[Mr. Sharkey]

A conventional 4 stroke piston engine will send a signal (a 12 volt signal IIRC) to the tach one time for every 2 rotations of the crank.

That's true of a one-cylinder engine, but more than likely, KiwiEV's engine was a four cylinder, which would produce two ignition pulses per crankshaft revolution.

Bypassing the factory electronics is the first step towards building your own custom tachometer. Personally, I prefer to not install optical devices under the hood of a car. They will eventually get dirty and need to be disassembled and cleaned. When I was looking around for a way to put a tach on my diesel Golf (Rabbit here in the US), I came upon the idea of using the variable reluctance pickup from an 1980's Audi 5000 ABS wheel sensor. It's a nice, sealed stainless steel can with an integral wire attached. The wrecking yard guys are always fascinated by my crackpot projects, so they let me take it home without charging me.

My intent was to train the inductive pickup so that it sensed the teeth on the camshaft/injection pump sprockets on my engine.

Here's a photo of my test jig, one of the sprockets chucked up in the drill press, with a temporary wooden jig holding the sensor about 10mm from the spinning sprocket:

Once I had confirmed that the pickup was producing a strong signal from the sprocket using an oscilloscope, I fabricated a mount to hold the pickup in the proper location on my engine. Here it's fitted up on one of my spare engines:

The reason I'm bringing it up is that the inductive pickup could be pointed at nearly anything, a disc with holes drilled in it, a splined shaft, or (importantly), the teeth on an EV's flywheel (assuming it's still installed), with the pickup bracket pointing through the hole in the bell housing that the starter used to occupy. The inductive pickup can look directly into the teeth, or it can look across, no matter, it just needs to see an alternating pattern of ferrous material when rotation is occurring. Bolt heads would even work, but the more of them there are, the better low RPM accuracy you can expect.

There's more to this story, including calculations based on the number of teeth (dimples, holes, etc) that you are sensing, and a one integrated circuit electronic assembly project that can connect the VR pickup directly to your now available tachometer instrument movement. The end result is a tach that is dead-on accurate when calibrated, and is unaffected by water, dirt, heat, etc.

Interested in additional details?

 

[Mr. Sharkey]

I wondered if anyone would ever be interested in this...

Every ABS-equipped vehicle is going to have a wheel speed/rotation sensor of some type. I chose Audi because the wrecking yard where I frequent has lots of them, and they are arranged next to the VW's, which is my main vehicle of interest.

ABS braking systems require a signal that indicates that the wheels are turning. Usually, this is a variable reluctance sensor on the stationary part of the caliper assembly (or brake drum backing plate). This "looks" at a serrated "tone ring" that is either attached to or part of the brake rotor/drum. It will be about the only thing at the end of an axle that will have wires attached to it.

I didn't look at other ABS vehicles, but the Audi unit was a cylindrical tube about 3/4" in diameter, with an integral boot and attached cable with about 2 feet of length. It was held in place with a single screw, if I recall correctly, and wasn't difficult to remove, because I was wearing clean clothes and only had a Leatherman tool to remove it. It wasn't any more difficult to find and pull than simply bending down and pulling it off.

There may be variable reluctance pickups that are better suited to adaptation to tachometer use, a few hours in a wrecking yard can be a very educational experience. Personally, I find a strange aura of contentment among the parted out and demolished hulks, but then I don't make it a point to get too intimate with those which have met a bloody and violent end.

 

[Mr. Sharkey]

Your description of the pickup is essentially correct. The point is to induce a signal that is quasi-sine wave and directly proportional in frequency to the speed of the gear teeth. The voltage level of the signal coming out of the pickup is irrelevant as long as it is sufficient to excite the input of a simple frequency-to-voltage converter. In fact, an accurate frequency conversion circuit will correctly ignore voltage level variations in the pickup circuit, within a certain range of operation.

The DC output voltage of the frequency-to-voltage converter is directly proportional to the frequency of the pulses going in. This makes it easy to connect it to an analog meter movement of the type that instrument panel tachometers commonly use.

Fortunately, there is a $2 integrated circuit that does just this and uses a minumum of external parts. Here's the circuit that I designed and built for my diesel:

Note that this circuit has been designed specifically for my engine's sprocket teeth and sprocket speed, and calibrated to a VW GTI tachometer movement. Different tooth counts, rotational speeds and meter movements will require adjustments to the indicated component values, particularly the 1uf capacitor, 15k resistor and the 220 ohm resistor in the meter circuit. Google around for a data sheet for the LM2917 IC and you'll get a lot if info to help build a circuit that is specific to your needs.

[edit] Actually, I just looked on my hard drive and found the applications sheet for the LM2917, so here it is:

http://www.mrsharkey.com/d2gfx2/lm2907_apps.pdf

I also have the full data sheet if that's desired (it's more than twice the file size). [/edit]