Race Engine Forum UK

Hi All, I'm trying to gather a few opinions on how to fix a problem I have. The Tacho on my car is dead. My car is now running a Pectel SQ6 ECU. The original ignition system no longer exists so the RPM signal is being sent directly from the ECU rather than from the coil. The gauge cluster is good, and there is definately a signal there as I'm running a sequential shift light from the Tach wire.

I've tried a simple fix of installing a 12v supply via a 10k resistor to the RPM signal to up the voltage, but this hasn't worked. Am I on the right track? Would an MSD 8920 Tach Adapter sort the problem out?

Any info on wether you've had or seen a problem like this before would be of great help.

Thanks,
Ant

Try looking at this link http://www.msextra.com/manuals/MS_Extra ... m#tachoout this is manual for megasquirt but maybe described circuits will be useful

Thanks, will give that a look and report back.

Ant

This is very similar to the method I was given to try at the weekend. I was advised to put in a 10k resistor from a 12v feed into the tacho wire to boost the voltage. I gave it a try on monday and it didn't work unfortunately.

I've just found some more information which states that the stock Tacho requires a 20v signal as its expecting it from the Ignitor/Coil. As far as I'm aware there is no way the SQ6 (or any aftermarket ECU) can output this much voltage as an RPM signal.

Is there a relatively simple way of upping the power of that signal?

You need a circuit which will provide a higher voltage than it's being supplied, and then to use a transistor which is controlled by the ECU input to switch this voltage at the output (which will then go to the tacho). It's not too difficult a circuit to make, I'm sure, but you'll need either to spend some time experimenting or get someone to make the circuit for you. Something like the LT3433 (google it) is what you need to supply the voltage, and the switching transistor could be just about anything as it's not going to be a high current load.

I've pulled off a schematic for a Tacho driver that has been designed for Nissan Skyline RB25/26 transplants into the Australian Holden Commodore. I figured that its not going to be that dis-similar.

I've also been chatting with someone who tunes a lot of MR2 and he's thinking that there is something that can be done with the ballasts on the back of the tacho to make it work with a lower voltage. I'm awaiting his findings on this at the moment. It will probaly be the course I take as I have spare gauge clusters laying around that I can fiddle with.