MG-Cars.info

Welcome to our Site for MG, Triumph and Austin-Healey Car Information.

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Coil query for MGB 1980

Hi, I am Allan. I have an MGB 1980, one owner (me!). The car had most of the mechanics overhauled or replaced 1400 miles ago. Rebuilt engine, lead free head, recon carbs, standard distributor etc, new coil, I think. It ran well for a while, then it was laid up for a few years. Since then, it runs well for about 7 miles or so, then the power gets weaker and weaker until it stops. It appears that after it cools down for a while, it runs (at least it runs!). My query is this. The coil gets very hot - hot enough to burn skin. I think that when it was overhauled, an additional white ceramic resistor was fitted alongside the coil. The car should have a resistor wire in the loom? Am I running with 2 resistors? Is this a problem? How do I check if the resistor in the loom is working? And lastly, why is the coil getting so hot?
All answers will be most welcome!!
Allan

A Reynolds

Allan - The most usual cause of a hot coil is the points being too close together, causing an excessive dwell time. That said, a friend with the same problem and it appears that a defective capacitor (condenser). Defective capacitors have become a real problem in recent years, so that would be something to check. Cheers - Dave
DW DuBois

Many thanks, Dave. I will check out those 2 things. Still needing help with the slowing down and stopping!
Regards, Allan
A Reynolds

Too much dwell, as suggested by Dave will cause the heat, as will running a 6 volt coil permanently wired to 12v.
Just check the resistance of the primary circuit of the coil i.e., across the + &- terminals. 3ohms is a 12v coil, 1.5 is a 6/7 volt for a ballasted circuit and around 1 ohm is an electronic coil.
Then check your wiring. The ballast wire, which is actually pink, BUT White and light green at the coil + and white and brown at the fuse box. But if you had 2 resistors I doubt you would get much of a spark anyway.
Allan Reeling

"But if you had 2 resistors I doubt you would get much of a spark anyway."

Indeed, and it would result in a cooler coil not a hotter. A pal had some work done on his V8 but a well known company who will remain nameless, and became an absolute beggar to hot start. Eventually he discovered they had fitted a 12v coil plus a ballast resistance to already ballasted wiring.

Take the wiring off the coil spades before measuring or you will get incorrect results.

Then with the wiring back on, and the points closed or shorted out, turn on the ignition and measure the voltage on the coil +ve which should be two white/light greens. The correct combination should give 6v, if it's higher than that you probably have a 12v coil, if it's lower then you probably have two ballast resistances in series, but neither of those combinations will make the coil run hot. If you see 12v then there is no ballast in circuit, which if you have a 6v coil i.e. one that measures 1.5 ohms between the spades, then that definitely will run hot.

Bear in mind that a coil will always run hotter than the air surrounding it, which being after the radiator will be quite a bit hotter than ambient. The other week my V8 engine compartment got up to 64C and the coil (ballasted system) was 68C a few minutes after switch-off - which is hot. In January I've measured it at 40C which is barely warm.
paulh4

Hi Dave, Allan and Paul. Many thanks for your advice. You have no idea how grateful I am. Over the years, the car has been in to numerous garages in an attempt to solve the problems, with little success. I now understand much more about the ignition side of the car. I will get the time to set the car up following your advices. At the moment, it appears that the condenser I have just removed was faulty (no ohms at all. The new one shows very low ohms, but at least there is a circuit.) I have measured all the various resistances. They are as follows:-
Resistor loom = 12.9 ohms
White ceramic resistor = 1.8 ohms
Existing coil + to - = 1.5 ohms
Existing coil + to centre = 8.9 ohms.
I also have a new ballasted 12v coil which reads:-
New ballasted 12v coil + to - = 1.5 ohms
New ballasted 12V coil + to centre = 6.3 ohms.
It looks to me as if I have been running 2 resistors on a 6v coil. I intend to put the new ballasted 12v coil on, with no resistors. Any further comments would be most welcome! Regards to you all, Allan
A Reynolds

"I intend to put the new ballasted 12v coil on, with no resistors." This is still a 6 volt coil, intended to run on the 6v feed at all times other than when cranking.
I hope you mean just without the ceramic resistor, but still on the ballast resistor circuit!
Check by using Paul's voltage check, i.e., "with the wiring back on, and the points closed or shorted out, turn on the ignition and measure the voltage on the coil +ve which should be two white/light greens. This should be 6 volts + or - a volt.
Allan Reeling

"it appears that the condenser I have just removed was faulty (no ohms at all. The new one shows very low ohms"

If you have that the right way round, then the *new* condenser is faulty - short-circuit.

A condenser consists of two conducting sheets, with a thin insulating sheet in between them, rolled up into a cylinder. Therefore there can be no continuity between them. The only time you might see something, if you are quick enough, is to connect the ohmeter one way, than rapidly connect it the other way. In that case you may see a 'kick' of the needle on a analogue meter, or a reduction from open-circuit on a digital. The voltage from the ohmeter imparts a charge to the condenser, which leaks away over a short time, but if you can do the change-over fast enough it will discharge into the meter.

What is 'Resistor loom'? To measure the harness ballast resistance measure between the white on the ignition relay and the white/light-green removed from the coil +ve. Instead of the relay you could use the white/brown at one of the in-line fuses under the fusebox, but as there are two you have to find the correct one. It should measure about 1.5 ohms i.e. similar to the coil.

It could be that is faulty, which is why someone has provided a discrete resistor, but at 1.8 ohms it is a bit high which will reduce your spark slightly. If that is the case it should be connected to the white or white/brown at the fusebox or similar i.e. a 12v switched ignition supply.

The 8.9 and 6.3 ohms readings are OK, they are the HT winding.

And to repeat what Allan says, both your coils are 6v coils and must be used with a ballast resistance.
paulh4

Hello everyone. I have been away, hence there being nothing from me for a while. Now, in short, I think I am sorted!! I have new points with a 16thou gap and a dwell angle of 52 deg, a condenser with no ohms reading, timing of 14 deg at 1000revs advance removed and plugged, carbs balanced and tuned with a balancer and Colourtune. I have been for a run of 15 miles with no problems and with the coil just warm. The plugs are the correct gaps and colour. You have no idea how happy this has made me, so a great big thank you to you all!!
I intend to put a new resistor of 1.5 Ohms in place of the present one of 1.8 Ohms just for good measure. The downside to all this is I now have a fuel leak somewhere at the back of the engine block, so it is going in to the local garage tomorrow. Once again, many thanks for the good advice. Allan
A Reynolds

If you have an external 1.8 ohm resistor now, how is it wired?

I presume one end goes to the coil +ve, but where does the other end go?

If it goes to a white/light-green then you may have two ballast resistances in circuit which will give a low HT.

If it goes direct to an ignition switch source such as the white or white/brown at the fusebox, then you may well not be getting the boost on cranking. As I discovered through a faulty starter over quite a few years the boost isn't essential, but it can make the difference between starting and not starting under adverse conditions.

If you have the white/light-greens on the coil +ve as well as the wire from your 1.8 ohms then you may be getting the boost but may also have two ballast resistances in parallel, which would overheat the coil (cool coil noted).
paulh4

Hi Paul,
"If you have an external 1.8 ohm resistor now, how is it wired?

I presume one end goes to the coil +ve, but where does the other end go?

Yes, white light green goes from resistor to +ve on the coil. Another white light green seems to go to either the starter or ignition (I can't tell).
The other end of the resistor has a white wire to a relay (below the fuse block.) There is also another wire, white brown with an inline fuse going to the fuse box. (The outgoing wire from there is green.)

If it goes to a white/light-green then you may have two ballast resistances in circuit which will give a low HT.

If it goes direct to an ignition switch source such as the white or white/brown at the fusebox, then you may well not be getting the boost on cranking. As I discovered through a faulty starter over quite a few years the boost isn't essential, but it can make the difference between starting and not starting under adverse conditions.

If you have the white/light-greens on the coil +ve as well as the wire from your 1.8 ohms then you may be getting the boost but may also have two ballast resistances in parallel, which would overheat the coil (cool coil noted).

While tracing the wires, I removed quite a lot of the black tape and found two wires not connected up. One was white light green, and one was pink white. See the photo. I assume that the pink white is the resistor in the wiring loom. Should it have been connected to something, say the white light green? Both of them come from the wiring to the front of the car. If this is so, then the car was not wired up when originally built. It is worth noting that originally, the points used to burn out regularly and even welded themselves together more than once. Over to you Paul!!

A Reynolds

Yes the pink/white is the original ballast resistance, one end of which would be connected to a white/light-green, which would go to the coil +ve, that would be the ignition supply for running.

There would be a second white/light-green on the coil +ve which would go to the starter solenoid, and that is the boost wire and the ignition supply for cranking.

The other end of the pink/white (which goes across the front of the car and back again to near the fusebox) would go to a white to the ignition relay and the ignition switch, as well as to a white/brown to an in-line relay with green on the other side, similar to how you describe.

It looks as if the supply end of the pink/white has been cut from its white and white/brown, and the white/light-green on it's other end has been cut before it reached the coil. It also looks as if your 1.8 ohm resistor has been wired the the other half of each of those cuts, which would be correct.

Another symptom of a 6v coil running without a ballast is that the points burn, as well as the coil overheating. If that was happening after the ceramic resistor was fitted, then the implication is that either the wiring changes were made incorrectly, or the cut wires weren't fully insulated and were able to come together in some way.

When did the points burning stop? If no wiring changes were made immediately prior to that, then I would suspect the latter. It's also possible that the unwanted connection went high-resistance, which stopped the points burning, but still allowed the coil to overheat, until your most recent work disconnected it altogether.
paulh4

Hi Paul
"It looks as if the supply end of the pink/white has been cut from its white and white/brown, and the white/light-green on it's other end has been cut before it reached the coil. It also looks as if your 1.8 ohm resistor has been wired the other half of each of those cuts, which would be correct."

Because I stripped back the black tape from the original wiring (remember, I have owned the car from new)and found the white/pink and white/light green wires unconnected, I think that the wires were never connected when the car was built. So the car never had an effective resistor for the first 40 - 50,000 miles of its life. More recently, I took it to quite a few garages to look at the burning points problem and one of them obviously found the problem and fitted the ceramic resistor. I vaguely remember someone mentioning that they had fitted one, but it meant little to me at the time.
I intend to re-tape the 2 wires back into the bundle, as they were. I DO NOT INTEND CONNECTING THE TWO WIRES! As a matter of interest, the two wires complete a circuit when tested with the meter.
The recent overheating of the coil etc seemed to be an unrelated problem of the points gap being nearly closed, together possibly with a defective condenser. As an aside, I have found evidence of some wiring investigation work being done by the radiator, where some of the black tape has been removed and the wires inspected.

I have checked the resistance of the new ceramic resistor which is supposed to be 1.5 Ohms and it is 1.8 or 1.9 Ohms depending which meter I use. I intend to shorten the resistor wire somewhat, to bring it down to 1.5 Ohms. Please correct me if I am wrong!
Many thanks again for all your help. Allan
A Reynolds

I'd be very surprised if the factory butchered a harness like that. It's possible the harness ballast was faulty and shorted out (wiring investigation by the radiator?), it would originally have measured about 1.5 ohms i.e. between those two cut wires.

In theory shortening the resistor wire will lower the resistance, but you may have problems soldering it back to the tag and getting a good connection. For the sake of a fiver you would be better off getting a proper one from the likes of Accuspark or Stealth (other suppliers are available ...).

But 0.3 ohms extra is only 10%, you would be very unlikely to detect that except perhaps on a rolling road.
paulh4

Many thanks - all the help has been much appreciated.

Regards

Allan.

Church Stretton
A Reynolds

This thread was discussed between 29/07/2016 and 24/08/2016

MG MGB Technical index

This thread is from the archives. Join the live MG MGB Technical BBS now