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MG MGB Technical - Engine died!

Was driving along in my 1973 MGB today, when suddenly the Revs drop to zero, and I gently lose power (pressing accelerator doesn't do anything) and slow down to a stop. There are no odd noises prior to the revs going down.I had a look at my in line fuel filter, just before the carbs and there was fuel in it.
Waited a couple of minutes, then was able to restart the car and drive it.

Engine started earlier as normal (choke out a bit and a couple of turns of the ignition). I had the engine rebuilt just over a year ago and apart from a bit of running on haven't had any difficulties.

What could be the cause? I'm a bit worried now that I could end up stranded somewhere inconvenient...


If the engine just dies and the rev counter switches off it must be ignition related.
Mike Howlett

"suddenly the Revs drop to zero"

Ignition LT. Did the ignition warning light come on as well while the engine was still spinning? Or only after it had come to a halt?

If the former then you have lost power from the ignition switch.

If the latter then the switch is OK, but there is probably a break in the rest of the circuit to the coil +ve and through that, the points, and the distributor earth wire.

As a 73 I'm assuming it has an RVC tach (on the dial), if it is the earlier RVI then that is in series with the coil so the break could be there as well.

Possibly the condenser short-circuit, although less likely as it restarted soon after.

As an intermittent problem it will be tricky to resolve unless you can find and fix an obviously loose connection somewhere in that circuit. If it stays on long enough to diagnose with a voltmeter, first check you have 12v on the coil +ve (white wire). Then with the points open check you have 12v on the coil -ve as well, but zero volts with then closed.

If no 12v on the +ve then the break is back towards the ignition switch.

If no 12v on the coil -ve with the points open then either the coil has gone open-circuit, or the condenser has gone short-circuit, or the points wire is shorting to earth. Remove the wires from the coil -ve and if you still see no 12v on the -ve then it is the coil, otherwise it is the condenser or points wire.

If you see 12v on the coil -ve with the points closed then the path through the points and distributor earth wire is open-circuit.

Nat, the same thing happened to me ('80 B) - sudden loss of power and gradually ground to a stop. I turned off the ignition and just sat there for a little while trying to figure it out, then I tried a re-start. It started up ok and I drove on, but it happened another couple of times that morning. Turned out to be the fuel pump (points).
Brian Shaw

Fuel pump won't cause the tach to suddenly drop to zero while the momentum of the car is still spinning the engine, the tach will only slowly drop as the engine slows. Sudden tach drop is ignition LT failure.

And, to add to what Paul said, fuel pump failure or any form of fuel starvation doesn't cause a sudden and complete engine stop. As the float chambers become empty the engine loses power and pressing the pedal makes it worse, but it keeps running for a short while until the chambers are empty. Ignition failure is just like turning the key off.
Mike Howlett

Nat. Did it just "drop dead" with no sputter, or bucking? If so, I would cast a jaundiced eye to the ignition switch.


79 MGB
gary hansen

The ignition light didn't come on immediately when I noticed the rev counter go to zero. It did go on when I slowed to a stop...

No bucking or any other unusual behaviour once counter went to zero. Car just become unresponsive to accelerator pedal input and slowed gently (I was going down a hill at that point and had reached the bottom and then just slowed)

"The ignition light didn't come on immediately when I noticed the rev counter go to zero. It did go on when I slowed to a stop..."

That shows you had power from the ignition switch on at least one of the white wires when the problem occurred, so the fault lies from there through the coil, distributor etc.

On a UK 73 there are three white wires at the ignition switch. One goes to the ignition warning light and did have power as above, to keep the warning light off until the alternator had stopped charging. Of the other two one goes to the overdrive manual switch, the third goes to a double bullet connector in the mass by the pedal box.

It comes out of the main harness, and another white goes back in to feed the fusebox and the coil +ve. The other side of that connector has one white wire from the rear harness for the fuel pump.

So the fault could be in that connector i.e. power coming out on one wire but not going back in on the other.

It's unlikely to be at the fusebox as the two white wires there _should_ be in a single spade connector. However if the fuel gauge started dropping, and other fused ignition stuff stopped working while the ignition switch was still on, then the fault lies between the fusebox and the ignition switch, i.e. almost certainly that double connector.

But if the fuel gauge was still registering while the car wouldn't restart, then power is reaching there, so the fault lies onwards to the coil, distributor and their connections.

All diagnosed before getting out of the car :o)

Paul, I'm impressed!

Thanks everyone, will check it out.

This was an issue I had with my 67 GT for a few months. It would just die and then restart after a few minutes. Tack would drop to zero. No slow decrease in power or RPM.

Turned out to be a loose connection for the coil wire to the distributor.

I tightened up all the blade fittings and put in a new LT wire to the distributor.
Bruce Cunha

This thread was discussed between 09/06/2017 and 15/06/2017

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