Rheostat not functioning?

[jagnut66]

Hi,
My heater rheostat has an odd issue. I know it's the rheostat because when I swapped back to the standard on / off switch that had been fitted in place of the original, the fan switches on / off without issue.
Therefore I know there's power going to it and, using my electrical screwdriver, I can see power at the other side (output to the heater fan motor) when it's switched on.
However unlike with the switch, when the fan motor is powered up by the rheostat it does nothing!......
With the rheostat switched to the off position there's no power travelling through it (just as it should be), when I switch the power through I get power at the output side but, as stated above, no matter how far I wind it through the rheostats settings (supposedly increasing power / fan speed) nothing happens.....
Except that the rheostat gets hot......
I've opened it up to have a look and made sure the connections are being made inside, which they are.
Put it back together, same as before, nothing.....
Swap back to the switch (which is back in place for now) and it works first time again!.....
There's nothing really to go wrong inside there, so I'm flummoxed.......
Any suggestions will be gratefully received.......
Though I expect the answer is 'buy a new one'............ not cheap considering there's nothing to them!.....
Many thanks in advance,
Mike.

Rheostats innards:

DSCF3417(2)_LI.jpg (2.62 MiB) Viewed 1811 times

[Edward1949]

First thoughts are: is there good spring pressure from the moving contact blade onto the windings; are the contact areas (blade onto winding) clean & shiny? There could be decades of deposit which just need cutting through with an electrical contact cleaning fluid.

[jagnut66]

Hi Edward,
I can pull it apart and give it a bit of a clean, I've nothing to loose by doing so.
They look reasonably clean but maybe not enough.
Should there be a link wire between the winding and the output post?
Best wishes,
Mike.

[amgrave]

No, the connection is made by the wiper to supply the output connection. If I remember the rheostat is providing the negative supply, this might affect your readings when testing earlier.

[Sleeper]

Mike

" no matter how far I wind it through the rheostats settings (supposedly increasing power / fan speed) nothing happens..... "

you have it the wrong way round , it starts off on full power,decreasing the fan speed when rotated..

John

[jagnut66]

you have it the wrong way round , it starts off on full power,decreasing the fan speed when rotated..

Hi John,
Are you saying I have my theory of how it works (plus the scribblings on my picture) the wrong way round or my connections are the wrong way round?......
Best wishes,
Mike.

[don58van]

Are you saying I have my theory of how it works (plus the scribblings on my picture) the wrong way round or my connections are the wrong way round?......

I think John is right. The more resistance coil between the input and the wiper, the slower the fan will go.

Don

[jagnut66]

Cleaned it all up, made it nice and shiney, didn't take long at all.
Reversed the connections, as suggested.......
Nothing, not a blip nor a hum............
I believe sir they have sold me a dead rheostat.
There is no life in it. It has expired, become extinct, departed this sphere of consciousness, shuffled off its Minors coil, devoid of the slightest spark of life.......
It will not resurrect and start whistling Dixie......
Right!
I'm off to buy a Parrot instead!........

[Sleeper]

Mike
Back from the pub , your scribblings were the wrong way round ,as Don says , the problem would appear to be the wiper and its connection to the centre brass ring...the wiper wipes across the coil , picking up a decreasing voltage and passes this to the output terminal via the centre brass ring(s)...is the centre brass arm "spring-loaded" ?

John

Edit
Sorry Edward , did not see your ref. to the spring pressure...

[don58van]

I believe sir they have sold me a dead rheostat.
There is no life in it. It has expired, become extinct, departed this sphere of consciousness, shuffled off its Minors coil, devoid of the slightest spark of life.......
It will not resurrect and start whistling Dixie......

No mate--It's just sleepin'. Beautiful plumage.

I can understand your frustration.

This is an extremely simple device. Not much can go wrong. In principle, it is a long, straight piece of resistance wire with a connection on one end and a contact that you can slide to any point along the wire. It has been made more compact and given a rotary control. A couple of things that occur...

1) the resistance coil isn't making proper contact with the terminal assembly. They look like they should be making good contact, but looks can be deceiving in cases like this. Easy to test with any continuity tester.
2) the contact ring for the wiper isn't making contact with the terminal. As above.
2) As Edward and John have both mentioned--the wiper and central contact aren't making simultaneous contact as they must. As mentioned whatever spring loading is provided may not keeping them both in simultaneous contact. Perhaps the springiness of the wiper itself is intended to do the job. Check that both are making contact. You may have carefully fettle the bend(s) in the wiper. Is there some sort of spring arrangement on the shaft, behind the wiper? If so, check that it is doing its job.

I hope it gets back on its perch soon.

Edit. I just had another look at your original post. If the rheostat is getting hot when it is wired up, all the connections within it must be OK.

So, is this item one that is designed to be used as a fan motor controller? Could it be one that is intended for such jobs as controlling dash light intensity?

Is it intended to be used in series with or in parallel with the motor?

Don

[Sleeper]

Series.

[jagnut66]

Firstly, thanks for all your replies. (Glad someone else on here gets 'Monty' )

2) As Edward and John have both mentioned--the wiper and central contact aren't making simultaneous contact as they must. As mentioned whatever spring loading is provided may not keeping them both in simultaneous contact. Perhaps the springiness of the wiper itself is intended to do the job. Check that both are making contact. You may have carefully fettle the bend(s) in the wiper. Is there some sort of spring arrangement on the shaft, behind the wiper? If so, check that it is doing its job.

Secondly, when I read this it did tie in with my impression of the spring, as in being 'a bit tired and dusty', when I inspected it whilst giving the unit the good clean up, recommended earlier. I put it back though, as I don't have a suitable replacement and it is still 'springy' and enabling the rotating control arm to make contact. Once assembled it clicks in and out of it's top positions and you can hear / feel the switch, as it runs over the top of the winding on it's way round to its other stop position. (Note: I did this whilst it was disconnected)
Then I read this part:

Edit. I just had another look at your original post. If the rheostat is getting hot when it is wired up, all the connections within it must be OK.

So, is this item one that is designed to be used as a fan motor controller? Could it be one that is intended for such jobs as controlling dash light intensity?

Well, it was sold to me as a working 'Heater' switch......
But that doesn't mean that's what it is of course.............
Though it looks basically the same as the new heater ones advertised on the ESM site.
Is there any way of telling the difference?
Having taken it apart and pictured it on here in bits, I take it the answer is 'not easily' otherwise someone else on here would have seen and been jumping all over the above comments....
Best wishes,
Mike.

[Edward1949]

What about the connection of the winding wire to the "power in" terminal ("slow fan" in your picture). It looks like a fairly basic metal plate held against the wire by the compressed metal grommet which is your power-in terminal. There is potential here for looseness to have developed over 60 or so years causing a poor connection.

[don58van]

Hi. Its me again.

I have had further thoughts about this.

I added an edit to my previous post saying that if it is getting hot, all the internal connections must be good.

I now realise that is not necessarily true... It could be that the connection between the input and the resistance wire might be making contact--but poorly--giving high resistance. This could lead to localised heating at the connection and very weak current available at the wiper (output).

So I retract my earlier edit.

Don
(Norwegian Blue lover)

[jagnut66]

I think I need another switch, perhaps the one from ESM beckons.............

https://www.google.com/url?sa=i&source= ... 5632014326

I have revised my notations on the picture but as of this morning I am just getting the yellow triangle instead of the green tick whenever I try to upload a picture from my PC onto here.
I have created a separate post on this but I hope the moderators / site administrators spot it, as there have been several posts from people with picture problems and it might get lost amongst them......
Best wishes,
Mike.

Edited rheostat (in bits) image with what I hope is correctly revised notations: