Mazda Blower Motor Resistor

[Matty F]

A friend and I have similar cars. The blower motor resistor has failed, yet again, in both cars, so the air conditioning does not work.

There are coils of resistance wire inside the air stream, and the thinnest wire corrodes and goes open circuit. The new part if available costs about GBP 100. Second hand parts cost about half that but are likely to fail. On the resistor pack it says "Do not reuse by repair".

Well I'm going to ignore that. I propose finding some resistance wire of the same gauge and putting that in, for a few cents. Or I could measure the resistance of the two bits of wire and buy a ready made resistor of huge wattage so it never goes wrong again. But there would be a difficulty in connecting that somewhere. I'm not quite sure why Mazda don't just have larger reliable resistors elsewhere where there is planty of room.

Anyway, I wish to express my displeasure at Mazda for installing a part that often goes wrong and costs so much for a replacement. The stupid thing is that it's almost the only part to go wrong on otherwise good cars. The replacement should be free or very cheap to stop us moaning in here!

[Harry Bloomfield]

Matty F explained :

The slow speed on my 100w cooling fan had failed, so for the sake of £12 I replaced that with a standard commercial heatsinked resistor of three times the wattage rating of the original. All you need to know is the resistance value and the wattage it needs to dissipate.

Another and more efficient way to achieve slow speeds on DC motors, is to use pulse width modulation control - basically turn the DC supply on and off rapidly and electronically. The speed will then be related to the on time period versus the off time period.

[Tabby]

If you have a parts box, you can also use a string of diodes. Don't make the mistake of thinking each drops 0.6v, 1 - 1.5v is nearer the mark for power diodes.

NT

[Donwill]

Are you sure that the resistor is purely a resistance? The fact that you describe it as "coils of resistance wire inside the air stream" rings alarm bells for me, is it possible that the wire changes resistance with the temperature of the air stream and is therefore not a pure resistance, coils of wire in the air stream seems a very clumsy way of introducing a pure resistance in there. Just a thought, Don

[Tabby]

A 12v lightbulb is also a good option. Its R at 3-6v is a lot less than its operating R.

NT

[Matty F]

Well it looks sort of like his:

[Matty F]

I assume that it will be OK if I use a resistor of a very much bigger wattage. In my job I repair tram resistor banks that disippate 60,000 wats. They are rather too big to fit in the car and they glow red hot!

[Andy Dingley]

A Citroen favourite.

It's quite hard to repair these - it's a very high power resistor and it often only survives because it's forced-air cooled (they're mounted in the heater airstream). It's just hard to find the parts to repair it - even if you have some nichrome handy, it's not usually thick enough. On some cars it's not the resistor that fails, it's the motor

- a now-sluggish motor running permanently on the slowest speed was no longer cooling the resistor pack.

I've seen three repairs to these that lasted. One used six strands of (thinner) nichrome in parallel and needed to be wound on a new former (ceramic hexagonal cage, old radio bits) to fit them. Another swapped the resistor for a PWM gadget. The simplest to do was actually the most complex - a whole new RAF surplus fan & motor, with a 400Hz 3 phase inverter to drive it (at the time I was building 400Hz inverters for flight sims).

[Harry Bloomfield]

It is the usual way to do it, the air flow helps cool the resistor down.

[Harry Bloomfield]

[Donwill]

Ahh I see, I understand why they did it now.

PWM would be the way to go for the manufacturers I think.

Good Luck Don

[Steve Walker]

Some do that. My sister's Peugeot had that 20 years ago. I rather liked that the speed was continuously variable rather than a small number of fixed steps.

Incidentally one feature that I've never seen on a production car (although it may be out there) is the semi-automatic, continuously variable, intermittent wipe - push a button and it wipes, wait 'til the screen needs wiping and press again and it keeps repeating. If conditions change, repeat. No need to fiddle with a variable speed switch or pot to get just the right speed for the conditions.

SteveW

[Harry Bloomfield]

Many have already. Resistors are the cheap to install solution.

[Harry Bloomfield]

It happens that Steve Walker formulated :

My company Vauxhall had that system of about 5 years ago. You put it on intermit ant and if you needed to push the button to get a wipe earlier than its setting, that became the new delay setting.

[Steve Walker]

Ah, I did wonder if anyone had used it, but just hadn't seen it myself.

I added that to my first car, having seen it in an electronics mag at the time (that'd be over 20 years ago) and was suprised not to see it fitted as standard by many manufacturers.

SteveW

[ARWadsworth]

It used to just involve swapping the relay on most cars.

[Matty F]

I can buy the resistor part for 100 GBP. I can buy four fans for 40 GBP. Why not just have four fans? Or the manufacturers stop charging a horrendous price for the resistors? Or make them more reliable? e.g. thicker longer wire.

[Derek G.]

Vauxhall & others also use them.

The assy. also carries a fusible temp. cut out, that notionally cannot be replaced. Mine was spot welded (sldering not possible) to the

*iron* frame that supported the big green tapped wirewound resistor.

One cause of failure can be a dirty pollen filter reducing airflow but I suspect it will also "blow" if the fan motor is stalled.

Derek G

[Andy Dingley]

Mid-late '90s Volkswagen Golf (mine was a VR6)

Last time I bought one, the module was about twenty quid and easily retrofitted to anything with a convenient pushbutton. Marvellously convenient.

[Andy Champ]

My son's Fiesta had that.

At least, it did until some guy in a land rover with traler thought the lights on the pedestrian crossing were the ones on the nearby roundabout. The collision moved the suspension tower.

Andy.