MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

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 - Sudden loss of ignition.

I took our '73 BGT to a car show 4 hours' drive away at the weekend. She ran beautifully 'til the halfway mark; I stopped for a cuppa, and - here's the rub - plugged a 3-point adapter into the cigarette lighter socket, to supply power to a "Head-Up" digital speed reader and to power up my phone. After about 20 minutes, sudden loss of what I assume is ignition power - ie motor trying to stop without notice, tacho dropping to zero ... and then after a couple of seconds, recovery to more or less normal; then repeat every few minutes. I was miles from anywhere, and it was dark. Not good.
So I unplugged the triple-socket gadget, whereupon the car resumed normal service, and remained good all weekend. On the return 4-hour journey, I proved that it runs fine with just the GPS speed reader plugged in.
Anyone have an idea of a possible cause? - as far as I know, the power supply to the lighter socket is as original.
Any advice gratefully received. Regards, John.
J P Hall

Sounds like the adapter may be dodgy- did you check internal connections.
Mike
Mike Ellsmore

John:

"ie motor trying to stop without notice, tacho dropping to zero..."

Do you still have headlights and other accessories??

When the cars quits, does the starter NOT work when you turn the key to "start" position?

I had similar experience with my 79 B. Turned out to be ignition switch was on its way out.

Cheers

Gary
79 MGB
gary hansen

I'd have a look at the fuse block , remove the fuses and give them a little pull to make sure the end caps are secure and give the fuse holder 'tangs' a little tweek inwards to make sure they are getting good contact with the fuses
If anything there is a bit on the loose side the heat from the extra load can open the tangs a bit and open the circuit
BUT, if it only happens with the adapter in line, there is an issue with some adapters in that they are built for power outlet sockets which are slightly different to some older cigatette lighters and they short out if you push them right in --although if this was the problem you would think the fuse would have blown
I'd check the fuses and connections first and if everything is ok there, then have a good look at how good a fit the adapter plug fits up or try another adapter
willy
William Revit

That's a very odd one to say the least. The cigar/power outlet is a permanent live (purple) taken almost directly from the battery, all be it, through a fuse box fuse. Hence difficult to see a direct connection to the ignition circuit, except that the other (Brown) end of the fuse does feed amongst other things the ignition switch and the lighting switch.
Apart from requiring a 12v supply to power it, what other feeds are there to the HUD?
Allan Reeling

As said the lighter socket is fed from the purple circuit via the fusebox and is live all the time.

When you say it's a 73 does it have the RVI tach or the RVC tach? It changed between 72 and 73 and the difference is significant where ignition problems are concerned as with the earlier RVI tach the feed to the coil goes via the tach. With the later RVC system it goes direct to the coil and a trigger wire comes back to the tach.

With the tach dropping to zero instantly as well as the engine cutting out it shows it is power to the white circuit (and hence the green via the fusebox) from the ignition switch that has been lost.

Did you notice if the ignition warning light came on as well? If so then that shows power has been lost from the ignition switch itself as there are three white from there - one to the warning light, one to the OD, and one to everything else.

If the warning light remained dark - and it normally works as it should - then the switch itself is OK and the break is closer to the coil i.e. the 'everything else' circuit.

For a 73 i.e. with an RVC tach that goes to a 4-way bullet connector in the mass by the master cylinders, with two wires from the main harness and one to the rear harness for the fusebox. One of the two from the main harness is the feed from the switch and the other is the feed on to the fusebox and the coil.

All that is how things are _supposed_ to be wired, so a common-ish point is where the white and purple circuits sit side by side. It's faintly possible that if the socket were faulty, and putting too much current through the fusebox but not enough to blow the fuse (possible with tarnished contacts) that it could be impacting on the white. But then the two white wires on the fusebox should be in the same spade connector, which means you could pull those off the fusebox and the engine would still run, even though you lost all the fused ignition circuits.

Which leaves the brown circuit as the only true common point. For that a feed comes from the solenoid to a sealed 3-way branch - one to the lighting switch, one to the starter relay, and the other to the fusebox. That branch is highly unlikely to fail, which leaves the connections up from the solenoid, and I have known those to go intermittent and cause total loss of power. So as Gary asks - did the lights and starter fail?
paulh4

Thanks for helping: I did not notice any dimming of lights, and I don't believe the ignition warning light came on; it was all very sudden so it was "action stations", being in the dark with traffic behind me.
I didn't test the starter because I kept the car going (just) until, each time, it recovered a reasonable amount of power; and then when I unplugged the triple-adaptor and normal power returned, I just maintained a steady speed (100kph) for another hour until destination.
No other feeds, Allan, just the 12v cigarette plug.
Paul, I'm at work for a few more hours but will look at the tacho type when I get home.Is RVI or RVC indicated on the face, or do I just trace the wires?
More soon ... thank you all. John.
J P Hall

John - it should be written on the face.

With no dimming of lights this is a weird one ... subject to what any PO may have done with the wiring!
paulh4

Tacho is RVC Paul. Car ran fine again today; I like Willy's advice of checking fuse connections as well as the wiring to and from the lighter socket. Because of the sudden but intermittent interruption to ignition,I had been trying to figure out what connection there might be between coil and lighter socket, including possibly a shared earth. Until the weekend, as the car is functioning normally with nothing plugged in to the socket, I'll leave well alone. I'll come back with an update after further investigation. Thanks all until then. John.
J P Hall

John,
As a species engineers and classic car owners both tend to be over analytical, but there are times when you just have accept, that the removal of the (probably) Chinese device, which has resulted in ending of symptoms, IS the cure!!! I know we need to know, but as we have said it's weird and not logical and I for one have been around auto and domestic electrical systems long enough to realise "weird" things do happen, as does coincidence!!!
Allan Reeling

Originally any sharing of a 12v supply was marginal at best as described above, but what it is now is anyone's guess. Again any shared earth is (was) marginal as the coil earths through the distributor to the engine to the body and the lighter socket to the body, the only common point being the battery earth cable. If there were a problem with that it would show up with cranking first of all.

Do you have electronic ignition? That could be powered in some strange way, and be suffering interference from this other third-party device.

You could leave well alone, but personally I'd be plugging it back in and looking for how voltages change to cause the cutting-out. It may be symptomatic of a worsening fault that could cause total ignition failure at some point. But that's just me :o)
paulh4

Allan, there's no doubt that what you say is true. Guilty as charged.
However, I DO have electronic ignition, a fairly old Lumenition setup - I plan to have an auto electrician check the circuits to see if we can find a root cause - precisely because of what Paul says, I don't want my wife's daily driver to let her down somewhere.
No-one has come back and said "I had exactly the same problem .. and the cause was x or y" - so I'm happy to plug on (no pun intended) and try to find the root cause. Regards, John.
J P Hall

John,
Oh I know, the need to know!!!!! I would be exactly the same AND the old electronic system you didn't initially mention. Apart from having it's coil connections I presume it has a 12 supply...............from where?
Allan Reeling

I do apologise - the electronic ignition clearly could be a factor, and I clean forgot until Paul mentioned it. I'll have a closer look and report back - I generally only post a problem if I think it (and its solution) might help others out there. John.
J P Hall

If the car runs OK without the adapter in place it would appear that it is that unit that is source of the problem.
Take all the cars wiring out of equation by running temporary supply direct from the battery. Use a length of twin cable (household) fused at either one end or the other. This will make sure Her daily transport does not fail. If this solves the problem then do the job properly with suitable coloured cables, make sure you mark the cables as to their function at both ends for any subsequent owners ie "supply to cig skt" and "supply from bat". Suggest that you insulate any "bare" spades with shrink sleeve, not tape it degrades over time, and also mark the unused cable as "old cig skt supply
W Scott

Good advice, WS - I can confirm that the car hasn't missed a beat since I took out the 4-prong adaptor; I like the idea of running separate power supply as a test rig, and yes for some time now I've used heat-shrink in place of insulating tape.
I'm thinking the cheap power adaptor will be consigned to the bin, once I've confirmed by separate wiring that it is at fault. Regards, John.
J P Hall

Not sure about 'Take all the cars wiring out of the equation'. The battery to starter cable would have to be left or it wouldn't start, and the temporary wiring would have to be extended to every component that has a brown wire, and from all of those components to the components they connect to, and so on. That would double-up on the wiring and connections at least, and unless you did something particular with the alternator all the brown wires would still be live. Way over-kill, IMHO.
paulh4

This thread was discussed between 04/06/2018 and 07/07/2018

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.