Club 80-90 Forums

The revcounter on my upgrade kit works with all VW ECUs.. all Subaru ECUs.. coil based engines such as WBX.. and mechanical diesel engines with tach driven by the alternator with adjustment to cope with varying pulley sizes.

My interface board uses a low-pass T filter on the coil input with a diode for reverse voltage protection and a switching transistor. The voltage spikes on the input can be ~200v so the components need an appropriate rating.

HTH

Thanks synroandy that confirms my thoughts about low pass filters. It's also useful to know about the spikes you get too, which can't be very good for a tachometer with a poor filter circuit.

I've come up with this:



It's basically a 2 pass filter which cleans up the interference of frequencies higher than the LT pulses. The diodes limit the output voltage to between 0 and your battery voltage ~14V which should sort out those nasty spikes to be kinder to the cheap rev counter.

To test it out I've temporarily made it on a breadboard and it results in a completely stable reading from the rev counter.



I just need to transfer the circuit to a bit of vero-board and it's fixed.

Hopefully the circuit may be of use to anyone else with a twitchy rev counter needle. Arguably this circuitry should be inside the rev counter, but I guess you get what you pay for!

My ideas are based partially on syncroandy's and this article: http://flowcon.us/td/VdoTachometerFix/" onclick="window.open(this.href);return false;

Nice fix and admirable perseverance.


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