LED Taillights

918 Tach Internals

So hear are some pics of the inside of my stock tach that was converted to v8 and MSD. The large capacitor in the middle of the fourth picture is 0.15 µF and the second capacitor is a 0.047 uF. You also need to change a resistor. Basically you need 3 new componenets:

1) 0.15µF capacitor
2) 0.047µF capacitor
3) 5600Ω resistor
 

PDF of wiring diagram of tach, out of '72/'73 914

How to:
914 tach recalibration:

These steps are for a tach that came out of a '72 914. I looked at the internals of a tach out of a '73 and it looks the same.

1) You want to first take off the face (3 rings and a piece of glass). Pic
2) Then take off the needle by gently prying up on it with equal pressure on all sides. Pic
3) Undue the screw that holds on the plastic terminal insulator. Pic Pic
4) Undue the 3 remaining screws with your hand holding the face so that when the screws are undone the internals don't fall out. Separate the face and casing. Pic Pic
5) Remove the face and be careful since the circuit board is now lose. Pic
6) Remove and replace the components one at a time, use the PDF as a guide.
7) Reassemble the tach to a point that you can check whether it works.

This is an addition sent to the pors-chev group (http://pors-chev.com) by Jonathan. I did edit it a little, just to get rid of the unnecessary stuff (last names, redundant information). Thanks Jonathan.

"I adjusted my tach for V8 use by using part of the instructions that Paul posted.
These instructions are for the early tachs.  Since I'm using HEI, I didn't
change the 150KΩ resistor on the input signal to the 5.6KΩ + cap.  I'm
guessing that this is needed to compensate for the lower voltage (or
duty cycle) of the MSD tach signal.

I just replaced the 0.33 µF cap.  Instead of using just a 0.15 µF, I got
an assortment including a 0.15, two 0.01, a 0.0047, a 0.0022 and two
0.001.  That way I figured I could adjust the value incrementally to
give an accurate reading.

I took the tach apart by removing the front bezel (it has to be pried
off by going around and loosening the crimp).  I then proceeded to take
the needle off and broke it (it's rather brittle plastic, not metal like
I thought it was).  Here's another thing I would change in Paul's
instructions: there is no need to take the needle or front plate of the
tach off.  Just take the four screws off the back and the unit will
slide forward and you can find the 0.33 µF cap pretty easy.

I saw a hint somewhere about using a car battery charger to get a half
wave rectified signal that could be used to generate an accurate tach
signal (3600 RPM for a 4 cyl and 1800 for an 8 cyl).  However, this did
not work on my tach.  I had another tach (for my Buick V6 510) that I
set to 8 cyl and used to calibrate the 914 tach.  I connected both tachs
at the same time using some jumper wires.

I started with just the 0.15 µF and as expected the value was a little
low and the RPM's read high on the 914 tach.  I added the 0.01 and it
was still high, then another 0.01.  It was still a little high so I
added the 0.0047 and it was pretty close.  The changes were getting
pretty small so I don't think you would need the smaller caps to do the
adjustment.  I ended up with about 0.175 µF (of course caps are not
usually that precise - these were probably 5% tolerance).  It read the
same at 1000, 2000 and 3000 RPM.

For the late model tachs with the IC, there is an thread on the
Pelican Parts BBS that someone already provided a link to."

Blue LED Gauges

So I had an idea and had a couple extra LED's and thought this might be fun to try. I gutted a the housings of two regular incandescent light's and put in an LED with a resistor. I must say that it turned out pretty cool. The problem is that the light isn't dispersed enough to cover the whole gauge. The pictures didn't come out that well but here they are anyways. Maybe if I had a better camera.

914 LED Taillights

This is my first array of LEDs for taillights. The LED's are really bright!!!

So I have a completed prototype installed in my car. I still have to figure out what it would cost to duplicate it. Here are some pics of the process I took to make them. I first designed a circuit to run the LEDs. I was going to use a oscillating circuit but found that it was not necessary. The current to the LEDs is reduced by resistors and the LEDs receive are driven by a constant current circuit. This is done so there can be a variation in voltage but the light output will not change.