I have a 1989 GMC Sierra that has a faulty throttle position sensor. The TPS only serves one function: to lock or unlock the torque converter, depending on gas pedal position. These sensors are over $100, and go back frequently, so I would like to replace the sensor with a switch.
So what I need is to reduce 5v, to 1.5v
The 5v is coming from the computer, so i’m not sure about amps in this situation. The sensor wouldn’t have drawn any appreciable amps that i can see.
Can anyone suggest a way they would go about this?
Depending on the current available and the current needed, a resistor bridge might work. On the other hand, it might overload the source, which is not a good idea if it is a computer.
There aren’t any 1.5 V. visible light LEDs, so a flashlight buck driver won’t do it. A DC to DC converter designed for those voltages and the right current range should do it, but I don’t know how to look for it. There could be electromagnetic interference problems, due to the necessary switching in the converter, if the converter is not properly filtered.
It seems that TPS is linear. so you can connect potentiometer (aka variable resistor) and find the resistance/voltage when required position settled.
TPS is simple resistor voltage divider.
if ECU (computer) need 1.5V, you can make it using following way
5V——10K——–1.5V_out_here———4.7K———-GND
As I understand, the original device was a variable resistor with only two connections.
Then the voltage drop in the resistor depended on the source and the voltage sensor it drove as well as on the resistance.
On way would be to imitate the original sensor if you can measure or analyze it. Perhaps there is enough left of it to get an proximate measure of its resistance.
If you can’t do that, I can’t really say what to do but can make a guess about the circuit. I don’t know if the current is direct, alternating or pulsed. It is likely that the source output acts like a constant voltage plus a resistor in series, and likely that the sink input sensor acts like a resistor in series with a short. In that case, the current is the source open circuit voltage divided by the total resistance, which is the sum of the source equivalent resistance, the sensor resistance (which depended on throttle position) and the input resistance of the voltage sensor. The input voltage, at the connection to the voltage sensor, was the source voltage (5V?) minus that current times the source resistance plus the sensor resistance. Perhaps the sensor resistance is larger than the other two, so the other two can be ignored.
Please post pictures of the internals. If it’s using a slide potentiometer you may be able to replace that component by itself.
It sounds like you realize this, but having the TPS in place would be much more convenient. You want the torque convert locked as much of the time as possible, but never when shifting gears…
1.5v is my guess at what the TPS’ vout would be to lock the converter; it’s likely a range, but i’m bettin 1.5 is in the range.
aww I tore up the part. and i think it was a slide potentiometer. It was supplied both power (5v) and ground from the computer, and returned an adulterated 5v to the computer. Power was attached to one side of a dark piece of film, and ground on the other, and some copper fingers slide across the film from ground towards power. The copper fingers attached to the return wire.
So the pinout would have been, GRD, 5V, output.
I know it would be better to have the TPS do the work, but tbh it only sends the signal… the computer decides when to actually lock; it won’t lock if you go to slow, if you aren’t in 4th gear, if you are depressing the brake … and there might be more, such as line pressure, but that’s a guess (that i’ll likely find the answer to today.)
About locked-while-shifting… IDK if that would occur. But i’m hoping the intended use will be once we hit 50+mph for any length of time, flip the switch and lock. If forget to unlock, it’s ok, computer will do the work as she slows down. Worst case, speed back up, and locks too early (40mph). And frankly that could be overcome by accelerating to 50mph a bit more aggressively, which would cause the 700r4 to retain 3rd to that point. Then ease off, 4th comes and the TCC locks right after.
Unfortunately i destroyed it before i could measure it’s resistance, and my motoralldata.com subscription got changed (too many ex workers still knew the p/w i think)