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]

Now the fun stuff. You will need one of these.


This is a adapter bracket that will mount to the modern circuit board, but will also allow you to mount the 914 face dial to it.

Below is the specs for this bracket. Take this drawing to any machine shop, they should have no problem making you one.



I am very fortunate to have a family member that happens to own a machine shop. This particular bracket was made via a waterjet machine.
After you have obtained said bracket, you will need to remove the plastic bracket on the sunpro's motor. See picture below. Remove this bracket and discard of it.



Now grab the old tachometer that you pulled apart earlier. Carefully, (can't stress this enough) pull the needle straight up. Some force might be involved to remove this needle. Next, remove the 2 screws (some have 3) that are on the 914's face dial. With the face dial removed, go ahead and wipe it off and remove any oily finger prints. The sunpro tachometer will need to have its threads re-tapped to accommodate the new screws for the mounting bracket.

Use this tap on the sunpro's motor plastic bracket: 8305A77 Metric high speed steel hand tap 2x.4mm, D3 pitch diameter, 3 flute After these 2 holes are re-tapped go ahead and counter sink the 2 holes with this counter sinking bit: 2846A124 Single-Flute high speed steel countersink 90 degree angle, 3/8" body diameter, 1/4" shank diameter.

Next, mount the adapter plate on the sunpro tach mounting plate. Use these screws: 91698A202 metric 18-8 stainless steel flat head phillips machine screw black-oxide M2 size, 6mm length, .4mm pitch. Make sure the holes on the adapter plate are counter sunk, and the screws are flush with this bracket.


Now mount the 914 face dial to the adapter plate. You can re-use your old screws if they aren't too scratched up. Or you can use these screws: 95836A105 metric pan head phillips machine screw black-oxide 18-8 stainless steel M2 size, 4mm length, .4mm pitch

It should start looking pretty good. You should now have the modern circuit board mounted to the 914 face dial. There is one last thing you'll need to do with machining, the needle. The needle won't simply push back on the shaft. You will need to drill the hole a size bigger. Obtain a 0.037" drill bit, and chuck it up to a dremil. After the needle has been honed out, hold off from re-installing the needle. We will need to calibrate it first.

So now we need to assemble this back into the can. Use this hole to loom the wires thru the back of the can.


This mod was intended to re-use holes and doesn't require you to drill new ones which will ruin your gauge can. If you use this hole, you will lose one light port for your tach. This is a great time to upgrade to LED lighting. See my topic "custom gauge backlighting" for further explanation.

This is a picture with the wires running thru the back of the gauge can


Put a couple nylon washers between the back of the gauge can and the circuit board. This spaces the circuit board, so nothing shorts out. I ended up using 2 nylon washers per screw.