79 300sd: location of power window relay?

[linus9]

Hi there,

I'm trying to diagnose the cause of multiple power windows failing on my 79 300sd. I've tested the driver-side window motor (it was fine) and the driver-side console switch unit (also fine).

I'd like to take a look at the relay (or relays) for the power windows but I'm having trouble finding it.

Searched the online wiring diagram but diagram 8 (which covers the power windows) seems to be missing.

According to my Haynes manual (covering other diesel models of the era but not specifically the w116 300sd [which seems to have been produced only for the American market]) there are (in at least one other model) two relays for the power window motors (one for each side?). On the other hand, the wiring diagrams for the US model 300cd and 300td indicate that these models have only one power window relay so my guess is that the 300sd does as well.

Thanks for any insights you might have.

Best regards,

Linus

[tbomachines]

I'm having a similar problem with my 300SD, all don't work except the rear right. However, the soldered leads on the motor tested OK for power so my motor is bad. Did you test the leads at the motor? I'm assuming they are not getting power. I'd trace the wires back to see if they are mangled anywhere. After 30 years of loyal service, those things can take a beating (and from what I hear it is a fairly common problem, at least in the w126).

[Phil]

As tbomachines said, test the wiring. After 30 years of the wires bending back and forth at the door hinge they tend to break. One thing is you generally can't see the break because the insulation can still look ok but the wires seperate inside. I have had to splice in new wires for everything that is electric in the drivers door on my car.

[linus9]

Thanks. You were right about the front driver side window (although there's more to it which I'll get to) and I was just kind of dumb about the front passenger side and rear driver side windows (the fuse was old and spent; I hadn't even checked).

In any event I tested for conductivity in the driver side front circuit behind the kick panel; it was strong there. I replaced the wiring and found unfortunately that when the switch is pressed the motor sounds for a couple of seconds then the window goes down slowly and seizes about a third the way. I'm thinking the regulator may be bad. Does anyone have experience with this problem and if it would seem likely to be the regulator what exactly makes it fail in this kind of way?

[Diesel911]

Originally Posted by linus9

Thanks. You were right about the front driver side window (although there's more to it which I'll get to) and I was just kind of dumb about the front passenger side and rear driver side windows (the fuse was old and spent; I hadn't even checked).

In any event I tested for conductivity in the driver side front circuit behind the kick panel; it was strong there. I replaced the wiring and found unfortunately that when the switch is pressed the motor sounds for a couple of seconds then the window goes down slowly and seizes about a third the way. I'm thinking the regulator may be bad. Does anyone have experience with this problem and if it would seem likely to be the regulator what exactly makes it fail in this kind of way?

Different year and model but on both of may rear windows the Regulators were bent and twisted. On one the teeth were rounded of on the regulator gear.
One of the windows would go up but not down; even after the new regulator was installed. It turned out that the contact points in the switch were a little burned.
Taking apart the switch and cleaning the contact points cured that.

Have not had to mess with the front ones so far.