classic mini tachometer converter

Re your earlier reply which I missed (we seem to be out of sync... ha ha), it's the same with spark. Injectors might fire as often as a spark, but some are sequential and some fire "wasted injection" or even every half rotation (ie, 4 cylinder). Some may change their squirts per cycle (eg, high RPM). And as I said, at startup they may fire at fixed intervals - eg every 12.5mS equivalent to 160RPM or maybe other intervals. Remote starts usually need a mere "is the engine running?" signal as opposed to accurately knowing the RPM as a tacho does. Certainly some vehicles use injector signals for their tachos - especially diesels - but they be a constant pulse number per cycle (excluding perhaps when cranking). I'd probably try a single coil source initially. To get the right pulse to RPM I'd first see if the tacho can be modified. It may be a simple as adjusting its calibration trimpot or changing a resistor (easy if you can switch select the number of cylinders or a capacitor. As to pulse multiplication, division is usually easy. Multiplication is tricky (read: PITA & complex).

Sorry Peter I was wrong and I take your point about tach sensing.

Fine, but it is IMO a valid - and oft overlooked - solution. (Especially when injectors are easily accessible... I know some IgCoils etc are a pain to get to!) PS - I added the last line to my last reply after you posted your reply.

Thanks for the suggestions. I have ordered a multimeter that will measure frequency so I can do some testing. Some info i've read about the coil pack suggests it has one wire for each spark A&B from the ECU (pin 1 & pin2) and a ignition live (pin 3) so hopefully I can just tap pin 2 which will give me the signal for the second coil. Then using the diode method to tie the two together as tach input.

For a 4 cyl tacho you'd want to diode both signals - eg, pin 1 & 2 or A &B (switched side). It should be the same diode connection to ECU pins 1 & 2 since ECUs/CPUs are usually common collector output - ie, 0V/GND when on, and not connected to 0V/GND when off. For the ECU any 1N400x diodes will do, but I still suggest grabbing 1N4004 or 1N4007 as they can also be used for relay coil spike suppression.

A quick update I have purchased a Smiths tach on eBay that will suit the look of my car but it is for a 4 cylinder positive earth and obviously not suitable for my car but neither would a neg earth unit without modification. I am going to be building the circuit found on this website LINK that will replace the circuit board in the tach allowing any configuration of input i.e. -ve from coil or ECU and it can be calibrated to suit my needs. I could have purchased a ready made conversion board for about £43 but that would be the easy route and I wanted to learn as much as solve the problem. I know that I haven't designed my own circuit but building one based on someone else's design will still be satisfying for me (if it works!) I will report back with how I get on in case anyone finds this post and wants to know for their own project. Thanks for all the help everyone. Luke.

Have a look at the LM2907/LM2917 Frequency to Voltage Converter instead. But have you confirmed it does need a pulse-ratio modification? If not, it should be simple enough to invert the signal (maybe just a cap, else a transistor). But as before, changing ratio is probably just a cap or resistor change. Of course old tachos often have failed diodes and (electrolytic) caps.

oldspark wrote: Have a look at the LM2907/LM2917 Frequency to Voltage Converter instead. But have you confirmed it does need a pulse-ratio modification? If not, it should be simple enough to invert the signal (maybe just a cap, else a transistor). But as before, changing ratio is probably just a cap or resistor change. Of course old tachos often have failed diodes and (electrolytic) caps.

I will be testing the signal hopefully at the weekend to confirm whether it needs modification. The main reason I have decided to go with a new circuit board is that the tach I picked up is RVI and will need converting anyway, so the circuit I linked will give me the flexibility to calibrate how I want. Thanks, I will take a look at the LM2907/LM2917 circuits before committing. Luke.

Bluddy RVIs! (I had to look up wth that meant, but then "ah - them!". Primitive things - good at the time - but my mid-1960s Jap cars had non-transformer types and so too my bikes - even my 1972 Ducati!) See dummett's electronic tacho driver using LM2917N. Ultimately only the moving coil (meter & needle) is of interest, all the rest can be ditched. I liked the LM2907 or more 'universal' LM2917 as they should have been adaptable for any situation (ie, universal), and (from memory) damping can be set independently of the actual meter used. Tho I designed the odd LM29x7 circuit, I never built one because I ended up using other tachos (staring with an $8 K-Mart tacho the replaced my temp etc combined "meter", to later 1980-1990 OEM tachos).