W123 300D Window won't open - Page 2 - PeachParts Mercedes-Benz Forum

Graham · #16 06-15-2012, 09:21 PM

Decided to see if I could find out why motor failed.

Checked continuity in armature and it was good.

Checked wiring in brush module. There is an unidentified component in that module that has an open circuit. It is the item marked Otter 1 in attached diagram. The copper coils are no doubt chokes and would act as a noise suppression filter.

I did a search for Otter + UK (Item says "British Made"!). As it turns out, they make overload devices for automotive motors. So I guess that is what that part is. But it is supposed to reset! It would be easy enough to replace if we knew specs & source. Maybe get to that once I have used up my 3 spares!

Well at least replacement motor is working fine!

Graham · #17 06-16-2012, 09:42 PM

I have a question about how the window motor circuit works.

When the window winds up or down, it eventually comes to a stop even if we are still pushing the button. There does not seem to be anything in the external circuit to shut off the motor. See pdf below.

Only protection seems to be that little overload device within the motor (see above post).

Would that mean that that little device cuts power every time we hold the button down for a second too long?

ADDED: Seems Chris in this link has made similar conclusions.

qwerty · #18 06-16-2012, 10:11 PM

Quote:

Originally Posted by Graham

Would that mean that that little device cuts power every time we hold the button down for a second too long?

That depends on how you define "too long." I would submit that if "we" held the button down long enough to trip the relay, then the button was held down too long. Whether that was one second too long or 1 nanosecond too long might be subject to debate.

If you are suggesting that the relay trips within one second of the window reaching its limit of travel, it does not under normal conditions.

Graham · #19 06-16-2012, 11:32 PM

Quote:

Originally Posted by qwerty

If you are suggesting that the relay trips within one second of the window reaching its limit of travel, it does not under normal conditions.

Interested to know how you determined that. Did you do a test? If so, why not post the results? I am sure many here would be interested.

I suspect, but really don't know for sure, that current rises very rapidly once motor stalls and will be cut off very quickly by the circuit breaker to protect the motor windings.

jay_bob · #20 06-18-2012, 09:19 AM

Yep, if the motor has electricity applied and the motor is not spinning, the motor will quickly overhead.

Conservation of energy; you are putting energy (electricity) in to the windings, but not removing the energy in the form of mechanical work. The energy has to go somewhere, this is what makes the motor get hot.

In case someone is still confused on the switching, first thing to know is a dc motor can be reversed by flipping the polarity of the applied voltage.

The motor has 2 wires, neither of which are grounded when the switch is in the resting position. The switch in the console has both power and ground connections for the motor.

Inside the switch assembly are two sets of contacts. One set is made with the switch at rest or in the up mode. The other has continuity at rest or in down mode. With the switch at rest there is 12 V applied to both sets of contacts so the motor has 12 v on both ends of the winding. Therefore the motor does not run because it has 0 V across it.

When you push the switch to the up or down position what you are doing is taking one side of the switch away from 12 V and instead grounding it. Depending on which contact is made, the motor now sees 12 V across it. The polarity determined by the contacts decides whether the window goes up or down.

The Otter is a thermal overload device. If the motor current increases beyond limits, due to over travel, jammed linkage, etc. the metal heats up and bends, which breaks the circuit. When it cools off it bends back and reconnects the circuit. Apparently from what I have been reading these tend to corrode and fail. However please do not bypass this, it is there to prevent overheating and potential fire. Not sure whether the fuse will clear fast enough to protect the motor/wiring/switch in a stall situation.

qwerty · #21 06-18-2012, 10:10 AM

Quote:

Originally Posted by jay_bob

The motor has 2 wires, neither of which are grounded.

The motor has two wires. One or the other is grounded when the window switch is actuated.

jay_bob · #22 06-18-2012, 10:28 AM

You are correct, I should have said not grounded when the switch is in the resting position.

I edited text above to reflect this.

Graham · #23 06-18-2012, 12:38 PM

Quote:

Originally Posted by jay_bob

Not sure whether the fuse will clear fast enough to protect the motor/wiring/switch in a stall situation.

Good summary jay_bob.

I agree that Otter should not be jumpered unless another circuit breaker is added to circuit (preferably that will self reset). I suspect that even if fuse is fast enough to provide protection, it would blow and may have to be replaced quite often. There is no other type of limit switch in the circuit. In a separate thread re W124, Chris is going to see if those Otters can be purchased.

Interesting that on cars (at least ones I own!), fuses are still used. On boats, they have gone almost entirely to circuit breaker-switches that can easily be reset. Same in our homes. Probably a cost thing? Or perhaps space?

ADDED: It seems replacement circuit breakers can be obtained. Perhaps the Type IIs might work on the window motors. Opps forgot to add link and now I have lost it! But Digikey have auto-reset breakers that might work for anyone wanting to try and repair their window motor. http://www.digikey.com/us/en/ph/ETA/1610.html