Ignition coil 'hot' wire

[minisweeper]

Hi (again!),

Today I thought I'd embark on the challenge of fitting my new gadget to the westfield.  One of it's talents is to measure RPM to control a shift light which requires me to splice into the ignition coil 'hot' wire.  Now i've worked out where the ignition coil is (cylindrical thing next to the fuse boxes, right?) and there are the following connections on it:

1 terminal with 1 wire coming out marked +

1 terminal with 2 wires coming off marked -

A main centre plug (goes to where the spark plug wires all connect to (distributor?))

The gadget needs a connection splicing into the 'hot' (as described by the gadgets manual) wire of the ignition coil which connects the ignition coil and the ECU.  I've tried splicing into all 3 wires, one does nothing at all, the other two give me a stupid reading (about 13krpm idle, dropping when i put my foot down!) (once it gave me what seemed like the right reading but the idle reading seemed a bit high then when i turned the ignition off and back on it gave no reading at all).

Hopefully all this makes sense to someone out there.  If anyone could try and point me in the right direction I'd much appreciate it!

[DickieB]

The hot wire to me would imply the +ve feed to the coil (ie your + terminal).  However rev counters normally work off the -ve terminal (ie your - terminal).  No need to splice them to anything, just put a ring terminal on the end and attach to the coil terminals.

[peterg]

Does this 'gadget' have the facility to be configured for differing numbers of cylinders? If so, the 13k rpm may have been 'cos it thought it was a single cyl engine or similar....

Most shift lights have jumpers on them so they can be set for number of cylinders as well as the shift point    May be worth a call to the makers if that's an option  

[Neil Purdie]

1 terminal with 1 wire coming out marked +

1 terminal with 2 wires coming off marked -

A main centre plug (goes to where the spark plug wires all connect to (distributor?))

I've tried splicing into all 3 wires, one does nothing at all, the other two give me a stupid reading (about 13krpm idle, dropping when i put my foot down!) (once it gave me what seemed like the right reading but the idle reading seemed a bit high then when i turned the ignition off and back on it gave no reading at all).

DONT try again connecting onto the coils center plug (coil to distributor 7 or 8mm silicone cable)  the voltage from this is enough to jump the gap at the spark plugs, a few thousand volts.  This will probably blow your new toy or has potential to give you a sore hand at the very least.

[minisweeper]

Haha.  Sorry: when i said 'all 3 wires' I meant the 1 off the + terminal and the 2 off the - terminal.  I didnt touch the centre wire (splicing bracket was too small anyway but i got the impression that wouldnt be the right wire).  Thanks for the heads up though

I've finally got it going again just hooked onto the - terminal of the coil.  I'll have to make a clip or something for it but for now it's doing the job - in the end I just had to fiddle with the configuration settings (thanks for the pointer peterg).  

It's certainly pretty spot on with the idle speed (~1000rpm), but as i go higher up the revs the difference between the gadget and the rev counter increases quite exponentially.  I'm assuming the rev counter is out (seems most likely to be wrong as I've never been too confidant in it - i can rev right off the scale).  Does anyone know roughly what the red line should be on a Xflow engine?

[minisweeper]

Yeah the tacho on the dash is definitely fubar.  Just took the thing for a quick spin and while the needle on the dash was past the scale (which runs to 7krpm) the gadget was reading 6500 (i think, was trying to pay attention to the road too!) when it felt like the limiter was kicking in (which seems about right to me).  It also said i was using about ~3krpm at ~60mph as opposed to the 4.5krpm the tach was suggesting.

Any clues how to correct the tach?  RTFM?