Fan Motor Issue

[CraigD]

I'm trying to diagnose problems with the fan motor on my '70 280SL.  Currently, the motor only operates when the switch is rotated to the "high" position. The blower doesn't run at all in any other rotational positions of the switch.  So I thought the problem was the switch.  Scoot was kind enough to send me a switch (now NLA) that he had.  So I plug it in, rather confident that this would solve the problem... but low and behold... (and as Scoot cautioned might happen ) I have the same problem.  Again, the fan only runs on the high setting.

What else might cause this problem?

Thanks for your ideas...

[mdsalemi]

Craig,

(Scientific Method!) Did you check the switch before installation? I think that the blower motor is a DC one, and that its speed is controlled by voltage. Thus, the switch has a full off position, full on position, and variability of voltage in-between. This is probably akin to (have not taken it apart so I don't know for certain) the small rheostat on the instrument cluster to control the brightness of the instrument lights. After 40+ years these usually fail and mostly the fix is to solder in the full on position (which isn't that bright anyway). However full on isn't what you want on the blower!

If your switch appears functional, with full on, full off, and variability (check with an ohmmeter) check the plug connectors and then check the wiring where it goes to the blower itself; ensure that you have full on, full off, and variability there. If all that seems to work--the switch/rheostat, connectors AND wiring, then you have pretty much pinpointed to the blower motor.

As a final check, you could power the blower with a 12VDC variable power supply (easy to wire one up) external to the car and see how it responds…that's the procedure I'd follow.

Yeah I know easier said than done. They suspended the blower motor with fishing line in the middle of the assembly line and built the car around it…access is a royal PIA.

[garymand]

Looking at the schematic, I see 2 resistors (think they are resistors, its a German diagram, I'm not trainined on German symbols.)  inside the motor.  As Mike states, the motor is far from easy to get to, so I have not seen what's in there.  I would guess its like older American cars where they had 2 wirewound high wattage resisistord in series with the motor to slow it down.  The risistors would be inside the heater near the motor not inside the motor itself.  The dash control has 3 connections: Fused 12V, a reostat and a single pole switch.  The switch is the high speed position and it feeds one of the two resistors.  The reostat feeds both resistors in series.  Looks to me that the motor never gets the full 12V.  High speed is a reduced voltage and slower adds a fixed resistor and the reoastat.

A Green w/red stripe wire feeds the motor from the reostat and the Green Grey wire is high speed.  Sounds like the resistor fed by the Green Red wire is open.  Hopefiully someone has dug into the heater and knows where those resistors are.  AND, they are not difficult to get to.  I would hope for an access panel specifically for the resisitors.  I remember seeing a pair of round wire wound heater resistors on a common board (like small spools of wire) on EBAY for a 70 280SL.

I'm thinking you could Jerry rig a resistor parallel to the open resistor at the motor connector outside the housing, as long as iot is the right resistance and high enough current rating, it will solve the problem of giving you control over the motor from the dash switch.

[CraigD]

Thanks for the input from both of you.  I've already done a pin voltage analysis on the switch in all it's possible positions, and it still wasn't clear to me how it should work.  With the lack of clarity, trying a second switch seemed the most effective next step.  Then when the second switch performed exactly as the first switch, I began to wonder if there was in fact some other possible source of the problem.  Discussions with local Mercedes parts folks suggested that there might be another resistor in the circuit, exactly as you are suggesting, Gary.  I think I'll focus on diagnosing in that direction now, and let you know what I come up with.

Thank to you both!

Craig

[garymand]

The switch is simple: three pins, one is common to the next 2 functions.  one is a variable resistor: as you rotate the knob, the resistance decreases.  the other is the single pole switch for high fan.  You can see the resistor on the switch.  Its the cioil cemmented in the ceramic.  the wiper is the sencond pin I spoke of.  You can easily test the switch.  Test the one you have out.  My guess is its good.