long story short; wife complaining about T-I-C-K-I-N-G mechanical tach on her 1973; even though i had replaced it once already (and the cable too). The used replacement tach wasn't as noisy; but still bothersome enough for her to still complain about it.

Solution: upgrade to 3 wire electronic tach from 1975 Corvette.

Since many here have already mentioned the conversion process; I won't go into every detail.



first problem; electronic tach meter movement - mounting hole spacing wider than original 1973 face plate holes. I really didn't want to drill out/oval the 1973 face plate holes; so instead I moved the holes inward on the electronic meter.

I cut notches along the sides of both mounting ears using a dremel with a thin cut-off wheel:


then with a large pair of pliers, I squeezed together both mounting ears until the holes were closer together; enough to match the spacing of the 1973 mechanical face plate. Obviously the mounting ears were no longer flat with the face (sticking up on a slight angle at their tips). A smaller pair of pliers were used to bend the tabs level again.


check to be certain the bent-in areas are not making contact with the meters electrical posts (colored yellow in this pic):


calibration: 25 years ago I was repairing these tachs for GM at a warranty repair station. We would test/calibrate tachs using an audio frequency generator - that was switched to square wave output.
Although I have long forgotten the specifics, I kinda remembered that 350hz of square wave equalled 5000 on the tachs scale. So I set the pointer to 5000 with the signal generator connected; set to 350hz. While i had the test generator connected, I swung the output from 60 to 400hz, watching the pointer move around, making certain it moved freely.
To verify , I connected the electronic tach to the 1973 and ran it along with the (replacement) mechanical tach still in the car. With the mechanical tach reading 5000, the electronic tach showed 4900. I must have remembered correctly.
I think there was a fudge factor if plus-or-minus 150 rpm, so the newer converted tach seems to be within calibration.

I need to spend more time and verify the square wave CPS calibration setting; ya' know... they say the memory is the first thing to go.

Hope this helps anyone thinking of this conversion.
The 1975 tach came from the flea market area of Carlisle this year; I got it for $20 - though the dial face was bent and the pointer was missing. But that was just fine as my intentions all along were to use these parts from the original tach.

I am holding up on final installation until I have a reason to take the dash apart again.

 

Mike, that was very interestring. thanks

 

Nice technical post Mike! I never got the impression of you as a hands on guy...guess I was wrong. Hands on and attention to detail. I can't lend much advice here, but good luck!

 

interesting... as I am so cheap that I do most car work on my own. I usually hang in the C5 section; that is why my posts are few in the C3 section. You shoudl visit my C5 web page:
http://mikemercury.home.att.net
and check out my 3.42 rear axle DIY upgrade.

 

ttt one last time.

 

Thanks Mike
Good guidance... I too, need to do the conversion on my 69 and 70 that I have converted to EFI. I was considering sending two electronic tachs (75 &76) that I scrounged up to Corvette Instruments who would re-screen the faceplate for $40 each plus shipping. I also have a reputation of pinching a dime or two... so I need to give some serious consideration to your do it yourself approach......anybody have experience with the quality of the re-screening approach. The only plus I see for re-screen is I would be able to change the redline to 6000 rpm.

Bullshark