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GMC CCKW on-going ignition problem


TooTallMike

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Hi folks,

 

I'm being plagued by an ignition problem that is eluding all of us and probably has a simple fix. I hope someone may be able to offer some thoughts?

 

The GMC was converted from 6V to 12V by the installation of a 75A alternator and replacing the coil with a 12V one. The alternator is wired with the main charge wire to the stud on the starter and the exciter wire to the ignition switch. Initially all other components were un-changed. At the same time the truck was also changed from +ve to -ve earth and the coil was reversed to reflect this. The coil is standard type (as was the 6V one). The coil is wired as ignition switch to '+', '-' to distributor.

 

The truck ran ok on 6V & -ve earth so is believed to be mechanically ok.

 

On its first trip out it managed approx. 150 miles before breaking down with an intermittent cutting-out lasting maybe half a second before it would recover. This would get worse and worse until it became un-driveable. After leaving it for around 10mins to cool down it would go another 10 miles or so before starting to fail again. The frequency became worse and worse until the truck eventually had to be towed in. The truck will usually re-start and tick over but dies when you attempt to pull away.

 

Over the course of problem-solving we have changed in numerous coils, points, caps, rotor arms and condensers. We also tried running a live wire from the battery straight to the coil. Clive Elliot generously gave his time at W&PR last week to go through the whole stock of spare ignition components to weed out anything faulty (in the end this was only one of the spare condensers and a wrong-spec coil.). We fitted the best set of components and Clive was then kind enough to check them in-situ and was as satisfied as he could be that they were ok as presented. Understandably he couldn't comment on their performance under load or when hot but said these were standard tests for these components and should be conclusive. Unfortunately the truck failed after only 5 miles on the way home from W&PR and over the course of several stops every ignition component was changed for new items purchased at the show from Rex Ward. Nothing made any difference and it was again towed home.

 

The most important symptom is that the points get very hot and are losing material across the contacts. Normally this would be attributed to a failed condenser but as per the above tests all components appear to be fine.

 

My friends and I have experience of numerous other vehicles with 12V alternators and standard coil ignition systems which give no trouble at all so what on earth is going on?

 

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

 

Thanks in advance for any ideas!

 

- MG

Edited by TooTallMike
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Regulator box ?

 

I know that changing from positive to negative earth can kill the regulator over time. The contact materials are specifically set for either negative or positive. Running them the wrong way causes pickup transfer on the regulator box contacts.

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Regulator box ?

 

I know that changing from positive to negative earth can kill the regulator over time. The contact materials are specifically set for either negative or positive. Running them the wrong way causes pickup transfer on the regulator box contacts.

Thanks Gordon but it's a brand new alternator with built-in rectifier pack so the original regulator unit isn't connected.

 

Saying that, I'd better check the old regulator is fully isolated in case there's some earthing thing happening there...

 

- MG

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Does your new coil have an internal ballast resistor or does it need an external ballast resistor? It could be that.

Good luck and take care, Tim

The purpose of a Ballast resistor coil is purely to assist in cold starting, this truck is fitted with conventional coil as standard

Edited by Nick Johns
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Sorry to hear that it is still plaguing you Mike.

 

I suppose one step would be to wire in a voltmeter for the time being & keep an eye on the voltage actually being delivered under all conditions.

 

Just a thought. Have you tried another distributor? I was wondering if it was something with the weights sticking or the springs deteriorating & not doing well with heat?

 

Perhaps get a can of freezer & zap a suspect component & see if it produces an instant recovery.

 

http://www.ebay.co.uk/itm/Rothenberger-Quick-Pipe-Freezing-Freeze-Spray-300gms-/120318605243?pt=UK_DIY_Materials_Plumbing_MJ&hash=item1c038c37bb

 

Incidentally thank you Mike for your investigations on my half shaft problems. It was very impressive over the week seeing people helping each other out in times of need.

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I once had a problem with a french made distributor (Ducellier?) on a 270, the base plate was crimped on to the shaft and this had worn so that there was slop, when cold the balance weights would advance and retard as they should, after warming 10+ miles or the shaft would expand and the plate would stick in the fully advanced position. Truck would pull like a train until the throttle was closed than it would miss and die and not restart until everything cooled down. The problem took a long time to locate. Like you I had changed everything on the ignition circuit several times, I finally found the problem when I striped the distributor right down, two tacks with the Mig and the problem was solved.

 

Pete

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Yes we are all waiting for a satisfactory diagnosis for this Mike. Not just your peace of mind, but it is a puzzler that is rather frustrating that any of us might get caught out with on our own vehicles.

 

I commented that one of the rotor arms looked to have lost metal suggesting a positive spark. If that was so it would require 10% more HT to get spark but would probably only be significant if other things go critical.

 

This is the basis of that comment from an old training course from Lucas Technical Services.

 

Rotor arm polarity.jpg

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Hi Mike, the small wire from the alternator should go to an ignition light ( charge light ) not sure how the alternator would react with a direct supply ! I did this on my GMC all I did was fit a new alternator and a coil off a straight 6 4.2 Jag car, closest match to the 270 cc wise and number of cylinders, did nothing else with the distributor at all.

 

With ref to coils it is very important that if you use a coil that should have a ballast and feed it direct you can get some very funny symptoms I speak from experiance on a 3.0 Ltr v6 transit a friend had !!!!!!

 

A ballasted coil runs through the resistor ( ballast ) normally all the time the only time the resistor is bypassed is when cranking the starter the idea being the heavy load of the starter dropps the volts by quite alot and so by bypassing the resistor the coils gets a much closer to opperating voltage than with out it so giving a good spark to get the engine started.

 

 

Try getting a 6v battery and feed just the old 6v coil and see what happens should run fine if so its the new coil you are using !!

 

 

Good luck

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Hi folks,

 

I'm being plagued by an ignition problem that is eluding all of us and probably has a simple fix. I hope someone may be able to offer some thoughts?

 

The GMC was converted from 6V to 12V by the installation of a 75A alternator and replacing the coil with a 12V one. The alternator is wired with the main charge wire to the stud on the starter and the exciter wire to the ignition switch. Initially all other components were un-changed. At the same time the truck was also changed from +ve to -ve earth and the coil was reversed to reflect this. The coil is standard type (as was the 6V one). The coil is wired as ignition switch to '+', '-' to distributor.

 

The truck ran ok on 6V & -ve earth so is believed to be mechanically ok.

 

On its first trip out it managed approx. 150 miles before breaking down with an intermittent cutting-out lasting maybe half a second before it would recover. This would get worse and worse until it became un-driveable. After leaving it for around 10mins to cool down it would go another 10 miles or so before starting to fail again. The frequency became worse and worse until the truck eventually had to be towed in. The truck will usually re-start and tick over but dies when you attempt to pull away.

 

Over the course of problem-solving we have changed in numerous coils, points, caps, rotor arms and condensers. We also tried running a live wire from the battery straight to the coil. Clive Elliot generously gave his time at W&PR last week to go through the whole stock of spare ignition components to weed out anything faulty (in the end this was only one of the spare condensers and a wrong-spec coil.). We fitted the best set of components and Clive was then kind enough to check them in-situ and was as satisfied as he could be that they were ok as presented. Understandably he couldn't comment on their performance under load or when hot but said these were standard tests for these components and should be conclusive. Unfortunately the truck failed after only 5 miles on the way home from W&PR and over the course of several stops every ignition component was changed for new items purchased at the show from Rex Ward. Nothing made any difference and it was again towed home.

 

The most important symptom is that the points get very hot and are losing material across the contacts. Normally this would be attributed to a failed condenser but as per the above tests all components appear to be fine.

 

My friends and I have experience of numerous other vehicles with 12V alternators and standard coil ignition systems which give no trouble at all so what on earth is going on?

 

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

 

Thanks in advance for any ideas!

 

- MG

 

Just had another thought I could bring up all my bits off my old 270 engine as I know they all work together and then you will have a base to work from for testing at least ?

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As Nick has pointed out Mike is using a conventional 12v coil & needs no ballast resistor.

 

The primary of this coil measured 3 ohms which seems right for a standard 12v coil. Using my coil testing machine it was able to demonstrate that it had a healthy output across a pointed spark gap of 12mm equivalent to about 8Kv.

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As Nick has pointed out Mike is using a conventional 12v coil & needs no ballast resistor.

 

The primary of this coil measured 3 ohms which seems right for a standard 12v coil. Using my coil testing machine it was able to demonstrate that it had a healthy output across a pointed spark gap of 12mm equivalent to about 8Kv.

 

All well and good I bow to your knowledge, but it is still playing up though !! there is not much to replace however in this situation when you are desparatly swapping things back and forth you can easily start fitting other bits which might be problematic and not be the actual cause for the fault.

 

Thats why I have offered all the bits on my 270 as I know that was fully sorted and used for probably 8 years with no problems at all.

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Thanks Richard,

 

The alternator exciter feed does go via a warning light - I forgot to mention that! Alternator has a healthy 13.8V at the battery.

 

As stated the coil is 12V standard ('standard' meaning non-ballasted). During the course of the search for the problem it has a had 5 coils including 3 brand new ones.

 

Thank you for your offer of parts. I'll look at the distributor tomorrow and go from there.

 

- MG

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Thanks Richard,

 

The alternator exciter feed does go via a warning light - I forgot to mention that! Alternator has a healthy 13.8V at the battery.

 

As stated the coil is 12V standard ('standard' meaning non-ballasted). During the course of the search for the problem it has a had 5 coils including 3 brand new ones.

 

Thank you for your offer of parts. I'll look at the distributor tomorrow and go from there.

 

- MG

 

Just a thought how did select the "standard coil" I decided to go for a coil which was off an engine as close to the spec as mine so the 4.2 6 pot jag ws the choice the thinking behind this was that the time duration for the coil to re charge before the distributor triggers the coil to fire down the next lead would be the same as they are both 6 cylinders and the cc was just an extra bonus as it should give a simiar size spark.

 

Let us know how it goes, all the 270 bits are in the garage now as I have the other beasty in the GMC, only GMC I know which returns about 18 to the gallon :cool2:

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Just to add to the discussion , in case this has any importance, even with swapping a new coil on the road the GMC only covered no more than 7 miles before failing again, the coil was nearly too hot to touch, which in such a short distance does not seem right, :banghead: .

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Just to add to the discussion , in case this has any importance, even with swapping a new coil on the road the GMC only covered no more than 7 miles before failing again, the coil was nearly too hot to touch, which in such a short distance does not seem right, :banghead: .

It should be noted though that on many occasions when it has broken down the coil hasn't even been warm, even when that coil is a known new one freshly installed. The symptoms at breakdown do not always seem to be the same.

 

As previously mentioned the truck drove over 150 miles after the initial install before this problem manifested itself and yet the mean time between failures now when starting from cold is only around 10miles.

 

- MG

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I have seen problems with ignition switches that intermittently halt the flow of current. . Might be worth checking. The coil getting hot by someone's reckoning, yet John said it failed and was not hot, maybe the coil issue is a red herring.

 

I could not see any mention of the distributor cap, are the contacts inside the cap clean? I came across a problem recently where a coil had been connected in reverse (for years) and caused a build up on the contacts, scraped off and been fine since, (after changing the coil and connecting correctly).

 

Watch any coils that are only marked CB and SW as you cannot be sure if they are for Neg or Pos Earth systems.

 

regards, Richard

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Dear Mike,

Very interesting!

 

Check the wire between the coil and the distributor very carefully. It could have a poor connection between the wire and the terminations. Alternatively there could be a short between the wire and another live wire or the metal work. The short could be very subtle or due to insulation breakdown. Perhaps do a substitution test with a new wire.

 

If the fault is present, with the coil disconnected and the contact breaker closed, measure the resistance between the end of the wire normally attached to the coil (and going to the distributor) and the negative terminal of the battery. The resistance should be very low (compare with a running vehicle).

 

Does not sound as if you have an alternator problem. If the alternator is producing far too much voltage the bulbs will start blowing before the ignition system starts playing up.

 

I won't insult you and everyone else by suggesting all the standard tests because you obviously know what you are doing!

 

We are all surely dying to know what the fault is!

 

John Attlee

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POSSIBLE INTERNAL SHORT HAPPENING INSIDE THE DISTRIBUTOR. Check for frayed wiring. I belive the heat on the points is caused by a short.

 

John G

 

John but surely if that was the case then the short would be carrying the current, in effect bypassing the points. One side of the points is at earth of course, so if the other side of the points was earthed by a short, then the current would not be carried through the points so they wouldn't get hotter.

 

But I think you may be right that something untoward is going on in the distributor. Something like the base plate or weights sticking when hot?

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I came across a problem recently where a coil had been connected in reverse (for years) and caused a build up on the contacts, scraped off and been fine since, (after changing the coil and connecting correctly).

 

 

As Richard says. This is from a Lucas training course:

 

Rotor arm polarity.jpg

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Just a thought how did select the "standard coil" I decided to go for a coil which was off an engine as close to the spec as mine so the 4.2 6 pot jag ws the choice the thinking behind this was that the time duration for the coil to re charge before the distributor triggers the coil to fire down the next lead would be the same as they are both 6 cylinders and the cc was just an extra bonus as it should give a simiar size spark.

 

Richard,

I see your theory but in reality, the Lucas specified coil for a 4.2 straight six Jag is the same one as fitted to a Reliant Robin, Land Rover 2a or 3, and a good number of other British and European vehicles, largest engine listed with same coil being a Rolls B81 straight 8 in a Dennis. There is no matter whether it is a 4 or 8 cyl and how large the engine is. ;)

 

cheers Richard

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