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MG MGB Technical - Dead MGBGT

My 1971 BGT packed up on Sunday night, I'll explain what went wrong and would like some thoughts as to what the problem might be before I make a start on it.
I was driving with the headlights on and noticed them dimming every so often, the car also had a slight missfire now and then, this went on for 20 miles until it died.
The lights and engine cut out and I came to a halt. The ignition switch was dead, as if the battery was disconected, the only thing that still worked was the horn and the interior lights, everything else was dead, the horn was still full volume so the 2 x 6 volt batterys are ok. The car has run superbly until now so there has been no sign of things deteriating.
I have gave it some thought and beleive the ignition switch could have packed up.
If anyone has had the same problem or has any other ideas then feedback would be most welcome.

Many thanks!
S CREWE

Similar things have happened to me, check:

1) Battery connection/ condition. Sometimes the connection works itself loose. "Out of sight, out of mind" applys. Rattling around in a remote location will do that.

2) White wire dual pin connector (near fuse box). this is on the circuit for the ignition. wires can come loose, the dual pin connection is prone to corrosion and crumbling away (happened on both my '72 roadster and '73 GT)

3) Points wire in distributor came in contact with distributor cam shaft.

4) Points locking screw came loose, points closed up.

5) Distributor locking nut came loose, distributor rotated.

6) Connections at solenoid crud up, and need cleaning.

These are the simple, visual things to start with.

(You know, until I started this list, I was under the impression that my cars were somewhat reliable.)
John Z

I've had big short circuits cause those symptoms on a couple of occassions, I'd look around for something making contact with something that ought not to be. Since you didn't say you smelled anything, maybe start in the engine compartment instead of around dash and ign switch, especially examine all the big wires running to the alt, the fuse box and the distributor wire.
Greg
Greg

I vote for the wires at the solonoid. Headlights are not on the white wire circuit so that rules out anything at the distributor or the double bullet on the white wire. The problem is upstream from the ignition switch. I'll bet the four-way flashers don't work either. Along with the headlights, interior lights and horn, the four-ways are the only items that work independently of the ignition switch. The headlights are unfused; i.e., they work directly off the brown wire circuit. I don't have a wiring diagram handy but I'm guessing this problem must exist somewhere between the junction on the solonoid and the brown wire connection to the fuse box.

Sometimes the twisting movement of the engine, after 33 years, can cause wires to the solonoid to weaken and/or break. My '71 GT (now sold) did this. It apparently had done it before I owned the car and it looked like the PO had cut the old terminals off, stripped the wire back and fit new terminals. This shortened the wires, making them even less tolerant of engine motion, and the likelihood of recurrence increased.

If this is your problem, you can fix it easily, but expect a recurrence unless you exten the leads by an inch or two (solder all connections!). ' Hope this helps. - Allen
Allen

One I have learned the hard way, but easy to check: clamp a jumper cable to the block and the other end to the frame. Bad grounds always make for interesting symptoms.

Pete
Pete

I second Pete's comment. I would definately try Allen's idea first however, and take apart the connections at the starter solenoid first. Check, clean and/or repair those as well as clean and repair ALL battery connections - especially the ground wires. Replace any battery cable ends that may apear slightly blackened or old. Replace the cables if there is any corrosion into the wires. Once that maintenance work is done, try the jumper cable routine. If your car runs better with the cables connected, you know there are still grounding problems. For safety's sake, I use a battery cable to ground the engine to the body rather than the fragile braided wire that can eventually split and break from flexing.
Jeff Schlemmer

If the horns are working while the ignition and lights aren't (do they still sound properly when these are switched on but not working?) then the brown at the solenoid for the cars loads *must* be OK, as must the battery connections and the battery itself. Of course, this assumes factory wiring and not something that has been mucked about with by a PO.

There is a sealed four-way connection in the brown behind the dash with one wire coming from the solenoid, one to the fusebox and starter relay, one to the main lighting switch, and one to the ignition switch. It's possible that this sealed connection has failed for the lights and ignition conenction, but that would be something I have never heard of before. Check the brown connectors at fusebox, ignition switch and main lighting switch for 12v with everything switched off, then with everything switched on. If you get voltage at the fusebox but not at the other two, in either case, then that sealed conenction has indeed failed. Of course, if you don't get voltage on *any* of them, in either case, you need to check the solenoid conenctions.

Take care when testing the main lighting switch as pulling it forward through the tin dash may short out the brown which is unfused. You may well have to get the dash out to get enough access to remake the sealed brown connection, and disconnect the battery ground strap before you start, reconnecting it last, just tapping it momentarily on to the battery post initially in case you have left anything shorting out.

The problem with diving in and cleaning this and that without fully diagnosing the problem is that you may well disturb the real problem but not fix it, everything works for a while afterwards, then the same thing happens again. Most frustrating.

The lights don't use the engine ground strap, and in practice there are enough alternative ground paths through accelerator, choke and heater cables to allow the engine to continue to run and the alternator to charge even with the engine ground strap disconnected, often the starter will even crank slowly like this, causing said cables to smoke and get very hot. One point about the engine ground strap, it is *supposed* to be braided and not a battery cable with relatively thick and straight conductors in a plastic sheath, as the braided cable resists the continual flexing of the engine better.

The lights *dimming* could be indicating that the alternator is cutting out, which would mean you were running on battery, which would eventually run down causing a missfire before dying altogether. However with those switched on and not working a flat battery would mean that the horns wouldn't work either. The lights dimming could also be a good connection becoming a slightly bad connection, just before it broke altogether.

A missfire, especially if accompanied by the tach flickering about and the ignition warning light flicking on indicates that power is being intermittently lost through the ignition switch onto the white.

FWIW a UK 71 doesn't have four/way aka hazard flashers.

Paul Hunt

Thank for the replies so far, just to highlight some points.

1) The ignition is dead as if the battery was disconected. The only thing I find still working is the horn and interior lights. The batteries are fully charged.

2) There was no smell of burning when the engine died.

Regards

Steve.
S.Crewe (owner)

Steve,
to echo Paul's comments: working horn and interior lights mean that 12 Volts is reaching the fuse box, the fuse is not blown and the connections of the purple wires on the fusebox are good.
The 4-way connector is the most likely suspect, IMHO. The dimming lights could be explained by the connector heating up. A good look behind the dashboard should reveal more.
I must confess I like electrical puzzles...
HTH,
Rufus
Rufus Pool

Many thanks for all the feedback, good news, the BGT has now burst into life again.

The fault was loose and dirty conections on the starter motor, they were cleaned , coper greased and put back together, I also cleaned and coper greased the battery terminals and earth from the battery to the body to be thorough but I am 99% sure it was the starter terminals that was causing the problem.

Thanks again.

Stephen.
stephen crewe(owner)

This thread was discussed between 07/12/2004 and 10/12/2004

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