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MG MGB Technical - Cuts out very briefly

1978 1800 with good electrics - Accuspark ignition and coil otherwise standard - cuts out occasionaly - usually when accelerating - tacho dies momentarily - anyone recognise ?
R
Roger Walker

Check your ignition switch and the wires attached to it. Cheers - Dave
DW DuBois

Roger, I second the ignition switch. Pretty much the same thing happened with my 79 MGB and indeed it was the ignition switch.

Cheers

Gary
79 MGB
gary hansen

Good advice - thanks
Roger Walker

If the ignition warning light comes on briefly as well then power has been lost from the ignition switch or depending on the exact point of production it could be the ignition relay.

77/78 was a cross-over period for ignition wiring, if you have a single in-line fuse below the fusebox it is a 77, if you have two it is a 78, white/brown to green in all cases.

On the 77 all ignition switched power comes from the ignition relay (operated by the ignition switch), so the relay could be briefly releasing (back to checking switch connections), or it could be the relay itself or its connections.

On the 78 power for the ignition (and the tach, heater fan and indicators), came direct off the ignition switch. The switch also operates the ignition relay, so again this could be ignition switch or connections, but if the relay is failing that won't stop the engine.

Ignition switch wiring could be the brown or the white. Check the multi-plug for the switch sub-harness is fully pushed together, and none of the pins have been partially pushed out.

If the warning light doesn't come on, then it will be a break in the ignition LT circuit somewhere else. This could be through the harness ballast (if still in circuit), the ignition system, the coil, the wiring between them, and assuming your Accuspark fits entirely under the distributor cap it could be the earth wire between the 'points' plate and the distributor body.

As it seems to be when accelerating I would suspect either fractured conductors inside the insulation of the two Accuspark wires coming out of the cap, or the earth wire. As you move the throttle pedal the points plate twists back and fore under changing vacuum, which flexes these wires, and they will eventually fail. It could be the earth wire missing or broken altogether. The moving plate has a sliding connection to the fixed plate, which can give a good enough earth under static conditions, but gets erratic when the moving plate moves. I had a Scimitar GTE many years ago which by design didn't have an earth wire, and that had the habit of cutting-out just as I pulled out from a side road which was most disconcerting. In that case the owners club recommended fitting one, and it cured the problem.
paulh4

Paul
VMT. It's a 78.
I am interested in Dave and Gary's advice because the ignition switch wires have in the past been butchered and I had to repair them when we fitted a new loom. However, the problem first showed before the loom was fitted and your thoughts about the distributor earth resonate because a touch on the accelerator seems to start it again.
The dizzi had lumenition contacts before and may not have been susceptible to an earthing fault - will investigate.
The red ignition light is disconnected still because I can't work out which terminal on the high output non OE alternator to use. That's another little job !
Thanks again to everyone
Roger
Roger Walker

Roger, I don't know if your alternator is similar to the one I fitted on my Lotus Elan, but just in case it is I am attaching a schematic of how that was wired. The switched battery connection is to energise the field coils so the alternator starts charging as soon as the engine starts.

Mike Howlett

Here's another schematic of the same thing - slightly easier to understand.


Mike Howlett

"The dizzi had lumenition contacts before"

If this is with the external electronics module and only the trigger under that cap, then that type don't use the earth wire. So if for some reason it was removed when that was fitted, it would need to be replaced for points or a fully under-cap unit.

"The switched battery connection is to energise the field coils"

Odd, on Lucas alts it is the warning light circuit that initially energises the field coils. Even odder considering that the 'single wire' version doesn't use either of the other two wires. Also, on Lucas alts at least, even without a warning light connection it will start to charge normally once revved to about 3k rpm, then charge normally down to about 600 rpm.
paulh4

Yes Paul, I thought it odd when I bought and fitted that alternator, but I think the idea is that it starts charging right from the moment the engine fires. On my old 4-pot MGB (long since sold on) the warning light didn't always go out immediately, but once out it stayed out.

I've read elsewhere that the third wire also monitors system voltage and adjusts the charging rate to system load. Whether this is true or not I can't say, but I'm not sure that an automotive alternator is that clever a gadget. Perhaps modern circuitry makes it better than I imagine. I am after all stuck firmly in the 70s when it comes to car technology.

If you think about it, modern cars are extremely high geared to get fuel economy, and I have certainly been with drivers who never get above 2000 rpm, changing up as soon as possible, and in 5th gear at 30 mph. Under those conditions an old fashioned alternator might never start to operate!
Mike Howlett

Yes, one usually does have to blip the throttle when starting a warm engine, to something over 900rpm, to start the alternator charging.

"the third wire also monitors system voltage and adjusts the charging rate to system load."

Alternators always (but see the final paragraph) adjust to system load, but MGBs for a while did use a third wire to the alternator. This was a standard gauge brown wire that went back to the solenoid along with the main output wire and hence was live all the time, and monitored the voltage at the solenoid which can be said to be the battery voltage. In the alternator this was used to control the voltage regulator, and under conditions of high load where there was a voltage drop in the main output wire, it ensured the battery was kept up to the correct system voltage. These are known as 'battery sensing' alternators.

Before that the voltage regulator monitored the voltage at the alternator output terminal, and so could not take account of any volt-drops in the cabling, these are known as 'machine sensing' alternators.

For whatever reason the battery sensing system was dropped and reverted to machine sensing, but two heavy gauge wires were used from the alternator to the solenoid, and also from the solenoid up to a branching point for the rest of the vehicles electrics. Doubling-up like this also has the effect of reducing volt-drops under heavy load conditions.

I say 'for whatever reason', but I can imagine that if the voltage sensing wire becomes disconnected, the alternator would switch to maximum output, which would result in a much higher system voltage than should be present, over-charging the battery, and maybe blowing bulbs.
paulh4

Mike - many thanks for putting up the diagram - as it turns out, mine is principally used in a small Ford diesel - which is different but I found a terminal which seems to do the job while still charging at 14volts.

Paul, I did take her round the block today - she stalled on two occasions - the tacho dropped off but the ignition light stayed off. Tomorrow's job is the dizzi earth but I have a new ignition switch on its way.

We will get there in the end. I am less confident about my son's 2.0ltr diesel Volvo which is leaking diesel into the luboil. German pump, French engine manufactured by Ford and fitted in a Swedish Volvo - the buck stops where ????
Roger Walker

Looks like it was the electronic ignition
Roger Walker

Damned electronic ignition ...
paulh4

This thread was discussed between 19/10/2016 and 08/11/2016

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