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MG MGB Technical - ignition coil gets very hot

I have a 1975 MGB, and the starter coil gets very hot when I try to start it. I would appreciate any suggested remedies.
Christine Kestner

Christine

When you say "starter coil" are you referring to the ignition coil the ballast resistor the starter solenoid or starter relay.

If you're referring to the solenoid on the starter motor, does the starter operate normally? How long are you operating the starter before the coil gets hot? How hot is it getting? If something is getting hotter than normal generally it's because of an increased current flowing through the device. It can also be cause by an increase in resistance of the solenoid contacts or terminals. If the starter is operating normally I would suspect a fault in the solenoid. If the starter is operating slowly it could indicate a fault in the starter causing the increased current. It could also be caused by the contacts in the solenoid starting to fail or the two main terminals on the solenoid not being tight.

The below link may be of assistance.

http://www.venselenterprises.com/techtipsfromdick_files/solenoidrebuildingarticle.pdf
Bob Davis

I meant the ignition coil, and I do not think it has an external ballast. The car had not been started in awhile, so it was taking several cranks to get it going, but the coil got very, very hot.
Christine Kestner

Christine. Your 1975 MGB should be equipped with a Lucas 45D4 distributor, a points type distributor. It is possible that, over the last 35 years, this has been changed out or some form of points replacement system has been installed. This would be a good thing to know when trying to diagnose the problem.

The coil is a Lucas 16C6 coil and should show 1.43 to 1.58 ohms resistance across the low tension terminals (the ones having the white/light green wire going to the positive terminal and the wire going to the distributor. This is a 6 volt coil and the white/light green wire is a resistor wire which drops the voltage down to near 6 volts.

Your model, according to both of the wiring diagrams I have consulted, has only a single wire going to the input (+) terminal of the coil. Later models, due to hard starting with all of the pollution control additions to the basic engine, had a second wire going to the positive terminal of the coil. This second wire was connected to a second spade terminal on the starter solenoid which provided 12 volt input to the coil when the starter was engaged. It is possible that your car has been modified in this manner over the years. Knowing if it has would help diagnose the problem.

In any event, do a resistance check across the primary terminals and, if the resistance is out of specification, replace the coil. Your coil does use a resistance wire which functions in the same manner that a ballast resistor does. Thus, a check of the input voltage on the white/light green wire would also be worth knowing.

Les
Les Bengtson

Christine

If you don't have a ballast resistor (and it may not be obvious as some are part of the wiring harness) the coil should be 12v. The only reason a 12v coil would overheat during starting is because the current is flowing through it for to long a period. The current through coil should be pulsed by the points (assuming you don’t have electronic ignition) and is only on for a proportion of the time. I'd put a 12v lamp in series with coil and try to start it, you should see the lamp pulsing. If it doesn’t I'd look at the points and condenser. A short circuit condenser would cause this problem.

You don't say if the car actually starts.
Bob Davis

A 75 i.e. a rubber bumper did originally have an external ballast and a 6v coil. However the ballast is a length of resistance wire concealed in the harness so is not immediately obvious, except that there was a white wire feeding it from the fusebox end and two white/light-green wires at the coil +ve end. This could have been bypassed for whatever reason, and if you still have a 6v coil then this will overheat.

The only way to determine what you have, and what you *should* have, is by a combination of voltage and resistance measurements, although even that is complicated by whether you have points, the early 45DE4 distributor with attached electronic module, or the later 45DM4 distributor with remote electronic module. The 45DE4 is complicated by the fact that is has it's own ballast resistance as a separately identifiable component as well as the coil ballast. The 45DM4 only uses the coil ballast.

Easiest to start with the coil, measure the resistance between the two spade connections with the wires removed. If that measures from 2.4 to 3 ohms it is a 12v coil and should not have a ballast, or it will give low spark energy. If it measures from 1.2 to 1.5 ohms it is a 6v coil and *must* have a ballast in series with it or it will overheat.

If you have points or the 45DE4 distributor then turn the engine over manually with the ignition on, and measure the voltage on the coil +ve terminal, wires connected again. If this stays at 12v as the pistons go up and down then there is no ballast in circuit, and you must have a 12v i.e. 2.4 to 3 ohm coil. If there is a ballast in circuit then the voltage will switch between 12v and some other voltage as the pistons go up and down. If it switches between 12v and 6v, and you have a 6v coil going by the resistance measurement, then you have the correct combination. If it switches between 12v and 9v then the chances are you have a 12v coil when you should have a 6v, but this is unlikely if you think the coil is getting too hot.

If you have a 45DM4 distributor then you will have to measure the resistance between the white/light-green wires removed from the coil +ve terminal and the fusebox white terminal, as this distributor only supplies a pulse of energy to the coil while the engine is turning, unlike the points and 45DE4 systems. If you see zero resistance there is no ballast in circuit so you must use a 12v coil (or find and remove the wire that is short-circuiting it so the correct 6v coil can be used). If you see about 1.5 ohms then there is ballast in circuit and you need the 6v coil.

The 6v coil with ballast is a better system as it should be giving a boost to the coil during cranking to make starting easier.

Finally the coil *does* get pretty warm with the engine running, and it will in fact overheat with points or the 45DE4 distributor if you leave the ignition on with the engine stopped.
PaulH Solihull

Paul. "The 6v coil with ballast is a better system as it should be giving a boost to the coil during cranking to make starting easier."

As I noted, the wiring diagrams for the North American specification 1975 MGB only have a single white/light green wire going to the positive terminal of the coil. Hence, no full 12V input when starting. The information I provided came directly from the factory workshop manual for the NA spec 1975 cars.

Les
Les Bengtson

As I've always said I'm still learning, and this is a case in point. The schematic for the NA 75 does *not* have the boost circuit as you say, but the UK does from 74 1/2 until the end of production, and it appears on the NA for 76 again to the end of production. As the only reason for having an external ballast is so that it *can* be bypassed to give a boost during cranking, I suspect it is an error on the schematic unless actual examples show otherwise. I can think of no reason why that year for NA should be different.
PaulH Solihull

My ex- USA '75 did indeed have the 6 volt coil and the infamous "hidden in the front loom" ballast resistor. Infamous because this pink and white wire often melts its insulation shorting wires nearby (mine did). It runs from near the coil, across the front of the car and back again and changes to a green and white wire just before it reaches the coil. Prince of darkness stuff. It's a very good idea to substitute a ceramic after-market ballast resistor ($10) . Some DPO may have shorted the ballast resistor and be running the 6 volt coil on 12 Volts all the time instead of just when starting. That will warm it up a bit!
Peter Sherman

Often? Never heard of it before and all RB cars and V8s have it. More likely to be because the coil, or the feed wire at that end, shorted to earth.
PaulH Solihull

This thread was discussed between 11/11/2010 and 17/11/2010

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