Mk2 rs2000 tacho testing

[Andywellon86]

Have got hold of a rs2000 dash and put it in my car but everything works apart from the rev counter/ tachometer. It's a lesser spec escort so have swapped all wires according to diagrams on here ect and just can't get the tacho to work (everything else works as should). Have ran a wire to the coil negative as needed. Putting a volt meter on the wires on the small plug, the 12v power is reading 12v and so is the negative wire I've ran (I'm not sure if it's meant to) I'm wondering if there is something I've missed/done wrong or a way of shorting it out/by passing it to test or that everything is correct and it just doesn't work. I'm not auto sparky so would really appreciate the knowledge of someone who'd had issues/ had the same problem and solved it. Much appreciated if you can help. Thanks

[Miniliteman]

Have you checked the circuitboard (foil) ?

[Andywellon86]

For breakages? I've looked and had a light that wouldn't work due to it being folded over. I've not notice anything out of place on the tacho side. Nothing looks stuffed when I've taken it apart to look at the electrics at the rear. Maybe I need to just send irbid for repair and see what they come back with.

[telf]

Have you got a spare rev counter to try that you know to be working? Would give you a start in the right direction.

I think the negative wire from the coil should be giving a negative reading on your volt meter with respect to earth (cars chassis).

[Miniliteman]

I think the negative wire from the coil should be giving a negative reading on your volt meter with respect to earth (cars chassis).

Not correct. With a points distributor the voltmeter should read 0 - 0.5 Volt with points closed or +12 Volt with points open.

[telf]

Not correct. With a points distributor the voltmeter should read 0 - 0.5 Volt with points closed or +12 Volt with points open.

This is more for my own understanding....

From the negative side of the coil you will get +12V to earth? So the negative side is just a reference not an actual indication of polarity? It could of been called '2' for example and the positive side '1'?

[Miniliteman]

Yes and no.
With the points closed current flows through the coils primary winding from + to -. (definition)
+ is at +12 Volts (or 9 Volts if there is a series resistor), - is at 0 - 0.5 Volts.
With the points open there is no current through coil and with no current the - side of coil is at the sam voltage as the + side of coil. With no current through series resistor the + side of coil is at +12 Volts so is the - side.
The construction of a coil is such that the - side of the coil is connected to the primary winding and the secondary winding.
When the points open current stops immediately due to which the voltage at the - side of coil and also the HT side of coil (other end of secondary winding) goes through the roof which creates a spark.
If you would connect a coil the other way around you would still get a high voltage peak (spark) but it would be a negative spike and you would get interference on the battery + side.

[Andywellon86]

Only just had time to get back on here. Thanks very much for your info. I'm tucked away in a far corner of Australia so parts arent too easy to come by but have a tacho in the post that is believed to be working. In the mean time I can turn the engine over to open and close to points on the dizzy and check my readings on the wires. Hopefully it's just a faulty tacho but your information has enlightened me to the actual workings of the coil ect so thank very much. Il keep you posted!

[telf]

Good bit of information there. Thanks for taking the time to write it all