Convert 6 cyl tacho for use with 4 cyl

[Shawn Lin]

Peter Gargano wrote:
> 
> Two questions:
> 
> 1. What's the cheapest way to convert an analogue electronic tacho,
> set up for a 6 cylinder, so it reads correctly for a 4 cylinder.
> Assuming there is no access to the internal electronics, just the
> external signal from the coil. (logically, the tacho requires 3
> pulses for every two pulses the coil generates).

This seemed easy until you assume no access to internal electronics.

> 2. What's the easiest way?

IMO, the easiest way is to change the value of the calibration resistor
for the tach.  On the early 90's GM cluster I've played with (from a
Grand Prix), the cal resistor is laser-etched on the top of a white
14-pin DIP.  Less resistance across the terminals, and the tach reads
higher.  More resistance, and the tach reads lower.  I noticed my speedo
read about 25% too high, so I jumpered a 1 megohm (or was it 10 megohm?)
resistor across the original cal resistor and was able to get it 100%
accurate (compared to the digital HUD and my multimeter's frequency
counter).  I played with the tach a little, and it worked the same way. 
I think this would be easier than building any kind of conversion
circuit.

Shawn
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