Upgrading the tachometer, to remove that bounce Options

[timothy_nd28]

First I would like to give thanks to Terry, my uncle, for his time on this and my other RGB backlighting project. He has spent more time working on this than he cares to admit. He's more of a Corvette guy, but his vast knowledge and willingness to help, transcends which benefits our community.

Okay, so I have been told that our tachometer has a problem with bouncing. I personally have never seen this problem and I have to admit that I've never really paid much attention to the tach while driving, so I decided to put the signal generator on the tach and see how it performs. This signal generator bypasses the ignition system and provides a clean signal. This also serves as a control because a bad condenser on the dizzy can cause problems as well.

This video shows the untouched tach in its original state running on a function generator. As you will see, it does bounce and over shoot during the sweeps.

https://www.youtube.com/watch?v=rbC37l1z_oA


This next video shows our tach but with a cheap modern 30 dollar tach stuffed inside. The bounce seems to have disappeared, and there is no more overshoot during the sweeps.

https://www.youtube.com/watch?v=cmk1lSop_iE



The VDO tach was designed for mass production and had a very simple but effective design. This modern tach uses an air core massless motor which is controlled by sine/cosine inputs which tends to be a bit more responsive and accurate. The original vdo design is a motor/spring contraption which will always have some bounce. Over time, capacitors and dampening fluid may leak or fatigue which will aggravate the problem.

In this write up, I will show how to incorporate a off the shelf 30 dollar tachometer commonly found at the auto parts store, and make it work with our VDO gauge can.

[timothy_nd28]

After the jumper has been soldered, we need to change out a resistor (R9). R9 is shown in the picture below.


So remove the current resistor in this location and replace it with a 499k ohm resistor. The reason why this must be done, you will notice that our VDO tach face has the tic marks at certain RPM's and different degree marks than the 30 dollar tach. The VDO face dial is 4 some inches in diameter, and the sunpro tach has a tiny 1.5 inch dial face. The numbers for each RPM simply don't line up, so changing the resistor will fix the gain on the needle sweep and correct with the corresponding angle.



This resistor has been carefully calculated for solving this problem at no extra charge to you! (IMG:style_emoticons/default/biggrin.gif) However tips are appreciated! (IMG:style_emoticons/default/slap.gif)

Edit: R9 resistor values:
499k ohm for a 4 cylinder
340k ohm for a 6 cylinder
243k ohm for a 8 cylinder


Now go ahead and cut the white wire near the circuit board. This wire is used for lighting, which you will not need for this project. You will also need to pull all the other wires so they come thru the top of the circuit board. If you leave the wires where they are at, the circuit board will stick too far out from the back of the gauge can.

It should look something like this when you are done. Go ahead and zip tie the wires to the hole in the circuit board. This will be a good strain relief for the mice that might be pulling on the wires under your dash.



The electrical portion of this build is now complete. The backside of the circuit board is very clean with no wires in the way. The circuit board should sit nicely inside the gauge can.