Tachometer (RVI type) drops to zero at 5.500rpm

Dominik · Post by **Dominik** » Sun Jun 20, 2010 8:53 pm

Hello everybody,

as you can see in the "Powered by MJLJ Gallery", my Ford Crossflow powered Super Seven is working pretty well on Megajolt for two days now. But there is still an annoying issue with my tachometer.

It is about a quite antique Stewart Warner gauge. As far as I understand, the tachometer is a current driven RVI type similar to old Smith instruments: Originally its "sensor wire" supplies 12V to the ve+ pin (clamp 15) of the coil. Therefore I connected it to the switched 12V "power in" of the coilpack (pin 2). In principle this configuration works, but the tachometer drops to zero after reaching ~5.500rpm. On lower revs there is no problem.

Do you have some hints for me to solve this issue?

Regards from germany,
Dominik.

B@man · Post by **B@man** » Mon Jun 21, 2010 8:31 pm

Hi There,

I had the same problem with my SW / TVR Tach, If you take the unit apart there should be transformer which senses the current spikes and generates a voltage output.

I Cut the track going to the transformer and connected the output of the transformer to input cable with a 1K resistor and also fitted a 9V Zener from the chip end of the resistor to ground just to prevent the input voltage of to the chip exceeding 12v.

The white wire is now connected to the MJ tach output and the ignition coils are fed by the MJ rev limiter PCB.

The only problem I have is that I now know how bad the calibration of the tach is

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Dominik · Post by **Dominik** » Mon Jun 28, 2010 10:41 am

Hi B@man,

thanks A LOT for your hint! Honestly I didn't really get it, but my Dad reformed the tacho this way. And what should I say? IT WORKS!!!!

I guess he had too much time, because he made a drawing of his modifications. Curiously electrical engineers like him enjoy designing this kind of drawings

Greetings,
Dominik.

NITROPIXIE · Post by **NITROPIXIE** » Mon Jun 28, 2010 9:25 pm

Thanks gents that a really useful topic, i think i should make it a sticky for future referencing.

Oh and thank you domink's dad

B@man · Post by **B@man** » Sun Jul 04, 2010 6:33 pm

Hi Dominik,

I'm glad the mod worked after looking at the datasheet of the chip used I came up with the simple hack and was to be honest quite surprised when it worked.

Good drawing too, as you say your dad must have too much spare time.