Tacho tech issue

[rovamota]

I have a rev counter which was taken from an Allegro fitted to my 1980 Princess 2000. It is wired via earth, ignition and straight to the LT side of the coil and works perfectly well and has done for a few years. However, the car refuses to start from cold with it connected. I have to disconnect it from the coil and the engine will fire with the first flick of the key and once it's running the tacho can be reconnected and it will work perfectly well. The timing, ignition and carbs are all tuned and working properly. I have seen some wiring diagrams showing a fuse in-line on the LT wire to the tacho. Not sure that would help.

 

I am totally perplexed by this problem, but then most things in life perplex me and I don't even know what perplex means.

[Tamworthbay]

A fuse wouldn’t mate any difference but a diode could. One of them perhaps?

[AMC Rebel]

An inline fuse won't help.  Wondering if either the Allegro to the Princess used ballast resistor in the ignition and the other one didn't?  That might explain the different behaviour on start vs run depending on how it's wired.

 

Also wondering if one or other had/has electronic ignition?

 

We need a bit more info.

 

Also outside chance tacho is faulty of course.

[rovamota]

Yes, Accuspark electronic ignition and coil are fitted.

[AMC Rebel]

I think we need a diagram of how it's wired and pics of the tacho if poss - do you know if it's an RVI tacho? (it may be printed on the dial).

[rovamota]

It's a Smiths tacho - standard fit in Allegro 1750. The red wire goes to coil, blue to ignition, brown to earth.

 

 

*The wiring has been tidied up since this 2015 photo.

[AMC Rebel]

How is it wired at the coil end?

 

(feel free to tell me to sod off with all the questions but I doubt we can figure it out without all the info).

[rovamota]

It's a spade connector direct onto a spare LT side coil connection.

[Rocket88]

Does Sir Vulg [or anyone else] have a spare tacho you could try?