Testing Replacement Electronic Tach
Thread Starter
Pro
Joined: May 2007
Posts: 593
Likes: 0
From: Warrenville IL
Testing Replacement Electronic Tach
The tach in my '75 functions like the 'big wheel' on wheel of foturne, it goes all the way around and then does nothing when I start the car. So I figure the board is fried. I picked up a used one from a rollover (thanks vettes by design) that is supposed to be working but really don't want to put my whole dash and steering column back in only to find out the new one is dead also. I am also reading that a new board would have to be calibrated once installed, so it really appears I need to rig this up with the column (or at least ignition) and get it set.
So here is what I am thinking:
1 - Pull the 'new' used tach from its bezel and plug it in, without the dash pad.
2 - Lay the column back in the car on the floor and reconnect the electronics
3 - Reconnect the battery
4 - Start the car and check that the tach reads and reacts to the gas peddle.
5 - Turn the key to the on position (engine off) - check for 0 reading - adjust per standard 'calibration instructions' (pull needle and rest to proper position, etc.)
Any other method of bench testing or otherwise testing this?
So here is what I am thinking:
1 - Pull the 'new' used tach from its bezel and plug it in, without the dash pad.
2 - Lay the column back in the car on the floor and reconnect the electronics
3 - Reconnect the battery
4 - Start the car and check that the tach reads and reacts to the gas peddle.
5 - Turn the key to the on position (engine off) - check for 0 reading - adjust per standard 'calibration instructions' (pull needle and rest to proper position, etc.)
Any other method of bench testing or otherwise testing this?
Racer
Joined: Feb 2009
Posts: 343
Likes: 0
From: San Diego CA
When I did my dash a few yrs ago, I made the mistake of taking the '76 tach to the local Speedo-Tach shop to have 'em verify the tach since it had never worked. I think he messed with the pots in the circuit board because it's got crazy readings even with a new tach filter installed. I wish I had found a way to check it myself because it's a lotta work to get it out, and I wasted all that time. Now I'm tempted to just buy a Sun tach and strap it on to the column.
Le Mans Master
Joined: Aug 2003
Posts: 7,720
Likes: 12
From: Chattanooga TN
You could do that, but you can check it out of the car. Pink is ignition switched, Brown is tach (AFTER the filter), Black is ground. Hook it up with alagator clips in the engine compartment after the filter and it should work.
Former Vendor
Joined: Aug 2006
Posts: 76,656
Likes: 1,853
From: Jeffersonville Indiana 812-288-7103
St. Jude Donor '08-'09-'10-'11-'12-'13-'14-'15
Instead of doing this in the car, you might consider doing it by the engine. I’ll explain why below.
To do this, just run 12 volts to the tach, a ground and then loop in at the distributor cap connection (marked Tach) for your signal.
The signal to the tach is a square wave signal which can be split over and over. So, get an old fashion Actron dwell/tach/volt meter and you can test away. On the upper corner of the board is an adjustable pot. You can use this to dial in the readings.
If you go to our site, we have a picture of the board with the inputs labeled so you should not have any problem with this.
As mentioned above, the tach filter is not needed for testing and you can get 12 volts off the back of the alternator, a ground off the engine.
If you do it this way, you will eliminate any variables in the wiring. Now just to be sure once you have the new board in place and tested and before you put it back in the car, plug it up on the inside. If your tach has problems on the inside and not on the outside, you have a wire issue.
If you don't have any luck, send it to me and I'll dail it in on our machine.
Willcox Inc.
To do this, just run 12 volts to the tach, a ground and then loop in at the distributor cap connection (marked Tach) for your signal.
The signal to the tach is a square wave signal which can be split over and over. So, get an old fashion Actron dwell/tach/volt meter and you can test away. On the upper corner of the board is an adjustable pot. You can use this to dial in the readings.
If you go to our site, we have a picture of the board with the inputs labeled so you should not have any problem with this.
As mentioned above, the tach filter is not needed for testing and you can get 12 volts off the back of the alternator, a ground off the engine.
If you do it this way, you will eliminate any variables in the wiring. Now just to be sure once you have the new board in place and tested and before you put it back in the car, plug it up on the inside. If your tach has problems on the inside and not on the outside, you have a wire issue.
If you don't have any luck, send it to me and I'll dail it in on our machine.
Willcox Inc.
Le Mans Master
Joined: Sep 2002
Posts: 5,236
Likes: 898
From: Myrtle Beach SC
Sullivan,
You've got the right idea by bench testing especially on those 75-77s with dash issues and Wilcox offers some great suggestions.
I might add that Wilcox was a great help to another CVer who had problems with his 76. Owner sent tach and board to Wilcox who than troubleshot and returned in order.
Illustrates great service for this type thing.
You've got the right idea by bench testing especially on those 75-77s with dash issues and Wilcox offers some great suggestions.
I might add that Wilcox was a great help to another CVer who had problems with his 76. Owner sent tach and board to Wilcox who than troubleshot and returned in order.
Illustrates great service for this type thing.
Last edited by hunt4cleanair; Mar 26, 2009 at 03:53 AM. Reason: add
Thread Starter
Pro
Joined: May 2007
Posts: 593
Likes: 0
From: Warrenville IL
After much back and forth I broke down and laid the column back on the floor and plugged in the tach as you normally would. Got everything calibrated and up and running.
Then the fun part, putting the dash and steering column back in.
Then the fun part, putting the dash and steering column back in.
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Top 10 Most Expensive Corvettes Ever Sold on Bring A Trailer
Brett Foote
10 Things Every Corvette Owner Needs (2026 Edition)
Michael S. Palmer
8 Most "Only Corvette Owners Understand" Quirks and Problems
Pouria Savadkouei
10 Reasons the C6 Z06 is Still A Performance Benchmark After 20 Years
Joe Kucinski
How Much Horsepower Every Corvette Engine "LOST" in 1972
Joe Kucinski
Top 10 DOs and DON'Ts for Protecting Your Convertible Top!
Michael S. Palmer
Top 10 Most Explosive Corvettes Ever Made: Power-to-Weight Ratio Ranked!
Joe Kucinski
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski