Testing Throttle Position Sensor

[Gnomonic]

Could anybody let me know if I'm diagnosing a bad TPS properly.

Engine off, ignition off.

Remove TPS connector from TPS.

Place one lead of Multimeter (set on 20 Ohms) in center terminal of TPS and the other lead in one of the two outside terminals.

Ohms should be roughly 1.2 when throttle is closed.
As throttle is opened Ohms rise consistently to approx 5.3.
Release the throttle and it decreases back to 1.2.

Do the same thing except put the second lead in the other outside terminal, and the results should be the same.

Is that the correct procedure?

On mine, one side works fine...the other side has an Ohm reading that jumps all over the place, mainly between 10 and 20 Ohms. Occasionally it will hover around 15 Ohms and when the throttle is opened is DECREASES to about 10 Ohms.

Is the right? Does one terminal act differently than the other?

Thanks in advance.

[stuzman]

Originally Posted by Gnomonic

Could anybody let me know if I'm diagnosing a bad TPS properly.

Engine off, ignition off.

Remove TPS connector from TPS.

Place one lead of Multimeter (set on 20 Ohms) in center terminal of TPS and the other lead in one of the two outside terminals.

Ohms should be roughly 1.2 when throttle is closed.
As throttle is opened Ohms rise consistently to approx 5.3.
Release the throttle and it decreases back to 1.2.

Do the same thing except put the second lead in the other outside terminal, and the results should be the same.

Is that the correct procedure?

On mine, one side works fine...the other side has an Ohm reading that jumps all over the place, mainly between 10 and 20 Ohms. Occasionally it will hover around 15 Ohms and when the throttle is opened is DECREASES to about 10 Ohms.

Is the right? Does one terminal act differently than the other?

Thanks in advance.

Okay, first a little theory about a variable resistor or potentiometer. If you measure across the outside legs you will get the total resistance of the pot and turning it won't make any change in the resistance. Let's use a 5K Ohms pot as an example. If you measure across the outside legs, you should get approximately 5K Ohms. This total resistance can be off a little depending on the tolerance of the component. The TPS probably has a tolerance of +/- 10%. The center leg is the wiper and when you measure from this leg to one of the outside legs, you will get some resistance, let's say it's 1K Ohms. Now, if you measure from the wiper to the OTHER outside leg, you will get 4K Ohms. This value is derived by subtracting its value from the total resistance of the pot. As the pot is turned, the resistance should not jump erractically which indicates a loose connection of the wiper to the carbon resistance. If the resistance doesn't vary smoothly, then it's time for a new TPS.

Now in respone to your statements.

Place one lead of Multimeter (set on 20K Ohms) in center terminal of TPS and the other lead in one of the two outside
terminals. Ohms should be roughly 1.2 when throttle is closed.
As throttle is opened Ohms rise consistently to approx 5.3.
Release the throttle and it decreases back to 1.2.

This is true; however, remember, it depends on which outside leg that you measure. If you measure from the wiper to the other outside leg, you will get the difference that is left over. In the example above for a 5K, you will measure 5K-1.2K = 3.8K Ohms. And if this is the case, then the resistance will DECREASE as you turn the pot.

Do the same thing except put the second lead in the other outside terminal, and the results should be the same.

Yes, except as already mentioned the resistance will be higher or lower (depending on the outside leg that the wiper is referenced) and the resistance will increase or decrease; again depending on which outside leg that you start with. In your case, where the resistance jumps all over the place around 10 Ohms or so, you should see the same pattern if you measure to the other outside leg. The difference is now you will see the jumping in the higher resistance. So in your case, 5K-10=4990 Ohms. In any event, if you see the jumping of the resistance, it's time for another TPS as the resistance should vary SMOOTHLY and not jumping erractically.

Also, you could measure the voltage and get the same response (my preferred method as it checks the voltage delivered by the PCM, the TPS itself and its associated wiring back to the PCM). The PCM delivers a +5VDC reference to the pot. The 5V and Gnd are connected to the outside legs of the TPS. The center terminal (wiper) is fed back to the PCM where it determines the position of the throttle as the voltage is varied. When the throttle is closed, the voltage should be about 0.5V or so. Here again, this voltage depends on what outside leg you measure the wiper against. In this case, it's measured from the outside leg which connects to the PCM Gnd and the center terminal. If you was to measure from the wiper to the OTHER outside leg with the throttle closed, you would measure about 4.5V, yet the voltage would DECREASE as the throttle is opened. To keep all things simple, just remember to reference all voltages to GND when measuring. Getting back to the example, the voltage will increase to about 4.5V SMOOTHLY as the throttle is opened. By the way, I believe your resistance values of 1.2 and 5.3 should be in K Ohms. Hopefully, that gives you a little insight into how a TPS works.

[Gnomonic]

Very nice stuzman.
thank you very much.

I do understand a little better...and I do understand that my TPS was bad, the reverse change in Ohms was normal but the jumping around very far out of range was not. When I checked it again I couldn't get any reading at all on the bad side.

I swapped it out and I'm back to running fine (at least that problem is solved).

The TPS was only 3 weeks old however. I'm left wondering if there is some underlying cause that ruined both the original and the one that I put in 3 weeks ago. I guess if this new one also goes bad I'll need to dig a little deeper.

Thanks again.

[stuzman]

Originally Posted by Gnomonic

Very nice stuzman.
thank you very much.

I do understand a little better...and I do understand that my TPS was bad, the reverse change in Ohms was normal but the jumping around very far out of range was not. When I checked it again I couldn't get any reading at all on the bad side.

I swapped it out and I'm back to running fine (at least that problem is solved).

The TPS was only 3 weeks old however. I'm left wondering if there is some underlying cause that ruined both the original and the one that I put in 3 weeks ago. I guess if this new one also goes bad I'll need to dig a little deeper.

Thanks again.

Was the TPS that you replaced the first time replaced with an old one? If so, it was probably on its last leg? If not, then check to make sure that the TPS is in alignment with the throttle shaft and not having a binding issue which could accelerate the wear on the TPS. Glad to hear that you got it going?