2003 Dakota 4.7L 200k miles

Driving home from work yesterday and I stopped to drop off some mail. Came back to the truck and started it and it was running terribly rough. Was running beautifully up to that start. Truck was throwing the P0121 TPS code so I replaced the sensor and no improvement. The lead voltage tests fine and plug has a good ground but the new sensor matches the old one exactly when plugged in: (back probing center wire on plug to ground) .5v at idle and 3.6~3.7v with the throttle wide open (seems low). New sensor is aftermarket. Shouldn’t wide open be closer to the 4.8v supply voltage? I tried a couple resetting “procedures” to no effect but these sensors don’t need to be reset, or do they?

There were zero issues with throttle or idle prior to this.

Brendan

 

Check return voltage at the PCM.... If you have a scanner that can read live data, would be interesting to see what it has to say.

 

No scanner, just an OBDII reader and a multimeter. How would I check return voltage to the PCM?

 

Figure out which wire it is AT the pcm, and backprobe the wire there.

 

Yeah, same voltage range at the pcm connector as at the sensor.

 

See if one of the local parts stores has a scanner that can read live data. If the correct signal is making it all the way back to the PCM...... last step is too what the PCM is actually seeing.

 

What’s in the signal beyond voltage? Just curious.

 

TPS just modifies a 5 volt reference voltage by running it thru a variable resistor. As the throttle opens, resistance goes down, voltage goes up. PCM sees that as %throttle opening. (or, interprets it that way, that value is in the live data as well.) If the return signal is out of range, or 'bounces' on its way up/down, PCM gets confused, and sets a code.

 

That much I understand. But even my Chilton manual says the sensor is bad if the voltage isn’t closer to the supply voltage at WOT. I mean, isn’t 3.6v too low when the supply voltage is 4.8v? It says if it isn’t approximately 4.5v to replace the sensor. Should I really worry about the PCM if that voltage is not within range? Or, is that range wrong?

 

I have seen a lot of debate on this topic..... theoretically, if you reset the PCM, it will relearn the TPS base voltage. (throttle closed), and will extrapolate throttle position based on the signal it gets from the tps..... Is that a table of fixed values though? Not sure, but, probably. I know on the GM cars I used to work on, if TPS didn't hit >4 volts at WOT, replacing the sensor was the next step. Dodge may indeed be the same, but, the question becomes, will the new sensor be any different than the one you have? If you can warranty exchange the one ya got, probably worth a shot, though I would consider going with a different brand sensor for round 2.