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MG MGB Technical - Sudden engine stop

I knew this would happen eventually!
Out for a 20km round trip run on a 28degree (Celsius) day. (1963 Mk 1 with 600 miles since rebuild, standard ignition, points, coil, plugs).
1km from home on a busy highway intersection waiting patiently for a green light and it suddenly stopped. Went to start it (thinking it just stalled) and nothing - no ignition light, no fuel pump click, no indicator, no starter. Some friendly drivers pushed me to a safe spot (in the shade) where I stopped panicking and started thinking.
Battery cables firmly attached, I pushed the starter knob on the solenoid and I had power to turn the engine OK.
I jiggled the wires at the fuse holder and the regulator, then out of frustration, gave the regulator a smack with my hand - pushed solenoid button and she started immediately and I was able to drive home without further incident.
Ive looked through the archive, and others have experienced similar problems, so the question is whether -
- The coil suffered heat fatigue and 10 mins in the shade allowed it to cool down (conventional replacement for original, 600 miles old)
- The regulator suffered a meltdown (brand new 600 miles ago)
- A wire became loose

This would be the hottest day I have had it out since the rebuild.

Thoughts?
John Minchin

John, having had a similar experience with my 1979 MGB, I would suspect the ignition switch. Before you do fault the ignition switch, you need to check a few things.

Was the car running strong and just quit, or did it stumble and die?

Did you have headlights, horn, and other accessories which do not require the switch to be on the "ON" position?

Did the tachometer just drop to zero or did it fluctuate before it dropped to zero?


Check ALL connections. I replaced about 20 connector ends and cleaned the connector tabs. Doing that job alone cleared up other smaller electrical issues.



Cheers

Gary
1979 MGB





gary hansen

Hi John
I agree with Gary here, I'm thinking a loose wiring connector, probably around the fusebox or reg.
A loose connection will run warm which in turn will make it expand and become looser still
If you go around them and give them a little wriggle / pull and give the loose ones a squeeze up and refit to make them tight I reckon you will fix it, specially seeing that it drove home OK

Had it been the ign. coil the ignition light would have still been operating normally so I doubt that was your problem----Same with the reg. if they die they don't become undead
I'd be concentrating my efforts on going around all the connectors you can find and making sure they are a good tight fit
Cheers willy
William Revit

Definitely not coil, it would still have cranked as well as still having an ignition warning light.

Neither is it fusebox connections as none of them are involved in cranking.

I wouldn't expect the fuel pump to click (if a standard SU) as the carbs float chambers would still have been full, although it's possible to get just one.

It won't be the regulator having a 'meltdown' or it wouldn't have corrected itself shortly afterwards.

Almost certainly a loose connection, somewhere you jiggled wires immediately before it started.

It's possibly a battery connection. When driving, the dynamo would still supply power to the rest of the cars electrics. But when idling (unlike an alternator) it may not.

On a Mk1 power comes from the solenoid to the regulator, and is daisy-chained from there to the ignition switch, implying it could be a connection at the regulator. However the factory diagram shows both those browns on one terminal, implying they are in the same wiring connector, plus a separate brown on another regulator spade going to the fusebox. If that's the case then you should be able to remove the first two from the regulator, and as long as they are still connected together you should still be able to crank and start the engine, only charging would be affected.

That leaves the ignition switch, and its connections. There are no other inter-connections between battery and coil. As such it would have to be in the brown wire. Whilst a fault in the white would stop the engine and prevent the ignition warning light glowing, it wouldn't prevent cranking. A fault in white/red starter wire would only be discovered when you tried to start the engine and it failed to crank, that wire would not affect the ignition warning light.
paulh4

Thanks for the suggestions. I will study the circuit diagram again to ensure I fully understand what has been suggested before doing anything.

Further comments/answers to your questions -
It was a sudden cut out - no stumbling or rough running

Batter cables are very secure, as are cables at the solenoid.

Tacho is mechanical - it stopped with the engine

When ign light was still off, I was still able to crank with solenoid

Ign switch is the original - ie 53 yrs old

I had been poking about replacing the dual gauge recently so I may have disturbed the wires connected to the ign switch when reaching through the radio opening.

I will check all the connections - clean, tighten, etc.

Risk Management - What about a temporary jumper wire with alligator clips to get emergency power to the ign circuit in the event of a failed ign switch? Just to get home...

John
John Minchin

John. I had this issue with my 67 after a motor rebuild. It was a loose connection on the low tension wire from the coil to the distributor.

It would just die. Make it to the side of the road, open the hood, jiggle a few wires and it would start and run fine until it did it again.

While checking, I found the crimp connection on the low tension lead was loose.
Bruce Cunha

As soon as the engine died and you grabed the coil with your bare hand did you get a 2nd degree burn on your hand?

Prop
1 Paper

"What about a temporary jumper wire with alligator clips to get emergency power to the ign circuit in the event of a failed ign switch?"

Just clip the brown to the white at the fusebox ... or better still the purple to the green in case the green/white clip falls off. If on the brown and the white clips earths it will burn your harness, unless the jumper burns through first.
paulh4

Thanks Paul and bruce

The coil was hot, but not that hot!

John

John Minchin

Just sitting studying the circuit diagram and thinking things through bearing in all your comments.

Key issues-
Sudden engine stop - means no spark, no volts at coil
No ign light means no volts at ign switch (on position) - either no 12v to switch, OR faulty switch.
Cranked OK by manually operating solenoid - means ign switch would have 12v UNLESS there is a fault in the brown wire from the solenoid via reg to the switch.

As I havent touched the brown wire connection to the Ign switch, the likelihood that it is a bad connection that came right is low.

I may have jiggled the brown wire connection at the solenoid which could have been a bad connection. Or, I suppose there may be a fault in the brown wire itself (its the original 50 yr old wiring loom) so I could bypass it with a new wire. Otherwise I suspect a fault in the ign switch itself.

Any other possibility that I've missed?
John Minchin

The ignition switch, and the lighting switch for that matter, on early B's carry quite a high current load, so likely to be past their best. If replacing either switch it would be a good idea to introduce a relay. One on the starter circuit (White/Red) will take some of the load off the switch contacts. A relay on the headlight circuits takes the load off the lighting switch and has the added benefit of giving you improved light output.
Allan Reeling

Yes Allan, I remember using some light relays on a 67 MGB I had back in the 1970s.
John Minchin

Bad connections can develop all by themselves, and go away again, they don't always need encouragement, inside switches as well as at their spades.

Tight connections can go bad, they don't always need to be loose. Had that on my starter solenoid and that was bolted brown wires, not spades like yours, when the roadster was in for an MOT - absolutely no problems beforehand. Totally stumped the mechanics, couldn't understand why it was still dead when they put a jump-pack on the batteries.

Unlikely to be the wire itself unless it has previously suffered damage, and there are no connections between solenoid and switch shown on the diagram.

As it's neither here nor there I'd clean and check for tightness and general condition the brown spade connections at the solenoid and the switch.

I currently (no pun intended) have a very intermittent 'click but no crank' on my ZS180, and have an analogue battery sitting on the console wired to various points in the circuit. The battery connections showed good voltage when the problem happened, and again more recently (like today) so did the battery cable stud at the solenoid. The link between the solenoid and the motor is open, so that is the next place to see whether it is solenoid or motor. I suspect the former.
paulh4

I ran into the same problem with my '71B on my way home from storage. The car ran fine for the most part, hicupped but I attributed that to stale gas. Then the car went south. Missing, spitting, bucking, backfiring. It finally died and would not start though it would crank over. I called for a hook home and once there, I started cleaning all connections, including the multi-plugs under the dash. Once this was done, the car started right up. Took it for a test drive around the neighborhood and it started acting up again. I barely made it back home and rechecked my connections and wiggled the wiring loom under the dash. I checked my new connections and the car now runs fine, however, I'm hesitant about taking her out now. I'm not sure replacing the ignition switch is in the future and whether that will solve my issue. On the upside I never leave without a cell and have plenty of towing coverage.
Keith Yarbrough(TD 1275)

If it's the ignition switch or any connections in the circuit from that through the coil and points it will be obvious from the tachometer, either jumping around when the missing is happening, or suddenly dropping to zero when it cuts out altogether but the momentum of the car is still spinning the engine. That's the very first diagnositic.

As far as my ZS goes having moved the meter onto the output of the solenoid that is definitely the culprit. Waiting for some warmer weather to get the starter out ... but not too long, it's getting a bit more frequent.
paulh4

Make sure that the engine earth strap is bolted to the chassis side of the engine mount and not to the engine side. When the engine was refitted in my 1970, the strap got attached to the engine side stud so grounding was through the speedometer, choke, accelerator and heater cable. All of this finally came to a head when there was a short in the starter solenoid and these various cables became a direct part of it.

Glenn Mallory

I found my problem to be a bad wire from the ignition. Cleaned all connections and it started right up. Thanks for all help
Keith Yarbrough(TD 1275)

Update
After a few weeks enjoying summer and 3 weeks sick with a bug, I decided to finish what I had started .

Carbs - jets adjusted and a carb balancing device created and installed ( made from pieces of wire coat hangers )

Plan was to balance the carbs, set the idle and then adjust the choke.

Cranked the engine using the solenoid button - nothing
Couldn't hear the fuel pump click and thenthe penny dropped. Tried to start with the ignition key - nothing.

I am now narrowed on dodgy ignition switch first, and faulty wire from switch to fuse box. There are no volts at the end of the white wire from the ignition switch - hence no fuel pump operation, no coil op and no spark.

As the ignition switch is 55 years old I might treat myself to a new one, and if that doesn't solve the problem, a new wire to the fuse box.
Any recommendations for ignition switch manufacturers or suppliers? I see plenty of Lucas branded units ( I assume they are genuine) and despite criticisms of Lucas products, the switch has done pretty well I think.

John
John Minchin

If you mean by 'nothing' it cranked but didn't start, then your diagnosis could well be correct. However the fuel gauge would also not have moved and the ignition warning light not come on.

If either of those moved/came on then it's not the switch but a connection somewhere. Prior to 1973 the feed to the coil was a separate wire from the ignition switch via the tach. If that circuit is broken then you would get the warning light, fuel gauge, and cranking, but no start.

The feed to the fusebox is not used for the coil. If that circuit is broken (and nothing else) you get no fuel gauge (or brake lights, indicators, wipers, heater fan etc.), but it should start.

If you got no ignition light, no fuel gauge, no start, but it cranked then it almost certainly is the switch.

If it didn't crank either then it could be the switch, but it could also be a break in the brown circuit feeding 12v to the switch.

Diagnose fully before buying a switch, they are expensive (at least the later steering-lock types are), and regardless of the name on the box it's very unlikely to be an OE, but a remanufactured item from one of the many companies that have owned or licenced the Lucas name in the past decade or two.
paulh4

You are correct Paul.
No ign light, fuel gauge, wipers, fan, fuel pump.
I will remove the radio - again (it sounded like a good idea to install a period radio but it certainly gets in the way)
Then I can see if I have 12v on the brown wire at the switch and 12v on the White wire connection with ign on.
John Minchin

I have been mislead by the circuit diagram I have been using. It's one of the redrawn circuits that are very much easier to follow than the original diagrams.
What I discovered was that the new diagram shows the brown wire from the solenoid being twinned with the brown wire via the loom to the ign switch. This would mean that the state of the spade connector at the regulator would be irrelevant . The reality, and the original circuit shows it, is that the wire from the ign switch is twinned with a wire to the fuse holder, and a separate wire goes from the solenoid to the regulator. Hence there are two spade connections between the 12v at the solenoid, and the ign switch.
This morning I turned the ign on ( no light, pump, fuel gauge etc) then reached under the regulator to check the connections. I just barely touched the wires and the ign circuit came to life.
So I have some cleaning and tightening of the connections to do, and hopefully this will fix the problem ( I will still check the connections on the ign switch just in case)
John Minchin

This thread was discussed between 08/01/2017 and 26/02/2017

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