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gmorrone1214

S13 Throttle Body TPS issue

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It looks like with the early s13 engines, nissan used a tps with two plugs (the one you have separate). The plug attached to the body is (allegedly) a multi-position switch that triggers at different throttle positions for automatic transmission kickdown. The pigtail hanging off of it is the potentiometer (variable resistor) that can be used to calculate throttle position, which was used for EFI calculations (and is used for megasquirt).

 

I don't recognize the one attached to your throttle body (maybe it's an aftermarket unit for manual transmission only), but you can find out by measuring the resistance with an ohmmeter.

 

A throttle position sensor looks like this TPS.jpg 

 

Measure ohms between the center terminal and either of the side terminals (probably). as you rotate the sensor, the resistance should gradually change in a consistent, predictable way. If it does, it's a TPS and you can use it. If instead of gradually changing, the resistance goes from zero to open circuit (or vice versa), it's a throttle position switch, like used on the l series motors, and that won't work for what you want.

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Go read the manual.   Tuner studio gets one resistance reading when throttle closed, and one when fully open. As long as these are consistent it will work fine, and it is variable in between these two points. Used TPSs sometimes have issues, but that style is 100% useable as long as you have those values.

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From the Megamanual

  • To hook up your throttle position sensor (TPS), disconnect the TPS, and use a digital multi-meter. Switch it to measure resistance. The resistance between two of the connections will stay the same when the throttle is moved
  • If you read a high resistance which gets lower as you open the throttle, then disconnected wire is the one which goes to ground, the other one which had the continuous resistance goes to the +5 Vref from the MegaSquirt® EFI Controller, and the remaining wire is the TPS sense wire

So I have a high resistance which gets lower as I open the throttle, however the other one does not have the continuous resistance.  Instead it steadily increases when the throttle in opened. 

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Posted (edited)

Measure the top and bottom pin. the resistance between these two should not change when the throttle blade is moved.

 

Essentially what you have with a tps is a resistor between the two outside pins and a movable center tap on that resistor (the center pin). The total resistance should not change, but how it is divided by that center tap will change as the wiper is moved along.

 

If you want to get (somewhat) technical, the TPS is acting as a voltage divider, and is dividing the 5v reference voltage to a smaller percentage of that, which the ECM reads and uses to calculate its position.

 

 

Extra side note not really related to the function: I wouldn't use that particular tps because the clip is broken, so the harness won't necessarily stay connected and could lead to some troubleshooting headaches for a pretty dumb reason.

Edited by ZHoob2004

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On 3/28/2018 at 9:27 AM, ZHoob2004 said:

Measure the top and bottom pin. the resistance between these two should not change when the throttle blade is moved.

 

Essentially what you have with a tps is a resistor between the two outside pins and a movable center tap on that resistor (the center pin). The total resistance should not change, but how it is divided by that center tap will change as the wiper is moved along.

 

If you want to get (somewhat) technical, the TPS is acting as a voltage divider, and is dividing the 5v reference voltage to a smaller percentage of that, which the ECM reads and uses to calculate its position.

 

 

Extra side note not really related to the function: I wouldn't use that particular tps because the clip is broken, so the harness won't necessarily stay connected and could lead to some troubleshooting headaches for a pretty dumb reason.

 

How would I wire this to Megasquirt?  The middle pin should be the signal wire, but how do I know which pin is the ground and the other power?

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Either look up a diagram or measure it out (or just plug it in and see what happens). IIRC either one can be used as power or ground, but reversing them will reverse the direction of the sensor (making megasquirt think the throttle is closed when it's wot and vice versa)

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