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I have had problems with my Bedford OX for several years - basically, the ignition coil overheats after a few miles and then dies so that it is impossible to restart the engine until things cool down. Does anyone know if an electronic ignition setup would cure this problem? Has anyone fitted electronic ignition and what are your experiences of it? Who supplies them?

Any advice gratefully received.

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3 hours ago, mike30841 said:

I have had problems with my Bedford OX for several years - basically, the ignition coil overheats after a few miles and then dies so that it is impossible to restart the engine until things cool down. Does anyone know if an electronic ignition setup would cure this problem? Has anyone fitted electronic ignition and what are your experiences of it? Who supplies them?

Any advice gratefully received.

Hi Mike,

I would suggest that you have an incorrect coil. It is entirely possible that although the coil is for a 12 volt system, it could be a ballast resistor type used on a modern vehicle, possibly only designed to have 9 volts or so running through it. I have come across this a few times. If you do go down the electronic ignition route, then fit a new coil that is recommended. I have fitted Powerspark kits on a number of vehicles and they transform the running and starting.

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Hi Richard,

Thanks for your input. I am aware that some coils are designed to be used with a ballast resistor, and have always been careful to buy only the correct type which are not made for use with a ballast resistor. I have tried coils from various sources, including one of the well known Bedford parts suppliers, and have had the same problem with all of them, although some have lasted longer than others. I am pretty sure the problem lies elsewhere, but precisely what it is baffles me. I had similar problems with my QL but having replaced all the earth leads and some of the ignition circuit wiring a couple of years ago, the problem has not recurred.

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2 hours ago, mike30841 said:

Hi Richard,

Thanks for your input. I am aware that some coils are designed to be used with a ballast resistor, and have always been careful to buy only the correct type which are not made for use with a ballast resistor. I have tried coils from various sources, including one of the well known Bedford parts suppliers, and have had the same problem with all of them, although some have lasted longer than others. I am pretty sure the problem lies elsewhere, but precisely what it is baffles me. I had similar problems with my QL but having replaced all the earth leads and some of the ignition circuit wiring a couple of years ago, the problem has not recurred.

Hi Mike,

Another thought then, is the coil connected up correctly?  On a Negative Earth vehicle, the Neg connection on the coil goes to the distributor, having the coil reversed will cause problems.

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Hi Mike

Richard has covered all the options. If it is a good correct 12volt coil on a 12 volt supply wired correctly it should not get hot, warm yes. Does your points burn out ? if so this would  suggest a high current through a faulty coil or the capacitor/condenser being US. Any external heat from the engine making the coil get hot?

Good luck

Michael

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Hi Richard & Michael,

I can confirm that the various coils I have tried have all been connected correctly, with the negative terminal to the distributor ( vehicle is negative earth). The points are undamaged, which suggests that the condenser is functioning correctly, although it has been changed anyway, just in case! Ref Michael's point about heat transfer from the engine, Bedford originally mounted the coil on the side of the engine. I have run it with the coil in that position, although for some time I have had the coil mounted on the bulkhead in an attempt to keep it cooler. I suspect this problem was ongoing even before I purchased the vehicle, as when I bought it there was an illuminated switch wire across the + & - terminals on the coil, which I presume was to indicate whether or not there was a problem with it. 

I have just ordered myself a proper automotive tester, so next job is to work my way through the ignition circuit looking for any voltage drop or poor connections, and to take off and clean all the battery/engine earth points.

Cheers,

Mike.

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Just a few further thoughts.

What is the "cold" resistance of the primary? About 3 ohms?

Is the charging system delivering no more that 13.8V?

Is the dwell angle correct? ie Are the points closed for too long?

How old is the coil? The thermal conductivity of the oil deteriorates with time. Or is it quite old & the insulation medium a sort of pitch? Those coils will usually have the primary wound first ie on the inside as one would expect given the name primary! But modern coils have the secondary wound first so that the primary is nearest the can & will dissipate heat better than an older coil as the primary winding is the main source of heat.

From experiments I have done by heating coils by running them continuously up to working temperature & a bit beyond, I have found that the insulating properties of the oil deteriorates to such an extent that it is about a tenth of its cold insulation.

The insulation of any winding (as they are all connected to each other) relative the case should be many gigohms at say 5kV when cold, so that once heated it will still perform adequately as an insulator. In addition to this not only does the thermal conductivity of the oil deteriorate with time so does its insulating properties, so once heated things get even worse.

Another point, your vehicle is negative earth but is the coil designed for negative earth or is it a positive earth coil with the SW & CB connections reversed? This dodge is often done, because it is important to have a negative spark for good running. 

By using a positive earth coil the wrong way round you lose out on some HT. voltage. As the points open the magnetic field collapses inducing a high voltage in the secondary, but of course there is a voltage induced in the primary in the order of 300v. Any spark at the points should be minimised by an efficient condenser  contribute to an abrupt switch off that optimises the HT output.

This 300v is added to the HT output, but if you use a coil originally designed for a different earth polarity this 300v is out of phase & reduces the HT output by 300v. So using the wrong polarity coil means the HT is 600v less than would be achieved by a similar coil designed for the correct polarity.

If you search the internet you can find references to this with the explanation that the primary winding senses this difference & adjusts itself accordingly! Well without the primary unwinding itself & rewinding again it is not possible! This  is why there are coils designed specifically for either a negative earth system or a positive earth system. Sites that promote Prince of Darkness themes are usually the most likely to come up with these extraordinary suggestions.

If you are going to GDSF I will have my test stuff with me & can test insulation up to 50 gigohms at 5kV, if you want anything tested.

The other thing is that if you do decide to go for electronic ignition, you may well find that you have increased HT output. This on the face of it sounds good, but that can give rise to trouble as older style rotor arms, distributor caps, cables etc may not be able to withstand this extra voltage & the insulation may break down.

 

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Hi Clive,

Some interesting points you raise there. Unfortunately, I don't possess a dwell angle meter, and I'm pretty sure that the manuals don't give anything as exotic as the dwell angle - bear in mind these vehicles were designed to be maintained by squaddies with nothing more sophisticated than a hammer and a tyre lever.

All the many different coils I have tried have been brand new from reputable suppliers. As they are all marked + & - I presume that they are for negative earth (if I understand you correctly, coils with the terminals marked SW & CB are positive earth?). None of the suppliers I have used specify coils as being for +ve or -ve earth vehicles.

I did check the cold resistance of the primary windings on some of the coils before they were fitted, and they were around 3ohms, from memory, but I was only using a cheap multi-meter.

As soon as I receive my newly-ordered tester, I will check the output from the battery, as well as checking the circuitry for voltage drop.

It is very kind of you to offer to test a coil for me at GDSF, but as I'm in Yorkshire it's a but far to come at the moment (it is on my "bucket list, however).

Hoping to have a good look at it this week, so will post any developments.

Mike.

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Mike

Has your vehicle been converted to negative earth and alternator? as all my vehicles of that period are positive earth with dynamos. Just a though that's been nagging me on this problem. My Dingo had the same problem in the middle of Normandy  in 2004, wired as made in 1942 with positive earth and that was a faulty original engine mounted coil. Half an hour to cool down and it worked again but had to be replaced in the end with like for like.

I had a similar problem when helping an owner in Norway with an OY half way up a mounting, but his spare coil had a REME label attached with big letters stating "faulty coil dated 1943".

Cheers Michael

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Mike the immediate thing to do is to check that points gap is not too narrow. Broadly speaking a coil marked SW & CB is likely to be for +ve earth & those marked + & - for -ve earth, but there is no guarantee of this.

For instance most screened coils are for -ve earth are marked SW & CB. But some earlier screened coils marked this way were for +ve earth. Unfortunately such coils are polarity identified with a sticky label. Why on earth they can't be polarity marked on the base where the date of manufacture & part number is stamped I do not know.

The easiest way is to look up the part number & see its design polarity. I notice one company that seem to produce or at least sell what look to be high quality reproduction coils have the coils marked + & - so that you are told to simply connect it according to the polarity. With such advice & with no mention of design polarity, this seems to demonstrate a poor understanding of the products they have on offer.

There is one way you could determine the design earth polarity. Although the are two coils they are connected in series to act as an auto transformer.

In +ve earth coils CB is connected to the far end of the autotransformer from the HT terminal.

In -ve earth coils SW is connected to the far end of the autotransformer from the HT terminal.

You could very briefly connect say a PP3 9v battery across the primary & measure the polarity with respect to the HT terminal & SW & CB in turn. Mind your fingers though as the will be HT produced when the magnetic field collapses  & the will also be 300v or so on the primary winding. You can see better how systems differ on page 3  & lists of coil polarities on page 90 & there is stuff about coil insulation on page 83-85 in this:

 

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5 hours ago, 2691H said:

Mike

Has your vehicle been converted to negative earth and alternator? as all my vehicles of that period are positive earth with dynamos. Just a though that's been nagging me on this problem. My Dingo had the same problem in the middle of Normandy  in 2004, wired as made in 1942 with positive earth and that was a faulty original engine mounted coil. Half an hour to cool down and it worked again but had to be replaced in the end with like for like.

 

Hi Mike,

Regarding your Dingo, all of them were Negative Earth, so looks like some may have wired it up wrongly.

regards

Richard

 

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As far as I am aware all the wartime and subsequent Bedfords were built as negative earth. My OX is still running with a dynamo - given the minimal electrics on military vehicles,  I can see no advantage whatsoever in fitting alternators. That said, my QL had one fitted when I bought it and I have never bothered to change it back.

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Richard

Yes your correct can I plead loss of memory, it the sun not the beer.

Michael

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1 hour ago, 2691H said:

Richard

Yes your correct can I plead loss of memory, it the sun not the beer.

Michael

Michael

No worries, we all get a bit of brain fade at times 😉

regards Richard

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