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MG MGB Technical - Battery Issues

Okay, just picked up my very first MG, a '76 B. Had it delivered from an auction last night. Turned it over and drove it around the block this morning. Ran great, all things considered. Just a bunch of smaller issues.
Except that an hour later it wouldn't crank over. Thinking I somehow left something on, I went to put on my battery charger, and when I tried to apply the positive terminal, I gave my neighbor's a free fireworks show. This has me thinking that I might have a dead short somewhere? Anybody have any insight on how I can diagnose what I did?
E Ishmael

The first lesson here is connect the battery with the battery charger turned off or disconnected from the mains, that way there should be no sparks unless youíve got the connections back to front. The reason for doing this is that batteries produce hydrogen and oxygen, this is an explosive mix that only takes a spark to ignite it. I found out the hard way and it was probably only the fact that I wear glasses that stopped any eye damage, I only had a cut on my forehead to show for it.

Regarding your problem the first thing is to establish the state of charge of the battery, the easiest way is with a volt meter. With the engine stopped the battery should read approximately 12 volts it could be nearer 13 volts if itís fully charged and any less than 12 volts means itís fairly flat. If itís down to a really low voltage youíve got something discharging it or the battery is dead. Modern batteries seem to die quickly and can be ok now and dead in 5 minutes time, older batteries would die more gracefully.

Welcome to the MG club and donít let this put you off, these things are all part of the experience. When the sunís shining and youíve got the top down (assuming itís a roadster) youíll know why you own an MG.

R.A Davis

First thing to check is the condition of the main battery to starter cable and it's connection to the latter. After that the brown wire which is under the same nut as the battery cable on the starter, this feeds the starter switch, fuse box, starter relay, alternator and lighting switch. This brown wire is always live and un-fused. Look for the obvious worn insulation and disconnected, dangling spades and bullets. Failing to find anything obvious suspect the relay or indeed the starter solenoid itself. If you have a meter use that to check for a short (continuity) from these browns and the big lead, to ground, BUT WITH BATTERY DISCONNECTED, if you find there is a short, disconnect things one at a time to determine the culprit. The purple wires from the fuse box are also always live but ARE fused. A direct short on that circuit will have blown the "bottom" fuse.
Allan Reeling

Bearing in mind this is your first MG and you have just collected it, you need to look at some basics, no offence intended.

A dead short while the car was parked would have burnt the wiring.

You know that whilst early MGBs have the positive battery terminal connected to the body a 76 MGB should have the negative terminal connected to the body? I.e. the live terminal is positive? You can't go by any cable or cap colours that may or may not be on the cabling and battery terminals. AFAIK the both battery cables were always black, regardless of whether the battery was connected positive to earth or negative to earth.

Where did you connect the charger? Direct to the battery terminals? With the battery recessed under that shelf it's very easy to inadvertently bridge the 'live' i.e. positive terminal to the edge of the recess, and that WOULD create some massive arcing.

What is the power of the charger? Unless you have a jump pack capable of cranking an engine I doubt it would generate much by way of sparking at all, and I'd expect it to be fused anyway.

But going back to why it didn't start. The first thing to check is whether the ignition warning light comes on with the key or not, or whether you can turn any of the lights on. If neither then it could be nothing more than a bad connection at the starter solenoid or the battery. If you get external lights but no warning light or fuel gauge movement, then it's probably a problem with the ignition switch and its wiring.

If they do come on, turn the key to crank and see what happens. If they go out then either the battery is flat, or again it could be bad connections at the solenoid or battery.

If they stay on then the ignition switch isn't energising the starter, which requires diagnosis through the switch, starter relay and starter.

Shorting from battery to body is only possible when the battery is connected. A good reason for disconnecting before charging. Also when connecting a battery always connect live first and when disconnecting remove earth first. That way you can't create a dead short to the body and melt a spanner and your hand!!.
Allan Reeling

I was clearly an idiot and accidentally touched the edge of the battery compartment with the charger. Checked the battery, was low, charged it, and still just a click when trying to turn it over. I have the red ignition light. I checked and cleaned terminals, and checked all of those brown wires leading to the solenoid. Tried to jump the solenoid with my hammer, which also didn't work, so I bought a new starter/solenoid and will be swapping that out as soon as it stops raining.
E Ishmael

It was the starter. I'm back up and running.
E Ishmael

"accidentally touched the edge of the battery compartment with the charger."

It's easily done on the MGB, which is why many use a point in the engine compartment or cabin for charging, also more convenient than removing and replacing the battery cover.

In 1972 (optional) and from 1973 (standard) the cigar lighter socket was factory wired to the purple circuit (fused always live), which by putting a suitable plug on your charger wires makes it simplicity personified to connect, and it's fused so safe. Other than that you can use the purple circuit at the fusebox in the engine compartment.

If the battery was low as well as the starter problem then you may have a drain, so it will be something to keep an eye on.

Well, I'm officially stumped. Swapped out the starter last night, and it turned over like a charm. This morning turned over again right away, so I drove it around the block to get it in my driveway instead of the street. Went to turn it over an hour later, SAME ISSUE. Starter clicks but doesn't crank. This happened the exact same way. Drove it around the block, then let it sit. Return and it won't crank.
E Ishmael

I would double check all the battery terminals, including the earth strap and its grounding point. Also check the earth strap from the body to the engine. Check the battery cables for continuity and corrosion around the connections.
Recheck the battery voltage, maybe with the headlights turned on. A good full battery should show around 12.6 V, a bad one less.

H J Adler

As herb says you need to check the battery voltage then check the battery cables and earth straps, it only takes a small resistance at one of these connections to cause your problem. Having tried starting the car you can also feel the connections, the bad connection will probably be warmer than the rest.

If that doesnít help youíll need to be more methodical, measuring the voltage at each joint to ground until you find one that has 12v while not cranking and almost zero while cranking. Youíll also need to measure the voltage across the battery earth lead and engine earth strap in both cases the voltage should be zero, neither should measure more than a small voltage while cranking. When measuring these you should connect your meter between the earth lead and clean point on the body. You can also check the engine earth strap by measuring the voltage between the engine and the body, it should be virtually zero while cranking.

A friend had a similar issue that I traced to loose connections on a battery isolator.

R.A Davis

If lights dim right down when it clicks but won't crank then it will be a bad connection at the battery cable stud on the solenoid or the battery connectors. If not then power isn't even being applied to the starter motor.

If the lights dim right down you will have to use a meter across various points to see 12v, then see what happens when someone turns the key to crank. Such places are:

Between the battery posts - if this drops right down the battery is bad.

Between the battery connectors - if this drops right down one or both of the connections between connector and battery post is bad

Between the 12v battery connector and a good body earth else where - if this drops right down then the battery cable to body earth connection is bad.

Between the battery cable stud on the solenoid and a good body earth elsewhere - if this drops right down then there is a bad connection in the battery cable where it fits in the battery connector, or where it attaches to the solenoid stud.

Between the battery cable stud on the solenoid and a good earth on the engine - if this drops right down the engine earth is bad, although usually with this the engine tries to turn over and the accelerator, choke, heater cables and temp gauge capillary tube get hot and may well smoke.

If the lights don't dim what can you hear clicking? I.e. is it the starter relay by the fusebox? Or the solenoid on the starter?

If the former then you probably have a loose connection somewhere from the brown at the starter relay (always live, unfused so take care), through the relay onto the white/brown, and from there down to the solenoid.

If the latter then it implies a problem with the solenoid or the motor, but that seems unlikely given a replacement motor.


Gonna start from scratch and try to jump start the battery. When I test it it shows 12.35 volts, so obviously it's a little low. Where do you all attach the negative for the ground with it being in the backseat? There's not a great body spot for me to connect to.
E Ishmael

Paul, I'll be trying all of suggestions in the morning, but several of them I cannot do, as I seem to have the unfortunate additional problem of not currently having headlights. The headlights have been super intermittent, I can only occasionally get driving lights, and have only been able to grab the headlights once or twice. I can, however. Flash my brights from the turn switch.

I should also state that I've got an after market piranha ignition. Any chance that's could be messing with any of this?

I'm confident a lot of these problems are connected, I just can't find it for the life of me.
E Ishmael

Attach neg from jump lead last and to engine block, engine earth strap etc,, If you have no lights except flash that may be a problem with the lighting switch . The flash is fed by a different circuit than the lighting switch. As I said earlier the switch is fed from the brown, always live unfused, the flasher from purple, fused.
When you say you can hear a click but doesn't crank, it could be you are hearing the starter relay click. Some of these, especially the 6RA 'lookalike" replacement items are incredibly bad. The click is the electro-magnet pulling the contacts in, but these are the issue, often so little contact area they are not able to pass enough current to energise the solenoid, sometimes they weld up! Try by-passing the C1 to C2 terminals temporarily with a fly lead while someone turns the key, if it starts, immediately disconnect otherwise the starter will keep going. Modern square or round relays are labelled 87 and 30/51.
Allan Reeling

Any lights will do, doesn't have to be headlights, could be interior light for example, or ignition warning light, or parking lights if someone else can watch.

Testing the voltage of a battery off-load tells you very little unless it is very flat, it needs to be on load. If you can't get the headlights to work then brake lights and reversing lights (ignition on) should be enough to expose a weak battery.

A 76 should have a gearbox earth strap, round a left-hand rubber mount, rather than the left-hand engine mount earth strap on CB models.

Your ignition system is very unlikely to be affecting your ability to crank, or the headlights, unless someone has bodged some wires from it onto a starter or headlight circuit and left a poor connection.

There is no light dimming. Tossed in a new starter relay per the mg mechanic in my neighborhood. No luck. Replaced potential bad connection on both the negative battery cable as well as a wire that looked dodgy off my electric ignition. No luck.
Gonna putz around with it a bit over the weekend, but it looks like it'll be out of commission until my mechanic can look at it on the 13th.
Beyond bummed out.
E Ishmael

Alright. Tried jumping the starter and it wouldn't turn over. So I got a new starter coming in the morning and we'll see if it was a faulty starter.
E Ishmael

Old one and new one with the same fault? Possible, but ...

How did you jump the starter? If you link the battery cable stud to the solenoid operate spade, then it should crank.

However. There are two spades on the solenoid, one is the operate terminal, and the other is the bypass terminal that boosts the coil voltage during cranking. Originally they were different sizes - the operate terminal (that carries the thickish white/brown wire) was a standard size spade, and the bypass terminal (thinner white/light-green wire) was smaller. But I've had a rebuilt starter where both were the same size.

The first time it happens does mean you have to replace the connector on the end of the wire, but after that - if you get another starter with the same sized spade terminals, it's easy to get them mixed up. I've attached a (mucky) picture showing the terminals - albeit on a 73 which had the same starter, but the boost terminal wasn't used. A is the battery cable stud. B is a large 12v spade used for bown wires before they got the bolt-through terminals to fit on the battery cable stud. C is the solenoid operate terminal, and D is the bypass terminal, which is very close and just above the solenoid operate terminal.


From what I can tell, it is assembled the same as yours. Added a photo as well.
And I linked the battery cable stud to the solenoid operate spade with no luck.

E Ishmael

It was the replacement starter!!! After all that, I ripped the starter back off, brought it down to the parts store, had them test it, it failed, and grabbed a new one. Started right up. The real test is whether it was the starters fault, or something else in my wiring that fried it.
I guess I'll know soon enough.
E Ishmael

Well there you go, increasingly par for the course, whatever happened to manufacturer testing. I had a replacement motor on my V8 for years, and it was only when investigating a hot-start problem that I discovered there was no coil boost function. When I took the back off the solenoid the contact was bent backwards, away from the bar that should have been supplying 12v to it, and so had never worked from new.

Again, sticking my neck out a bit, I can't see what external wiring/electrical fault could cause the solenoid to fail to operate. The solenoid has two parallel windings - one very low-resistance, high current pull-in and another slightly higher resistance, lower current hold-in, and both are required to operate it. The pull-in is wired through the motor circuit, so that when the solenoid operates and its contacts connect 12v to the motor, they short-circuit the pull-in winding so the current through the solenoid drops. Without that the solenoid would burn out with relatively brief cranking. So either or both of those winding circuits could have been at fault, and I doubt anyone will be bothered to find out.

This thread was discussed between 30/05/2016 and 05/06/2016

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