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]

First we need to purchase this cheap 30 dollar tach. I have found them readily available at AutoZone.
This write up will implicitly describe how to use this style tach.


After buying this tach, go ahead and disassemble it. On the circuit board you will need to look for a revision letter. We are looking for revision letter D, if it's not a D go ahead and reassemble the tach and trade it in for another. Rev A has some problems, and is actually quite different than the D. The shaft size for the needle is smaller than rev D and the electronic layout seems to be dramatically different as well. The picture below shows the revision letter on the circuit board.



Next, we will need to remove the selector switch on the back of the circuit board. This selector switch, switches from 4 to 6 to 8 cylinders. This switch will be in the way and hits the back of the gauge can. You can either de-solder this switch, or you can use diagonal cutters to snip the leads.

After you have removed the selector switch, you will need to solder in a jumper wire as shown in the picture below.