Grant, the tacho does the full deflection thing when the ignition is initially turned on, but is synchronised with every misfire.

I tried to run a direct 12v source to the positive side of the ballast resistor, but it didn't make any difference? I'll go through the procedure again but this time direct from the battery to avoid any issues.

The first time I did this, I took the feed direct from the input side of the No1 fuse (where the tacho gets its feed from anyway). It's not the only thing that's powered from this fuse. Instrument power, horn relay, ignition protection relay and fuel pumps also come from the same ignition source, although these would be a lot less susceptible to the odd short?

I'll let you know how I get on.

 

So this morning I ran a direct 12v feed from the battery to the input side of the ballast resistor. No change? I then ran a new wire from the coil(-) to the Dizzy. No change. Both times I still had Tachometer deflection as soon as I switched the ignition on.

I had tested the starter circuit that provides 12v direct to coil for starting only and the voltages were all good. I then realised that if there was an intermittent short on this 12v once the car had started, it would earth the coil and cut the ignition. I decided to run a new wire to the ballast resistor and coil(+). I turned the ignition on and eureka, no tachometer deflection!

Process of elimination.

Thanks for the help along the way.

 

Fair Dinkum.

That is one serious FEMALE ca tehre.

What we drinking, I got a Canadian Club, headache coming.

 

Now this is this is the really weird bit. With the problem sorted, I head into town with a mate, leaving the Jag under cover, but no rain expected, so all good. When I come back, I think that I should tidy up the additional cable and ensure that nothing is going to rub/run on anything.

I then turn on the ignition again and get full Tacho deflection. The car runs great and confirmed during the drive, with none of the original symptoms returning. So I'm still calling the initial problem solved, but trying to work out why there is still a fault of some description.

Ultimate outcome is the same as the car runs great!

 

My thinking is a tad "fuzzy" on this issue. But, I do have an idea or two.
One based on a past travail with craning and other issues, similarly "unrelated".
Not quite. The ignition switches in our cars are multi tasked. Not only to provide V's to crank and run the engine, but for others! And, although most have served well for multiple decades, they are a tad "puny" and become errant. Mine did!! A good used one was much tighter and solved at least two mysteries??? No stop lights and radio drop outs!!!


Tachs have served as indicators of ignition flaws since the earliest electric units.
Not only on the - side, but the + as well. Takes a complete circuit to indicate pulses as well as fire the plugs!!!


And, there was a time that the little transistor radios did something like that.
Tune to a silent point. Fire the engine. A steady buzz in the radio meant all was well in the ignition system. Irregular buzz meant skips or misses!!!!


I've not tried it, but I suspect a dwell meter would detect a flaw in rthe puilses of the ignition!!


And, an inductance type timing strobe.


If your car has points and condenser, you do need the ballast resistor. If electronic, not so much.


If it has to be there, the things convert V's to heat. That tells me that they can "wear out". A fresh one might do a lot.


Sum up:


1. Worn ignition switch?
2. Worn ballast resistor?


Carl.

 

Carl, thanks for the advice, both sound reasonable to me. I've read a few threads now where the ignition switch has been at fault or contributed to it. Fair wear and tear over 42 years....

 

I went for a cheeky little Shiraz!

 

The car is still running great. Pick up is fantastic and fuel economy is.... .....exceptable!

The problem I saw today was a Tachometer sticking at 3500 rpm? No change to running conditions.

 

I reckon you got a tacho finding an odd earth.

The tacho feed as we call it, is via the neg of the coil, as is the earth of a points system, soooo, that is the earth of the tacho, with an ign feed being the other connection/path.

Like the oil sender, earth that wire and the gauge goes full scale. The sender varies the amount of earth, and that gives us the "warm and fuzzies", coz we got oil pressure. Other connection to that gauge is ign feed.

It may be inside the tacho, or the cars wiring, so drink some more and go looking.