Speedo Conversion Mk3 Supra

July 13, 2005

hi guys i am new to this but i hope you can help

i have a 1991 2.5 tt with a sports roof and the didgital dash can enyone tell me how to change the kmh to miles and remove the 112 mph speed limiter and also the cruise control limiter :(

July 13, 2005

hi guys i am new to this but i hope you can help
i have a 1991 2.5 tt with a sports roof and the didgital dash can enyone tell me how to change the kmh to miles and remove the 112 mph speed limiter and also the cruise control limiter :(
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There is only one bloke who makes the bit of kit to do this, and he's prob far too busy to knock another one up.. but you could always ask him.

Pete Betts at www.thor-racing.co.uk

I've got one and it works great (he used my car as the dummy).

The thing is with the 2.5tt with a digidash is that it uses a unique speed signal, basically a scaled down version of the normal speed signal seen by most ECU's. For this reason you need a particular kind of converter, and as far as i'm aware pete is the only person who makes one.

Once you've fitted one of petes units, you then have to take the digidash apart (not an easy job believe me) and change the pins to make the mph legend light up.

All in all its a !Removed! of a job.

Read this.. taken from a forum posting by pete...

PS pop over to us at the UK owners group at www.forum.mkiiisupra.net

"The MkIII 2.5TT digi-dash is very complicated to get apart in comparison

to the MkIV dash. The whole philosophy behind the design has changed to

make it easier to get into and repair the dash on the MkIV. That said the

MkIII 2.5 TT digi-dash is also a piece of mastery and ahead of it's time.

From my investigations I have found the following.....

The Speed signal enters the Speedometer head via a white wire (whey hey

I here you say) and is rebroadcast to the rest of the car via a pink with

silver stripe wire on the same connector.

Normally with KPH->MPH coversions one has to be aware of the potential

changes to the rest of the cars systems. Namely progressive power steering,

cruise and auto gearbox behaviour.

Changing from KPH->MPH, which I'm sure you're aware, is a 5/8ths scaling

of the original speed signal (which incidentally is devired from a mechanical

cable from the gearbox entering a signal converter box behind the dash

to produce an electrical signal which then drives the speedometer head etc)

One can therefore summise that the speed signal seen by the rest of the

cars systems is also 5/8ths of what it actually is

The rebroadcast signal was the UNIQUE factor on the MkIII 2.5tt as is was

1/5 of the original input signal. On the MkIV the rebroadcast signal is

the same as the input signal (same frequency) but on the 2.5tt it's 1/5.

(so I mean 100Hz signal fed into the speedo head produced a 20Hz output

freq' signal)

It was ONLY because of this that I have decided to reprogram a TRL DSC (http://www.trlperformance.com/dsc.html)

for Adi (in fact re-write the DSC software to make it more configurable

(which I almost finished last night))) to take account of the 1/5 output

from the normal speedo head.

So now the DSC will output a 5/8ths scaled version of the input signal and

feed into the input of the speedometer.

The output (pink/silver) wire of the speedometer wil be cut and the loom

side connected to the DSC new 1/5th output. On the ECU side the delimiter will be overcome by feeding a CLAMPED 1/5th scaled signal stopping short of 112MPH at 110MPH."