My 2004 car has a resistor module attached to the DC heater motor circuit that has three coiled resistance wires (2.1 Ohms, 1.5 Ohms and 1 Ohm), two which have burnt out as only two out of four speed positions work.

I'm a total EE novice and wondered if there is a solid state device perhaps using PWM that would be a suitable alternative to a resistor. Ideally one that would be an in-line replacement or requiring the addition of only a little more wiring to the car.

I've read up on PWM Controllers and Buck Converters but I quickly get out of my depth understanding the types, applications of each and their suitability.

With my basic understanding I hoped three DC-DC Step Down Buck Converters from 12v to 9v, 6v and 3v respectively would provide the necessary speed variation on the heater motor circuits to emulate/replace the resistors. 20A version would cost around the same as replacing the resistor module but would be more efficient and likely equally reliable.

Please can someone point me in the right direction as to the best way to go about such an upgrade.


  • \$\begingroup\$ Are you sure the resistors burned out? Typically, this modules contain a thermofuse which likes to blow up and costs just a few cents. While PWM sounds nice, replacing the fuse is unbeatable cheap. You just need to get it and replace the old one. (Don't solder!) \$\endgroup\$
    – sweber
    Dec 15, 2016 at 7:47
  • \$\begingroup\$ I've researched this with enthusiast websites for my car and apparently this type of resistor pack is not fused and uses exposed coil resistor wire located into the heater fan airflow for cooling... as a consequence the resistor wire corrodes and eventually breaks starting with the hottest wire (lowest speed) and ending with speed two or three becoming none functional, as in my case. There is an upgrade pack that uses ceramic resistors, but I thought the use of PWM might be a more elegant solution, being more power efficient and allowing control of each fan speed setting. \$\endgroup\$ Dec 16, 2016 at 8:57

1 Answer 1


Make or buy is a daily Engineering task.

In this case buy $5 bucks enter image description here

  • \$\begingroup\$ Hi, Thanks for you advice. The circuit for the car has a 4 way switch that directs the power through three, two or one resistors with a bypass for full power on the DC motor. In order to keep this switch function for varying the speed of the motor, would I need to buy one device per resistor circuit route and fix the speed assigned to each using the Rheostat? Or is there a way of wiring the circuit so that each path changes the motor speed using just one of the devices you indicate? Thanks again. \$\endgroup\$ Nov 16, 2016 at 11:18
  • \$\begingroup\$ or you could remove the pot and measure fixed resistor values. but your switch reliability may be suspect. I would just mount the Pot in or below the instrument panel. \$\endgroup\$ Nov 16, 2016 at 14:08
  • \$\begingroup\$ Mounting the pot would be the straight forward solution, but it would mean the Heater Speed Control is simply an off/on switch. I was hoping to keep the HSC function intact so your fixed resistor values idea might be the way to go. To set a particular speed, would it be as simple as putting a resistor across two of the wires used for the Pot, thus replacing it? If I tried to use the existing car circuit to select which resistor is used I presume I would need to use a relay for each HSC switch position that would activate each resistor on the 5v Pot circuit? Apologies for all the questions \$\endgroup\$ Nov 16, 2016 at 14:54
  • \$\begingroup\$ ...then again I could use three Pots set at different positions and switch between each using the HSC switch and relays again. That would allow me to adjust each speed setting individually and still maintain the HSC function. Is there a more elegant solution than this? \$\endgroup\$ Nov 16, 2016 at 15:01
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    \$\begingroup\$ @Tony Stewart Sunnyskyguy EE75 and regarding percentage - maybe 25% steps if there's 4 positions in total... I think I misunderstood your question before :) \$\endgroup\$ Oct 8, 2020 at 14:44

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