tacho cutting ignition?

[checkrail]

Hi all, I had the engine ticking over when it suddenly cut out and would not fire up, all the connections were fine and I swapped the coil over but no difference.
I checked for power at the coil terminals, with my test light and got 12v. But when I went to turn the ignition off the tacho needle had gone over to maximum,which returned to zero once I swiched off.
The engine would not then start.

Any ideas would be most welcome Thanks John.

[h20hamelan]

dynamo?
wire from tach to coil or key to tach
more likely, connection at tach

[Andy8421]

As with the apocryphal tale about ice cream sales and skirt lengths, correlation does not imply causation.

Sure, its possible that the tach has caused your problem, but it is unlikely. There are two types of tach - written on the front of the dial.

RVI tachs (by far the most common) don't have any direct contact with the ignition circuit, just a loop of the wire that supplies power to the ignition coil runs through a pulse transformer on the back of the tach. Tough to see how that could cause a problem - and you probably wouldn't measure +12V at the coil if there was a fault.

RVC tachs are directly connected to the wire that runs between the contact breaker and the coil, and potentially a fault in the tach could lead to your issue, but the circuit design is such that there is a large (10K ohms) resistor in the way that would have to fail to a short for your problem to occur (an unlikely failure mode). It could of course be a mechanical problem in the tach (a wire has worn through the insulation and shorted to the case or similar) but again, not a common failure.

edit: It is always possible that a PO has had the innards of the tach repaired, or replaced with modern circuitry, in which case the above analysis won't apply.

Good luck.

[checkrail]

Thanks for the replies, It's a RV1 tacho on a +2, I've checked behind the dash and the connections and the earths are in place. If I try to start, it fires, sometimes the tacho works sometimes not but everytime it will not rev and cuts out.
John

[mbell]

Do you have a ballast resistor car? If so likely the coil power for running is faulty but bypass is working.

[Hawksfield]

Hi checkrail
Try a +ve lead from the battery to the coil and see if it runs.
That will help to decide whether it is the tacho or something else

[512BB]

Sounds like a classic case of the condenser failed. Replace both the points and the condenser if you have them fitted.

Leslie

[checkrail]

The dizzy fitted is brand new bought from a trader on ebay, which I swapped over from the electronic one fitted.When I tried it the tacho did not work,spoke to the seller who said make sure the corrections are correct especially as its a Lotus! Later when I started the car the tacho was fine.
After all this trouble I having now,, I swapped back to the Accuspark electronic one and after slightly retarding it the engine ran up to idle at 2000 rpm which I quickly reduced.
So after all that the tacho and the coil are fine, Was it the condensor?

Thanks for your patience. John