Help with removing speed limiter on st205! Options

[Celica_kid_st205]

I have a swapped celica gt, with a st205 engine. I'm running 16 psi and for some reason my car tops out at 112 mph? I was wondering how I could get my top speed up. Help?

This post has been edited by Celica_kid_st205: Jun 5, 2013 - 9:46 PM

[qatar11]

So.... Here is how you would do it...

The Speedo is a 3 wire sensor (+, GND, Signal), The signal wire goes to both the instrument cluster and ECU, meaning that they are essentially in parallel. This is good because you can muck with the signal to the ECU while having the Speedo report the correct value


If you throw a cheap and cheerful scope across that signal, you should be able to see if it is analog (0V ~ 0 MPH to 5V ~ 200 MPH) or a square wave / pulse train (basically the frequency of the pulses will change with velocity)

If its a 0V-5V signal (unlikely) you can switch in a little high K resistor divider network to spoof the speed of the car to the ECU - 80% of the actual speed means that you could hit 140 MPH before the limiter kicks in

If its a pulse train (more likely), you would need a little circuit (inc a 555 timer) to do the same thing and report a slower frequency of pulses than what is really being reported. This would be a fun little project and not out of the realm of the first year electrical engineering student.

You could also possibly do one or both of these little hacks with an Audrino and some kind of shield, which would allow you to switch the hack in and out depending on your needs.


Some Risks:

- These cars predate CAN buses and more advanced OBD systems and the controllers probably are not smart enough to validate the speedometer against Wheel Speed Sensors... However, if you car is new enough to have OBD II and/or ABS - you may create signal quality faults to set in the software and the car may enter some kind of limp home - or disable ABS - or some kind of fault recovery / limp in action... Basically if you pulled this stunt in today's car - you would get Christmas lights on the cluster and possibly scaled back performance.

- Speaking of OBD II, If a signal is deemed to affect emissions, the controller must have the ability to detect a short to 12V, Short to GND, or a Open Wire... If you try to just cut the wire, and it has been identified as OBD relevant - you will set a MiL lamp which may spell bad news for you (depending on your state's laws)

This post has been edited by qatar11: Jun 7, 2013 - 4:06 PM