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MG MGB Technical - Momentary loss of power (electrical) 80MGB

My 1980 MGB (USA) has had this problem for a couple of years and I have not been able to solve it. Going down the road it will loss electrical power for a moment or 2 then start back up. I have had it backfire (blew out a muffler). It happens at low speeds, high speed, maybe on a bump on a smooth road. When it happens tach also goes dead. I found some information that if the igntion light was on, probably would be bad ignition switch and if the ignition light was not on then probably bad at the igntion relay. The igntion light does not come on when it dies. However, I checked and replace the ignition switch anyway since where the wires went in were a little loose. Took the old one apart and it was in pretty bad shape inside. Still having problem. Replaced the ignition relay and checked all the connections. Still having the problem. Seems to be happening more frequently. Went for about a 4 more drive around the neighborhood and it happened 5 times. Loses electrical power, engine dies, no tach, then starts back up.

I'm pulling my hair out.
PCF Farmer

As ignition light does not come on it suggests that alternator is still charging. Tacho takes its pulse from Low tension (i.e.negative ) side of coil, I would be looking at all components in that circuit, coil, condenser, points and all the wiring between coil and distributor earth
jim soutar

Whilst the alternator is what is powering everything when the engine is running, if that stops working everything will continue to work from the battery so the alternator itself is out of the equation.

If the ignition warning light doesn't come on when the engine dies that indicates the supply from the ignition switch is still there. That powers the relay, and the relay powers both the coil (directly) and the tach (via the green circuit fuse as well as coil pulses). So the most likely source of the problem is around the relay.

As you have replaced the relay itself I'd be looking at it's connections. This could be any of the four on the relay itself, where the black from the relay goes to earth nearby, where the brown for the relay comes off the fusebox, and where the white/brown from the relay connects to the fusebox before teeing off to the coil. However the browns and the white/browns *may* each have two wires in one spade connector, in which case a problem here is less likely unless the conductor has fractured inside the insulation (unlikely). As there are three white/browns at the fusebox, the third going to an in-line fuse for the cooling fans, there should be two spade connectors for three wires. With the engine running pull the spade with two wires off the fusebox, and if the engine continues to run you know those two wires are from the relay and to the ignition. If the engine stops, but bypassing the cooling fan switch causes the fans to start, then power for the ignition has to go through the two spades on the fusebox, which are riveted together as well as to the fuse holder, so it could be a problem there.

However all this depends on the wiring being as it left the factory. Many late model North American cars have had the wiring changed or a diode inserted in the warning light circuit as a result of problems with the emissions plumbing or its removal causing the engine to continue to run with the ignition switched off. In that case the conditions and possible causes could be very different.
PaulH Solihull

Silly me. It *could* be the relay connections of course, but it could equally well be something purely to do with the ignition.

If things like the washers, wipers, indicator etc. have stopped working as well it's the relay. If they are still working then it's probably just through the ballast resistor, coil, and whatever ignition trigger you have.
PaulH Solihull

Thanks for the thoughts. I'll work through those. I have an electronic ignition. The factory electronics have been replaced with a Crane Cams unit.
PCF Farmer

Unfortunately you can't do much about suspect electronic ignition other than replace it.
PaulH Solihull

Sometime in the early 70s, they started using the rectangular housing connectors, instead of the original "Lucar" ones with the soft covers. Lucar were top grade in material and function, the later ones are completely different and an order of magnitude toward fail from the day they were made. Much lighter material of lower grade, which looses tension much more easily, giving poor connections. Thing is, they have an incorporated latch, which keeps the connector from falling off the contact blade, but it does not do a thing for the electrical integrity of the actual connection.

You will note that when you remove one of these things the rectangular black plastic housing moves a freely a bit before the actual connector moves; that movement is actuating the latch. Result is that people "check all the connections" and since they are all in place report "all good".
But in fact, the internal metal bit is loose and making a high resistance and intermittent connection. Worse, this generates heat, which further degrades the crap material, reducing the contact pressure still more.
If these are not too cooked, you can get inside the connector with a tiny tool and bend things such that the actual contact pressure is increased. High load ones, like the IGN relay connectors, are usually too toasted to save; in fact, these crappy connectors are the usual first fail part that then cooks the relays.

The actual latching function is a metal tit on the female connector which fits into a hole in the male blade, which is lifted by a ramp in the plastic piece, when you pull on the plastic housing.
If you pull on the wire itself, this lock engages metal to metal and you then have contact for a time, but it soon vanishes. All this is what makes it fun!

I cut these stupid things off and replace them with good quality new ends without locks - I'd rather have the wire fall off if the connection is bad than stay there and lie to me.

(A couple of days ago I came across a lost and forgotten c1965 Lucar Connector Service Kit 54006040 - single and double connectors and boots in both 17.5 and 35A rated, and a couple of a special control box connector. And NO you can't have it!)

FR Millmore

Hi, just something from left field. I had a similar problem. Motor would not cut out but would run ruff and backfire while driving intermittently. Usually ok at idle. The car had been fitted with a battery isolation switch removable red key type for competing in hill climbs. The switch was the fault. It did not totally fail as car always started and had 12v power but arced across its contacts. I replaced the switch 9 months ago and all ok now after 12 months of frustration till I found the switch was the fault and not the tune up. In frustration I had let the experts at it who attacked the carbys and replaced the distributer but no change to the fault.
K Stuckey

Hopefully, I have solved the problem. The HT going into the coil was not fully engaged. The boot was fully on the tower, but the terminal was not seated fully down into the female connection. The boot was holding it metal to metal but apparently the wire was gradually working back through the boot which is why it was getting worse. I pulled the boot back, fully seated the terminal then pulled the boot back down the wire onto the tower. I've gone for 2 drives (about 20 and 10 miles) with noth loss problem. Hopefully, fingures crossed, this puts this one down.
PCF Farmer

That is not "losing electrical power" and it will not cause the tach to die. It will most certainly kill the coil after a while though.

FR Millmore

This thread was discussed between 24/06/2012 and 27/06/2012

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