Most bizzare ignition fault...

[sparky_spit]

...that I've ever come across.

A few weekends ago Scrapman and I did the Bedford CC treasure hunt event, and on the way there the car (Mk3 Spitfire) was misfiring on acceleration and, just as we got to the start, we noticed it was not charging properly. I changed the coil with another one cable-tied on top, which "cured" the misfire and we decided to do the event anyway just on the battery as nobody had a spare alternator. The battery lasted the 4 hours of the event and the 2 hours home with no problem.  When I checked later on, one of the alternator brushes was worn down to almost nothing.

Today I put another alternator on and put a new coil on properly (having tested it last weekend on another vehicle). The car wouldn't start. I immediately put the old coil back on and it wouldn't start with that either. I then checked everything I had done and found nothing amiss.

I noticed that after cranking it for a while it would sometimes start just as the ignition key was released back to the normal "on" position, but would never fire whilst cranking. So, I though the ignition switch had gone faulty. I checked the voltage at the +ve terminal of the coil and it was fine with the ign switch on normal and when cranking; so not that then. I then changed the coil for a 3rd one but didn't bother to reconnect the rev counter wire to the coil, and it started first time. Connect the rev counter wire and it went back to only starting sometimes as you release the key from cranking.

Although this is a Mk3 car it has a 1500 rev counter fitted. Testing the rev counter with a meter shows no low resistance between its coil and 12v contacts; no shorts to earth, and just a small kick when you reverse the meter polarity; so there must be a capacitor in the input circuitry I guess. One thing I did think about is that when it lived in a 1500 it would have had 6v pulsing on its input wire (as the 1500 runs a ballast resistor) wheras it now has 12v in this car. Would that have damaged it?

With the engine running, you can reconnect the rev counter wire to the coil and it works fine, the engine stays running fine, and the voltage at the coil terminal is the same as when it is disconnected. But you can't start the engine with it connected. I've even changed the condensor as this is electrically connected to the same point, but with no effect. Bizzare.....

Maybe the fluctuating charge voltage as the old alternator was dying (we could see this on the ammeter) killed the rev counter's input circuit? But why then does it all work once the engine is running and the wire is reconnected? And why do I not see a drop in coil voltage with it connected? Any ideas?

Maybe I just ought to stop therorising and go and see TheScrapman and borrow another 1500 rev counter and temporarily connect it up (this time with a 3 ohm resistor in the lead) and see if that fixes it.  This is certainly an odd fault though; it was only by chance that I disconnected that rev counter wire, it's not something you would expect to be at fault when diagnosing an ignition problem.

[Nick Jones]

Had similar on a hillclimb Mini many years ago.  The tacho was some smallish diameter Smiths RVC from I know not what.  The Mini was ALWAYS a bugger to start until we discovered the cause by chance one day.  It also had a persistent and previously uncurable misfire at very high rpm (7500+) which magically went away as soon as the tacho went in the bin.

I've never been able to find a decent circuit diagram for the RVC tacho but I suspect that there is a DC blocking capacitor and this sometimes goes well out of range due to old age.  This could mean dumping more voltage at low frequency (cranking) than at higher frequency (running).  Best try another tacho.  I doubt the voltage matters - the input will actually be seeing 40+v spikes with the engine running.  Never had a any other problems running RVC tachos with 12v coils.

Nick

[don cook1]

A few years ago I knocked a wire off the ballast resistor so that it only got power when cranking, when it would start, as soon as it stopped cranking it stopped; took me the best part of the afternoon to work that one out!

[steve_1360]

A friend of mine had a very similar fault, it would start at idle but as soon as you touch the throttle it stopped. It took a while to find that the points had partial melted, as soon as the revs increased the points stop closing and the engine stopped.