Classic cars forum & vehicle restoration.

[MikeEdwards]

I wonder if anyone can shed some light for me please, I'm trying to improve how my car runs and came across something I am struggling to understand.

It's a seventies Vauxhall, so it has a ballasted ignition system, originally using a resistance wire in the loom, but I bypassed that to use a ballast resistor on the side of the coil. The loom seemed to be getting a bit warm, so this was mainly for peace of mind. I realised this year that I hadn't done anything about the starter resistance by-pass, so I ran a cable from the solenoid to the second terminal on the coil.

I connected the car to a Crypton machine, and ran through the basic tests. Key on shows coil voltage of around 6v, and trying to start shows a coil voltage of around 8-9v (this is what showed me that I hadn't done the stater bypass circuit). When the car is running on the original, Lumenition-equipped distributor, the coil voltage shows 9-10v, and I don't know whether that's correct or not.

As an experiment, I've replaced the distributor with a spare which doesn't have Lumenition, just uses standard points, set it all up (first time I've done anything with points) and got the car running, and the only difference now is that the coil voltage when running is higher, more like 11-12v.

So the question I can't seem to find an answer for is, what should the coil voltage read when the engine is running? The Crypton book doesn't mention a value, doesn't even talk about testing it, so perhaps if the ignition and crank values are correct, it must be OK. I am just a little worried because the suggestion for a ballasted system is that the coil should normally get 6v. Is this some kind of feeback from the HT side of the coil? The coil doesn't get warm, in fact after about 20mins of fiddling around with the idle mixture and a Colourtune, the coil was cool to the touch.

So, any ideas anyone? And while we're at it, if I have the timing light pointed at the crank, when I rev the engine, why does the timing retard before it advances?

[ukdave2002]

Hi Mike

For a ballasted system , you should see 5-6V on the coil when the engine is running normally and a minimum of 9v when the engine is cranking. On a non ballasted system, with the ignition switched on, the coil voltage should be within 0.5v of the battery voltage.

The timing "retard blip" you see is the short amount of time that the distributor advance mechanism has to overcome inertia.

Cheers

Dave

[MikeEdwards]

Thanks Dave. So I don't know where to look now, to figure out where the "extra" voltage is coming from. Removing the starter bypass wire doesn't help, so I know that isn't staying live, unless there's something wrong with the ballast resistor itself.

Those pages are the same as the ones in my Crypton book - trouble is, it pointedly doesn't mention what should be showing when the engine actually starts up.

The retard thing is interesting - I was wondering if it was something along those lines, but you never hear anyone mention it. I've disconnected the vacuum advance because the Dellorto manifold takes it only from one inlet, so it doesn't work properly. So I've tried to compensate by adding a bit of extra static advance. I thought I'd sorted it when I found that the advance bobweight springs were very slack, but I replaced them and it didn't help much.

[MikeEdwards]

I have an idea that I've figured out why no-one talks about the coil voltage when the engine is running.

I turned the key to test again today, and noticed that when the points are closed, it shows around 6v which is what you'd expect from a ballasted system. If I do the test when the points are open, however, it shows around 12v.

Assuming the above is correct, then the reading when the engine is running will be nonsense as it will flicker between 6v and 12v as the points open and close, and the meter won't keep up.

[47Jag]

MIke,

It would flicker at more than 12 volts if it could keep up. From memory it goes up to about 30 volts in diminishing oscillations due to the back EMF (voltage) from the secondary collapsing. If the condenser is doing it's job it should only oscillate about 3 or 4 cycles. I've never used a Crypton but I have used an American Sun and it displays the primary circuit along with the dwell. Why are you concerned about this reading?

Art

[MikeEdwards]

Hi Art,

My concern about the reading was because everyone talks about not running the coil at more than 6v, and there we have a meter that seems to be showing that I'm running it at closer to 12v. My concern was that I'm going to drastically shorten the life of the coil. It's only been on the car 15 years, I haven't had my money's worth yet!

Your comment about the back EMF from the coil is kind of what I was imagining - I don't know how these things work (my Physics a-level was a long time ago, and I don't think I passed it), but as I'm only putting 6v in, it just couldn't be coming from anywhere else. It's good to know that there's some truth in it, even if I plucked it from thin air. I thought there had to be a reason that the manual doesn't mention it, though I'd have preferred a note to say "don't worry about the voltage when running as it's meaningless".

I don't have a condenser now, I've swapped back to the Lumenition distributor. I'd gone to points to see if a different set-up altered the retard effect that Dave explained, and/or made it run any better. It didn't - not as much as some new plugs, a new rotor arm and and a new cap.

Thanks for your comments. I will ignore that reading now.

[ukdave2002]

Hi Mike

the back emf will be in the order of 100's of volts, this is why condensers in ignition systems have to have a working voltage of 600V. You can feel it if you put your fingers across the LT side of the coil, it wont do you any damage (honest ) as the current is so low, and the vehicle battery effectively acts as a regulator ..

Physics A level....that was all neutrons and maths shudder shudder!

Dave