Diode in heater fan controller?

Thermal fuse?

Maybe it stops current coming back from the motor when in a speed setting that does not use that particular route to get to the motor.

if there is more than one could you test the other to see what it does? Thermal fuse is possible. are there any markings at all on it? Diodes usually are black with a line or two at one end. Thermal fuses usually have a temperature on them.

Thanks both,

I only looked at it briefly and initially it looked like a diode as one end was pointy and marked red (it was maybe 15mm long x 5mm diameter, generally silver and I think there were some markings but not easy to read as it was still within the frame thing. It was an axial device and free standing on it's own leads that were crimped into the system).

If he's still got it when I pop in later I'll have a closer look.

I didn't get chance to see the replacement unit or stick a meter on it (which might have been handy).

Cheers, T i m

One like this>?.

Assuming it is a diode.. It is there to prevent back EMF flowing into the wiring and knackering any electronics that may get damaged.

BUT, it could also be a thermal fuse. If its a Ford, it is a thermal fuse which fails usually due to the pollen filter being blocked.

That's what I suggested to the guy who said he's shorted his failed one out.

Ah, makes good sense. With the blower on low most the resistors would be dissipating the surplus energy and with poor airflow it all overheats.

Funny, I showed the Mrs a heat-pipe cooler off a failed laptop that had died through overheating for similar reasons (the whole fin area was blocked with fluff / dog hairs etc). Handy having an airline at home. ;-)

Cheers, T i m

Possibly to prevent a reverse surge from the coil when switched off

[...]

A classic case of a little knowledge being a dangerous thing...

Cars (and other consumer items) are built down to a price. Seldom is anything that can't be advertised as a "feature" fitted, unless it is absolutely necessary.

Or reduced airflow due to inlet vents blocked by leaves or snow, or the classic "never been changed" pollen filter.

Chris

Yes its amazing how often pollen filters are not changed during service , its a nice earner for the less scrupulous garages as many are not cheap

I suppose it could also be a thermal cut out, rather than a fuse per se

- ie stop running the heat when outside temp > reasonable.

Ie stop car cooking users.

In which case it should reset when temp drops. More likely to be one use tho.

I think preventing back EMF as the motor runs down from flat out when the ignition is turned off is very likely.

I recall an old car of mine where the heater fan would feed the radio for about a second as it wound down. Could be serious with modern electronics.

Agreed

[...}

I think that very unlikely!

It will be a thermal fuse, to prevent resistor pack overheating setting light to the car.

Bear in mind the OP said it is mounted on the resistor pack.

Chris

Any markings on it?

Then why "that sits near the coil/resistors (I believe the whole thing sits in one of the internal air hoses)."

A diode would sit on or across the motor..

80W fan motor at 1/4 speed means a lot of power dissipated in resistor, it's like having open an headlamp bulb filament in the duct. Toasted car. If the insurance find the cause to be the resistor pack and find evidence that the thermal fuse has been removed/bridged etc then no payout.

Not necessarily. It merely needs to be the next component working back from the motor. It makes more sense to put it on a PCB thats easily accessible than a motor which could require the whole dash to be stripped out to get at.

Oh indeed and why I have him the 'are you sure' sorta look.

My words to him exactly.

Yup.

Cheers, T i m