I suspect the earlier speedometer boards for the GT4 have a small design issue as they switch ground for discharging the 680uF cap into the odo stepper coil directly via the SAY IC, whereas VCC is switched via the BC327. If memory serves then this was the main difference compared to later Speedometer boards, which deployed one transistor for VCC and and one for ground switching.
Had some time this weekend to do the hardware design, next step is to build a prototype and write the embedded firmware. If all works as expected, i'll include a feature that allows to calibrate the speedo on the workbench by just setting a jumper an then adjust the trimmer until the speedo displays 130 kmh/80 mph, so no special tools required. There is also a small diagnose LED to check if the speed sensor is sending triggers. Image Unavailable, Please Login
Awesome, my speedo has suddenly burst back into life, but I have little confidence it will continue working.. I can't wait!!
I'm a bit behind schedule, spend some time trying to understand why the ODO in some speedos fails sometimes, then starts to work again eventually. Have the same symptoms with a speedo i fixed some weeks ago by installing a NOS SAY115Y. It looks like the ODO stops working because pin 5 of the SAY115Y fails to apply ground to the ODO motor for discharging the 680uF cap. No idea why that happens though, might be the first signs of a SAY115Y IC breaking down. Updated and cleaned up the prototype board design and started to write the firmware code. Image Unavailable, Please Login
Hi Adrian, as you know I swapped the SAY115 in my speedo for a brand new chip. it works fine on the speedo and 75% of the time on the odo, then the odo stops. If I switch the car off and restart the odo starts again. I tried this the other day whilst I was driving, the engine back fired so loudly the chap behind nearly had a heart attack! I cant wait until your board is ready.
Firmware development is done and the first PCB has been assembled and installed in my speedometer for testing. Works nicely so far! A Speedo calibration mode is also supported, so you can calibrate your Speedo with just a 12V source and a screwdriver as the board provides a high precision speed signal. I'll do some test drives and will then provide another board to Nick who offered to support with the final testing. The speedometer replacement board will be fully documented online as a DIY project if anyone wants to build one. The assembly is a bit more difficult due to the smaller SMD components though. The good news is that the bill of material is only around 8,00/$11 for the components sourced from standard electronic part suppliers excluding the PCB. Thanks to all the posters who covered this before, the posts were very helpful for getting started on this project! Image Unavailable, Please Login
Thread about the DIY speedo board replacement continues here http://www.ferrarichat.com/forum/308-328/419560-308-speedo-repair-rebuild.html
I'm super excited to hear about this! My odometer stopped working who knows how long ago and I'd love to have it work again.
SA_308, thanks for asking. The project turned out to be more difficult from a build perspective than anticipated, so i refrained from turning it into a DIY project as you need special tools to assemble and flash the board. You can implement the basic functionality of the Speedo Type 1 with discrete TTL/CMOS DIP components to generate the pulses required for driving the tacho and ODO motor, but i wanted to have the options of the onboard calibration generator and conversion between Miles and km/H as well as adaption for Type 1 and 2 sensors and optional wheel adaption/conversion. This made the final board a bit more complex than planned.
Thank you Adrian for answering, mine only the speedo side works odo is not. is there something something i could do or add to the board to get the odo to work?
If the ODO does not work but the Speedo does, there is a chance that just the BC327 output transistor is broken. Try to replace the BC327 as well as the 1N4148 diode next to it (marked by the yellow arrows below). Also check the large 680 uF capacitor, and replace if in doubt. The capacitor is being charged via the BC327 transistor and then discharged into the ODO motor. Image Unavailable, Please Login
