Smoke in the Cabin! W124 e220 (1994)

[Dieselman]

That looks to me as if it's just to short out any residual (back) EMF (voltage).

The switch connects the battery 12v to the motor.

 

[Ted]

That looks to me as if it's just to short out any residual (back) EMF (voltage).

Precisely!

 

[Dieselman]

Precisely!

But only as a secondary action.

It cuts the 12v to the motor. The Back EMF issue is just to stop the motor genrating power if the wiper is moved manually.

And it doesn't lock the motor, it can be moved manually.

 

[Ted]

When a motor spins, it generates a voltage.
Shorting out that voltage will attempt to turn the motor in the opposite direction, however there is not enough energy to do this so the motor just stops very quickly.
Try it. Spin up an electric motor (DC) and remove the power. Depending on a number of variables it will slow down.
Now do the same but short the wires together (after removing the power!) The motor will stop (almost) instantly.

Since my first post, I've googled it (not infallible, I know), and this bears out what I was taught at college as an auto electrician (a LONG time ago).

From the schematic, you can also see that if the park switch goes dodgy (technical term again), it can short the supply to ground.

I would also suggest, but I'm not coming from a position of knowledge on this, that even with the worm drive, on a wet screen, and with an old motor/drive with little friction, without a method of stopping the motor quickly, the inertia MAY take the wipers past their parked position.

So I completely agree with your first post.

 

[television]

First of all there is no where that a short can be applied, short out the field coils and that will burn out the armature, short out the armature and you will burn out the field coils. So you cant short it out anywhere , if you could there would be very high current being drawn the whole time that the short was applied.


The worm drive is roughly 300 to 1 so the stopping is not a problem

 

[television]

Deleted

 

[Dieselman]

When a motor spins, it generates a voltage.

Not Back EMF by any chance.

Try it. Spin up an electric motor (DC) and remove the power. Depending on a number of variables it will slow down.
Now do the same but short the wires together (after removing the power!) The motor will stop (almost) instantly.

Not an issue for a lightweight DC motor though. Agreed that this could be an issue on a heavy motor but not on one of this size. The permanet magnets will stop the motor dead anyway.

From the schematic, you can also see that if the park switch goes dodgy (technical term again), it can short the supply to ground.

It's not shorted to Gnd as the wiper motor runs, it won't stop.

I would also suggest, but I'm not coming from a position of knowledge on this, that even with the worm drive, on a wet screen, and with an old motor/drive with little friction, without a method of stopping the motor quickly, the inertia MAY take the wipers past their parked position.

Try turning off the ignition when the wiper sweeps, it stops dead.

Your theory is sound, but the principle point of the switch is to break the feed to the motor, the short to Gnd is just an added benefit there to protect other circuits from over voltage.

 

[Ted]

The we'll have to agree to disagree!

Apart from your first post!

Oh, and Television (and I greatly value your knowledge - seriously)

The schematic shows no field coils which suggest it is a pm motor
And surely the schematic shows the motor withe the switch turned off, abd the wipers parked, with a short across the armature?

 

[television]

A DC motor with field coils wil not generate any voltage if the armature is turned and it is not joined up to a battery, it cant as there is nothing to generate a flux field, A dc motor with magnets will produce a voltage but wiper motors have field coils.

As I have said it stops dead as there is 300 to 1 gear reduction on the worm wheel, so if the motor over spins say 200 times the movement will only be a few mm on the final drive, and that is neither here nor there

 

[CAT220]

The depth of knowledge demonstrated on this thread is fantastic! Makes me feel as though I know nothing!

So should I still be pulling the relay and if the wiper stops then its new relay time?

Also one final question , the diagram showing the location of the relay is this the box which is screwed in behind the fuse box?

Thanks,
C

 

[Dieselman]

So should I still be pulling the relay and if the wiper stops then its new relay time?

Yes, but as the smoke was from the wiper unit I think it's the park switch inside.

Also one final question , the diagram showing the location of the relay is this the box which is screwed in behind the fuse box?

Thanks,
C

Not sure, think so.

 

[television]

The number on the relay will be something like 124 820 10 26 and yes it is inside this box, just take it out and if they stop it should be the relay.


Its not knowledge, just 55 years of earning a living.


Just thinking, on some of these faults where one has dealt with say 50 and always been the relay, does one get egg on their face when the odd one comes along with something different

 

[CAT220]

The number on the relay will be something like 124 820 10 26 and yes it is inside this box, just take it out and if they stop it should be the relay.


Its not knowledge, just 55 years of earning a living.


Just thinking, on some of these faults where one has dealt with say 50 and always been the relay, does one get egg on their face when the odd one comes along with something different

Never managed online last night, pulled the relay out tonight and the wipers stopped so spot on "television" So planning on getting another relay unit from ECP. The wiper arm will still not budge but i'm assuming a new relay will sort that?

I really appreciate all your help on this guys!

Cheers,
C

 

[MangoMan]

Could be wrong, but it sounds more serious that just a blown fuse. More like a blown wiper motor or relay.....

Like I Said about 30 replies ago.........

 

[television]

I still worry about the smoke in the car or did it come from the relay box and find its way in, has it messed up the stalk wiring, I wish that I did not worry about these things, does the relay smell

 

[CAT220]

No real smell from the relay although never smelt one before looks fine, e.g no signs of smoke marking etc but it literally only smoked for 2 secs. Think i'll bite the bullet and replace the relay and hope that frees up the stalk.

 

[television]

The stalk ony grounds one contact, if the relay went short circuit it could have put a + voltage over the stalk, so I think that the relay has to be replaced as the original cause of the problem

 

[CAT220]

Eventaully sourced a new Relay, just installed it however the stalk is still rock solid and wont move, because its stuck on intermittant the wipers are back on. So looks like I need to replace the stalk. Help I feel like doing a Mr Bean and attacking the car!

 

[MangoMan]

I'm sorry, but I don't remember you saying that the stalk was jammed? You know the twist switch to activate the Wipers. Or maybe I just missed something, but if I had seen that, it would have been obvious to change the whole Stalk Assy.

Cheers.

 

[CAT220]

I'm sorry, but I don't remember you saying that the stalk was jammed? You know the twist switch to activate the Wipers. Or maybe I just missed something, but if I had seen that, it would have been obvious to change the whole Stalk Assy.

Cheers.

Cheers MangoMan, sorry maybe im not been very clear, the stalk still moves its only the twist switch to activate the wipers thats jammed.

Cheers,
C