Tachometer

[jimoshel]

Not wanting to re invent the wheel but does anyone have a schematic for the Renix era tachs? I think the 2.5 and 4.0 tachs can be swapped by changing a resister on the tach module. I have instrument panel for both and can look and see but just wondering if some ones already done it. Checked search but couldn't find what I wanted.

[Pete M]

I think some years had adjustable pots on the back.

[jimoshel]

From what I've been able to determine, they didn't have calibration feature until '91.

[HOrnbrod]

http://www.naxja.org/forum/showthread.php?t=22173&highlight=tach+cylinder

[jimoshel]

Don. Tnx for the link. It did help. I just got through tearing a '88 panel apart and it does have a cal pot. However it's only for fine tuning and cannot cover the range difference between the 2.5 and 4.0. Still need to know which resistor to change and what value. The first tach I ever had was a 'make your own' using directions from a popular mechanix and parts from a scrap TV. That was back in 1955. Put it in a '52 Pontiac. The principle's the same. Just the components got smaller.

[Geonovast]

Pretty sure 91+ tach is ECU controlled. I put a gauge cluster from a 92 4.0 XJ into a 91 2.5 MJ without changing a thing and the tach works correctly.

[HOrnbrod]

The 91 and up tachs have a selectable switch on them, at least all the ones I've worked on. On the earlier ones, the pot I believe is for adjusting full scale meter deflection, and the single resistor (IIRC) on the board can be changed to switch between 4/6 cylinders. You could get a similar value resistor and alligator clip it across the resistor on the board, effectively halving it's value, and see how it affects the meter movement.