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MG MGB Technical - No idea what this coil/system is - help!

Further to my recent coil related ramblings, I have gone about trying to identify my coil and loom according to the information on Paul Hunt’s great site.

Well, first off, there is little in the was of identification on the coil itself except the remains of a sticker which reads:

“..LLAST
..NLY”

Now then, I checked the voltage across the LT side with the distributor connected removed and the ignition ON and got 12.2V. However, when I then earthed the ponts-side LT terminal of the coil the voltage across the coil dropped to 10.1V

According to Paul’s info, that might signify that a 12V coil has been incorrectly fitted to a car with a ballast resistance in the loom.

The car is a 1972 so I don’t expect a ballasted loom and the wire from the ignition to the coil is (or at least once was) white.

So maybe someone has retrofitted a ballast resistance? I can’t see any obvious signs of loom surgery but I could be wrong. The previous owner did carry out an awful lot of modification to the car which has taken me all the time I have owned it to discover.

However, to confuse things more I measured the resistances across the coil and got the following results:

Between ignition terminal and LT distributor terminal – 1.5 ohms
Between HT distributor and either other terminal – 8.4 k ohms

What is going on there? The 1.5 ohms makes it look like a 6V coil (rather than a 12V wrongly fitted in a 6V system)

However, the 8.4k ohms makes it look like a sport coil of some sort. I understand that the primary and secondary resistances quoted on Paul’s site are not definitive by Paul’s own admission but can anyone help me fathom what type of coil and system I have here?

Could the fact that the voltage across the coil when earthed drops to 10.1V be a sign of a faulty coil rather than the wrong coil type for the system?

Any wise words appreciated.

Alec
Alec

Can't help much I'm afraid, other than suggesting that possibly the sticker used to read:

USE WITH BALLAST
RESISTOR ONLY

???

Cheers,
Tim
T Jenner

Hi Tim, thanks.

That is what I was beginning to suspect myself.

I'm just a little nervous about diving in and buying a 6V replacement without being sure - for fear of incucing the exact symptome I think I'm currently experiencing anyway, i.e. an overheating coil.

I can't really think what else that sticker might have said.
Alec

I'm no expert and forgive me if I'm off the mark as I didn't see your original postings, but here's a few random thoughts. The first thing would be to try to get a known good coil to try by substition. The next random thought is that if you are experiencing overheating that could because the oil has leaked out rather than it being the wrong type of coil - regardless, you'll still need to replace.

You could try a 12 volt coil (I think this would still work even with a ballast in circuit, though obviously not brilliantly) as this wouldn't be damaged whatever system you have. If you find that it's actually got a ballast you could then remove it and revert to 12volt. Another alternative could be to temporarily bypass the ballast by running a test lead direct - without scouring the wiring diagrams I don't know how you'd do this but it might either show that you do have a ballast or that you don't and the coil is pulling the voltage low.

Cheers,
Tim
T Jenner

Alec,

the item No. of the coil fitted during production was GCL 101 for the year of your car. Later cars with ballasted ignition used GCL 111.
The coil in your car should have a bright light alloy finish and should be stamped 16 C 6 at the underide of the can (if it is still the OEM part).
It is not uncommon, that coils have been changed by a po, so it might be ok to use another coil with jumper cables to do a second test and mesure out the figures you got from the one installed by now.

If in doubt, i would just try another coil from a Mini, midget, Austin 1300, Volvo, Ford..., that is known to be a 12 Volt version.

Ralph
Ralph

From your voltage and resistance measurements, I would say that you have a 6V (ballasted) coil incorrectly fitted to a non-ballasted system. The coil voltage falls to 10V because the coil is taking far more current than the thin wiring was designed for.

Do the points burn out quickly? I would expect them to.

If you are buying a new coil I would get the 12V non-ballasted sport coil.
Martin Layton

Tim, Ralph, Martin,

Thanks for your thoughts. Very interesting.... The coil gold coloured and was probably fitted to the car at the same time as the opus-style electronic ignition, which I removed some years ago. I haven't noticed particularly quick burning of points but now you mention it I'll check.

I say the car is a 72 but the wiring is basically what is described as 70/71 in Haynes manual (if that is anything to go by).

It is beginning to look as if this could actually be the cause of the problem I posted about a few weeks ago. Basically the car suffered an electrical cut out when I was driving at sustained speed on the motorway. The car then started up again fine a few minutes later so I started suspecting an overheating coil.

It is strange though because I have had the car for some years (with a lot of hard driving all across Europe) before this problem started raising it's head.

Strange.

I don't suppose it can hurt to try a new 12V coil.

Is it possible to tell for sure if there is a ballast in the loom by testing the resistance between the coil's power rail and, say, a 12V rail in the fusebox? (with the battery disconnected). If so, what should it read?
Alec

I think its a "sport" coil. I had one on for a week or so and it was anodised a gold colour. I took it off and went back to the original as it got far too hot and the car would not run well. I think the standard coil is about 3 ohms, so a ballasted system would have about 3 ohms in series to achieve 6v on the coil in normal running. The voltage you are measuring shows less 400 millohms in series, I would put this down to contacts and conductor resisatnce. I think a standard 12v coil is the way to go.
Stan Best

Your coil measurement definitely shows a 6v coil, which needs to be in series with a ballast resistance, and *should* give a voltage of about 6v at the coil +ve when you earth the -ve.

Your voltage measurement indicates that either there *is* a ballast resistance but it is of too low a value, or there isn't but you coincidentally have some bad connections in the circuit. Either will mean a 6v coil getting too hot, as it should be running with 6v and not 10v.

If you measure the resistance between the wire (should be plain white on a chrome bumper car) removed from the coil +ve and the white at the fusebox you should see zero resistance on a chrome bumper car, that means no ballast resistance, and you need a 12v coil i.e. one that measures 2.4 to 3 ohms.

If a car has had an external ballast resistance fitted at some point then being an after-market item it could be any value, but in the order of 1 to 2 ohms I'd say. And I've even seen an external ballast resistance used in series with the harness resistance, and then a 12v coil connected to that. The result is a coil working an about a quarter of its voltage! (But certainly not overheating). A coil can get hotter than normal if the dwell angle is too high i.e. points gap too small.

Unless you have some exotic electronic ignition system fitted I'd find and remove the ballast resistance to get a full 12v to the coil +ve, and fit a 12v (3 ohms) coil, as nature intended.
Paul Hunt 2

Stan, Paul,

Thanks for your advice.

I don't know which is worse - tring to find a hidden ballast resistance or trying to find a bad connection causing the voltage drop.

I suppose the process is essentially the same either way.

I suppose until I find the ballast/bad connection I can get a 12V coild and hot wire it off the fusebox.
Alec

Run a white wire direct from the ignition switch white to the coil, this will get rid of all the nasty connectors and voltage drops.
Be careful, as it will be unfused.
(This is how an ignition feed should be IMHO)
Martin Layton

Right then...

Now I think about what you are saying Paul it all makes perfect sense. All it really took was for me to scribble a picture of two resistances in series with 12.2V across both to see what was going on. I don't know why I didn't do that earlier.

So it seems that some where between the ignition and coil there is 0.3ohms resistance where there shouldn't be - too little for ballast and too much for a healthy circuit. I have checked most of the connections, including at the back of the tach and cleaned with contact cleaner but to no avail. I will go back and check the connections at the ignition swith because I am not sure I did them properly.

I have also unwound some of the loom by the coild and so far haven't found any ballast but maybe I just need to go further back.

However, in the meantime, before I find the cause of my 0.3ohms I bought a standard 12V coil and switched it in. Voltage across the coil with negative earthed is now 11.2 V (again consistent with 0.3ohms in the line somewhere) but I reckon that 11.2V isn't all that bad.

The first thing I noticed when I fired up the car was that it now ticked over at about 1100rpm where it had been ticking over at about 850rpm so I assume it is getting a better spark now.

I am trying to work out what on earth my old coil was.

It is Lucas, gold in colour, primary resistance 1.5ohm, secondary resistance 8.4kohms and is slightly heavier than my new standard coil.

I was not aware of any 6V sports coils which is what it would appear to be so maybe it is a sorts coil gone bad (i.e. a few primary windings fused short circuit reducing the primary resistance?) Who knows.

Next thing I am wondering is whether, if I was to continue using the loom's coil-ignition connector (with the resistance in line) would I benefit from using something like a (Sparkrite) dwell extender to strengthen my spark.

Also, Martin, if I hotwire the coil off the fusebox should the tach stop working?

Thanks again everyone.
Alec

I have heard of 6v i.e. 1.5 ohm primary resistance gold Sport coils but heven't had my hands on one to check the secondary resistance. I'd say that is what you have.

The proper harness for a 72 wouldn't have a ballast wire inside it. If someone had installed a later harness by mistake there would be two light-green/white wires for the coil +ve instead of one white, and it would measure 3 ohms from there to the white at the fusebox.

The white from the coil should go back via the tach to the ignition switch (all MGB ignition systems *are* unfused from the factory). On a 72 *model* there should be two bullet connectors on the back of the tach - one male and one female to ensure correct polarity - plus the pickup primary inside the tach which should only be a single turn of wire. If you connect an earth to the coil -ve as before and measure the voltage on these two spades as well as the coil +ve you will be closer to determining where the volt-drop is occuring. There are at least a couple of connectors where the ignition switch harness goes, plus the switch itself, giving a numer of other places for a volt-drop to occur. However a problem here would affect all the ignition powered circuits and be causing slow wipers, slow or non-flashing indicators, getting worse as more things were switched on, and the same volt-drop would be measurable at the fusebox white.

Wiring direct back to the fusebox or ignition switch *will* disable the tach on a 72 *model*, but not a 73 model (which doesn't have the tach wiring mentioned above) some of which were manufactured in 73. The cut-off was chassis number 294250 for the roadster and 296000 for the GT i.e. later than that are 73s.

The better spark with a 12v coil doesn't compute, as you have actually *reduced* the spark by fitting a 12v coil in place of the 6v. It maybe that there is a bad connection somewhere, and you have made it less bad during your investigations.

A dwell extender will do nothing useful, except make the coil hotter. With the relatively low-revving engine of the B the standard dwell is fine. Remember the same system is used on the V8 with half the dwell, and on Jaguar V12's with one third!
Paul Hunt 2

This thread was discussed between 07/08/2007 and 12/08/2007

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