Pulse width modulation PWM of fuel pump updates?

My rheostat died this morning, 3rd one to go to gasifier heaven. Looking to make more reliable and easier to control option.

I read Henry’s thread : http://forum.driveonwood.com/t/pwm-fuel-pump-controllers/1111/2

as well as Carl’s thread: http://forum.driveonwood.com/t/carls-gasifier/371/452



Anyone found a ready to use PWM to control the fuel pump? I found Henry’s description a little difficult to follow, if the brake controller is still the best option could someone walk me through it?

Probably henry’s thread answers my following question but why would something like this not work?




the best thing i have found is the 20 dollar reese brake controller, and they dont require independent grounds like most pwms (ive bought a few)

Hi Dustin, The one I use is still available on Ebay.
The one you listed requires an isolated ground on the fuel pump. Possible I guess, but never been there.
I’m currently building one for Marvin from a brake controller using a rotary switch to achieve approx. .5-.7 volt steps.
I like the steps, always the same, every time, but considerably more work to build.
The slider on the brake controller is the simplest. This voltmeter would take the guess work out of a reostat, or PWM slider system.

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Hi Carl, I just ordered the PWM you suggested as my Mule (Ford Ranger) needs it, My question regards the voltage meter, Doesn’t the PWM always put out 12V but at different pulses? If that is the case, the volt meter will always read 12V, correct? Only the pulses change from 100% to 0%
Gary in PA


thanks Carl and Joseph

Carl I watched your video and looked at your wiring schematic but am still uncertain in copying your design. I apologize before hand as I am no electronics expert or even novice.

first question: do I need to use the 8 position selector switch or can I use the PWM that you listed without the custom made 8 position selector switch? As I understand it you would just have an “infinite” amount of voltage selection which is why you listed the voltmeter so you could find/tune to the correct voltage for a given situation.

second: If I decide to tackle the 8 switch selector, I am confused about wiring the switch to the PWM. From the diagram listed in your thread (I won’t post it here as your thread I believe is listed in the premium section) but can be seen here http://forum.driveonwood.com/t/carls-gasifier/371/1068.
From what I understand from the schematic the stock “potentiometer?” on the PWM is replaced with the custom selector switch wired in it’s place. However on the photo in the ebay ad the potentiometer has 6 legs whereas the schematic only has three.
Also there is 2 switches in the schematic one with the resistors and the other looking like a regular 8 position switch, I don’t understand the non resistored switch function and did not see it in your videos either (just a simple SPST that you stated was for total disconnect but was not needed).

third: overall I’m pretty confused on modifying the PWM. I am competent in soldering and feel I could carry that aspect out but I really lack the understanding of how this PWM and its modifications work, however would put the effort into learning if you could direct me to the material to read or explain further etc.

Fourth: I probably shouldn’t be trying to build the selector switch if I need to ask this but again forgive my electrical ignorance, what kind of switch am I looking for? I’m thinking something like this:
1 pole, 8 positions?

I like the break controller idea but it seem bulky compared to your unit and I feel learning about these components would pay off greater in the long term.

thanks for the help and continued patience


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The way I understand it the meter shows an average voltage.

The brake controller works and is easy to hook up. The biggest drawback is you don’t have a lot of control over the output. Only the bottom 1/4 or so of the dial is useable trying to hybrid.
There’s my rough wiring diagram on how to hook it up. If you need any more help going that route just give me a shout.

Mr. Carl’s solution is a lot more elegant.

You’re right Marvin, full 12 volts at varying pulse widths. Most PWM operate around 10,000 - 20,000 cycles. The brake controller has an audible hum, so it must be a lot lower. The meter does read the average (more or less) and really is only an approx. reference. Got company this weekend, so I’ll have to be slow addressing your questions Dustin, sorry. The Ebay model gives poor resolution from the factory so modification is necessary.

O.K. Dustin, first, no need to apologize, it is very complicated but do-able.

  1. pot or switch. The stock potentiometer gives poor resolution, ie, 1/4 turn goes from 0 - 12 volts. You won’t like it. Also it doesn’t seem to be capable of shutting off completely. That’s the 2 electrical problems we solve in the mod.

  2. 6 legged pot. They use 2 pots in parallel. Know idea why.

  3. Non resistored switch. Very observant. We are addressing the “unable to shut completely off” problem. TWO SUB-POINTS HERE:
    a) On the original I did used the on-off for a total disconnect. Still a viable solution if you don’t mind fussing with 2 switches.
    b) The non-resistored pole simply powers the PWM off in position 1, by breaking the circuit to the fuel pump relay, and on in all other positions. That means you need a 2 pole, 7 or 8 position (2P7T) switch like found here: It’s big!

  4. Controlled demolition of the PWM. . Desolder the 6-leg, install a fixed 10K, and 2 wires to the selector switch. Shouldn’t take more than an hour or two, just kidding. Note the schematic is using a 7 position sw.

My turn to apologize for a poor description. Chapter two will attempt fuel pump relay mods.
P.S. I will send you resistors for a Christmas present if you decide to go that way.

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thank you Marvin for the schematic it really helps clarify the wiring for the break controller.

Carl thank you for your description It makes sense to me now. I will go ahead and order the parts and work to recreate your genius. Thank you for your offer to supply the resistors but I can source these while I order my other parts.

I already have a brake controller I’m not using so using Marvin’s advice I will wire this in first until I can source my parts and build the other unit.

thanks again for all the advice guys, greatly appreciated

Not exactly ready to run, but an Arduino Uno and a single power MOSFET will work perfectly. I do this on my automixer.

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i am ordering a few of these, indicates %, want to use them for the cooking fan but might work for the fuel pumps as well ?


My fuel pump pulls a little over 6 amps running and has a 20 amp fuse. Test the fuel pump your planning on using but I think that ones to small.

Also the pwm’s we use for fuel pumps have a single wire out for the motor like the one in Mr. Carl’s Ebay link and a trailer brake controller. I don’t think the one you listed will work with the two wires going out to the motor unless you could isolate the ground for the fuel pump and run it all the way back to the pwm.

I can confirm that the brake controller is a PWM. I put mine on the oscilloscope and it reads 12 volts with a stepped sine wave of varying duty cycles.


Me wanna see! Me wanna see! Does yours buzz??


ordered my parts for Carl’s PWM, I will keep studying


To control the power to the fuel pump, I used a brake controller.
My brake controller had a spring loaded slide resister that would let me manually apply power to the brakes. The further I pushed the slide, the more power went to the brakes
I replaced the slide resister with a variable resister that had an on and off switch built in. We will call this switch (B). Think of a radio that you turn the knob to turn the radio on and keep turning to increase the volume.
I remoted the switch (B) to the control panel next to the fuel pump switch.
The fuel pump switch, we will call switch (A) is a double poled double throw switch (DPDT). When running on gasoline, the fuel pump switch (A) is in the up position, making the normal path from the battery to the fuel pump by way of side ONE of the switch. The input power to this switch is wired to the center lug of switch (A) side ONE. The Fuel pump is wired to the lower lug of side ONE of switch (A). When the (A) switch is down, power is removed from the fuel pump and is applied to the upper lug of switch (A) side ONE. The upper lug of switch (A) side ONE is wired to one lug of the on and off switch on switch (B). The other lug on switch (B) is wired to the power input to the brake controller. The output of the brake controller is wired to switch (A) side TWO center lug. The lower lug on switch (A) side ONE is also wired to the upper lug on switch (A) side TWO
When the fuel pump switch (A) is down and the brake controller switch (B) is off, there is no power going to the fuel pump. When I turn on switch (B), then I start to trickle in power to the fuel pump. The more I turn switch (B) the more power the fuel pump receives.
When Switch (A) is in the up position, there is no power going to the brake controller.
I know (clear as mud)


Why not just turn the brake controller all the way up for running on gasoline? That’s what I do.

I do have a switch in line so that I know that OFF is OFF.


If I just ran the fuel pump through the brake controller and something happened to the brake controller, I could not run on gasoline until I fixed the brake controller. I am sure there are a lot of ways to feed fuel into the system and most work.
I tried about four ways before choosing the system I have.


using the parts listed and Carls schematic and help I was able to put together “Carls PWM” and installed it into the truck. Ended up wiring separate switch for total disconnect. Works Great! very easy to use and dial in for hybrid driving. Never ended up using the break controller so can’t comment on that but gotta give two thumbs up to Carl and his awesome switch/PWM.


What you guys think of this ? http://www.ebay.com/itm/381476735571?_trksid=p2055119.m1438.l2649&ssPageName=STRK%3AMEBIDX%