75 tachometer does not work

[Greg S.]

75 tachs have a tach filter and electronic circuit board to function. My tach is stuck at 1800 RPM with motor on or off, key on or off. Need help to troubleshoot. Tach filter registers 18000 ohms and has continuity (not internally shorted). So tach filter appears to be OK. There is a circuit board in the tach as the 75 is the first HEI system which is not distributor driven. Should I replace the circuit board or is it more likely that the tach has some physical problem? Either way the tach has to come out.

What is the procedure for removing the tach in a C3? Where is the best place to have it rebuilt?

[Tom R.]

Re: 75 tachometer does not work

Greg

It is estimated that 95 percent of the time...its the circuit board but replacement is no easy task on 1975-77 Corvettes. They require dash removal! I have listed some links that can provide some background information that can help you decide the best course of action.

There is a circuit board in the tach as the 75 is the first HEI system which is not distributor driven. Should I replace the circuit board or is it more likely that the tach has some physical problem? Either way the tach has to come out.

Replace the board! The first tach boards built for the 1975s burned out easily because the copper paths were not very thick. Also there was no fuse to protect them so they took a hit when there was a short in the system. Your best option is to replace the board, test the system to ensure it works before you button up the dash. This link to an article published in Corvette Enthusiast offers some background on late model C3 tach boards

http://www.hunt4cleanair.net/Articles/tachboards.pdf

What is the procedure for removing the tach in a C3? Where is the best place to have it rebuilt?

The tach in your 1975 has the tach face, a floppy printed circuit card and the tach board. You are looking to replace the tach board but do some research so you'll understand what makes up the HEI tach. Willcox has put together some nice videos to assist 1975-77 owners with dash removal. He also has put together some other videos to understand 21st century tach board technology in contrast to what went into those 1975 boards. His tach boards are made in the US and pass muster in terms of quality and service.

http://willcoxcorvette.com/repairandinstallhelp.php?hID=318