I have a '79 with a bad tachometer. The catalogs I've looked in sell the circuit board that is the problem "90% of the time". However, when I trace the tachometer wiring to the distributor, there is also a wired cylinder I assume to be a capacitor of some sort in the curcuit. I cannot determine which of these two components is the problem and worse, I cannot seem to find the capacitor in any of my parts catalogs. First, is this a capacitor? Second, how do I tell if it, or the cicuit board is the faulty part? Finally, if it is the capacitor, where can I get one?
When I drive, the tachometer increases with increased RPM, but the reading is not accurate. There is no way I'm red lining at 50 mph. When I shut the car off, the tachometer sometimes pegs out all the way to the right and then drops to zero. I'm no automotive electronics expert, but that seems like a bad capacitor to me.
If anyone out there has experinced similar problems, or knows how to troubleshoot a tachometer to figure out exactly what the problem is, I'd appreciate hearing from you.

 

It is called a "tach filter" and is used to clean up the signal to the tach. My tach works the same whether the signal wire is going through the filter or not. Does your tach go to zero with the key on and engine not running?

 

The tach filter stops the tach from being spiked! When powered up (when you turn the key on) the tach should go to zero as mentioned.

You can hardwire this tach right off the distributor if you have it out of the car to see if it works. I just did a post on this the other day which will tell you how to do it. The boards are easy to install and as you have read more than likely the problem.

If you need help with this, just email me at Willcoxcustomerservice@willcoxcorvette.c om

If you need the board, here is the link. Our boards are pre-calibrated and will work just fine when installed.

http://willcoxcorvette.com/product_i...ducts_id=24036

 

i made the mistake of not using the Willcox circuit board and my tach is not accurate.

 

Thanks...I'll check it out.

 

I have an old original 75-7 tach in my '72 for the HEI, with typical filter yadda yadda.....

SO I wanted verification of tach accuracy and also by the tables therefore speedo accuracy....as I don't beleive people are flying the way they apparently ARE, cops I know say they ARE, no biggie...

oh well, too keep it to a dull roar...

at any rate, I fired up a digital audio oscillator and through a transistor into the hot/tach side of my coil...powered up the mess with ignition on, and ran the freq/time base through my scope on line synch that probed the generator was dead nutz on ....damn nearly zero drift....

ok, so think about it....600 rpm at idle speeds......10 up/down per second for a piston, making 5 sparks/second....DAMN, it's buzzzzzy.....

so times 8 that 40 sparks/second from the coil.....it's even buzzzier....

so to read the obvious tach reading to what is known truth on the time base....

tach read 300 rpm HIGH out of 6 grand, same 300 rpm high out of 4500 rpm....about 200 rpm high at 3 grand, and dead nutz right on at 1500 rpm.....

I figger it's the needle weight is slightly off, thereby tossing cal out a slight amount....not messed with it, as it's a PIA to do so....

now this was run through a stockish tach filter, but MY buildup by schematics from part that will NOT fail....EVER....maybe shotgun....


so yes 5% accuracy is fairly good for a stock tach that is some 35? years old now.....

SUUU PRIZE, SOOO PRIZZZE.....

so yes the tach accuracy can be fairly easy verified in your garage, IF you are a geek, if not....well what can I tell you, wear glasses, talk funny, be awkward with women, and have a calculator on your side....

NERD....