Tachometer reading too high

[Marvin Gruber]

I've had a high reading tachometer on my 68 OTS for years. I recently bought another 68 (2+2 rusty car). I thought I would swap out the tach's and have a good one in my OTS. It works but it reads high as well. The first read at least two thousand RPMs high and would be on red line at 60mph. The second tach reads about a thousand too high. It goes to 4500 rpm at 60. The wiring is correct ( with loop). I have pertronics, but that shouldn't mean anything to output signal. Any ideas?

Marv

 

[LarryK]

Is the tach mechanical or electrical? Maybe voltage stabilizer in dash. Mechanical, do they have adjustments on back?

 

[Marvin Gruber]

Tach is electrical with the white wire loop. Since both tachs read high I thinking signal is wrong.

Marv

 

[mikephillips]

Try swapping the tach generator off the back of the intake cam. Three screws hold it in place, probably need to pull the cam cover to be sure you don't drop the little drive ear that slides over the shaft and into the cam slot. Should be an easy job.

 

[AHS]

I am not familiar with Jaguar tachs but "Electrical with the white wire loop" sounds exactly like the Smiths tachs on 60's MGs I know. Every one of these I've ever seen reads 1000-2000 RPM high as you've described. The circuit inside wasn't the greatest and the calibration drifts over time. It's a simple matter to remove the 2 screws in the back holding the can on (noth the 2 in clearance holes) and there's an obvious adjustment potentiometer inside that has brought all of them back into the ballpark. I can usually get them within +/-100 from 1000 through 6000. It's not perfection, they still change a couple hundred with temperature, but it does turn it back into a useful instrument.
I'm not sure it's the same movement inside but hopefully that helps.
Andy