Cab rear window demister heater

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ricardo2019
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Post Mon Nov 25, 2019 1:28 pm

Next thing.....hope your not all getting fed up with me with so many posts....

The rear window demister blower in my '87 works but the small heating element inside does not :(...ideally like to get it going but not essential. Ive taken it apart and all looks good inside,(although very crude design but probably effective enough), it has power getting across the elements. I guess the small contact strip in there is an overheat device? .....any ideas...looking on the OEM parts website its NLA.
Thank you as always!
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paultv
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Post Tue Nov 26, 2019 2:08 pm

I've never been inside the blower, but it looks like a thermal trip - and it looks "open"

The metal plate probably gets hot - too hot and it melts the solder and goes open circuit.

Does the trip unscrew - doesn't look like it's riveted in place, you may be able to get it out and re solder the trip elements.

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Brianmoooore
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Post Tue Nov 26, 2019 7:07 pm

I've never looked inside one either, so I'm also going by what I can see in the pic.
The thing on the far side looks like a bi metal thermal trip, similar to the one in the resistor pack for the front heater/demister fan. These are normally, but not necessarily, self resetting when they cool down.
BUT there's another component in your pic. which I think post dates your car. The thing that looks like a bullet with wires at each end is a one shot, non resetable, thermal trip, which, I think, wasn't in production when your car was made. I'm wondering if the bi metal trip has gone wrong, and someone has bypassed it, but added the 'bullet' trip to maintain safety. These 'bullet' trips are readily available, in dozens of different trip temperatures, and the temp. for yours should be written on it in degrees Celsius.
If yours is blown it will measure open circuit with a meter, or short circuit if it is good.
ricardo2019
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Post Tue Nov 26, 2019 8:41 pm

Ok, thank you for that information. Ill do a bit more investigating. Do you know a source for the 'bullet trip'......tried Googling and all I get is Royal Enfield road trips to India...sounds fun though!
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Brianmoooore
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Post Tue Nov 26, 2019 10:54 pm

CPC-Farnell will have what you need, if it is blown.
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Brianmoooore
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Post Tue Nov 26, 2019 11:06 pm

Wire ended or axial thermal fuses are what they are officially called. Took me a while to find them myself: https://cpc.farnell.com/c/electronic-el ... rmal-fuses
ricardo2019
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Post Tue Dec 17, 2019 3:46 pm

Fantastic information Brian thank you......now revisiting this......(probably should not of done this but I carefully shorted across the fuse terminals and it worked..even bi metal thermal strip cut seemed to work between off and on. I cant see any information on the old fuse, so any idea of what temperature rating (they seem to range from just above 70 to around 240 deg C.?!) I should get. They are not too expensive so could try say 100 for starters ? cheers !
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Brianmoooore
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Post Tue Dec 17, 2019 6:47 pm

I thought I could almost make out writing on the fuse in your pic., but you're the one with the offending item in front of you.
The rating won't be less than 100 degrees, but will be less than 200. You could try a 100, but don't be surprised if it fails after a short time, Better failing when it shouldn't, rather than not failing when it should.
Does it look like BMW put it there?
ricardo2019
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Post Tue Dec 17, 2019 7:16 pm

Thanks Brian, it does look like a proper job and not a later mod...I could make out some writing on it and when I wiped it to clean it the remainder of the writing dissapeared !! Maybe ill go for around 150 or below.
ricardo2019
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Post Fri Dec 20, 2019 1:27 pm

New thermal fuses here from CPC Farnell who have a very good delivery service,..ill keep you posted once fitted.
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Brianmoooore
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Post Fri Dec 20, 2019 1:58 pm

For anyone wanting to know more about these devices: http://www.farnell.com/datasheets/629601.pdf
I don't know how the original is fixed in place, or how you intend to connect the replacement, but its important, if solder is involved, that you don't let excessive heat travel along the leads into the fuse.
ricardo2019
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Post Fri Dec 20, 2019 8:22 pm

The original was soldered in situ. Hadn't thought of that, maybe some low heat solder or some kind of heat sink crudely attached prior to the fuse?
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Brianmoooore
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Post Fri Dec 20, 2019 8:31 pm

I'd use ordinary lead/tin solder (190 degrees) rather than the lead free rubbish (217 degrees) we're gradually being forced to use, keep the leads as long as possible without pushing the fuse too far from where it should be, and only introduce the fuse's lead when the solder on the connection is already melted.
ricardo2019
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Post Tue Jan 21, 2020 8:36 pm

Now completed this fix and all seems to be working ok...demisted the rear window perfectly at the weekend ! Thanks for your help on this guys.
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Brianmoooore
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Post Tue Jan 21, 2020 8:40 pm

:thumb:
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aimlessrock
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Post Wed Jan 22, 2020 8:54 am

this would be a worth addition to the wiki for us cab owners.
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