Technical Tach Calibration

I got a *****in older Sun tach, but it seems far out of calibration. I don't have another gauge to check it against. So, with some searching I found this on another forum (but I trust you guys):

Use a 12 volt battery charger to check the tach (+ve to tach +ve and -ve to gnd). The theory goes that a 120 Vac 60 Hz battery charger puts out an imperfect DC voltage. It is actually a pulsating DC voltage that corresponds to

* 1800 rpm for an 8 cylinder,
* 2400 for a 6 cylinder
* 3600 for a 4 cylinder

Do you recommend doing this? Am I going to eat my new gauge if I hook it up to my charger? Any better way? Thanks!

 

Here is a shot of it, by the way

 

neat idea. I guess you want me to try it first?

btw a point ignition will see pulses of a couple hundred volts on the primary side, where the tach is connected....if you tach can't deal with the roughly 20v spikes from a battery charger, it won't survive long in normal use

 

Yes, Squirrel - you must have a bunch of old tachs laying around. Don't you want to try it out for me? : )

 

the problem is I only have one old electronic tach that I don't care about, and I don't know if it works at all. It read 0 when I connected it to the battery charger. but I also connected it's power lead to the battery charger, and I don't have any extra batteries laying around that I could power the tach with separately.

Sorry I'm not as much help as I could be.

You can also play the game of gear ratio, tire size, mph, and guess at how much the torque converter is slipping (or not, if you have a manual trans)

 

How about hooking up a cheap dwell meter, the few I have have tachs in them.

 

Then you can guess at which tach, if either, is calibrated. But if you have several to play with, then you might get somewhere.

 

You could connect a pulse generator to the input. Then the problem becomes how to adjust the reading on the dial.

 

Move the needle

 

Of course, dumb that I didn't think to just do that. I've got a standard 1939 Ford trans and my speedo is right on (based on GPS). So, I should just be able to do the math. Thanks everyone.

Also, there is an adjustment screw on the back of the Sun that lets me adjust calibration.