GMC CCKW on-going ignition problem

Amongst all the other swaps, have you tried changing the distributor?

Andy

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

Oh well got that wrong too, however it worked without any problems right up untill I put the Cummins 6BT in it.....

Oopps that might have let the cat out of the bag :wow:

So, we had yet another look at the truck today. The distributor was taken apart on the bench and nothing problematic was found other than the screw holding the condenser to the base plate had been too long and had rubbed on the advance weight securing nuts. This presumably happened ages ago (long before the recent work) and was no longer contacting anyway. I cannot see how it could be related to the current problem. Nonetheless the screw was shortened appropriately. The distributor was found to rotate freely and without any play and the advance weights move freely. There is no vacuum advance so nothing to check there. It was all re-assembled and timed up again. After that I took the truck on a 20 mile test drive during which I was able to do a more scientific assessment of the problem. It got 10 miles before the problem started again. The truck began to kangaroo randomly as the ignition seemed to cut out entirely for maybe half a second at a time. This was accompanied by a small amount of backfiring. Once the problem had begun, the more it was driven the worse it got. Once the truck was pulled over and allowed to tick over for a few minutes it then drove fine for maybe half a mile before going bad again.

The interesting thing is that the problem only occurs during what I would describe as a 'cruising' rev range. The engine will 'lug' quite well even up hills without failing and if it is driven very gently it will not fail, but if driven normally it starts to fail as soon as the engine is in the sort of rev range used for general rural driving. Although I cannot be sure, I think it was also failing when on overrun going downhill which I would have thought was unusual as there is much less load on.

Even after it had got really bad I was still able to drive all the way home very slowly easing it along without putting any major load on the engine.

I must apologise for my description of the problem but it really is very hard to explain such an odd set of circumstances!

Regards - MG

Oh well got that wrong too, however it worked without any problems right up untill I put the Cummins 6BT in it.....

 

 

So are you saying that since you did the conversion it now has loads of problems...? :cool2:

Amongst all the other swaps, have you tried changing the distributor?

 

Andy

Andy,

This will probably have to be the next step although I cannot for the life of me see what difference it will make!

Thanks - MG

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

Thanks John,

I agree it is not likely to be an alternator-related issue as there are thousands of 12V standard coil ignition systems with 13.8V alternators out there. Incidentally we checked the battery voltage today with the truck running and can confirm 13.9V.

Oddly I don't know if I've tried replacing the wire from coil to distributor so I'll certainly give that a go although it looks and feels fine. Unfortunately the fault only happens for half a second or so at a time and only when on the road so your other test is not feasible at the moment. I'd actually prefer the thing to fail completely right now as it would be a damn sight easier to diagnose!

Regards - MG

Have you checked and cleaned the earth path to the battery? It may be worth checking the resistance from the block to the battery.

I ran a wire directly from the star washer under one of the coil mounting bolts directly to the battery earth but it made no difference.

There are also several of the braided radio bonding earth straps from the engine and ancillaries back to the chassis.

- MG

Mike the next diagnostic suggestion for the electrics would be to temporarily mount two panel lights in the cab.

Each earthed on one side, then one light to the SW terminal of the coil & the other to the CB terminal.

Under normal running the SW bulb will be illuminated. The CB bulb will probably glow dimly as the points rapidly open & close.

Get used to how the bulbs look when you are running normally. When it starts to go peculiar see what the bulbs are doing.

SW bulb should be on constantly.

Dimming or extinguishing shows a supply lead or ignition switch problem. There will be parallel behaviour for the CB bulb.

CB bulb should flicker/dim.

If it brightens up & stays on the points are not closing or the coil CB to distributor lead is failing.

If it does not light up (& the SW bulb is still bright) then the points are stuck closed or the lead inside is shorting.

That will give you an immediate idea of the behaviour of the LT circuit when it has a failure/ funny turn.

Mike;

What you describe is sorta of a "load" condition when in the cruising range. We had a newer vehicle (electronic ignition) buck. It turned out that one one the secondary (spark plug leads) was bad, so when the engine was under load it had an intermittent miss. Have you checked the resistance in the leads, I think it should so many ohms per foot. Also, do you have the right condenser installed, I know you have changed out condensers, but it might be worth a second look.

Have you considered ballasting down to 8 volts to "save" the points.

John G

It is going to interesting to find out what the culprit is.

I would try a condenser of the correct type.( not the standard one for 6 volt pos earth)

When changing from pos earth to neg earth will kill the condenser.

You may need a condenser of a different value too.

I would try a condenser from a 12 volt neg earth 6 cylinder vehicle and not a standard one for 6 volt and pos earth.

A faulty condenser will do the following.

Run OK for a short while after a new set of points have been fitted.

Will after a while start to run rough and miss fire and then be hard to start and then not start all .

Fit new points and it will run OK again for a short while.

With a sparkplug earthed on the motor and the ignition on with a HT lead running from the coil HT to the sparkplug which has a wider than normal gap....say 35 thou flick the points open and close with a small screw driver and carefully look at the spark being thrown.

A incorrect or faulty condenser will throw a red spark on the plug and sometimes weak spark too.

A correct spark will be big fat and very blue in colour.

Take note

points are very much a thing of the past and not made and supplied by the big names in Auto electrical.

Most condensors are now made in Chinese sweat shops and sold under big name brands.

I still work on a lot old points ignition industrial vehicles and is finding new condensers out of the packet are not what they used to be and I now find it is best to fit a good second hand item or a new genuine from the original vehicle dealer.

If pos/neg earth has been changed.........a different type of condenser must be fitted......changing poles on a electrolytic condenser/capacitor affects its operation.

If pos/neg earth has been changed.........a different type of condenser must be fitted......changing poles on a electrolytic condenser/capacitor affects its operation.

Ron I think you will find its not an electrolytic capacitor & is not polarised. As far as the capacitor is concerned it is not interested whether the initial energising voltage is 6, 12 or 24 volts. It is not dependant on the earth polarity & has no memory effect of any previous charge, so does not need to be changed per se. It should be rated at 500v to absorb the back emf induced in the primary as the magnetic field collapses as the points open. The effect of the capacitor is to cause a more abrupt switch off & consequently a greater HT output. This occurs at the rate of 5kV per ten thousandths of a second.

The capacitors Mike has were in excellent condition when I tested them on a Megger.

Ignition system condensors...or the modern correct term capacitor is electrolytic and sensitive to polarity.

The capacity of the capacitor/condenser is decided by the number of cylinders 2.2 microfarad is common for a small 4 cylinder motor, I cannot remember the normal size of a 6 cylinder motor.

The size of the ignition coil ,dwell angle and vehicle design and low tension supply voltage affect the size of condenser needed.

The early posts state the points are burning/pitting out on one side which indicates the condensor size needs to be changed.

At the end of the day if the points are wearing evenly the condenser is the correct size for the job and design of that ignition system as it stands.

The low tension voltage does climb to 500volts on a 6 volt or 12 volt base voltage, but it is the size of the condenser to suit the ignition set up at hand is everything.

Mike,

Sounds like the problem I had with my dodge a few years ago. Changed the distributor and all was fine! Jimmy's could be completely different but just thought I'd mention it.

Nick

out of interest mike you didnt let jack near it at w&p did you

Mike this has just come to me what type of plugs are you using in the GMC, see my motorcycle thread !!!

out of interest mike you didnt let jack near it at w&p did you

I dont think the issue is to do with no fuel I think Jack has learnt his lesson.

Mike this has just come to me what type of plugs are you using in the GMC, see my motorcycle thread !!!

And did you pay any attention to the posts on your own thread about GMC plugs all those years ago?

:nono:

:n00b:

:-D

P.S. Although I think the conclusion was that pretty much any make goes - it is all down to personal experience/preference. All things being equal that is i.e. not counterfeit plugs!

The capacity of the capacitor/condenser is decided by the number of cylinders 2.2 microfarad is common for a small 4 cylinder motor, I cannot remember the normal size of a 6 cylinder motor.

The size of the ignition coil ,dwell angle and vehicle design and low tension supply voltage affect the size of condenser needed.

 

Hi Ron,

Afraid I do not agree on the above. The condenser used on a Rolls B range engine is 0.2mfd. That is for 4, 6 or 8 cylinders. It matters not whether it is 6 volt or 12 and to how many cylinders the engine has. Just a quick look at a Lucas catalogue will show you one particular condenser that was used on anything from a Mini right through to an Aston Martin DBS V8.

cheers Richard

Mike,

I believe the GMC originally had an ignition filter in the line from the ignition switch to the coil. If it is still in circuit, I would bypass it as I have known these filters on other vehicles to break down and leak to earth. Worth checking.

regards, Richard

Mike,

I believe the GMC originally had an ignition filter in the line from the ignition switch to the coil. If it is still in circuit, I would bypass it as I have known these filters on other vehicles to break down and leak to earth. Worth checking.

 

regards, Richard

I have experienced similar problems with my 6 volt GMC due to fuel starvation..a blockage between the fuel tank (LWB ) and filter in the pipe that runs along the chassis rail...this was fine silt and a few small bits of fuel gauge float breaking up in the tank...blowing the pipe out cured the problem, but it did return a week before Normandy and had to be blown out again....Very similar symptoms that you describe. It behaved itself for the 975 mile round trip to Normandy..

but returning to ignition I would put another 12 volt battery in the cab, and wire it up directly to the coil and distributor body to isolate your alternator and existing wiring and see what happens..you will eliminate overcharging and a lot of existing fault posibilites..

or even put your old 6 volt coil back on and used your old 6 volt battery in the cab jury rigged up to run the ignition...and let the 12volt system crank it over. Ed Abbott

Hi Ron,

Afraid I do not agree on the above. The condenser used on a Rolls B range engine is 0.2mfd. That is for 4, 6 or 8 cylinders. It matters not whether it is 6 volt or 12 and to how many cylinders the engine has. Just a quick look at a Lucas catalogue will show you one particular condenser that was used on anything from a Mini right through to an Aston Martin DBS V8.

 

cheers Richard

I stand corrected.

I checked out a few condenser application and specs books etc and found.

Mr Lucas tended to use the same size condenser in 4cyl to 8 cyl motors where possible.

The Americans tended to use different condensors for the number of cylinders, but not always.

The Japanese use different size condensors for the number of cylinders most of the time.

I also found a Japanese industrial motor of the same make and distributor etc can have a different condenser capacity of say 2.2 MFD to 2.5 MFD the only real difference being the motor fitting of one motor having a ballast resistor and the other not.

I also found a range of 4 cyl industrial motors of different brands having condensors anything from 2.0 to 2.8 MFD.

I guess if the points wear even and don't ark is what counts at the end of the day.

May explain why same vehicles tend to wear points unevenly though transfer of material and some times need the condenser value changed.

Condensors I know from practical experience don't like the earths being swapped from pos to Neg.

Mike,

I believe the GMC originally had an ignition filter in the line from the ignition switch to the coil. If it is still in circuit, I would bypass it as I have known these filters on other vehicles to break down and leak to earth. Worth checking.

 

regards, Richard

Thanks Richard,

Good point, although in fact this truck has previously been re-wired to take this filter out of circuit. For information I tried wiring directly from battery to coil +ve and this made no difference.

- MG

I have experienced similar problems with my 6 volt GMC due to fuel starvation..a blockage between the fuel tank (LWB ) and filter in the pipe that runs along the chassis rail...this was fine silt and a few small bits of fuel gauge float breaking up in the tank...blowing the pipe out cured the problem, but it did return a week before Normandy and had to be blown out again....Very similar symptoms that you describe. It behaved itself for the 975 mile round trip to Normandy..

 

but returning to ignition I would put another 12 volt battery in the cab, and wire it up directly to the coil and distributor body to isolate your alternator and existing wiring and see what happens..you will eliminate overcharging and a lot of existing fault posibilites..

 

or even put your old 6 volt coil back on and used your old 6 volt battery in the cab jury rigged up to run the ignition...and let the 12volt system crank it over. Ed Abbott

Thanks Ed,

The way in which it fails is strongly suggestive of an electrical fault in that it is a total and instantaneous failure and then instantaneous recovery. The only thing suggesting fuel issues to me is the popping from the exhaust but I put that down to accumulated fuel firing once the ignition recovers. I have been offered the use of another running truck to rob of components to help problem-solve so I may swap the carb over just to eliminate this.

I like your suggestion of rigging a second battery to create an independent ignition circuit and I will try this next time.

Thanks - MG

I agree with Don, it could be fuel related.

Many years ago, I modified my Honda C70 step-thru into a trail bike, and rode it happily around the UK. Then I took it on a trip to the Alps, where it started having major ignition problems. Eventually, at the side of a mountain road, I discovered the problem was the condenser - it was mounted to the charging/ignition backplate, on the other side of which was hot engine oil. All the time the oil temperature was normal, it was fine, but climb a mountain road and it would fail.

Anyway, the point I wanted to make is that while bench tests are important, some things won't fail until hot.

To find the fault, I actually stripped out all the wiring on the bike and then reinstated it, wire by wire, so I could discount any poor connections. I would consider fitting a temporary ignition wiring system, and also recommend checking that you still have your 12v to the points when the failure occurs.

And here is the C70 just before I stripped the whole wiring system out:

Ah, to be young and stupid again...

trevor

Folks,

A number of people both on here and elsewhere have suggested fitting a 'ballasted' coil. I would like to clarify my understanding of these vs. 'standard' coils to be sure I haven't missed something.

My understanding is that a 'standard' coil running at 12V/13.8V puts out approx. 30,000V.

My understanding is that what it known as a '9V' coil, 'low voltage' coil or 'ballasted' coil running through its ballast resistor where the input is 12V/13.8V also puts out approx 30,000V. The only functional difference between a standard coil and a ballasted coil is that for starting the 'ballasted' coil is actually run at full battery voltage, thus briefly giving in the region of 125% of its rated voltage, giving a better spark to aid starting. Therefore once running, there is no functional difference between the spark from either type of coil.

It seems to me the suggestion that the 'ballasted' coil would cause the points to run cooler is therefore incorrect since their output voltages are the same.

What could the benefit be or fitting a 'ballasted' coil, or have I misunderstood something?

Thanks - MG