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Rheostat getting very hot

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When I'm testing the heater motor on my 1955 Chevy that I am re-wiring the rheostat switch gets very hot when I put the heater motor on low speed, so hot that it starts smoking and you can't touch it. At the moment it has a seperate switch bypassing the original but I wanted to try and reconnect the original switch for authenticity. I assume it getting so hot isn't normal so why does this happen? As a new switch from the States is prohibitively expensive would it be OK to cut the resistor out and use it as a single speed switch?

Hi Tim,

Reason it will be getting hot is from current draw. The current being drawn through will be great than the rheostat is designed to take. Is it possible to find a higher rated version that you can use

My initial thought was it would be designed for 6V systems, but the '55 was the first to use 12V - could it be using a '54 speed controller?

They will get hot at lower fan speeds because of the way they work - biasing the resistance and voltage drop across the swith and reducing the voltage across the fan - this means a lot of electrical energy being released as heat.

However, depends on what you call expensive and how you value the fan working correctly. Assuming it's a genuine switch -

Thanks for the replies, it is the one in Gord's links but shipping to UK from Danchuk is around £100 for a £25 item so not really worth it plus would I be buying the same problem, it is after all the same coil of wire in the resistor. I'll get an auto electrician friend to check it over as soon as lockdown finishes.

Many thanks

I don't know if the UK postal service offers it, but we have a service from out Post Office that has the option of using a 'local' delivery address (useful for the 'free delivery' sometimes offered in the USoA), which then forwards the item - it can work out MUCH cheaper!

I was assuming it was the original heater motor, but it would seem not. If you have a friend in the biz, makes sense to just have him make up a solid state controller operated by the switch.

Thanks Gord, I've just googled the USA address for shipping and it puts it down to a reasonable price, around $30 against $115 direct.

They're still taking the piss, but it helps - one thing that you should also bear in mind is if there are any local (UK) import taxes or duty to be paid - that can be a nasty surprise as some countries base it on the total cost - with P&P, etc. rather than just the item.

Yeah we normally get hit with a £20 (NZ$ 40) tax as minimum plus our post office or Fedex want to screw us for a further £8 handling then it'll sit in Heathrow customs for a couple of weeks. I normally try and find consolidated shipping from the US where all taxes are paid upfront and the parcel then arrives straight to the door in under 5 days. The best company I've found for this is Rock Auto, unfortunately they don't stock this part.