Tachometer reads 2X High

[ggunder]

Things were going great to install a factory tachometer (white needle, inductive tach) into my '71 510 wagon.  It has the original 1.6 engine and a matchbox electronic distributor.  I read tons of helpful posts on wiring the tach (and even headlight/wiper switch know removal).  Now it reads 2X too high at any rpm!  I have an external ballast resistor on the coil. 

 

I am wondering if the problem is one of the following:

• tach needs to be adjusted using the adjuster screw on the back
• tach needs a better ground
• the problem is with the use of the electronic distributor/coil combo

This problem has me stumped (no surprise there) but has two of my smarter friends stumped too. 

 

Thanks for your help on this.  I'm trying to show the little wagon some love since it has been promoted to daily driver.

[datzenmike]

Don't mess with the adjustment. Try it on another 4 cylinder engine. If the same.... it's the tach.

[racerx]

or you connect a tach/dwell meter and see if reading is same if not your tach is bad.

[ggunder]

We did connect it to another 510 for comparison purposes, and the tach still read about 2X what it should have compared to a tach/dwell meter.  As a last resort, we did try the adjustment on the back.  There was a lot of adjustment available with only a small change in the adjusting screw position.  We were able to dial it in and it is pretty accurate over a 3,000 rpm range compared to the tach/dwell meter.  A good used speedometer was installed at the same time, so everything works on the dash!

 

Thanks for the tips!