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MG TD TF 1500 - Lucas Q12 coil specs

What should the coil resistance measure? I show 4.5 ohms on the primary and 17.08K on the secondary.

Thanks for your info.
Brian W.
ZBMan

3 ohm primary. 8 - 10 k ohm secondary I think.
Chris Couper

Mine shows 3.9 ohms on the primary & only 4,500 ohms on the secondary. There is a thread on the Y site that suggests 4,500 on the secondary isn't enough. Probably explains why I have a later sports coil fitted to the car & the original is in my box of bits & pieces. The one on the car looks different to the BR12 that I just measured. Joe Olson has posted a pic of his original Lucas BR12 on the "Original Dizzy etc" thread below which is identical to my original. Judging by Willem's investigations on the Y thread a higher reading on the secondary seems desirable.
Anyone know if the Moss replacement for the BR12 looks the same as the BR12 or if, like most AM parts from my favourite supplier, it "just sorter looks kinda like it?" Also is it possible to have the original BR12 rewound? Cheers.
Peter TD 5801
P Hehir

Peter - I believe you have misunderstood Joe's post. The photo that he posted is labeled "the original coil from my '53" - his factory-original Q12 coil, NOT a BR12. The eBay listing I posted is indeed a BR12 coil (mis-read by the seller as ER12). Period coils are all identification-stamped on the base of the coil with model and date.

As far as I know, Moss never sold a reproduction or replacement BR12 coil. I know they sell the following:

143-210 Moss $69.96, a reproduction of the Q12.

215-630 Lucas Sports coil- visually unlike either Q12 or BR12 - has spade connectors, is a different shape and color, has a different bracket.

Hope this helps clarify things.

Tom Lange
MGT REpair
t lange

Tom, it's not at all unusual that I misunderstand things. I put it down to the advancment of time. Joe posted two pics, one that he believes was the original LUCAS embossed coil fitted to his car, with the bakelite top & threaded terminals (which looks the same as/identical to the Ebay pic you posted & also appears to be identical to the original coil I have in my hand), as well as the pic of the Moss replacement.

The markings on the base of the LUCAS embossed coil I'm holding reads as follows:

Q12
L O
EP 12V 652
45020A

Is the Q12 not the original TD coil? What is the significance of the BR12? Also do you happen to know if the 143-210 Moss coil that you mention is an accurate reproduction in every detail of the Q12, including the word LUCAS embossed into the cover? As rewinding the original coil is difficult because of the problem in removing & replacing the cover without damaging the bakelite, if the Moss effort is accurate, at $69:96 it is worth getting. Do you or does anyone happen to know the resistance of the Moss, Q12 & BR12 secondary winding? THIA.
Peter TD 5801
P Hehir

Peter - I seem not be be expressing myself very clearly! The Q12 coil was the standard TD coil, and the BR12 coil was the optional higher-performance coil available for the Mark II or TD/C cars. The shape and finish of both the original-equipment Q12 coil and the BR12 coil are pretty much the same. Joe's coil is indeed just like your Q12 coil, the standard TD coil. The BR12 coil for which I provided an eBay reference is slightly larger in diameter, and is the higher-performance coil, the one I recommended to you. They occasionally appear on eBay, sometimes NOS.

The Moss reproduction does not strive to duplicate every detail of the Q12 coil, but it is the same general shape with a lip at the base, and screw-on terminals. It cannot have the embossed LUCAS name because Moss does not own Lucas or pay it royalties (Moss sells a Lucas sticker - also not original - that can be applied), the bracket is in a slightly different position, and the Lucas identification stamping on the end is (I believe) absent.

I'm sure it is a perfectly fine coil, no matter where it is made, and as close to an original Q12 coil is appearance as you will find. It just won't have the Lucas embossed name.

I'm sorry I am away from the shop, and cannot take any resistance measurements of the two coils.

Tom Lange
MGT Repair
t lange

Thanks for the clarification Tom.
Peter TD 5801
P Hehir

O.K. Interesting stuff about the BR coil and the original Q12....however, now back to my question. Can anyone tell me if my coil resistance measurements are within spec? I know coils can test fine on the bench but then when in use fail due to heat, however I just need to know about the resistance specs for my old, cold, original Q12 coil.
Regards,
Brian W.
ZBMan

I don't know what the exact specs are for the Q12 coil but if you read the text on the link below, you'll conclude that the primary 4.5 is on the high(ish) side, as is should be around 3 Ohms. The secondary should be around 7.5K-10K, so again a rather high reading.

Are you sure you are making a good contact when measuring?

http://mgaguru.com/mgtech/ignition/ig108.htm
Willem van der Veer

Yes, good connections. I have also factored in the resistance for my test leads as well. I have heard, but cannot confirm, these original Q12 coils typically had a higher primary and secondary resistance from the more modern oil-filled non-ballasted coils. But don't know for sure...anybody out there with a known good Q12 coil ( not a repro)' who could measure the resistance figures and get back?

Regards,
Brian W.
ZBMan

Hi Brian,

Do bear in mind that the windings within the Q12 coil are 'solid', the better oil cooled coils only coming in towards the end of 1954, and used in the very late TFs. By all means keep your Q12 coil but get a good quality oil filled coil as a spare, or preferably for normal use. You can spray the body satin black if necessary, and many are now supplied with Lucar spade and screw on LT connections for originality.

Personally I use S/H 1960s -70s Lucas coils, made in England, and sprayed black. Despite green packaged 'Lucas' equipment currently advertised, the Lucas Birmingham factory closed in the late 1980s, and todays 'Lucas' name is merely a franchise, made anywhere in the world.

Lots of info in the Archives,

Cheers,

John
J C Mitchell

I'm still a bit confused about what the secondary coil resistance should be. My analog multi-tester shows about 16K ohms on the secondary of my new Q12 repro coil. Is that too high, and if so, why? My original coil shows about 6K.

Joe
Joe Olson

Later Lucas coil:

3.0 primary, 5.5k secondary.

Seems to run well.

JE Carroll

Hi JE,

A coil of that period was still made in the UK and could well last many years. I wish the same could be said for many of the modern and repro coils!

Cheers,

John
J C Mitchell

Hi Joe. I'll try & answer your query re high secondary resistance. I'm not an auto electrician but I do have a basic understanding of primitive electrical theory. As you know a coil is simply a transformer turning 12 V into many thousands of volts, sufficient to create a spark. V=IR, so as resistance is inversely proportional to current, the greater the resistance the smaller the current. Less current, less induced voltage in the secondary winding, so the weaker the spark. A point will be reached where the coil ceases to be effective.

There is an interesting post on back EMF's in the coil on the BR12 thread below, originally posted by E.B.Wesson, along with a simple test, both of which are well worth a read. Lucas coils with the markings CB (contact breaker) & SW (switch) are almost impossible to connect incorrectly. However the same can't be said for those marked + & -. Cheers.
Peter TD 5801.
P Hehir

Thanks Peter. I’m no auto electrician either, but I have a little knowledge of electricity, too. Of course a little knowledge can be dangerous so it’s probably best that I don’t work at a nuclear power plant.

My understanding is that the secondary voltage is determined by the difference in the number of turns of wire in the primary and secondary windings, so the greater the difference, the greater the secondary voltage. The secondary voltage will always be the same for a given coil and a set primary voltage. The more secondary windings there are in a coil, the greater the resistance and the lower the current, so as you point out there is a sweet spot where the voltage is highest and the current is still adequate to cause the plugs to spark.

The heart of my question is just what does the measured secondary resistance really tell us about all this? What am I to make of the fact that my new coil resistance is 16k ohms and my original 61 year old one reads 6k? The new one sparks ok. I replaced the old one on general principal just because it was old but haven’t tested it to see if it still works.

Joe
Joe Olson

Its the RATIO of the primary turns to the secondary turns not the difference.

If the secondary was wound with a smaller diameter wire, that could account for the increase in resistance.

Jim B.
JA Benjamin

This thread was discussed between 17/03/2014 and 22/03/2014

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