Which Potentiometer to use with Carb TPS?
 

I fabricated a TPS setup for my Demon several years ago but always struggled with dialing in the full voltage range required for accuracy of the throttle position reading.

Most typical TPS setups should indicate approx. 0.5v @ closed throttle and 4.5v @ WOT. In my setup when I adjust the closed throttle sensor position for 0.5v I end up with a max of only 3.6v @ WOT (I do have the correct 5v supply at the sensor). I also have a separate adjustable potentiometer wired to the TPS however it doesn’t seem to change the resulting voltage readings, maybe it’s not the proper potentiometer.

How do I know if this is the correct potentiometer and if not which one would work best for this application?

What potentiometer are you using? Need resistance specs. Do you have a wiring diagram? (Schematics)

George

Did you calibrate closed to be with the butterflies seated closed, or where it's set to idle? If the latter, maybe the lost range of motion is where the lost voltage comes from? What's reading this tps signal?

It sounds like you don't have enough throw on the tps. Try moving it either up or down to match the movement of the throttle with the movement of the tps's pivot.

My thought is one possibility is the total value of the potentiometer may be too low (ohms) where it may be loading the 5 volt source to the potentiometer. In this case, the potentiometer may actually be at the end of travel but the sourcing voltage is too low due to loading. If, for example, the total value is 500 ohms and it's loading the 5 volt source, try a higher value pot such a 2000 ohms, or 5000 ohms.

George

I won't be able to post a pic and some specs until after Christmas.

The TPS has been calibrated in multiple positions including fully closed throttle, very little variation in voltage. I can rotate the TPS for 0.5V at closed throttle but max's out at approx. 3.5v for WOT. If I rotate the TPS in the opposite direction the result is the reverse with approx. 1.2v closed throttle and 4.5v WOT.

George K, I don't know much about electronics but your explanation makes total sense and will provide some specs next week for further recommendations.

Looks like it’s 50k, is that 5000 or 50,000 ohms?

Looks like 50k (50000)). Measure from end terminal to end terminal should be 50K ohms. Measuring from one end to the center terminal will vary with rotation.

50k should be high enuf to not be loading the source.

george

Have to wait another week before I can test the ohms.

If potentiometer checks out ok any other suggestions that is preventing TPS from swinging through the full 5v range?

The degrees of sweep is what you need to figure out, it sounds like your potentiometer has too little, or too much sweep to be adjusted properly. You're either short on one end of the scale no matter how you adjust it.

Just a suggestion, but I would be looking at a TPS from an automotive source, rather than an electronics store to get the correct degrees of throttle opening, and resistance.

:2cents:

The TPS I’m using is OEM style from an auto parts store.

Then is the sweep on your TPS the same, or similar to the carb you're using. The sweep has to be close to the same number of degrees in the OEM application as your application. You might need to use a protractor to see how many degrees the TPS needs to reach those ranges, as opposed to how many degrees the carb goes from idle, to WOT. I'm guessing, but I believe the degrees of sweep are mismatched. Loosing a few degrees is going to throw everything off. From your description it sounds like your off roughly 20%.

If you're going to use it for a GM transmission, the resistance would need to be the same as the the OEM to get proper voltage for the transmission to see.

I would think that the aftermarket makes TPS switches for holley, and holley derivatives

Jegs has about a dozen listings for TPS for holley carbs, not sure if you've been there yet, or not.

This and This
 

Your mounting bracket has the TPS too close to the throttle shaft. Needs moved away from the throttle shaft to get more range of motion on the TPS.

Can't tell looking at your pics... But might need a longer lever on the throttle shaft to get the right amount of throw/sweep/travel on the TPS.

The further away from throttle shaft to the contact point is, between TPS stem and actuating lever, the more travel the TPS will have.

Clay

The small actuator arm I fabricated is attached to the end of the throttle shaft and rests on top of TPS lever near the edge. If I move the TPS further away from carb as suggested then the throttle shaft actuator arm will not reach the TPS lever.

I've not seen idle to WOT measurements listed for TPS's. You need one with less travel for your fabbed parts to work.

Or make a longer arm and modify your mounting bracket so you can slide the TPS further away from the throttle shaft.

Clay

With that pot, I think it is 270 degree turn pot, so to get 0V to full voltage (5v probably) it needs to turn 270 degrees. Anything less will reduce the voltage range you are seeing.

George

I believe it is a 270 degree Pot but even when it's cranked all the way to 270 I still don't get the complete range of 0V-5V. Will need to test the ohms as you suggested earlier to make sure it produces 50K.

After spending some time trying to figure out my birds nest wiring I have a confession to make:

The TPS is wired directly to my data logger and not through a POT as we have been discussing up to now, dumb-dumb (my apologize to all trying to help me here).

This doesn't change the status of my original issue which was not having full voltage range (0V-5V) required for the TPS to transmit accurate information.

As for the POT, if memory serves my right I had installed it to help dial in the tach signal for data logging. For the heck of it I checked this 50K POT with a multimeter connected to one 'End' and the centre 'Wiper' and found when turning the dial ohms would peak at approx. midpoint of total dial turn and as I continue turning the dial ohms would retreating back toward 0.

Are some POT engineered this way, or is this a faulty POT?

I've never seen one that switched resistance in the middle, but my experience is automotive, and aerial lift work platforms. That said, my guess is it's faulty.

I have the exact same spare POT purchased at the same time, only difference is it's 100K. Tested it and works as normal so your probably right that the 50K is faulty.