Code 51, and TPS question.

[lewis15498]

AKA no such thing. I have disassembled the ECU code. Your car computes fuel primarily from the AFM and RPM. Everything else is a correction. TPS is used only for transient throttle events. Don't believe me, disconnect the VTA signal wire and go for a drive. You will not run rich, you won't even know its disconnected because it will drive normal. Most of these cars no longer even have a good VTA signal due to flatspots worn in the TPS potentiometer trace.

I agree with everything said above but wouldnt that result in a excessively rich correction that would be temporarily experienced during throttle increase?

 

[jetjock]

On the contrary. Assuming the TPS was already off idle it results in a temporary but minor lean condition. Think of the TPS as a replacement for the accelerator pump in a carb. And btw a TPS doesn't decrease in resistance as it wears. At least I've never seen one do it. The wiper increases in resistance due to the thin film being worn off the ceramic substrate...

 

[lewis15498]

On the contrary. Assuming the TPS was already off idle it results in a temporary but minor lean condition. Think of the TPS as a replacement for the accelerator pump in a carb. And btw a TPS doesn't decrease in resistance as it wears. At least I've never seen one do it. The wiper increases in resistance due to the thin film being worn off the ceramic substrate...

I understand the comparison to an accelerator pump, but isn't the thin film what creates the resistance? My train of thought was that the wear in the film would decrease resistance, resulting in the equivilent of an addition of fuel for a greater amount of throttle than that of which is actually present.

 

[jetjock]

It doesn't follow that wear decreases resistance. The film (technically it's thick film btw, my bad) is being removed. Look at it this way: after it's gone and the wiper is on the underlying substrate there's no connection at all. Infinite resistance. Besides, have you ever seen any form of analog potentiometer that decreased resistance on it's wiper connection as it wore? Does a worn volume knob on a radio make the volume increase or does it tend to cut out altogether?

 

[lewis15498]

Ok that makes sense. I was thinking that the film was there to create resistance between the wiper and the track under the film. I am speaking more generally because I have yet to take a supra TPS apart.
Thanks for clearing that up! I now know better.

 

[jetjock]

Ok, I see what you were thinking. If that were true your assumption of decreased resistance with wear would be correct. However the film is there to create resistance between itself and the wiper as the wiper moves along it. The more film there is between the wiper and one end of the film the more resistance will exist between the wiper and that end. The opposite is true for the other end of the film. The ceramic substrate serves only as a supporting structure ie; it's an insulated place for the film to be deposited on. That's why it's called a substrate.

 

[lewis15498]

Ok, I see what you were thinking. If that were true your assumption of decreased resistance with wear would be correct. However the film is there to create resistance between itself and the wiper as the wiper moves along it. The more film there is between the wiper and one end of the film the more resistance will exist between the wiper and that end. The opposite is true for the other end of the film. The ceramic substrate serves only as a supporting structure ie; it's an insulated place for the film to be deposited on. That's why it's called a substrate.

Makes sense, that was just the conclusion I had drawn from taking them apart and studying them. Better to know the right answer. Thanks again!