Recalibrating your S10 tach for a V8

With a V8 engine, the 6 cylinder tach will read 33% too high - i.e. 4000 rpm indicated is really 3000 rpm. A 4 cylinder tach will read double - 4000 rpm indicated is really 2000 rpm.

Two different circuit boards were used on the '82 to '85 tachs. Generally, '82-'83 tachs (white pointer) used the circuit boards shown on the left, and '84-'85 tachs (orange pointer) used the circuit board on the right. To calibrate the tach for a V8, the capacitor shown (arrow) will have to be replaced with a capacitor of a different rating. For the board on the left, use a .0082mf (microfarad) polyester capacitor with a 100 volt rating. For the board on the right, use a .0063mf polyester capacitor with a 100 volt rating.

This is the circuit board for a '86-'90 tach. To calibrate the tach for a V8, replace the capacitor shown (arrow) with a .0033mf polyester capacitor with a 100 volt rating.

Sorry guys! No info was given on the '91-'93 models - you're on your own here, good luck!

To calibrate the '94 tach, replace "C103" with a .0040mf capacitor. The stock V6 capacitor is about.0055mf.

To calibrate the '95-97 tach to a V8, replace the "C103" capacitor with a .0022 to .0018mf. The stock V6 tach has about a.0030mf capacitor and the 4 cylinder tach has about a .0038mf capacitor.

Stripping the dash cluster down to get to the circuit board is the hardest part. On my '96 cluster, I removed just about everything that I didn't need to! It was much easier than I first thought - don't take out any screws, just put a screwdriver blade between the faceplate and the white plastic back-piece and gently pry up. You'll need to move the screwdriver around as the faceplate starts to come off. The faceplate will lift off (it's only held in by the metal "spring clips" that make electrical contact with the "stud" posts of the gauges). The circuit board can now be accessed. In my case, I was calibrating from a 4 cylinder to a V6 (Buick GN turbo motor), so you'd think a .0030mf would work. BUT, there are manufacturing tolerances in these capacitors, thus mine ended up using two .0010mf's and a .0001mf for a total of .0021mf. This was verified by reassembling the cluster and connecting the battery +, -, and tach signal wires to my '87 Buick GN and checking against a good Autometer tach at various RPM levels. Needless to say, it took several attempts to get it right!

Remember - if your tach reads too high, use a capacitor with a lower rating - if it reads too low, use a higher capacitance. NOTE: You'll probably need to experiment by testing against a tach that's known to be good. To get a value not available, add capacitors in parallel: the values are totalled. If you want to add them in series, good luck - the formula for figuring the value is beyond me.