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classic mini tachometer converter


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lukeh 
Member - Posts: 8
Member spacespace
Joined: October 22, 2014
Location: United Kingdom
Posted: October 22, 2014 at 2:07 PM / IP Logged  
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
oldspark 
Gold - Posts: 4,913
Gold spacespace
Joined: November 03, 2008
Location: Australia
Posted: October 22, 2014 at 3:45 PM / IP Logged  
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.
lukeh 
Member - Posts: 8
Member spacespace
Joined: October 22, 2014
Location: United Kingdom
Posted: October 22, 2014 at 4:23 PM / IP Logged  
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?
lukeh 
Member - Posts: 8
Member spacespace
Joined: October 22, 2014
Location: United Kingdom
Posted: October 22, 2014 at 4:32 PM / IP Logged  
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?
oldspark 
Gold - Posts: 4,913
Gold spacespace
Joined: November 03, 2008
Location: Australia
Posted: October 22, 2014 at 4:55 PM / IP Logged  
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.
howie ll 
Pot Metal - Posts: 16,466
Pot Metal spacespace
Joined: January 09, 2007
Location: United Kingdom
Posted: October 23, 2014 at 5:12 AM / IP Logged  
Or tag one of the uncommon injector wires, straight to the meter.
lukeh 
Member - Posts: 8
Member spacespace
Joined: October 22, 2014
Location: United Kingdom
Posted: October 23, 2014 at 6:00 AM / IP Logged  
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;
classic mini tachometer converter -- posted image.
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.
howie ll 
Pot Metal - Posts: 16,466
Pot Metal spacespace
Joined: January 09, 2007
Location: United Kingdom
Posted: October 23, 2014 at 6:32 AM / IP Logged  
Again, tag an injector wire. How do you think we pick up tach on remote starts?
lukeh 
Member - Posts: 8
Member spacespace
Joined: October 22, 2014
Location: United Kingdom
Posted: October 23, 2014 at 7:12 AM / IP Logged  
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.
oldspark 
Gold - Posts: 4,913
Gold spacespace
Joined: November 03, 2008
Location: Australia
Posted: October 23, 2014 at 7:18 AM / IP Logged  
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.
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