Hello,

Here's an interesting one for you. I recently bought into the world of Datsun, a nice orange 1200 coupe and the tach wasn't working but it came with a replacement. I put the replacement in with no joy, still no tach action. So then I pulled the wiring loom apart and checked all the connections with everything going where it should. So then I thought I'd plug the tach in directly before the coil and start the engine and here it works.

The car has a performance coil and a wire wound ballast resistor on it and I can only guess that there is enough inductance in the ballast resistor to filter or smooth the current pulses through the rest of the loom enough that the tach doesn't pick it up. It's definately the ballast resitor since the tach works when plugged in after it but not before it. SO if anyone else out there is having a similar problem I hope this helps.

I can see a few solutions, use a few carbon film resistors to match wattage and resistance spec. and hope the inductance is low enough that it works. Use a high energy ignition that doesn't require a ballast resistor. Move the ballast resistor in before the tach and finally run two new wires back to the tach from the coil. I'm going for the carbon film resistors for now, 7x 10R 1W resistors in parallel gets real close to the 1R5 wire wond job. At 44 cents for two it's a pretty cheap fix. I'll let you all know how it goes.

If anyone else has been here maybe you'll have some advise or pick up on something I've missed.

Aftermarket tachometers get the signal from the NEG side of the coil

Stock Datsun 1200 tachometers get the signal from the POS side of the coil wiring and there should be a factory TACHO loop in your wiring harness (B/W wire).

See the TACHO loop wire in this Factory Service Manual wiring diagram:

I'm being slow with this one. TO clarify I'm dealing with the standard tacho which uses an inductive pick up to measure the current pulsations in the positive supply to the coil. I tried to get my tacho working by replacing the wire coil ballast resistor with carbon resistors without success. In the mean time I picked up a silicon chip high energy ignition kit which allows the removal of the ballast resistor as dwell is programed and compensated for rpm, voltage etc. to form a nice hot spark.

With the high energy ignition setup the tacho works between about 1500 and 3000 rpm, below this range it is dead and above it there is lots of noise. The High Energy Ignition (HEI) has a tach output which would be a nice clean square wave but the problem with this, as illustrated by ddgonzal is the standard tach measures the current pulse, not a voltage pulse.

I see two possible solutions. 1, buy an after market tach and a standard one that is broken and weave the two together, or, 2, use the voltage signal to drive a signal transistor to switch a small current through a coil wound around the standard tach pick up point where the ignition wire goes. For option number two I suppose you would need to know the current that the tach is designed for, then divide this current by the number of wraps in the coil and set the current by choosing a series resistance.

Has anyone else been here before? Any one know what kind of current signal goes through the positive ignition wire? My guess would be a square wave with smothed edges swinging between about 0 and 6 amps.

My tachometer stopped working when I fit the crank trigger ignition

So i'd like to know too how to make it work.

G'day,

I finally got around to finishing and testing my circuit. I replaced the single coil of wire that goes to the ignition coil +ve with 50 plus coils of fine enamelled wire. I found that a piece of 6mm (0.25") black pvc pipe fits over the tach pick up and is pretty easy to wrap the wire onto. Using a 56 Ohm resistor in series it will flow about 200mA at 12V giving a magnetic pulse equivalent to about 10 amps through the original setup. The Tach signal is used to switch a signal transistor that controls the current through the small coil. Tested it today on the Lancer tach signal (since my coupe's locked away) and it works. The design's not optimum and full of guesses but seems to work. It may need tuning, i.e. the number of coils, resistance etc.

Included are some pictures for others who may wish to emulate this idea to get their original tachs working off a standard voltage signal like you get from most ECU, HEI and so on.

Be sure to avoid any shorts and to mount the board I used an M4 nylon nut that fits loosly over the standard M3 screw that goes through the Tach body at that point. I also replaced the ~12mm M3 with a 15mm M3 screw to make up for the board and spacer in between. You cant see in the photos but the board also has a good whack of gascuit silicone on the back to stop it vibrating, it's also a good idea to use a little silicone on the 5W wire wound resistor to stop the legs fatiging off with vibration.

If anyone knows more specific information about the standard ignition current signal then it would be much easier to design the correct magnetic field strength. It would be pretty cool if someone has a standard set up with a current clamp and an oscilliscope, then the standard signal could be measured directly.

Good luck to anyone else who wishes to take on this relatively easy mod. I'm no electrical engineer but I can try help with questions.

Edit: The transistor in the after shot is underneath the board so you can't see it, it's only there because it got a little messy when I was prototyping.

Attach file:

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  TachCircuit.jpg (31.46 KB)


  Tach_Before.jpg (43.18 KB)


  Tach_After.jpg (47.61 KB)

I HAV STD COUPE TACO RUNNING ON A CA18DET
IT PICKS UP PULSE IN IGNITER SUPPLY WIRE[BLUE,RED TRACE]
WORKS FINE OUT 200 AT 7000 SHOULD READ 6800
CLOSE ENOUGH

I found problems with noise and hence irratic needle movement with my HEI coil supply wire and with the points and after market coil and ballast resistor nothing at all, I guess due to a kind of lag filtering effect smoothing the current signal edges which is what drove me to do this. The wiring loom has been fiddled with which may also effect the signal. But for sure, if it aint broke don't fix it!

ALSO HAV A UTE WIT TACO RUNNING E15 DIZZY NO RESISTOR
RUNS FINE ALSO
SOUNDS LIKE A LOOM PROBLEM

What would happen if you 'deleted' the positive factory wire feed from the tach, and replaced it (with loop intact) with the wire going from the ignition module to the positive side of the coil (or the negative side - I haven't slept since work last nigt, I'm getting dopey) - ran it from the module to the 'loop' in the tach then back out and to the coil - so in theory it would once again pick up the current pulses (which no longer 'show' on the original positive wire since it is now powering the module, not just to the coil, with the points switching it on and off like in the std one)

Sorry if that sounds like I've lost the plot, if it doesn't make sense, I'll take a look after I get some sleep after the next night shift)

Interesting idea.