MGB Breakdown advice - ideas needed
Discussion
Hi - I have a 1978 standard spec. rubber bumper MGB GT that keeps "cutting out". The car will run perfectly for many miles and then, all of a sudden, the ignition light comes on, fuel gauge falls, wipers stop if in use, etc. and the car rolls to a quiet stop at the verge - no lurches, splutters. It then takes 10 minutes or so before it will restart, usually after a few goes, up goes the gauge, etc. and it drives away for another 30,40, 50....miles perfectly. Starts cold first time, idles lovely, etc. Recent new plugs, battery,...Any ideas...?? Help much appreciated..
That's an odd one!
At first I suspected a variety of faults till you mentioned you lose all electrics at the same time.
Just try one thing. While it is all working fine (no need for engine running as it seems your fuel gauge etc. are linked.) pull the wires off the back of the Rev counter, this would definitely kill your engine (hence when the rev counter dies why the engine will not run and it's a bugger to track down) see if this kills all the electrics too, if so it could well be a duff rev counter!
Failing that it's really strange the way it is happening, if it were a battery terminal coming off for example then provided the alternator was charging the engine would carry on running, likewise if the alternator is dead the battery will keep the engine running.
I'm not sure that a fusebox issue could cause this, however it could be a direct short to body and that would be my first suspicion, it could be from fuse box or from the coil to points lead running from the dizzy, these are sometimes too long and if not insulated can short out. Just not sure that would cause you to lose everything.
You anywhere in Yorkshire?
At first I suspected a variety of faults till you mentioned you lose all electrics at the same time.
Just try one thing. While it is all working fine (no need for engine running as it seems your fuel gauge etc. are linked.) pull the wires off the back of the Rev counter, this would definitely kill your engine (hence when the rev counter dies why the engine will not run and it's a bugger to track down) see if this kills all the electrics too, if so it could well be a duff rev counter!
Failing that it's really strange the way it is happening, if it were a battery terminal coming off for example then provided the alternator was charging the engine would carry on running, likewise if the alternator is dead the battery will keep the engine running.
I'm not sure that a fusebox issue could cause this, however it could be a direct short to body and that would be my first suspicion, it could be from fuse box or from the coil to points lead running from the dizzy, these are sometimes too long and if not insulated can short out. Just not sure that would cause you to lose everything.
You anywhere in Yorkshire?
The most frustrating thing to pin down - an intermittent electrical fault! Points to something related to the ignition controlled circuits if the lights stayed on and wipers stopped. If all of those circuits cease functioning at the same time, points to a component that controls the lot - like the ignition switch earth or internals.
As well as the other suggestions, check:
Battery earth lead AND the engine earth strap - both should be secure and the lead/straps in good order.
Fuse box connections and fuses - everything should be in good order, no loose terminals or loose fuses in holders.
Wiring connections to the ignition switch and the barrel of the switch with the key- same deal as the fuse box.
Trace connections to components, switches and in-line bullet connections and make sure they are all firmly made and that the wiring isn't hanging on by a thread.
If that deosn't fix it, jot down all of the electrical components into 2 lists - one for non-ignition controlled and one for ignition controlled. Remember the rev couter and ignition light too Take a voltmeter/continuity tester out with you and when the problem recurs, leave the ignition switched on and check what is and is not working - that will pin it down to a general or specific ignition circuit problem or not.
MG Mark
As well as the other suggestions, check:
Battery earth lead AND the engine earth strap - both should be secure and the lead/straps in good order.
Fuse box connections and fuses - everything should be in good order, no loose terminals or loose fuses in holders.
Wiring connections to the ignition switch and the barrel of the switch with the key- same deal as the fuse box.
Trace connections to components, switches and in-line bullet connections and make sure they are all firmly made and that the wiring isn't hanging on by a thread.
If that deosn't fix it, jot down all of the electrical components into 2 lists - one for non-ignition controlled and one for ignition controlled. Remember the rev couter and ignition light too Take a voltmeter/continuity tester out with you and when the problem recurs, leave the ignition switched on and check what is and is not working - that will pin it down to a general or specific ignition circuit problem or not.
MG Mark
Just another thought it could be the ignition switch itself, although I would expect it to come back to life when you jiggled the key.
It won't be earth straps, they wouldn't cause this issue at all, not turning over cleanly yes, dying when running no.
Ignition light isn't in series with the engine/main electrics so you can discount that.
Fusebox/rev counter/distributor would be my first points to check, it could be (unlikely though) that the wire going in to the dizzy has frayed and is catching on the dizzy body where it passes through.
It won't be earth straps, they wouldn't cause this issue at all, not turning over cleanly yes, dying when running no.
Ignition light isn't in series with the engine/main electrics so you can discount that.
Fusebox/rev counter/distributor would be my first points to check, it could be (unlikely though) that the wire going in to the dizzy has frayed and is catching on the dizzy body where it passes through.
Mine did something similar and the problem turned out to be corrosion in one of the spade terminals connected to the fuse box (left hand side under bonnet, viewed from the front)
I replaced the spade and a couple of others, cleaned the fuse, fuse holder and it's been fine since.
If I remember there is an odd combination of things that run through the ignition circuit which causes a few problems locating the fault.
HTH and good luck with sorting it.
I replaced the spade and a couple of others, cleaned the fuse, fuse holder and it's been fine since.
If I remember there is an odd combination of things that run through the ignition circuit which causes a few problems locating the fault.
HTH and good luck with sorting it.
Re; starter relay, I'm no mechanic but, there are two relays by the fuse box under the bonnet. They are the black cylindrical ones about 50mm long, (not sure if you meant informatiom like this or their function or symptoms their failure they might cause). My previous 78 Gt done exactly the same, it would just cut out without warning. I replaced the petrol pump, ignition switch, plugs, leads and numerous other bits only to have the same problem a few weeks later. Opening the relay up exposed a lot of corrosion, so was replaced and cured the problem.
Hope this helps.
Hope this helps.
Tony the relays your referring to are probably flasher cans for indicators and hazards, or relay for electric washer and halogen lights or maybe driving lights.
The gent who came up with the faulty starter solenoid might have hit it on the head though and I'm ashamed I didn't think of it, that is a seriously likely contender. On your car it will be on the starter motor it's a cylinder sticking out below it.
The gent who came up with the faulty starter solenoid might have hit it on the head though and I'm ashamed I didn't think of it, that is a seriously likely contender. On your car it will be on the starter motor it's a cylinder sticking out below it.
Oliver, it is definately the starter motor relay, link is to one for sale on ebay. Would'nt the stater motor solonoid only stop the car from starting and not cut it out while the engine is actually running?
What do you think?
Tony
http://cgi.ebay.co.uk/CHM68-STARTER-MOTOR-RELAY--M...
What do you think?
Tony
http://cgi.ebay.co.uk/CHM68-STARTER-MOTOR-RELAY--M...
The solenoid holds the main power from battery and from memory also the ignition light trace comes off it too, without purposely shorting one across it's hard to see what result could happen, I'm sure some models take their main feed from the solenoid too, whether that would cause the engine to cut who knows, electrics can be a right pain to track down and must be honest this one sounds unusual.
Relay every time. 2 round black relays on inner wing near fuse box. One (front from memory) is starter, other controls alternator & various electrics. Cost under a £10, swapped in minutes. I've had so many of these fail I'm thinking of changing them as a matter of course. If in doubt a sharp tap will often get it working again - for a while, but get another one before it dies on you in the dark/rain etc.
Hi I just wondered if a fix was found to the original poster's issue on this topic?
I have also just started experiencing the same problems with my MGB GT '78.
I wondered if anyone could please help as my issue is very similar but may be slightly different.
I've had the car cut out a couple of times recently, at a junction and also pulling up at a round about. Both bad times for the car to die, with lots of angry traffic behind!!!
The first time the car wouldn't start and I had to use a breakdown service, the second time it started up straight away again and I set back off no trouble.
I'm just worried it happens again or gets more frequent - the car runs fantastic, but my rev counter is intermittent. 90% of the time it works ok, but will drop to zero or bounce about. If I then change gear or flick over drive on it will start working again and show correct revs - it'll then keep working well but then drop again out of the blue. The car still runs fine though when the rev counter does this.
Another issue - though it may be unrelated and just coincidence is that when I press my brake pedal, the dash lights and clock light all come on and glow with the brake. Even when I have no lights turned on. Then I release the brake and they don't glow any more. But when I do have lights on they're all glowing all the time anyway as they should so you don't tend to notice as in the day time it's harder to see this happen anyway.
I've been round all the grounds/earths on the car checking and cleaning them and can't find anything wrong. I've also ran an earth from the brake housing to an extra earth in the boot behind the number plate which hasn't helped...
So the times the car has died has been when I've stopped. The engine just dies like I've stalled it, and the alternator red warning light comes on. I'm not sure if any other electrics stay on or go off as it's hard to say in the day time when driving without the radio on etc. The car sounds good then turning over and then just restarts or if it won't do like the first time this happened - just waiting over night it will then fire up again the next day or when the car has cooled off.
Another issue is I had an oil leak from my Oil Filter recently so I wondered if this may have caused this and the oil has all landed on the starter motor...
Or could it be a relay as mentioned in the thread. Any advice much appreciated, I didn't want to order anything to try replace yet or spend any money if it's not necessary or there may be a simple fix.
Thanks again for any help at all.
I have also just started experiencing the same problems with my MGB GT '78.
I wondered if anyone could please help as my issue is very similar but may be slightly different.
I've had the car cut out a couple of times recently, at a junction and also pulling up at a round about. Both bad times for the car to die, with lots of angry traffic behind!!!
The first time the car wouldn't start and I had to use a breakdown service, the second time it started up straight away again and I set back off no trouble.
I'm just worried it happens again or gets more frequent - the car runs fantastic, but my rev counter is intermittent. 90% of the time it works ok, but will drop to zero or bounce about. If I then change gear or flick over drive on it will start working again and show correct revs - it'll then keep working well but then drop again out of the blue. The car still runs fine though when the rev counter does this.
Another issue - though it may be unrelated and just coincidence is that when I press my brake pedal, the dash lights and clock light all come on and glow with the brake. Even when I have no lights turned on. Then I release the brake and they don't glow any more. But when I do have lights on they're all glowing all the time anyway as they should so you don't tend to notice as in the day time it's harder to see this happen anyway.
I've been round all the grounds/earths on the car checking and cleaning them and can't find anything wrong. I've also ran an earth from the brake housing to an extra earth in the boot behind the number plate which hasn't helped...
So the times the car has died has been when I've stopped. The engine just dies like I've stalled it, and the alternator red warning light comes on. I'm not sure if any other electrics stay on or go off as it's hard to say in the day time when driving without the radio on etc. The car sounds good then turning over and then just restarts or if it won't do like the first time this happened - just waiting over night it will then fire up again the next day or when the car has cooled off.
Another issue is I had an oil leak from my Oil Filter recently so I wondered if this may have caused this and the oil has all landed on the starter motor...
Or could it be a relay as mentioned in the thread. Any advice much appreciated, I didn't want to order anything to try replace yet or spend any money if it's not necessary or there may be a simple fix.
Thanks again for any help at all.
Whatever the problem is you can discount the starter relay because that does absolutely nothing once the car is running, you could disconnect it completely and it would make no difference.
The glowing lights are suggestive of an earth issue or possibly some mistake in the wiring.
Your rev counter is fed directly from the points or negative side of the coil which should give some sort of clue as to where the problem lays.
Next time it happens try connecting 12v directly to the + side of the coil and if that works transfer it to the white wire on the ballast resistor.
If it still does then your problem is in the ignition switch wiring to the resistor somewhere.
There are some excellent wiring diagrams here: http://www.advanceautowire.com/mgb.pdf
The glowing lights are suggestive of an earth issue or possibly some mistake in the wiring.
Your rev counter is fed directly from the points or negative side of the coil which should give some sort of clue as to where the problem lays.
Next time it happens try connecting 12v directly to the + side of the coil and if that works transfer it to the white wire on the ballast resistor.
If it still does then your problem is in the ignition switch wiring to the resistor somewhere.
There are some excellent wiring diagrams here: http://www.advanceautowire.com/mgb.pdf
soydemadrid said:
Another issue - though it may be unrelated and just coincidence is that when I press my brake pedal, the dash lights and clock light all come on and glow with the brake. Even when I have no lights turned on. Then I release the brake and they don't glow any more. But when I do have lights on they're all glowing all the time anyway as they should so you don't tend to notice as in the day time it's harder to see this happen anyway.
I've been round all the grounds/earths on the car checking and cleaning them and can't find anything wrong. I've also ran an earth from the brake housing to an extra earth in the boot behind the number plate which hasn't helped...
You haven't changed one of the rear bulbs lately have you and maybe put the wrong type in?I've been round all the grounds/earths on the car checking and cleaning them and can't find anything wrong. I've also ran an earth from the brake housing to an extra earth in the boot behind the number plate which hasn't helped...
I've just come across this thread about the MGB cutting out. I had a similarly 'haunted' MGB 40 years ago. Without warning everything would go black and the car would coast to a stop. It once happened to me on the Forth Road Bridge just as I was about to reach the crest. Fortunately, I was doing enough knots that I managed to freewheel to the Fife side and abandon the car on a grass verge. When I went back the next morning, with a friend and a tow rope, it started ! Me and several garages had a go at tracing the fault without success. One evening at the end of a weekend shift of duty, the car was dead. I made a gesture at looking again for the cause by lying down and reaching under the car. I had left the ignition switched on because the sudden switching on of the radio was a signal that power had been restored ! My arm brushed the back of the starter, and I heard and saw a spark, On closer inspection, the main lead from the back of the starter to the rest of the car electrics was separated from the spade connector and was being held in contact, or not, by the rubber cover. A 5 minute job in the end to solve a problem that was making the car almost unusable.
Gassing Station | MG | Top of Page | What's New | My Stuff