v positive on one end and a ground on the other to work.
This would be the same as bringing battery 12 volt , pos and neg., to the motor. It can be done by simply grounding the blue wire as the motor already has 12v positive at the red terminal........
also...make sure the blade turns freely..

OK, had the red side hooked up normally and grounded the other side on the motor as you said. Blower WORKS and turns nicely. What's next? Any way to t/shoot the regulator...or....?

[Arthur Dalton]

OK

Now you can see how the reg switches the ground . That is all a reg does ..Iit gives the fan ground, but it does it in a regulated fashion to give variable fan speed... ie-kinda like a light dimmer
How does it do that ??
It has a switching transistor circuit that is triggered by a third
variable voltage wire [ I think yours is yellow..]
Anyway , you now look for voltage at this wire [ it is somewhere coming thru the firewall ,a 3 wire plug. Usually near the brake booster One wire is 12v +, one is 12v-, and the other is from 1 to7 or so V +, depending on where the fan speed control is set in the acc/control panel]
So , this is called the trigger voltage and if you have that going to the reg and no fan, you need a reg. If no variable voltage here , the reg can not get triggered and the results are no fan.
Usually, it is a bad reg...but check the trigger circuit first...

I'll do my old "sewing needle thru wire" trick to meter the yellow wire for any voltage at the regulator. Thanks a lot Arthur. I appreciate your help, sir.

[Arthur Dalton]

Don't forget to have the other meter lead on a ground....

Well, had both blue/red leads off the b/motor and tested the yellow wire. Meter shows voltage. I normalized everything back up and the b/motor WORKS! I wiggled the wires around at the regulator and the b/motor and nothing happened. I cleaned up the contacts at the b/motor with some sandpaper and normalized the wires again. It still works. I'm going to normalize everything now and go for a small spin around the block and see what happens. Thanks again, Arthur.

[Arthur Dalton]

Cool..
as a side note to your progress..

Be aware that when one jumpers the blue wire to ground [eliminating the reg., as I had you do] , the blower gets full bat. voltage.. When everything seems to work well after this blower full voltage jump, it is usually an early sign of possible worn motor brushes..
Just something to keep an eye on.............