GMC CCKW on-going ignition problem

Mike I wouldn't worry about the suggestions to use a ballasted coil at this stage.

But in reply to your points raised.

30Kv is unlikely to be achieved it is more likely to be 5-20Kv with the average conventional set up.

The widely quoted advantage of a ballasted set up is to apply the full battery volts to this lower voltage coil. Interestingly FFR Land Rovers that have a ballasted system do not use this facility for start up!

The reason that these vehicles do not is that the ballast resistor is for another purpose. The purpose was to overcome some of the effect of capacitance of the screened system. This was achieved by loading the primary with resistance to offset some of the effect of the inductance of the coil this reduced the time constant of the circuit.

This benefit is obtained with any ignition circuit that is ballasted. A reduced time constant means that the coil can "charge up" with magnetism in a shorter time. Therefore at high revs, accelerating & under load the ballasted system will perform better. But for many ordinary sort of vehicles a ballasted system may not offer an improved performance.

So you might later like to experiment to see if a ballasted system improves performance. But I think at this stage it is a complication you could do without & concentrate on getting to function in the original manner.

Mike from my experience on these coils ,a 9v coil gives what it says whilst running , the ballast resistor part increases start voltage by 125% so back to 12v when starting then runs the ignition at 9 volt, it's over 20 years ago my Mini Coopers had these but I am sure this was to save points life etc .

Mike from my experience on these coils ,a 9v coil gives what it says whilst running , the ballast resistor part increases start voltage by 125% so back to 12v when starting then runs the ignition at 9 volt, it's over 20 years ago my Mini Coopers had these but I am sure this was to save points life etc .

Agreed that when running the coil will be at 9V but my understanding is that the transformer is wound differently to a 'standard' coil such that the spark power comes out the same? Are you saying the the ballasted coil gives a weaker spark when running? If so, surely that would impair performance on a vehicle designed for a full-power spark?

Out of interest, would the original 6V system have produced a weaker spark than a 12V arrangement? Surely that would also have been designed to produce a full-power spark?

- MG

Out of interest, would the original 6V system have produced a weaker spark than a 12V arrangement? Surely that would also have been designed to produce a full-power spark? - MG

Mike whether it is 6v or 12v it is simply the means to create the magnetic field. With a coil designed for a particular voltage it should give adequate output.

It is not necessarily beneficial to strive to increase the HT above what the system was designed to handle. Because the insulating properties of the distributor, rotor arm etc may not be able to handle a higher voltage as the insulation breaks down. Particularly true of older systems.

Thanks Clive,

I'm sorry for all the questions but I want to be sure I've got my head around this and I'm struggling to ask the right question.

What I'd like to establish is a theoretical comparison between the sparks achieved by a 'standard' 6V system, a 'standard' 12V system, and a 'ballasted' 12V system once the engine is running. Would there be any difference in spark from each of these or could they each be expected to produce effectively the same power?

- MG

 

My only thought is that I have missed something in the conversion from +ve to -ve earth, but what would that be?

 

 

 

Mike,

I have been looking again at what you have done and come across this sentence above, where you say you have changed polarity from Positive Earth to Negative Earth. Am I correct so far? You say you have been getting your components from Rex, so the coil would assumedly be for an original +ve earth GMC? Coils are made to be on or the other. They do not perform correctly if reversed. You may be connecting it with switch wire to + and - to distributor, but if it a +ve earth coil then there could be the problem. You need to try a known Neg Earth 12 coil.

Maybe you have done this already and I missed it

Just occurred to me that the GMC was originally 6volt ........... you would not be using 6v coil instead of 12v? This will cause the points to get hot.

Mike;

On a 12 volt system, a ballast brings the voltage down to around 8.5 to 9 volts and its purpose is to prolong point life. You can use either an internally ballasted 12 volt coil or an external ballast. DO not use both in the same system. You can run a straight 12 volts to the distributor without a ballast however the point life will be shortened, by how much I do not know. Back in the day the standard was every 12 to 15 thousand miles you changed points, condenser, cap and rotor and plugs.

On a 6 volt system, you do not use a ballast as your are putting 6 volts to the points. They would each produce the same "power"

I hope this helps you out.

John G

I've never come across a positive earth system CCKW.

All wiring diagrams I've ever seen in the various TMs and later reference works (e.g. GMC CCKW by Becker & Dentzer) all show wiring diagrams for the various trucks dating from 1941 onwards with an earth lead on the negative post of the battery.

TM9-801 dated 1944 clearly shows a negative earth system, so I guess it is reasonable to assume that the mass of original and replacement CCKW components around are compatible with neg earth systems.

If this is correct, I wonder why this truck had been converted to positive earth?

Is this a clue in tracing the problem?

Mike the precise comparison will vary with the design of each particular coil but I imagine a 6v average system & 12v average system of the same era would give broadly similar HT outputs.

However I'm tempted to think 6v coil design technology did not progress much beyond WW2, whereas 12v coil design has improved. So it is possible that a WW2 6v system may give less output than a modern (high inductance secondary) 12v coil.

The only way to compare is to do measurements of the HT as I did at W&P.

But do you necessarily want that, given the insulation problem I mentioned earlier? I have heard of instances where electronic ignition has been fitted that has given a sharper cut off of power from the coil hence higher HT but that has caused problems because the insulation of the WW2 rotor arm could not handle the extra power.

Just throwing in a googly here. Just suppose your new 12v system was giving more output than the original 6v system. Just suppose as things got nice & warm the HT output increased BUT to a point where the insulation in distributor cap or rotor arm could not handle it?

As far as the 12v with ballasted system goes, the coil current will rise more rapidly at high revs than the non-ballasted system. So the magnetic saturation will be greater at higher these speeds so can give more output than the non-ballasted at these higher speeds. 90% saturation is all that is required for optimum output. Excessive current by increasing the voltage cannot increase magnetic saturation beyond the 100% saturation it was designed for, it will just produce more heat.

I've never come across a positive earth system CCKW.

 

All wiring diagrams I've ever seen in the various TMs and later reference works all show wiring diagrams for the various trucks dating from 1941 onwards with an earth lead on the negative post of the battery.

 

TM9-801 dated 1944 clearly shows a negative earth system, so I guess it is reasonable to assume that all CCKW components around are for neg earth systems.

 

I wonder why this truck had been converted to positive earth?

 

Is this a clue in tracing the problem?

I thought that was the case, but Mike saying he changed the polarity made me wonder. Just added to my last post too, because if you go and ask for a GMC coil they would be 6v, and thereby may be the problem.

I think Mike is using a 12v coil as that measured about 3 ohms which seemed about right.

With talk of polarity does make me wonder. Because the rotor arm edge was lovely & clean & looked as if had been losing metal. That suggests a +ve spark rather than a -ve one.

Although we did consider that & when I used the LED HT voltmeter switching to reversed polarity setting it didn't seem shoot up in that setting to show that fault. But it was a rather wishy-washy set of readings.

What I should have done was to connect the analogue voltmeter say on the 1Kv range attach +ve to chassis & -ve to coil output & flicked the points to confirm an upward movement. I think that is the next thing to do so as to get out of the way this possibility of a reversed HT.

Coils are made to be on or the other. They do not perform correctly if reversed. You may be connecting it with switch wire to + and - to distributor, but if it a +ve earth coil then there could be the problem. You need to try a known Neg Earth 12 coil.

Just expanding on what Richard quite correctly says.

You need a negative spark from a negative earth coil. That way the back EMF of the primary, typically 300v, is added to the output of the secondary. This is because it is an autotransformer.

If you have a positive earth coil & want to use it to produce a negative spark in a negative earth system then you simply reverse the SW & CB connections. This will work BUT the polarity of the primary back EMF is now opposite to that of the secondary, so the HT voltage has 300v subtracted from it.

The system will still work but not as well as it might because a coil designed for positive earth used in a negative earth to get a negative spark will in fact be 600v worse off than using a negative earth coil.

As regards ballasted coils it is sometimes said that because of the reduced voltage then there will be reduced wear at the points. Apart from the wear that occurs by arcing from the back EMF which the capacitor should be dealing with, the electrical wear relates principally from the amount of current switched rather than the voltage as such.

Comparing a 12v ballasted system with a 12v non-ballasted system, it is true that the voltage will be less at the ballasted coil but the current in both systems will be much the same, as will be the wear on the points.

The purpose of the ballasted system is to allow a boost to a cold start & increase the rate at which the primary can be magnetised at high revs.

Comparing a 12v ballasted system with a 12v non-ballasted system, it is true that the voltage will be less at the ballasted coil but the current in both systems will be much the same, as will be the wear on the points.

 

The purpose of the ballasted system is to allow a boost to a cold start & increase the rate at which the primary can be magnetised at high revs.

Thanks Clive,

This is what I was hoping to see, and it suggests there should be no functional difference between fitting a standard coil and a ballasted one.

- MG

Mike,

I have been looking again at what you have done and come across this sentence above, where you say you have changed polarity from Positive Earth to Negative Earth. Am I correct so far? You say you have been getting your components from Rex, so the coil would assumedly be for an original +ve earth GMC? Coils are made to be on or the other. They do not perform correctly if reversed. You may be connecting it with switch wire to + and - to distributor, but if it a +ve earth coil then there could be the problem. You need to try a known Neg Earth 12 coil.

 

Maybe you have done this already and I missed it

 

Just occurred to me that the GMC was originally 6volt ........... you would not be using 6v coil instead of 12v? This will cause the points to get hot.

The coil I am using for all my tests was bought brand new from our local motor factors and is an Intermotor brand. I assume it is suitable for -ve earth since +ve earth would very much be a specialist application.

- MG

Mike;

 

On a 12 volt system, a ballast brings the voltage down to around 8.5 to 9 volts and its purpose is to prolong point life. You can use either an internally ballasted 12 volt coil or an external ballast. DO not use both in the same system. You can run a straight 12 volts to the distributor without a ballast however the point life will be shortened, by how much I do not know. Back in the day the standard was every 12 to 15 thousand miles you changed points, condenser, cap and rotor and plugs.

On a 6 volt system, you do not use a ballast as your are putting 6 volts to the points. They would each produce the same "power"

I hope this helps you out.

 

John G

Thanks John,

In our fleet we have numerous vehicles which run standard coils at 12V with no problems at all and no awareness of shortened points life. But then it's probably 5 years' motoring in a collector's vehicle to achieve the 12 to 15k miles you mention!

- MG

I've never come across a positive earth system CCKW.

 

All wiring diagrams I've ever seen in the various TMs and later reference works (e.g. GMC CCKW by Becker & Dentzer) all show wiring diagrams for the various trucks dating from 1941 onwards with an earth lead on the negative post of the battery.

 

TM9-801 dated 1944 clearly shows a negative earth system, so I guess it is reasonable to assume that the mass of original and replacement CCKW components around are compatible with neg earth systems.

 

If this is correct, I wonder why this truck had been converted to positive earth?

 

Is this a clue in tracing the problem?

Thanks Tony,

This isn't the first +ve earth GMC I've come across so it looks like some were either built differently or were converted for some reason. Certainly this truck was +ve earth when I first got to it.

Purely out of interest the 1940 Maintenance Manual for an ACKWX, the immediate predecessor to the CCKW, shows that as +ve earth.

- MG

As far as the 12v with ballasted system goes, the coil current will rise more rapidly at high revs than the non-ballasted system. So the magnetic saturation will be greater at higher these speeds so can give more output than the non-ballasted at these higher speeds. 90% saturation is all that is required for optimum output. Excessive current by increasing the voltage cannot increase magnetic saturation beyond the 100% saturation it was designed for, it will just produce more heat.

Clive,

Does this mean it could actually be worse with a ballasted coil?

- MG

The way I see it, the problem has to be a simple one. What is different to standard? Alternator fitted ........ can you be sure that it is not starting to overcharge after a few miles and thus higher voltage to coil creating heat?

I still would be suspicious of the coil though. Many years back I had a call to a Dennis fire tender with Jag 4.2 engine. On several occasions the AA had been called out because of failing ignition and points were found to be closing. On investigation I saw it had Lucas Quickfit points (the ones with red plastic heel) and the heel was distorted, obviously getting hot. I removed the coil and checked the part number, although it appeared to be for a 12v vehicle, it was in fact for one with a ballast. The owner said the coil had been one as long as he had known but the points had been renewed prior to the fault occurring, previous points had hard fibre heel which heat did not effect. My point is you cannot discount the coil unless you can be 100% sure it is for a true 12v ignition set up. I would cross check the Intermotor part number to see what vehicles it fits.

The way I see it, the problem has to be a simple one. ....

I woke up this morning with the very same thought! The only components which could have an effect on primary circuit are coil, wiring, condenser and points themselves - the high tension circuit can do what it likes. The only other variable is the voltage applied to the coil.

Oh and the ignition switch. What if it had dirty contacts - could this cause heavier arcing at points? You could run a wire direct from battery to coil and isolate wire from switch to rule this one out.

 

Oh and the ignition switch. What if it had dirty contacts - could this cause heavier arcing at points?

I would not have thought so, more likely a loss of current, but well worth checking.

I find it interesting that the problems seems to occur at part load, and it is summer. This problem does sound a bit like fuel, especially as so many electrical parts have been changed. I have no idea what carburettor CCKW has, but I would be interested in checking the part-load jet system, or simply pulling a sparkplug when the problem occurs to see its colour.

Another issue is when were the valve clearances last checked?

trevor

...This problem does sound a bit like fuel, especially as so many electrical parts have been changed. ….

It certainly does Trevor, but this would not account for the excessive arcing at points. Which one imagines could only be caused by the electrical components themselves (or some unusual oscillation of the distributer driveshaft, caused by excessive wear?…..).

It certainly does Trevor, but this would not account for the excessive arcing at points. Which one imagines could only be caused by the electrical components themselves (or some unusual oscillation of the distributer driveshaft, caused by excessive wear?…..).

Yes, but I am not really sure what is meant by 'excessive' here. Back when I used to service cars some of the points I took out had what I call excessive wear, but the owners had not complained about any driving issues. So could this be a bit of a red herring? If the points are indeed wearing fast, surely it is the condensor's job to keep this under control. It is possible to remove the condensor and fit a temporary one into the circuit outside the distributor - where it will not get hot.

trevor

Clive,

 

Does this mean it could actually be worse with a ballasted coil?

 

- MG

You seem not to have a ballasted coil as it measured 3 ohms resistance in the primary which suggests to me it is a 12v coil. It is unfortunate that modern coils are no longer stamped with their part number on the base, often have no label on them & only come from a box with a part number on.

Like you I tend to assume coils marked SW & CB are for a positive earth system. Generally coils marked + & - will be for a negative earth system, but this may not be the case. So I agree with Richard you should check the application of the coil.

It is possible you have been miss-sold the coil or been misunderstood, given that the other coil you had from them was only 0.8 ohms & clearly not a normal 12v coil that you had asked for.

If you search the net for discussions about using a positive earth coil in a negative earth vehicle the predominant advice is that you simply swap the LT connections over. A few brave souls will say this is not ideal but they are shouted down by people who say it works so it must be ok.

Well it will work to a large degree but as I mentioned earlier you will lose the benefit of the autotransformer & it will work against the system. So that you lose 600v of HT compared to a negative earth coil in a negative earth vehicle.

So this may be critical when under load, high revs, accelerating or with a lean mixture when you need all the volts you can get. Your coil only seemed to create a reliable spark across 12mm generally, I find 12v coils are are approaching a 20mm gap.

So it maybe only on the cusp of running ok. Unfortunate such a large gap in air is not necessarily translated to the spark in the plug where the gas density limits conduction.

As has previouly stated and I will agree with, is a bad (suspect) coil and at times a (suspect) condensor will act up with hot temperatures in the engine compartment. Also, early CCKW's were positive earth and had 25 amp regulators, when the change was made to negative earth they went to 40 amp sysytems. A coil, points, condensor are not specific positive or negative earth, but make sure you have the leads going to the rate posts on the coil.

Here is anothe avenue to check, your valve adjustment. We had an engine that had zero lash on a mechanical lifters, when the engine got hot, the valves would not close and the engine would die. However, on short recovery, sometimes right after it died it would fire back up, thats all it took for the metal surfaces to cool to allow the valves to close.

John G

Mike you need to rule out a few things first.

1 Did it run and drive fine before you turned it to 12 volts ?

If so then you can rule out engine valve adjustment, distrobutor and points and fuel ect.

So why not put it back to 6 volts and try it on the road.

then you have a true starting point.

My offer of all the equipment off my 270 which was 12 volts you can borrow to prove a point one way or the other !!!!

Although everyone has a welth of experience with sugestions it does not help that you have so many things which people are suggesting could be wrong, you need a base to work from, then go through one thing at a time.

Good luck.