Engine only starts with direct 12v to coil

[db7gtgrigio]

its fixed!

problem was that the coil feed was attached to terminal 4 (5) which is isolated during starting. Its been wired this way for 15years and was starting because of the momentum of the spinning engine firing when key released and 12v to coil restored. Te reason I had to attach a direct feed to coil appears to be that the 9 year old battery, despite showing good on an optimate charger, was on its last legs and not spinning engine fast enough

I note however that all the wiring diagrams I have seen show incorrect terminal wiring for the switch. I found the attached from Jag Lovers which I can confirm is correct. I have re-installed the original 39679A switch which I erroneously believed to be faulty but I can confirm that the Lucas 39415 is configured exactly the same way and works fine.

There are 2 wires attached to T2 - 12v from battery and 12v to starter solenoid.
There are 2 wires attached to T3, one of which is in fact 2 wires into 1 connector , so in effect 3 wires. These are attached to fuses 6 and 7 - coil, fuel pump instruments etc
There is 1 wire attached to T1 - starter relay
Nothing on T4 unless you have accessories

This is not what the wiring diagrams show.

I hope this helps - thanks to all those who provided assistance

[DWW]

Kudos for the application of logic in combined with common sense

[mgcjag]

Hi Chris...Great that your up and running....so if I'm reading this correct in all the time you have had the car it's never fired whilst cranking...but only fired as you released the key.....obviously becoming more noticeable with the flatter battery....Steve

[db7gtgrigio]

Hi Chris...Great that your up and running....so if I'm reading this correct in all the time you have had the car it's never fired whilst cranking...but only fired as you released the key.....obviously becoming more noticeable with the flatter battery....Steve

I never really thought about it or noticed it but I have never had the ignition switch off in my ownership so I suppose it must have been the case

[Series1 Stu]

So it was a failing battery that highlighted a wiring error. Who'd have thought it?

Regards

[MarekH]

So what you appear to be saying is that you haven't been charging the coil whilst cranking. That's either because the switch internals are dirty or corroded and were only contacting the brown to white/yellow when "on the edge", rather than when fully in the start position, or because of a wiring error/fault.

It's a bit harsh to say the battery is bad if it is good enough to crank the engine. The bit about not cranking fast enough isn't accurate - you should be able to start the car long before the battery depletes if you actually charge the coil during cranking, rather than in the few milliseconds as the cranking is slowing once the key is released.

kind regards
Marek

[db7gtgrigio]

It was a wiring error. The wire connected to the coil was on the accessories terminal which is isolated during cranking. This meant that there was a 12v connection to the coil when in position 2 on the switch (ignition on) but when turning to position 3 (start) the 12v feed to coil was turned off. For years the car started, which I can only surmise was because of a strong battery providing plenty of spinning momentum to allow the engine to fire when key returned to position 2 and 12v feed to coil recommenced

As battery declined (it was NINE years old) the car would not start until we attached a direct 12 v wire from battery to coil.

with switch wiring corrected (as I note above, the wiring diagrams are wrong), and a new battery, the car starts almost immediately cranking commences. Even though it is 9 years old, I suspect as you say that now I have corrected the wiring the old battery would still start the car, but at nine years old I felt I had got my money's worth