Tacho problem

Rich550 · Post by **Rich550** » Mon May 16, 2011 4:45 pm

I'm using a 1968 Porsche 912 (VDO) tacho with a Alfa Romeo flat 4. It's mid engined, and the MJLJ and Edis modules are behind me in the cockpit, with a long wire going to the tach in the dash

Before installing Megajolt, I ran the tachometer off the coil in the usual way and it worked fine. Since installing megajolt a year ago, I've done about 2K miles.

When installing it, the tacho would not run off the tach out pin from the MJLJ unit, so I used the suggested tachometer adapter circuit (bought from Trigger Wheels) to combine the output from the two coil pack wires to drive the tacho.

Initially this worked fine, and the tacho matched the revs shown on the laptop. However, after the first few hundred miles, the tacho started to behave erratically and has gradually got worse. For example, the revs on the tacho often climb and then just drop back to zero, or the needle just sits near zero. (The ignition system works perfectly otherwise, with the revs shown on the laptop reading correctly).

I think I've checked everything. The VR and pip/saw wires are shielded and grounded (Edis pin7 = VR earth. pin4 = pip/saw earth. Pin9 goes to the same ground as the MJLJ unit.

I have kept the Edis unit tucked away in the sill, through which runs the loom, main cable to the battery and coolant pipe to the radiator at the front. I though one of these might be causing problem (heat/interference), but I have driven the car with the Edis unit outside of the sill area and the problem is still there.

Other than my tacho being broken (hopefully it isn't) I can't think of what might be wrong.

Any suggestions?!

Thanks, Rich

Post by **brentp** » Mon May 16, 2011 6:15 pm

We installed a Porsche 914 tachometer into our race car and immediately experienced a variation of the problems you described-- brief operation, and then no activity.

Initially we built the diode- only tach adapter. This works for most tachometers, but some require the addition of a resistor to the circuit.

See this page. This might help you out
http://www.gadgetjq.com/tach_install.htm

Rich550 · Post by **Rich550** » Mon May 16, 2011 7:19 pm

Thanks Brent, that makes sense. I'll try it. (I assume the resistors have to be in parallel with the diodes and not just in series?)

Do you know what is included in the Tach driver sold by Trigger Wheels? It's difficult to tell under the heavy shrink tubing and it's not stated on their website. It look likes just two diodes.

Thanks, I was hoping it wouldn't be anything serious.
Richard

Post by **brentp** » Mon May 16, 2011 8:05 pm

You're welcome.

I'll let Chris speak for the tacho driver. I'm sure he will have a quick answer.

Rich550 · Post by **Rich550** » Fri May 20, 2011 4:03 pm

Thanks for that link Brent - It works fine now!

(I spoke to Chris at Trigger Wheels and there are two diodes and a zener, as expected.)

Instead of using the Trigger Wheels circuit, I made up a new circuit, as per the link above, to include two 110K Ohm resistors and it works fine - revved smoothly up to 5,500rpm.

What is the benefit of the zener diode, is it really needed? (Any way to make circuitry more simple and robust!)

Thanks again
Rich