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MG TD TF 1500 - coil, hot to touch

I have 1250 TF. The coil is VERY hot after driving. Is this normal? (too hot to touch more than briefly)
J H Crighton

Have you had the engine suddenly stop and then after sitting a short time, you can start it again?

A couple of us in our local club have had the same symptom and shortly after the car shut down. If it was me, I would replace the coil as I am guessing that it is on its way out. Hopefully you will get some other good responses.

Brian Smith

Brian, thanks. No, I have had no problems with the coil giving up from heat exhaustion, but I am keen to avoid the situation so I thought I would check to see if it is normal for a coil to get hot enough to be uncomfortable to touch. cheers, john c
J H Crighton

John - Check the dwell angle (amount points gap) of your points. too small of a dwell angle (or too small a gap) will cause the coil to run hot. Cheers - Dave
David DuBois

I once heart that a too hot coil points at a defect or loose condensor. Any comments? Greetings, huib
Huib Bruijstens


Dave D. is correct in mentioning that the heat dissipated in the coil depends on its on/off time (duty cycle), however his statement
“too small of a dwell angle (or too small a gap) will cause the coil to run hot.” is INCORRECT, it should be:
“too LARGE of a dwell angle (or too small a gap) will cause the coil to run hot.”

Dave D.

I am sorry to be pedantic, but your error can cause confusion, I am sure that it was an oversight on your part, not lack of knowledge.


52 TD
J Scragg

Dave D --- I agree with John Scragg. The more time a coil is "ON"(larger dwell) the hotter it will run. I have found the same situation on a couple of cars. So far they continue to run and run well.

R. K. Jeffers

Sorry, my bad. I was thinking points gap when I was writing dwell angle. I will be writing "larger dwell - smaller gap 50- times on the north wall of my garage.
Cheers - Dave
David DuBois

David D....
"North Wall" ...shame on you, now get it right and do it 100 tyme's on the East wall you are facing Abingdon as you repent!
Regards & Cheers
David 55 TF1500 #7427

By the way my old coil ran hot when it went into John Twist shop years ago...he replaced it with a sports coil (told me it was on it's "last leg") and I put in a pertronix ...hasn't been "hot" since.
David Sheward

David S. It will be hot when the pertronix is on "its last leg".
Len Fanelli

David D.
Is that typing error caused by the wood splitting error?
John Hambleton


Wasn't the wood splitting error a couple of years ago? If so, I need to ask Dave D. how to milk similar occurnances to my personal benefit at home.

"Sorry, I can't help with xxxxxxxx because of my ancient injury to my yyyyyyyyyy but I'll be in the garage under the TD doing nnnnnnnnnnn."

Dave Braun

thanks for the advice guys. What a great resource this is.

checked the dwell which at is 89! But I am running Petronix, so no points to adjust. maybe I just have to live with it

john c
J H Crighton

"Is that typing error caused by the wood splitting error?"

Nope - ever since the wood splitting error, that finger no longer gets in the way of anything :(
Cheers - Dave
David DuBois

John, I'd suggest that you measure the resistance of the primary of your coil. Pertronix prefers to be working with a coil whose primary resistance is about 3 ohms. See
Bud Krueger

John, keep a breaker plate, with points adjusted, & lubed, & tools, handy. Not for if, but when the Pertronix starts going bad. Oh,and keep your receipts from Pertronix, you will need them.
Len Fanelli

Gee, sounds like I'm going to keep my old points system?

Bud, thanks. Primary resistance of the coil is 1.7 ohm. I guess this means I need to replace it.

Is there any recommended type/specification for a TF with Petronix? Under what circumstances is ballast required?


john c
J H Crighton

Late TF coil is stamped LA-12 on the bottom and the primary measures 3.3 ohms. Secondary measures 8,250 ohms

Orig.TD type stamped Q-12 pri. 4.39 ohms sec. 5,190 ohms
Stamped SA-12 pri. 2.6 ohms sec. 10,050 ohms
SP-12 (sports coil) pri. 3.2 ohms sec. 8,660 ohms

MGB coil stamped HA.12 pri.3.1-3.5 ohms sec. ???

These resistances are a reflection of the amount and the gauge of the wire. The real numbers are the inductance of the primary and the secondary. They can tell you what the turns ratio is.

Notice that the D.C. resistance of the LA-12 and the SP-12 are quite similar. The inductance of the two are:-
LA-12 pri. 11 mH. sec. 55.3 Hy.
SP-12 pri. 8.09 mH. sec. 64.2 Hy.

Turns ratio calculates to---
LA-12 70.9:1
SP-12 89.1:1

If we assume 400 volts peak on the primary the max. secondary voltages are:-
LA-12 400(70.9)= 28,360 volts peak
SP-12 400(89.1)= 35,640 volts peak

For what it's worth! !

R. K. Jeffers

Thanks for the specs, Bob. John , I'd suggest that you locate a coil with primary resistance in the vicinity of 3 ohms.

Contrary to the esteemed Len, I have found my Pertronix to be absolutely reliable and haven't had to adjust the timing in at least three years. I do keep the old points plate in the tool box as a 'just in case' part.
Bud Krueger

I'm with Bud here 3 ohm sports coil and running a Pertronix for years with "no~worries" ...but keep a set of preset points on board as a spare!
I have noticed more complaints about Prtronix on here lately. Hope this is not a sign of yet another popular product that has been outsourced to meet increased demands and has lost the quality in the process!
Cheers & Best Regards,
David 55 TF 1500 #7427
David Sheward

I love the Pertronix. I did go with the coil that they recommended however and have noticed no hot spots. Some in our clubb have run the P. for years and with no problems. I too however carry the spare points in my tool box.
TRM Maine

Guys, Sorry to jump in here so late in the thread. My TD's rebuild is now complete and we just finished the re-wiring. New harnas (loom) and wired for indicators too. When we tested the circuitry, (all OK), with the ignition switched on, and engine not started therefore not running, we found that the brand new Lucas coil becomes hot. I mean so hot, that the paint of the clamp started to crack !!!. I found that there was oil coming out of the centre point of the coil. This coil is definitely defective.
So John, maybe just check it with the ignition on and the engine not running. You might just have the same problem.
I de Clerk

I'm just curious here, has anyone ever had to replace their Pertronix on the side of the road? I've always heard that if your running with a Pertronix that you should carry a spare set of points but I've never heard of anyone having to use them on the side of the road.

Cheers - Dennis
D L Rainey

Sounds like you have a wiring problem somewhere! IMHO I would certainly not leave the battery hooked up on that car if unattended in the garage!
David 55 TF 1500 #7427
David Sheward

Ivan, and John- Just scanned the thread and didn't see this mentioned, but if the ignition points happened to be closed with the switch left on, you will have these symptoms. This was the death of many ignition coils.

D C Congleton

Ivan, the coil is not designed to have power applied to it on a 100% basis. When running, the power is intermittently applied as the points (or solid state switch, e.g., Pertronix) open and close. A formula for electrical power is voltage^2/resistance. With a 3 ohm coil that computes to 12*12/3=48 watts. Yes, the coil will get quite hot if the points are closed when power is applied. Picture a 50 watt light bulb. If the engine is running you'll have not only the lower duty cycle of power applied to the coil, you'll also have the air blowing over the coil to help cool it.
Bud Krueger

Unlike some, I carry a spare petronixs and not spare points & condensor...the reason I went to Petronixs is to get rid of the points...why would I go back! As to changing out on the road...i once checked under the cap for moisture after going through a puddle...replaced the cap but wasn't careful about the wires...started the car and the rotor cut the petronixs wires! 5 minutes to put in the replacement petronixs...made sure to check the wires!!!
Bob Dougherty

Thanks Dave, DC and Bud. Yep, you're quite right. We did leave the ignition on for about an hour while we were testing !!! That could well have been the cause of the heat buildup. All is now sorted out and a new coil purchased, but I will take note of your suggestions.
I de Clerk

About Len's comment that the Pertronix will fail it's just a matter of when. That's probably true about just about everything on this planet (including you and me).

I've been working with electronics over 70 years and nobody has ever shown me any data that says a solid-state device, when operated within it's ratings, has a finite life. Now when a solid-state device gets subjected to a situation beyond it's ratings it will beat a fuse every time. Therefore if the designers put in protection for the odd case where there is a pulse not normally expected, the Pertronix will outlast us by a long margin.

Still, because of the cost, it would seem prudent to have a replacement set of points in the tool box, set and ready to go, in case there is a problem on the road.

A few years ago I had an ignition problem. Replaced the whole system. When I got home and was able to do some trouble shooting, found that the rotor, not the Pertronix or the coil was the problem. The new rotors available from "The ground up" would seem to be the answer to that problem.

Anyone else have any comments along this line ??


R. K. Jeffers

Should have been "From the frame up". Sorry about that!
R. K. Jeffers


I trust you've now got your coil problems sorted out. What I wanted to raise was an enquiry of Bob Jeffers who's coil specs are set out on this thread. My Nov 1954 TF 1500 came to me with what I think was the original Lucas coil, #Q12, black painted and dated August 1954. As far as I know this is correct for my car, but are you suggesting Bob that on TFs later than mine an #LA12 coil was used?

Any background info on this would be useful.

Cheers, John.
J C Mitchell

I also carry a ready to go points and plate but have never had the petronics fail me yet. Regardless, if you don't carry a spare something and it fails, you will either need to tow your car home or spend at least day or so in a town awaiting parts. Not like a lot of parts places carry mg spares anymore.

John --- As far as I have been able to find out the Q12 coil was original to the TD. The LA12 coil is what was on all the TF's I have seen that lay claim to being relatively original. That doesn't mean that that is the final word by any means.

My Lucas parts book says the Q12 standard short type coil was used on all MG's from 1939 till 1954 (Lucas P/N 45020A). It is not clear from the Lucas parts book whether there were some early TF's with the Q12 coil.

TD parts book says the coil was MG P/N 3H520.
TF parts book says the coil was MG P/N 2A536.

So Abingdon is saying that the TF's had a different coil than the TD's. Everything I have been able to find says that the TD coil was the Q12.

My conclusion from the above is that some PO changed the coil and all he could find was a TD coil.

The LA12 coils I have seen are all aluminum colored. All Q12 Coils I have seen are painted black.

I hope this helps John.

R. K. Jeffers

Hi Bob,

Thanks for your response and I appreciate that to some extent this is all academic, since I wouldn't normaly advocate running around on original coils, although I find 60s&70s second hand oil filled coils totally reliable!

I don't have access to Lucas parts books, but I am deeply suspicious of the paragraph in the TD/TF w. manual where it refers to the LA 12 coil being fitted to later cars, especially since the exact same wording is used in a Morris Minor w. manual. According to your Lucas book, TFs during 1954 should have the Q12 coil, and ties in with my August 54 coil, showing at the very least that the Q12 was still being made at that time. The LA12 was the first of the Lucas oil filled coils being used up to at least the late 60s, (eg Triumph Heralds) and although, as you say a PO may have fitted an 'obsolete' coil I think its unlikely.

As you say the Q12 was black, and narrower than more modern coils so easily distinguisable. Info from owners of very late TFs in 1955 would be helpful - perhaps a new thread?

Cheers, John.
J C Mitchell

This thread was discussed between 05/02/2009 and 22/02/2009

MG TD TF 1500 index

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