I have a 83 240d none of the windows work. I have replaced the switches and the relay. I have voltage to the door when motor is disconnected, but when I connect it I get nothing. Haven't quite been able to figure this out I'm not that good with electrical work.

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In my '79 (not sure if it's the same in other years), you have both wires going to the motor getting 12v when the key is on. My driver side window wasn't working and I found that I had no ground at the switch.

Again, not sure how later 240s are wired, but IIRC I had the hot wire to the window switch that provided power to the entire switch, and sent constant 12v to both sides of the motors. I was missing a ground on my driver side switch, but the rear window still worked because it grounded out from the window switch in the back door.

Where are you measuring voltage?

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Milan Brown 1979 240D, rebuilt OM617.952 turbo diesel, rebuilt 722.315 transmission - engine spun a rod bearing

1979 300SD, ~90k original miles, all stock

You are not mentioning the condition of the 2 high amp Fuses that go to the switches and motors.
I had a good looking window fuse until I pulled it and found that the tip was burned or eroded off. The strip was not melted but it was not making contact due to the tip being gone.

Note that the Relay also has a separate fuse. On my year and model it is #12 on the fuse chart.

Also check to see that one side of the fuses is also getting + voltage.
I had a situation where one of the wires that went to a whole row of fuses had become unsoldered. That whole row did not get voltage on either side of the fuse.

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84 300D, 82 Volvo 244Gl Diesel

There is a fuse chart at this site for an 82 240D: https://www.autogenius.info/mercedes-e-class-w123-240d-1982-fuse-box-diagram/

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84 300D, 82 Volvo 244Gl Diesel

Right at the door connection.

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Are you measuring voltage across the 2 terminals at the door connection? Those should both be 12v hot when the key is on, so measuring across those two will net you a 0v reading. You should ground out your voltmeter to a known ground, like the door check strap or the door latch.

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Milan Brown 1979 240D, rebuilt OM617.952 turbo diesel, rebuilt 722.315 transmission - engine spun a rod bearing

1979 300SD, ~90k original miles, all stock

Digital volt meters pickup stray voltage and can cloud diagnosis. Always use a incandescent test light not a LED as LED can pick up strays.

Basically there is a high resistance in the circuit that makes it look like you have voltage without a load but when a load is placed on the circuit, there isn't enough amperage.

Think of a garden hose that has a kink, the pressure will be high when the nozzle is closed but as soon as you open it, pressure drops and flow is minimal.

As was mentioned, the two leads normally have 12V, measured to a good ground. If one wire is cold, it's probably broken in the door post.



When the switch is flipped, one or the other lead will be switched to ground. If this isn't happening, then either the switch is bad or it has a bad ground.

Our guy has high resistance in the circuit, that is why he has voltage with the motor disconnected and no voltage when the motor is connected.

I don't think he's measuring correctly. The motor has 12V on both leads. So how would voltage go to zero? I think Maximan's post was to the point.

I am working under the presumption that, with the wire unplugged / plugged in, the OP is measuring across the harness that connects to the motor , is cycling the switch and observing a reading on his test equipment.

I will give that his statement " but when I connect it I get nothing. " is a bit vague and the " I get nothing " could mean no motor operation. Given he is testing for voltage, I take it as the voltage goes away when the motor it plugged in and switch operated.

Applying + 12V ( Or - 12V though I don't ever recall seeing this ) to both sides of the motor when in a non operational state then lifting one and grounding it when in operation performs 2 functions.

First it makes switches slightly less complicated because only one circuit needs to change state for a function.

The second less obvious function is shorting motor leads just after dropping the switch stalls the motor to prevent coasting / overshoot. As a motor is coasting down, it is acting as a generator so applying an electrical load creates mechanical load.

I'd apply power / ground directly to the motor terminals and see if it operates, reverse polarity to change direction as well . Use long wires so you don't get caught in the linkage and just brush the final connection in case there is a dead short. A fused / circuit breaker in the test leads is a god idea but sometimes we cheat.

OP should pull up the wiring diagram to better understand it. But what you say is what I also recall. (Here is link to one for my 85 300D)

On my car, the problem was that an internal thermal fuse in the motor had blown. Should be able to test by feeding 12v to motor directly. It is a special fuse that is not easily replaced. But can be. A junkyard motor may be easier! There are threads here about those fuses and the motors.

http://www.peachparts.com/shopforum/diesel-discussion/318503-repair-your-failed-w124-window-regulator-motor-maybe-others-too.html

http://www.peachparts.com/shopforum/2956367-post16.html

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5

This is true in the case of a fan or blower, which have considerable momentum and can coast for some time. But when a window motor reaches the end of it's travel, it's stopped dead, so it can't become a generator. At that point, it becomes an inductive load which consumes electricity and produces heat. The heat will trigger a thermal switch which breaks power in case the switch isn't released. When the switch opens, stored energy in the motor winding is either dumped back into the harness as a reverse spike or released as heat.

You are forgetting about a partial window movement where motor becoming a generator is relevent.

Think about the person that wants the now closed window open just a crack for air. Down , overshoot , up overshoot to fully closed, down . . . . Darn car. . .

A stalled brush type motor powered by DC is a restive load rather than an inductive load. It would be inductive only at initial power up / power off.

If this same stalled motor was powered by AC, there would be an inductive as well as a restive load. If this was a permanent magnet brush motor, the inductive load would go away once the " permanent " magnets became demagnetized. Winding failure might occur prior to this happening though.

At old job, we were stall testing 1 to 2 HP 90 VDC motors on 180 VDC. Every once in a while, the permanent magnets would flip polarity and motor would run in reverse.

From what I recall, some motors would run faster after a test due to field weakening but lose torque in the process.

IIRC, the gearbox that goes on the window motor is driven by a worm gear, so the window motors literally cannot act as a generator in these cars.

For what it's worth, I've never encountered a car where the window motor acted as a generator before.

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Milan Brown 1979 240D, rebuilt OM617.952 turbo diesel, rebuilt 722.315 transmission - engine spun a rod bearing

1979 300SD, ~90k original miles, all stock