Sparking problem

[Bigfish_74]

Any ideas please. Irregular spark on all plugs timing gun either doesn't flash or irregular, varies between plugs. Replaced all ignition over last 2 weeks to try and resolve, new distributor, leads, coil, plugs. Timing checked and ok, good earth to engine block, 12v at coil. Running out of ideas so thoughts would be greatly appreciated.

[Bigfish_74]

350i

[Bigfish_74]

Reading of 10.5v across the coil with that test. 12.5v at the battery. No strobe on the king lead although I have had previously but maybe not since I put the new coil on yesterday. Gun checked and working ok.

[Bigfish_74]

Job for the morning, thanks for the help

[Bigfish_74]

Tried to diagnose the voltage drop by disconnecting all from the coil other than the 12v ignition supply. So a direct earth to battery from coil negative and the single white wire from ignition which has a good 12.5v are the only connections but still get the voltage drop across + - to 10.5v. Also checked with the old coil and the same result, are we sure the coil isn't meant to drop the voltage under load?

[Bigfish_74]

You've convinced me, I'll work it back, maybe a fault at the relays. Thanks again.

[Bigfish_74]

Headlight bulb test, no discernible difference in brightness on the ignition feed/ direct to battery. Working through the fuse feeds visually checking connections and wires for breaks, any ideas how to better test without ripping the loom apart / rebuilding it?

[Bigfish_74]

It won't start, thinks about it but doesn't quite get there. As I've said replaced coil, dizzy, module, leads over last couple of weeks and made no difference so was thinking we must be onto something with the wiring as there's not much else left to try. What's baffling is you can get a spark laying the plug on the block but as soon as you plug spark back in the timing gun often won't flash at all or very irregular. It's had work on the earth's and all seem good.

[Bigfish_74]

I thought that but didn't like the idea of heating the coil up again and again. I had done a direct test to the coil and no drop, so as you say I do need to locate where the drop is. Just rebuilding the ignition relay as that was a mess.

[Bigfish_74]

New Lucas coil, shiny Ali, good earth braid from bracket to battery. I'll spend a few days testing all ignition wiring, it may or may not be the spark problem but does need fixing anyway. I'll post when I get further. All advice has been greatly appreciated.

[Bigfish_74]

Yes hoped it was the gun but works fine on other cars, also tried the bulb that connects to the spark plug which does give better results but still something not right. Still pursuing the wiring.

[Bigfish_74]

Penelope Stopit said:

When having 12.5 volts @ the battery and 10.5 volts at the coil

What voltage is there at the ignition switch supply in and ignition out?

[Bigfish_74]

Penelope Stopit said:

When having 12.5 volts @ the battery and 10.5 volts at the coil

What voltage is there at the ignition switch supply in and ignition out?

I think I've narrowed it down to the white supply from ignition switch to ignition relay 1and 2 ( different wiring to yours, maybe later 1988) but need to get the dash off as wire seems to reduce down from switch to relay so a join somewhere behind there. Just struggling for time on it with job, home tutoring and sunny weather

[Bigfish_74]

Bristol ave fag said:

From what you see it does suggest the high tension is low have you got another amp you could try it takes alot of stick to switch and pull the primary down and if its fitted remote it must ave a good earth on the back casing.

[Bigfish_74]

Bristol ave fag said:

From what you see it does suggest the high tension is low have you got another amp you could try it takes alot of stick to switch and pull the primary down and if its fitted remote it must ave a good earth on the back casing.

Had 2 new Amps on, last one along with new dizzy, back to the original at the moment but not ruling out there being more than one problem and may well be swapping them around again.

[Bigfish_74]

Bristol ave fag said:

Ok but you have checked the feed at 12v then you earth the coil. and measure it at 10.5v across the coil poles. disconnect the earth and it returns to 12v. It is the coil that is consuming that voltage as it should do, that is creating the load itself. There is no problem with the feed.

Fair point, but If it's not a wiring problem, I'm back to swapping out parts that I've previously swapped and nowhere to go. It's a real bamboozler this one 😵

[Bigfish_74]

Hi Guys, sorry not been around for a while as didn't realise the debate had continued. Update, I found a hot wire to ign fuse that I replaced also rebuilt a couple of relay connections that were iffy, still only 11v with coil earthed. Spent time swapping around amplifiers, coils (yes it is a12v coil) plugs etc. The new amp had packed in so back to the original but I now have a good spark at all plugs earthed to plenum and with the inline spark tester. Still the anomaly with the timing gun but put this down to a poor quality gun. Unfortunately for the debate can't attribute it to any one thing. Managed to set the timing as gun works with plug earthed to engine.
Car still doesn't start...... Now also seems to be a fueling problem (previously had been fueling so not sure if I'd fried sonething) been testing other components and I'm only getting 0.75v at the throttle pot and AFM, articles say it should be 4.5v, ECU presumably drops the voltage so I sent both ECU and AFM for rebuild (everything else on the car has been rebuilt so what the heck) to ATP but still no fuel. Currently speaking to ATP to find out the results they got, will let you know and will start a new thread as this is long.

[Bigfish_74]

Your are very wrong, incredibly concerned about how much I'm spending on this bloody thing, but I'm commited now. Yes I will continue to search for the missing voltage (could it be the ignition barrel)?

[Bigfish_74]

Penelope Stopit said:

The above influenced my thought process

I wish your car was here by me now, would gladly fix it for free

There are always question marks about what's fitted to a vehicle that's not within reach

First prove that there is definitely a relay supplying the coil

If there is a relay supplying the coil, the answer to your question is - No, any ignition switch volt-drop will not be causing the volt-drop to the coil, the ignition switch will trigger the ignition relay and that same relay will supply the coil

There could be a problem with the supply from battery positive to the relay, at the fuse holder, from relay to rev counter if wired in or from rev counter to the coil positive

I have a hunch that the positive from the battery to the relay could be connected to the positive battery terminal or close by

The problem is that a low voltage supply to a v8 engines coil is a proper pain, the coil is so busy (2 x 4cylinder work load) that it needs to have a good voltage supply to build up the magnetic field as quickly as possible

You can easily verify what I am now posting and have posted to this topic by contacting one of the following

They are very helpful

In no particular order

spitfire4v8 https://www.pistonheads.com/gassing/profile.asp?h=...

Zener https://www.pistonheads.com/gassing/profile.asp?h=...

blitzracing https://www.pistonheads.com/gassing/profile.asp?h=...

If you need any advice in carrying out a wiring job to improve that supply voltage, please do feel free to ask

No offence taken stopit just don't suggest to a Yorkshireman that money is no concern🤣😂
Although spark is now ok I too think there is a fundamental fault with the wiring that is also causing the ECU to drop voltage to t pot and afm

[Bigfish_74]

Adam, yes in my opinion the drop in voltage wasn't causing the original spark problem, it now reads 11v under load, had a great run yesterday and all fine and no fires yet.
As an aside I did lose it again last week it seemed to be flooding after running for short periods. Changed plugs and all good again, so looks like plugs deteriorated quite quickly probably lots of overfuelling while solving the problems.
Enjoying the wedge at last!