V8 tach adjustment, How to do it on an early tach Options

[jridder]

I adjusted my Tach for V8 use by using part of the instructions that Paul Smrz posted at http://www.pelicanparts.com/MotorCity/smrz...chinternals.htm . These instructions are for the early tachs. Since I'm using HEI, I didn't change the 150K resistor on the input signal to the 5.6K + 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 uF cap. Instead of using just a 0.15 uF, 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 uF 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 uF 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 uF (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 article on the pelicanparts BBS that someone already provdied a link to.

Jonathan

[Chris914n6]

Thanks for posting the info. I didn't know it was out there.

Chris