Street Rat

Hey crowd. I recently installed a rev limiter on my '81. I'm finding the factory tach to be off about 500 rpm in the upper rpms. 500 rpm too high.

Are these tachs notorious for being inaccurate?

Is there a way to correct it?

I may just add an aftermarket tach to the dash as so many of you have done. Anybody with an Autometer tach installed in the place of the factory tach? Let's see those if you've got 'em.

Craig

zwede

Factory tachs read high in the upper rpm range, most likely to decrease warranty claims after owners over revved them. 500 rpm sounds a bit much though.

I have a 75-77 electric tach in my 71. I got a replacement tach board that has an adjustment pot on it so I was able to calibrate it. It's within +-50 rpm all the way to 6,000 rpm.

Street Rat

Where did you pick up the adjustable tach board zwede?

zwede

Willcox

cv67

rev limiter is the way to go who really needs a tach?
Some of the aftermarkets arent good either, mines good til about 5 then reads low

7T1vette

Nothing "wrong" with the factory electric tachs...but the factory likely did set them to read a bit high for warranty "management". I'm sure it was adjusted when set at the factory, so it should still have a way to adjust for scaling. If you can set it to be accurate with another (more accurate) tach at 3K rpm, it should be plenty good at 6K. Most electric tachs have a 'zero' adjustment and a 'scaling' adjustment somewhere on the circuit board.

P.S. The tach 'filter' (on intake manifold next to dist) might also be acting poorly. The tach won't be accurate if that little gizmo is kaput.

carriljc

I have a 76 tach in my 68. It reads within about 50 rpm of my EZ-EFI 2.0 digital indication. I bought it from a forum member( aharte - ages ago, not sure he's around anymore) here 14+ years ago and I replaced the tach board after I received it.
The tach is hooked up to the Tach output on my MSD-6 Box....and I actually had to remove the tach filter because it would bounce above 3500-4000 rpm.
I never go above 6000 rpm, but it seems to track quite accurately all up and down from 0-6000-0 as seen by the EZ-EFI 2.0 digital indication. (It's not like I can track the sweep as I go from 1000->5000 rpm anyway.)

Street Rat

It seems that the tach board should be replaced from what I gather from the discussion. Let me give that a try.

mrvette

Long long LONG time ago, I made my own tach filter run off the HEI update back 22 years ago.....I forget the schematic/values of the parts but maybe you can find them on a search here.....

Got into a dispute with a good member here, long ago, and so it was about a fun chase over a local 3.5 mile long bridge 3-4 lanes each way.....AND it came out by me going after the tach with electronic pulses feeding into the dizzy and finding the tach was way in hell over indicating by about 3500 and at 5 or so it was up about 1k or so.....

then I went for the speedo....same story GPS confirmed the lie....BOTH were dead nutz right on up to a reasonable point....say 3k on the tach and 80 on the speedo......beyond that.....

fuggetaboutit......

Big2Bird

They are 40 year old Chevrolets built by guys in St. Louis about to lose their jobs.

Change the board. I think I'm on my second circuit board for the gauges, and it's dying too. (Not the tach, but the dash printed circuit.).

Street Rat

I remember changing the circuit board and the tach filter once.

I don't believe I have ever changed the tach board.

I'm contemplating getting an earlier model tach that has a higher red line on the face. I'm assuming a new tach would have the circuit board on it already?

Street Rat

Circuit Board approx. $70
Tach Rebuild Service approx. $100 60 day lead time.
New Tach approx. $180

7T1vette

The tachometer is just a simple galvanometer. If it reads throughout the scale (either from pulse generator or actual distributor signal), the gauge itself is probably fine. It could have mechanical problems in the mechanism; but it's way more likely to be a problem with the tach circuit board and/or the tach filter.