Thermal Fuse for a Blower Motor Resistor

aidanki

As per title I need a thermal fuse for a blower motor resistor, Mine blew as the resistor motor bearings were faulty and overloading the resistor. Never blew the 5amp 24V fuse powering the blower motor.

Its in an old volvo truck and the part is obsolete. Or maybe I should just solder a wire across it, not that i have the problem fixed.

Anyone know an AutoElectrician around Limerick who might have something like that ?

Its just the black fuse I need

10-10-20

Any markings on the fuse? Usually they are around the 80 to 250 degrees C mark and always spot-welded to the device. If you can find the thermal cut-off value and also the current rating you could easily replace it. If you replace it with a hard-link then you'll run the risk of a similar fault occurring again and not having thermal protection on that 1.8 ohm resistor.

https://ie.farnell.com/c/circuit-protection/fuses-fuse-accessories/fuses/thermal-fuses

aidanki

https://www.boards.ie/discussion/comment/120864774#Comment_120864774

unfortunately no markings on the fuse, it will run on a 5amp fuse on 24V DC, so I have a good idea of rating. and its located in the proximity of the fan so cool air blowing over it.

10-10-20

I'd guess it's rated around 125 C, maybe 150.

You could solder a replacement onto the pins providing that you use a pliers applied to the specific lead of the fuse which is being soldered as a heatsink so that the heat doesn't reach the fuse. Did you test the resistor with a MM, is it still 1.6 ohms?

Hellrazer

https://www.boards.ie/discussion/comment/120865839#Comment_120865839

I think its 118c myself - the 118 + / - 10% wrote on them is usually the thermal rating of these resistors if I remember right.

Radioincs https://ie.rs-online.com/web/p/thermal-fuses/7976042?gb=b

Thats a 10amp /121c one there^^^^^ Id go with that one.

Edit- actually looking at this closer the "109" at the end of the part number is the actual wattage of the resistor.

At 24v that gives you 4.5 amps.

Farnell have a 5 amp one / 121c one.

https://ie.farnell.com/thermodisc/g7f01121c/fuse-thermal-5a-121-c/dp/1869529?gclid=Cj0KCQjw8NilBhDOARIsAHzpbLBtmV_Q2G0HiZ69IKPyvacGL8aHDBkyFkWyxvZ8jm7j0D6zE7sTLQgaAhW0EALw_wcB&gross_price=true&CMP=KNC-GIE-GEN-SHOPPING-PerformanceMax-TEST-962

Joe1919

Hi. Its important to repair this properly. A similar fault/poor repairs has caused a number of fires with the opel Zafira's.

https://www.autocar.co.uk/car-news/industry/root-cause-found-vauxhall-zafira-fires

10-10-20

https://www.boards.ie/discussion/comment/120869092#Comment_120869092

I can't say I agree! The resistor stamps are as follows:

The critical values are: "1R8+/-10%"

These apply specifically to the resistor and not to the assembled package of the resistor and thermal fuse as each is a separate component.

High-powered resistor markings (specifically wire-wound and cement coated resistors like this one) are done like this: Resistance in ohms, tolerance.

Resistance in ohms is stated as "xRx" where the R takes the place of the decimal point and the tolerance in %.

An example is a 1ohm resistor which I have here:

That works out to be 1R (1.0 ohm), plus or minus 10%, with a package thermal rating of 2W.

Again, that does not tell us about the thermal fuse, those makings are well worn off, so we're left with guess-work or sleuthing to work that value out.

10-10-20

https://www.boards.ie/discussion/comment/120869158#Comment_120869158

A valid concern!

At 24v DC and with a guestimated load of maybe 3A, that 1.8ohm resistor will be dissipating 72W of power which needs to be carried away by the air flow.

zg3409

Go for a low temperature thermal fuse and if it blows again go slightly higher temperature.

Try not to solder the connections as that can blow the fuse. Instead use connectors if you can. Using a "chocolate block" extra small connector and unscrew both screws. This allows you to separate the metal from the plastic and use only the metal part to join the new component without any heat applied.

https://www.screwfix.ie/p/6a-12-terminal-terminal-strips-10-pack/60804

It will look something like that above with white plastic removed.

Go to any electrical wholesale and buy the smallest chocolate block connector you can, link to screwfix above.

aidanki

https://www.boards.ie/discussion/comment/120865839#Comment_120865839

multimeter reading 2ish or thereabouts

10-10-20

https://www.boards.ie/discussion/comment/120872877#Comment_120872877

It's fine then as the MM leads will have a small resistance.

MrCostington

https://www.boards.ie/discussion/comment/120869965#Comment_120869965

While I'm not familiar with blower motor resistors the above does not add up. The power dissipated by resistor is the current flowing through it by the voltage drop across it - the 24V is irrelevant. So using Ohms law, we see that 3A by 1R8 = 5.4V across the resistor, by 3A gives 16.2W

I assume the circuit is something like shown here https://www.elprocus.com/blower-motor-resistor/ in which case the 24V supply will be split between the motor and whatever resistors are in play as per the speed setting.

As to the original question, is seems from the Zafira fires this is a part you need to get right. Has the OP tried a breakers to get the correct part?

Hellrazer

https://www.boards.ie/discussion/comment/120869911#Comment_120869911

Thanks for that explanation 10-10-20. I was basing it on the "RWI109" at the end of the part number which is why I have the edit on my post.

A very good Polish mechanic / auto electrician friend of mine explained to me that the number at the end some car resistors part number should be the wattage that goes through the resistor IF its not marked as a separate number ie something like 50w marked separately on it.

Based on that and your resistance explanation we should be able to get there.

24v dc at 109 watts gives 4.5 amps or thereabouts so a 4 amp thermal fuse should do the job.

However in saying that most heater fans in cars (12v) are around 120- 180 watts and can run from 5-15 amps.

10-10-20

https://www.boards.ie/discussion/comment/120875194#Comment_120875194

5A will be fine for the job, it's the thermal break-point of the thermal fuse which is now a mystery. I still have my cash on around 120 deg C.🤛