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Blower motor resistors are used to reduce the speed of automotive blower motors.

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But they get hot, can burn out, are inefficient. This answer lists the benefits of PWM, yet there hardly seem any benefits of resistive speed control.

Why don't car manufacturers use PWM instead?

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    \$\begingroup\$ Resistors are much more reliable then an additional circuit system. Just a guess. \$\endgroup\$ – MadHatter Sep 13 '16 at 19:55
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    \$\begingroup\$ Also, a whole lot cheaper. \$\endgroup\$ – WhatRoughBeast Sep 13 '16 at 20:06
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    \$\begingroup\$ Yes, intuitively, one could argue for cost and reliability. But is a MOSFET really that much more expensive, unreliable or complex, than a timer+mosfet circuit? Even as a replacement part, it could easily fit the same form factor. With the added advantage of having better efficiency (lower fuel consumption by the alternator), gradual speed control etc. \$\endgroup\$ – user95482301 Sep 13 '16 at 20:23
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    \$\begingroup\$ It's hard to do PWM into an inductive load at that power level without introducing a lot of noise into nearby circuits such as a radio receiver or sound system. It requires a significant amount of additional circuitry to do it well. The resistor is simple and Just Works (TM). \$\endgroup\$ – Dave Tweed Sep 13 '16 at 20:37
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    \$\begingroup\$ I'd pile onto what @DaveTweed said, while cars do not need to pass any EMC requirements to my knowledge, everything tends to work better in a low noise environment. Also as for saving power, gas powered cars generate way more electricity then they typically use. It would be like changing one light in a building from incandescent to LED, would you notice at the end of the month? Most likely not. \$\endgroup\$ – MadHatter Sep 13 '16 at 21:05
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COST Targets, Performance and Reliability must all be met.

PWM also has to be filtered with expensive chokes and caps so it does not interfere with your AM rural reception from radiation from under your seat which now acts like an antenna. 15A Chokes can add significant cost.

When we made 100k/mo seat heater electronics for Chrysler's Jeep Cherokee, the qualification process was paid by Chrystler which included an Omron Relay Engineer who flew from Japan to qualify our Assembly process. The final Qualifying. test was Chrysler test drivers taking 2 vehicles up to Churchill and back to Detroit.

THat's not all but it is safe to say , they were well tested for reliability and our cost for all the simple electronics, conformally coated was around $12 so adding a PWM and filter would be a significant difference. That was back in the 90's so it may not be the same now , but cost and quality is everything. MTBF is less important as it supplements the spare parts business after 10yrs.

Ignition wire has long been carbon wire with high resistance to avoid similar potential EMC issues.

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