Low volts on ballast wire & misfire at speed

[Paul Dawson]

I've got a misfire at speed and under load. It clears up with a touch of choke, but then it's running rich. When I got the car the previous owner seemed to have been running it this way since the plugs were pretty sooty. Until last week I'd only driven it to the MOT place round the corner so I hadn't spotted the misfire, it was only when I took it out down the dual carriageway that the misfire showed

The plugs, points, condenser are all new. I am in the process of getting new HT leads. Fuel is low but is fresh, though there will be some old stuff in there from a couple of years ago. It fires first time, just misfires at speed.

But - just poking around looking at replacing the points and condenser with an Accuspark electronic module I got round to measuring some voltages. With the engine off, ignition on, the D4 connector on the fuse box (feed for the ballast wire to the coil) is only 4.8v. Surely even without the engine running that feed ought to be somewhere in the region of 9-10v? I am wondering if the misfire is due to the coil not getting a decent feed.

I was looking at bypassing the ballast wire anyway by moving the two wires in D4 to D5 as per David's thread on this very subject. The plan then was to install a 12v coil which is easier to connect to the electronic module since the two wires just go to either side of the coil rather than having to wire in a separate 12v supply for it.

Just thinking aloud really - hope this makes sense. Any comments or suggestions welcome.

[bedfordbad]

Sounds as if you are on to something there! What was the voltage at the coil with engine off and ign on...... mine with ballast wire was 8.2v and then 12.4v when bypassed for accuspark......

[Paul Dawson]

The voltage at the coil was the same, 4.8v.

Maybe there's an issue with the ballast wire - in which case all the more reason to bypass it. When I get time I'll get hold of a 12v coil and move the connection from D4 to D5. Presumably I could then disconnect the 12v feed wire (white) from the solenoid as it won't be needed.

[droopsnoot]

With the ignition on and the engine stopped (not cranking) your coil voltage should be around 6v, because the resistance wire is dropping the full 12v down to 6v for the normal running of the coil. While you crank the engine, the starter resistance bypass applies the full 12v to improve the spark, but the load from the starter means that you'll probably get around 9-10v measured at the coil. You need to check whether the points are open or closed, though, when checking the voltage.

I've been through a similar thing with mine just recently, luckily had the car on a Crypton tester which apart from anything else has much bigger readings than my usual voltmeter. At 4.8v yours is maybe a little low, but not wildly so. I would think having it showing ~8v for the normal running supply is going to shorten the life of the coil unless it's not the standard one.

I am confused by coil markings though - even those designed to run on 6v are often marked "12v" but have a note that they must be on a ballasted system, which really means they need to run off 6v, so it would surely be easier to mark them "6v". Anyway, I digress.

[Paul Dawson]

I always thought the voltage on a ballast system was nearer 9v. Annoyingly there's nowhere it says what it should be, and the coil itself is marked 12v. So if it's 6v then the drop isn't too drastic.

It is a ballast coil still fitted since it reads 1.3ohms on the primary circuit, which means I'll need a new coil anyway if I want to fit my Accuspark module because the instructions specify a minimum of 1.4ohms. The Haynes gives the range as 1.3-1.5 so there's nothing wrong with mine, it's just at the lower end.

There are Lucas 12v 3ohm coils on ebay for £12.45 at the moment with free delivery so that's pretty good. I'll fit one of those, swap the pins in D4 to D5 (easier than mucking about under the dashboard) and fit the electronic module, then see how it goes.

[droopsnoot]

I always thought the voltage on a ballast system was nearer 9v. Annoyingly there's nowhere it says what it should be, and the coil itself is marked 12v. So if it's 6v then the drop isn't too drastic.

Have a look at this thread where I was discussing my question (what voltage should it read while running), you'll see one person has scanned the two Crypton pages that talk about coil voltage: http://www.oldclassiccar.co.uk/forum/ph ... hp?t=15621

[Paul Dawson]

That's interesting. It explains why I'd got it into my head the coil voltage was 9v rather than 6v. My first car was a 1972 Triumph with a ballast system so that's what I learnt on - and I remember the coil showing 9v running. I guess I never measured it with the engine off.

[thomas]

If you're measuring coil voltage with the coil in circuit, it'll vary with the position of the points. You'll get a different voltage reading between earth and the coil SW connection with the points closed, than with the points open, due to the voltage drop in the completed circuit. The voltage with the points open will be the same as if/when the coil wasn't in circuit i.e. measuring the SW wire when it's disconnected from the coil itself. When running it'll fluctuate finding a mid-point as the points are opening and closing the circuit repeatedly, if the condensor isn't great some back-emf higher voltages on the primary side might interfere with readings. Unloaded voltage on the unconnected SW (white wire) or when connected but with the points open and assuming no leakage within the coil to the earthed can, is the only stable reading to work with.

I would go for the leccy ignition and a 12V all-the-time coil. If with electronic kit in the depths of winter you find the car firing into life at the point you release the key to stop turning it over on the starter, either your starter motor is taking excessive current or the battery is less than great. As to the bypass wire system it might be thought a shorter path and thus have less voltage drop than the long route through the ignition switch and so on and thus still offer some slight advantage even without any ballast wire or resistor present, but it isn't and doesn't as the bypass wire feed of the solenoid isn't fed by the big juicy ring terminal connection but from the operating trigger (white-red) wire and thus has taken as convoluted a path as the white wire has to finally reach the coil, through the ignition barrel switch etc.

Good luck, hope you find the solution that works best for you.

[droopsnoot]

Yes, running voltage confused me (and others, as you see in that thread) as it just wasn't mentioned anywhere.

Didn't entirely solve my original problem (very fast turning over when trying to start the engine when hot) but interesting to know there was nothing wrong.

[Paul Dawson]

Since electrickery is made by the cloud giants I'm not sure we ought to be messing about with it anyway. But if it's the only way to get petrol to burn then so be it.

I have on the bench - a spare dizzy, a new dizzy cap and an Accuspark electronic module (to replace points & condensor)
I have on order from Lucas - a 12volt 3ohm coil
I have on order from club spares - a set of new HT leads

Hopefully everything will come together early next week and I can put it all together. Until then a little bit of choke does the trick.