Anybody know how to change the calibration of a '89 Firebird tachometer? I need to tweak it to read more accurate at high engine speed so I can better judge when to shift.


I had a friend many years ago that calibrated my tachometer on a bench but I have no clue what he did and it isn't reading so accurate in the car.

 

While I can't help your specific issue, the 'birds seemed to share a common tach board with other cars (not the camaro)

This might at least get you on a start ... http://www.edaboard.com/thread197687.html to fixing your issue. I found that on my quest to trying to solve my tach/electrical issue in my 'bird.

 

You have to cut the leads for pins 4 and 10 on the resistor pack

https://www.thirdgen.org/forums/atta...or-network.jpg

Then install a potentiometer 250Kohm across both pins and tune it with a sig gen on a bench

IIRC RPM = (((input in Hz) * 60) / number of cylinders) *2

So for a V8 the table would looks something like this adjust the pot until it is right. I did find on one of my L69 tachs to get more accuracy down low above 5500rpm it suffered. I am wondering if it has something to do with the RC time constant. Probably would not hurt to replace all the caps while you have it apart.

There are three hookups on the back of the tach. +12V , GND and signal. IIRC I used a 12VPP square wave with a 6V DC offset so the peak was the same was Vin

Also good information in the sticky

 

Interesting you should say that. My tach is very accurate at normal driving speeds but is running low at higher rpms. It is about 600 rpm off at 7000 rpm. I thought I would just change it to favor accuracy up high but maybe there is a better way.


Frankly, I'd rather just have a Speedhut tach but I think it will stick out like a sore thumb at night when the lighting doesn't match other gauges.

 

The sticky has little relevance as the camaro and 'bird tachometer driver boards are not the same.

 

Weird the two firebird tachometers I messed with and have in my 84 TA were just like the sticky

 

I assume the change was with the electronic speedo cluster but, I agree completely... GM then used a similar resistor 'chip' on the 4thgen clusters which are air core gauges...

 

Unfortunately I dont have a good pic of my 84 but it is exactly like the ones in that thread. Must be a pre electric thing