Incorrect Tach Readings

[Jr. Corvette]

Hey guys.

So every since I bought my car my tach had been reading incorrectly. It would read properly but wouldn’t read over 2800 rpm. Last year I replaced the tach board and set the needle to perfect zero using my battery. When I turn my ingnition to ‘on’ the needle goes to zero. However my idle reads around 1200 when it’s only at 900 and every other reading is off too but it does read throughout the sweep. As far as I remember it’s about 300-400 rpm off through the whole sweep. I have to watch my Fitech screen to see my current rpms going through the range. Now at the same time I bought a new tach filter and upgraded my HEI to a high output coil and a flame thrower 3 module which does a dual spark up to 3000 rpm. Could my modified HEI be screwing up the signal? I’ve thought about a dakota digital SGI-8E to correct the reading as you can adjust the tach signal with it. Or did I just get a bad board? I made sure that I used those plastic washers when installing the board.

Any help is greatly appreciated!

 

[nascar03]

I’ve reading about tacos lately too.. I have to do the board on my ‘75.

First apparently 77s may be different..it might be that early 77/late77 thing?
I just saw a note saying this..

Did you pull the needle off the shaft and reset it to zero with 12volts applied?

I thought WILCOX Corvettes had a whole instruction fir this, but I can’t find it?

Good luck.

Graham

 

[Jr. Corvette]

I’ve reading about tacos lately too.. I have to do the board on my ‘75.

First apparently 77s may be different..it might be that early 77/late77 thing?
I just saw a note saying this..

Did you pull the needle off the shaft and reset it to zero with 12volts applied?

I thought WILCOX Corvettes had a whole instruction fir this, but I can’t find it?

Good luck.

Graham

Yup. I did reset the needle after replacing the board whilst applying 12V. I posted a picture of me zeroing the needle in my first post. The board I bought did come with instructions which I followed to the T. I’ve heard some people get bad boards but that doesn’t make much sense to me. I never heard about the early 77 or late 77 tach board but I’ll check and see

Update - Just checked and it’s a board from 75-77. No specific board for early or late corvette.

 

[nascar03]

Hi Jr...
This article I found seemed to think the 77 was different for some reason?.


The tach needle will need to be re-zeroed on any '75 or '76 Corvette. If it is not, it will read about 3000 rpm off. To do this, carefully remove the needle. Set the tach up so you can apply 12 volts and a ground to the correct terminals on the back of the tach. Apply 12 volts to the tach. Now re-install the needle at 0. Carefully remove the 12-volt connections. Now the tach is zeroed. On 1977 models, there is no need to re-zero the tach, but if you reuse your old unit, you may want to install a new needle. Carefully pull the old needle off using your thumb and forefinger.


I surmised it is the early/late 77 thing.. my bad,..

Graham

 

[Riley P]

Just a quick question with zeroing the needle with 12 supplied. Is there a rectifier or something to make sure the tach is getting 12.0 volts? I would imagine so, as a car battery is typically 12.6 volts when sitting and 14-ish volts when the car is running?

Would that have any affect?

 

[Jr. Corvette]

Just a quick question with zeroing the needle with 12 supplied. Is there a rectifier or something to make sure the tach is getting 12.0 volts? I would imagine so, as a car battery is typically 12.6 volts when sitting and 14-ish volts when the car is running?

Would that have any affect?

It shouldn’t as all summer my tach zeros fine when I put it into the on position. It also shouldn’t make much difference as the information that Graham found says you don’t need to zero the needle but that was a good thought.

 

[Jr. Corvette]

I just watched a Wilcox video on installing the 75-77 tach board. They say to adjust the signal with the potentiometer thats on the board. Thing is the board I bought never had that but it just adjusts the signal. So with that being said I think that dakota digital box would work perfectly as you can adjust the tach signal with it.

 

[YFCvii]

Wondering if Ziggy has a comment for this ... he knows a thing or two about instrument calibration. Not sure if this is a "fit" for him to comment.

 

[turbozig]

Wondering if Ziggy has a comment for this ... he knows a thing or two about instrument calibration. Not sure if this is a "fit" for him to comment.

No experience with these specific tach's, but very logical to do the pointer removal and reposition. This is the way most mechanical gauges are calibrated. My 70 Vette is in cal, so no adjustment was required. As long as the readings are out linearly by a constant value across the scale, you should be able to adjust it. If your gauge is electronic and has an adjustment potentiometer, it is a simple adjustment. If it is electrical with no adjustment, the needle R & R should also work fine. If you have access to a shop manual I would consult this first.

I recommend to use a needle puller of some sort to remove the needle however. A simple fork works well in most cases to use as leverage to remove. Be sure to pull straight up upon removal. If you google gauge needle removal you will see many example pictures. If you have a digital timing light or tachometer to hook up, you can use this as your reference while idling to reposition the needle correctly. Work slow and careful. A very simple and gratifying project for sure. Best of luck!

 

[Rruuff Day]

errrrrrr..... Like he said!

 

[Jr. Corvette]

No experience with these specific tach's, but very logical to do the pointer removal and reposition. This is the way most mechanical gauges are calibrated. My 70 Vette is in cal, so no adjustment was required. As long as the readings are out linearly by a constant value across the scale, you should be able to adjust it. If your gauge is electronic and has an adjustment potentiometer, it is a simple adjustment. If it is electrical with no adjustment, the needle R & R should also work fine. If you have access to a shop manual I would consult this first.

I recommend to use a needle puller of some sort to remove the needle however. A simple fork works well in most cases to use as leverage to remove. Be sure to pull straight up upon removal. If you google gauge needle removal you will see many example pictures. If you have a digital timing light or tachometer to hook up, you can use this as your reference while idling to reposition the needle correctly. Work slow and careful. A very simple and gratifying project for sure. Best of luck!

Thanks Turbozig! Ya, no clue why my board doesn’t have that potentiometer. Obviously not the best board out there but that’s all they sold at Corvettedepot. That’s great to hear. I think I’m just going to get that dakota digital box that has the potentiometer in it to calibrate the tach. I really don’t want to pull apart the dash again to get the tach out.