It sounds like the board is the issue.... by design the tach board should remember the last signal sent to it and in the old days when we'd buy used tachometers we'd look for them with a 700-1000 rpm reading. To us (when we didn't make the boards), that would indicate the board was working when the tach was removed.

But you issue sure sounds like the chip is failing on the board.

There is a ton of tach info on our tech site at this link. http://repairs.willcoxcorvette.com/?yt=1979&s=tach
And we actually make the boards in the USA at this link https://willcoxcorvette.com/corvette...78-79-usa-made

Willcox

 

 

My pleasure.. if you want you can send in the tach and we will match calibrate it to the analog movement.
it's not cheap but it's as accurate as can be had.. we match the board to your movement.

https://willcoxcorvette.com/corvette...-service-75-82

Willcox

 

If it doesn't end up being the board, I have an oddball solution. My tach did this same thing recently, and it ended up being a partially burned up fusible link. Somehow the car still ran and acted normally, but the loose connection at the fusible link was making the tach bounce erratically.

 

 

There isn't much variance in board calibration, but what we are finding more and more is analog movement failure. We have established what we deem as an acceptable range within what we also deem a close margin for error. This range has been established over 20 years ago in testing original movements over and over and over again.... So we don't have much of an issue when selling the boards. In one of my original videos I did on installing the boards it's evident that the OEM original board was less accurate than the one we placed on the tach..... The OEM original GM tach's were close but they were no work of art that's for sure.

It's always best to be able to have the original tach and board in hand and be able to dial them into match each other.. The issue is that for a customer to do this is next to impossible without a signal generator. The tach's are calibrated at 4000 RPM's... so for someone at home to do this they'd need a way to read the signal and also hold their tach to 4K, once you dial in 4K, the rest will fall in place. The changes at 4k are huge, but at the lower end of the scale they are very small. So tweaking one at 700 could make your tach way off at say... 3k rpms.

In the old days, maybe 1 analog movement a year would be bad..... but in the last few years we've seen an drastic increase in analog movement failures.

We just had a customer send his tach in and the movement is failing... There is a short inside the movement and the cost to repair is more than the cost to just purchase a new tach.. On this particular movement, we can put a board on it... wiggle one of the studs and the board works... then it tanks.. wiggle the stud again and it works for a minute, then tanks again....

The cost to the customer to take the movement apart and attempt to fix is no where near the cost to just purchase a new (as bad as I hate to say it) imported tach. The funny thing is that we changed our tach board to work as a replacement for both the original boards as well as the imported tach boards. Original boards shared a common ground, the new imported boards have a separate ground (dual).... We're just preparing our selves for when the new imported tach boards start failing, we'll be ready.

Willcox