DazW

iTrader: (1)

I think ive found a cheaper alternative to the usual route (Speedo cable step down converter) of de-restricting a classic Import.

I've had a spare speedo sitting on my desk for while ...so i decided to take it apart as I couldn't figure out how a mechanical speedo was getting information to the ECU to restrict it to 180 kph or 112 mph.

I hooked up a multimeter to the back of the speedo & got a pulse signal when spinning the cable (with a cordless drill!) ...that varied depending on the drill speed.

To fool the ECU i 'thought' it would be pretty easy to just bridge the contacts on the back of the speedo ...this threw up a code 33 (speedo sensor error) & put the car into 'limp' home mode ...cunningly I 'thought' an open circuit would be the way to go! ...again, I got a code 33

So basically the speedo needs to send a pulse at some point to show the ECU that's it's working normally ...further investigation of the speedo itself shows how it actually works:

The speedo cable connects to a circular magnet within the speedo ...the magnet spins, but has a gap in it & so creates an oscillating magnetic field ...this then passes over two very fine filament contacts (enclosed in a tiny glass tube) & as the magnet passes over the filaments they create a circuit ...the gap in the magnet breaks the circuit & the pulse is then sent to the ECU.

By soldering a divider into the circuit Ive halved the number of pulses to the ECU & thus have a theoretical 'limit' of 224 mph

Ive had no error codes as yet ...& the only down side seems to be that needle can go off the clock (well hit's the kilometer 'trip' unless you file the needle by about 1mm ) ...but for a £1 fix i can live with that

Sorry if it's SIAL ...but i did do a search & couldn't find anything

WRX_Dazza

black marker pen over the K in KM/H
and a second hand speedo cable worked for me

fatsweaty

Sorry to hijack this....but if it is a mechanical cable spinning could that explain why i get a sort of whirring clicking noise which increases and decreases with my speed and gets louder when warm?it seems to come from behind the dash instruments and if it is any ideas how to fix it because it is annoying me!!