Electronics help with TPS circuit.

[PlacentaJuan]

hey,

As you may or may not know, a common problem with 2jz auto cressidas (all 3 of them) is that the rev limiter kicks in at WOT regardless of the rpm of the engine.

Now southo had this problem fixed by having a resistor placed somewhere in the tps circuit, that limits the signal the ecu receives to a maximum of 90% or so WOT.

Unfortunately we do not know where or the value of this resistor and after a few days of going through wiring diagrams etc and finally working out what I thought would be the solution, it still hasn't worked.

Below is the circuit diagram of the TPS sensor, and also my drawn diagram of the resistor values and voltages at WOT.



So at WOT the ECU signal (VTA)is 4.31v (within the manual range of 3.2 ~ 4.9), so therefor 90% of WOT would be 3.879v.

If i add a 500Ω at the 5v signal input (where the grey resistor is) then it should drop VTA to 3.922v, which i thought would be close enough.

Unfortunately this hasn't worked, nor has a 650, 720, 900, and 1kΩ resistor.

Now according to the 2jz sensor range chart: http://primitiveco.com/rdm/supra/doc...NGE_CHARTS.pdf WOT resistance between VTA and ground (E2) can be anywhere between 2400Ω and 11,200Ω.

So this makes me think that the ECU is NOT looking for a particular voltage, but a DIFFERENCE in voltage between open and closed throttle, which means by lowering the VTA voltage (like i did) wouldn't work. Because what would happen if you replaced the TPS and it had different resistance?

If anyone can understand this better than I, your input would be greatly appreciated.

[MacroP]

Just wondering, did you reset the ECU when doing the change?

Also you could maybe put a 27K(if using a 500R in series) resistor in parallel with GND and +5V to bring the overall impedance of the circuit back down to 5K without affecting the voltage dividing circuit. Maybe the ECU measure the current flow through the TPS circuit too, nominally 1mA but now 0.9mA with a 500R in series. I doubt the ECU would care that much though. I only thought they monitored the circuit as a voltage divider, hence why a 10 or 20K TPS should work just fine.

Good luck.

[PlacentaJuan]

yea i have reset the ecu.

thanks for your help, im pretty sure the ecu only measures voltage, as thats what the pin voltage char (http://primitiveco.com/rdm/supra/doc...AGE_CHARTS.pdf) says.

if its only voltage will the 27k parralell resistor still work?

[MacroP]

Just looking at those voltage specs, its mentions the WOT voltage specs for VTA1 is 3.2-4.9Volts. Even a 1K resistor is not enough to bring down the minimum spec for WOT which is 3.2Volts(pretty low). I reckon you may need your 90% of 3.2Volts or 2.9 Volt for WOT.
Did a quick calc, you need a total impedance of 7.5K so a 2.5K(2.7K resistor) in series for 2.9Volts full scale deflection(WOT). This may affect the 0% throttle minimum spec though(0.3V).
Try that. Best to put the resistor in series and power up the TPS(IG on) and check the voltage range from %0-100%.

As the total loading of the circuit is down to .66mA(instead of 1mA) a 35K-40K resistor in parallel with GND and V+ with bring the circuit current back up to 1mA. Probably not needed.

EDIT. If the %0(Idle) setting is too low, then the adjust the sensor itself on the throttle body to bring up the idle voltage. This will affect the top end voltage directly so some more resistance maybe needed in series to compensate.

[JetspeedCamry]

EDIT. If the %0(Idle) setting is too low, then the adjust the sensor itself on the throttle body to bring up the idle voltage. This will affect the top end voltage directly so some more resistance maybe needed in series to compensate.

This would also effect the calibration of the IDL switch wouldn't it?

[PlacentaJuan]

i haven't drawn the idle switch in my diagram, but you can see the original one, the idle is actually a 12v supply that just earths through E2 (the ground) so it shouldn't be affected by messing around with the rest of the circuit.

i am starting to understand, i will try the 2.7k resistor.

[MacroP]

This would also effect the calibration of the IDL switch wouldn't it?

Yes but there should hopefully be a little bit of play in the sensor without having the idle switch turn off.

[PlacentaJuan]

well after a bit more driving with the 1k resistor, i realise it is playing havoc with the tps signal.

for one the car will not downshift at all, it seems to even have disabled the kickdown switch.

so i wont be trying any higher values.

my next experiment is to use a diy fcd circuit to clamp the voltage where i want it.

this one i have used previously and seems to work really well:

[PlacentaJuan]

just an update for anyone else that might be having these problems:

the fcd seemed to do its job, but the max voltage required to suppress the rev limiter (about 3.2v) affects the gear shifts, and seems to disable the kickdown switch that i installed.

this is obviously not the idea way to stop rev limiter!!

i am undecides weather to keep this or just leave it standard and cop the rev limiter because i really want to gearbox to behave normally (ie: kicking down when i floor it).

interestingly when i accidently disabled the tps alltogether (the 5v VC wire 'fell out') the car did not hit rev limiter at all, and it did not also change gears at all.

with 3.2 volts, the car will only shift down into 3rd gear (overdrive off gear) and the kickdown switch has no effect so its kind of halfway there.

another option may be to wire up a momentary off switch to the steering wheel or something which i can push when i know i want to rev out the car (like at the drags) and shift manually (although im pretty sure the car still shifts without a tps signal).

3rd option is to keep the tps at 3.2v max with fcd and manually shift down gears when i accelerate.

i wish there was another way around this, as it is awesome being able to rev out first gear without hitting rev limiter!!

[brett_celicacoupe]

how is the kickdown triggered? im assuming all electronic for such a newish car.

if it is electronic, what voltage does it look for from the TPS?


so let me get this straight, you want to avoid the rev limiter by scaling the TPS signal back. but if you do it too far, the kickdown function starts to lack - because it doesnt think your near WOT?

[PlacentaJuan]

kickdown is triggered via microswitch that i mounted above the tps, so at wot the tps adjustment screw thingy hits the switch:



the gearbox has limited kickdown ability without the switch connected, and this will occur at part throttle (depending on weather the 'pwr' button is on or off: off = about 60% throttle, on = about 30% throttle) and ONLY kickdown from 4th to 3rd gear. at lower speeds the car wont even kick down into 2nd gear.

southo has apparently had this rev limiter problem fixed by adjusting tps signal to never go above 90% open throttle, but he doesn't have the kickdown switch connected.

using a fcd type clamping circuit, i am able to adjust the voltage very close to the point where the rev limiter does not kick in, and yes, it seems that unless the voltage is HIGHER than what i need to avoid rev limiter, the kickdown switch signal is ignored.

[brett_celicacoupe]

are they seperate pins at the ecu for the kickdown function and the rev limiter?

edit; maybe you could setup something to trigger the switch electronically when voltage = 0.9*3.2

[PlacentaJuan]

yea kickdown switch has its own pin at the ecu, and doesn't seem to trigger at all if the voltage is at or below 3.2v

i haven't looked at the rev limiter yet, i suppose disabling it would be another solution, and since the car is auto it shouldn't ever hit the 'real' rev limit of the engine anyway...?

[brett_celicacoupe]

well,

why cant you hook up the raw TPS output (0-5V) to the kickdown pin and leave the scaled signal from the TPS connected to the rev limiter pin?

[PlacentaJuan]

well the kickdown switch pin is 12v and needs an earth for kickdown to be activated.

im not sure what relationship the tps has with rev limiter but i will investigate it, thaniks mate