TooTallMike

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

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

Too late now but there were several of these for sale at W&PR. They were on the stall owned by the couple I think from the midlands with the coach. They were also selling the bowsers. Someone on here may be able to put you in touch with them.

- 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

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

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

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

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

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:

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

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

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

Thank you to Clive and Pete,

I'm going to have another look at the truck tomorrow and will pull the distributor out for inspection on the bench.

I will report back!

Thanks and regards - MG

Hi,

Steel cab, if original, dates it to pre-Jan 1943.

US army registration numbers up to 56910 were closed cab, 541581 onwards were open cab.

Overall approx 2700 examples of U8144T were built between 1941 and June 1945.

Other than that there seems to be very little info available on production numbers.

Regards - Mike

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

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

Not sure how I missed Andy's post the other so I'm sorry for not replying sooner. Firstly, thank you for the new times. I hope we can all step up and do some good showing-off!

I'd certainly like to do something with cars if nothing bigger can be found. From my experience at Beltring, the cars are delivered without engines/gearboxes and with the glass removed. Clearing up the mess afterwards is the biggest issue! I've no idea whether there will be tank on car action this year, but if there is we could just 'borrow' two of theirs.

- MG

Hi Howard,

If this is the chassis/front end that was bought from Sussex, you are correct that it was part restored by the previous, previous French owner but that was over 20 years ago and then it sat in an open-sided shed until we brought it over to the UK.

- MG

Well, it was contaminated fuel. We've no idea what the source was but the truck now starts and ticks over on new juice like the proverbial sewing machine.

Many thanks to all for thoughts and suggestions.

Cheers - MG

Thanks guys, someone else also suggested contaminated fuel so I'm going to rig up a fresh jerrycan as my first test this afternoon. Also hoping to borrow a known good carb if needed.

I'll let you know how it goes.

- MG

Don't know if you have access to a TM, but just had a quick look:

 

Assuming jets set correctly, what about sticking crankcase ventilating valve? Idling circuit in carb clogged?

 

White smoke indicates brake fluid but no idle / running without choke suggests poorly adjusted jets?

Thanks Tony,

I should have mentioned that the truck ran perfectly about 2 months ago when parked up for work. It hasn't been run at all in that time and the carb was not touched during any of the work.

I blew everything through with an air line when the carb was stripped but the only jet removed was the idle. I'll have a look at the others too tomorrow.

- MG

Dear esteemed team,

I am tearing my hair out with a GMC that needs to be at a D-day event at the end of the week. It has just had an overhaul consisting of a partial strip-down and re-paint, also conversion to 12V, alternator and positive earth, clutch change, and some brake work.

It starts eventually on 90% choke but then if let in even slightly it dies very quickly. It will rev up on 90% choke although it's a bit lumpy. If the choke is pushed in while revved up it sounds a lot better but very quickly and irrecoverably dies. There is a lot of white smoke when it's revved up. I've tested for air leaks and checked fuel supply. I've stripped and thoroughly blown out the carb. The regulator is missing its butterfly so that's not the problem.

When I saw the smoke I wondered about the hydrovac going bad, but it's the same even with it disconnected and the vacuum pipe blanked off. It has been suggested that if that is the problem there may still be residual brake fluid in the inlet manifold thus giving a false negative. This will be my first thing to look at tomorrow unless anyone has any better suggestions?

I'm sure I've seen this problem before but I'm stumped.

Thanks for any suggestions - MG

Parts for WW2 heavy trucks are still available, if not necessarily cheap. The most likely things you would need for a Ward laFrance are torque rods, which are still available NOS; brake diaphragms which you can get from the US; and transfer box parts as they are a known weak point due to poor lubrication. There are few if any NOS transfer boxes left, but rebuild isn't difficult and the bearings are off-the-shelf.

...and of course a modern diesel engine unless you like going up hills really slowly.

- MG

Didn't RR Services have some of these engines NOS a couple of years ago? Might be worth a phonecall.

- MG

Here is a promising-looking extract from the document referred to in this link:

 

https://www.gov.uk/government/publications/rules-on-drivers-hours-and-tachographs-goods-vehicles-in-gb

 

Note there is a nice disclaimer though meaning you can't be 100% certain it is legally correct!!!):

 

 

 

GB domestic rules

The GB domestic rules, as contained in the Transport Act 1968, apply to most goods vehicles that areexempt from the EU rules. Separate rules apply to Northern Ireland.

 

Domestic rules exemptions

The following groups are exempt from the domestic drivers’ hours rules:

u

drivers of vehicles used by the Armed Forces, the police and fire brigade;

u

drivers who always drive off the public road system; and

u

private driving, i.e. not in connection with a job or in any way to earn a living

.

 

 

Note also the distinction between EU rules and GB domestic rules - trouble is I can't see where the EU rules also allow exemption for the blanket clause shown in red, and I have no idea if EU rules trump GB domestic rules :nut:

 

In fact the EU rules only seem to allow exemption for vehicles over 25 years old.

 

There is also a possible distinction between rules requiring a vehicle to be fitted with a tacho and the exemptions which apply, and rules requiring drivers to maintain records and the exemptions which apply. One imagines these rules would tie up - but you never know! The document referred to above does however make the point that even if a vehicle is fitted with a tacho, it does not necessarily have to be used if the driver is exempt from keeping records.

I must admit the more I read the less I understand!

I think you'll also find that all vehicles over 7.5t GVW fall under the EU regs, and not the GB ones. The EU rules make no exemption for private use.

- MG