classic mini tachometer converter

Hi I have just registered on the forum in need of some advice/guidance. I have found the information on here very useful in the past but I can't seem to find the answer to this particular problem. I have a classic mini cooper mpi that has fuel injection using siaemse ports, so 2x injectors across 4 ports and the coil pack is wasted spark. I would like to fit an aftermarket tachometer but I am struggling to find something suitable and I like a challenge! I would like to build a circuit to convert from the ECU frequency output to analog voltage input so it can be used with a conventional tach measuring voltage (-ve). I found this information about the ECU output signal. Its a Rover mems 2j ECU. "The signal output, derived from the CKP sensor, has two pulses per engine revolution, at a frequency range of between 0 and 267Hz (0 to 8000 rev/min) and a duty ratio of 50% ± 5%." Would the use of a LM2917 in some sort of circuit allow me to convert from digital pulse to analogous voltage? I found this topic that is talking about the similar thing that I am trying to achieve although my output is 2 pulses per engine revolution. https://www.the12volt.com/installbay/forum_postsm.asp?tid=110209&tpn=1 I am perhaps being a little ambitious with this as I am no electronics expert. I can follow a digram and build circuits but I couldn't design one. I could buy a tach that's converted already but they are quite expensive and not much sense of achievement. Any help would be appreciated. Thanks. Luke

Conventional tachos are NOT voltage - they are frequency - ie pulse - eg, points or ignitor grounding outputs; 1 pulse per crank rotation per two cylinders (for a 4 stroke engine). Otherwise you are correct, and LM2917 is the prefect solution... Wasted spark should mean a normal tacho tho you may need two diodes if it uses two twin-spark coils - ie 1N400x - I'd suggest 1N4004 or 1N4007 as they are the now commonly manufactured version and suitable for all 12V applications including relay spike suppression/protection. The diodes would be on the tacho output with diode lines towards the coil ignitors (unless it's a CDI). Distributor or crank sensors are another source but the pulses per rev can depend on the particular method/system used. Same too for injecttors - some systems fire at fixed intervals during startup and some fire multiple times per cycle (2 revs) - eg, 1, 2, or 4 for 4 cylinder 4 strokes.

Thanks for the reply. My mistake re: voltage output I was confused by seeing the -ve reference during my research. So the problem then is converting the signal to one that a conventional tacho will read accurately. If its 2 pulses per cycle I assume it would make a conventional tacho read twice as fast? I believe the signal is sensed on only one of the ignition coils if that makes any difference?

Actually I got that slightly wrong. It reads off one coil so rpm would need to corrected to twice what is sensed to show true engine rpm. Is there a simple circuit I could build to change the sensed pulses to twice the original I.e doubling them to use an input for standard tach?

Yes - diodes as explained - one from each coil. A pull up resistor may be required, and maybe a series resistor if the tacho reads erratic. If it's only a single coil (which would imply a (spark) distributor), then a st'd 4 cyl 4 stroke tacho is fine. Another option for a two coil 4 cyl set up is to use an 8 cylinder tacho. Otherwise its often simplest to change a capacitor in the tacho itself to change the ratio - analog tachos are pulse to voltage (or current) converters.

Or tag one of the uncommon injector wires, straight to the meter.

oldspark wrote: Yes - diodes as explained - one from each coil. A pull up resistor may be required, and maybe a series resistor if the tacho reads erratic. If it's only a single coil (which would imply a (spark) distributor), then a st'd 4 cyl 4 stroke tacho is fine. Another option for a two coil 4 cyl set up is to use an 8 cylinder tacho. Otherwise its often simplest to change a capacitor in the tacho itself to change the ratio - analog tachos are pulse to voltage (or current) converters.

Sorry I should have been more specific. Its a two coil 4 cylinder engine. 1&4 are shared and 2&3 are shared. The wiring schematic for the ECU to ignition coil/tach is this; So what I am getting confused about is how I use the diode method if I only have a signal wire from one of the coils (Spark A). Do I tap into C197-2 WS wire? or could I make a circuit that takes the Spark A signal and simply doubles the pulse and outputs to Tach? Thanks for everyone's help btw. Luke.

Again, tag an injector wire. How do you think we pick up tach on remote starts?

howie ll wrote: Again, tag an injector wire. How do you think we pick up tach on remote starts?

Sorry I missed your reply when posting my last one. I have never thought about remote starts so I wouldn't have known the answer to your question. Surely I am still going to have the problem with the signal reading wrong. It's 2 injectors for 4 cylinders (siamese ports) so two pulses per engine revolution making the tach read double the speed.

Assuming the IgCoils share a common +12V and hence are GND switched (from ignitors, ECU, points etc) then diodes with their lines towards each trigger (ignitor etc) with other ends joined to the tach input. Add a pull up resistor (maybe 10k) to tacho +12V if the tacho isn't registering. Add a series resistor (maybe 1k) if the tacho is erratic.