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MG MGB Technical - Engine shutdown electrical

I have a 1973 MGBGT running a Crane XR700 and MSD Blaster 2 coil. I have run this set up for a number of years. The car has developed an intermittent Shutdown problem. About two weeks ago it just stopped and after a few tries got it running and back home. I checked and cleaned numerous connections. Been driving it, no problems. Started the car yesterday died from not enough choke. Went to re-start and nothing except dimming of interior light and alt. light. Tried a couple of more times and it started as normal. Went to drive and it died again got it restarted. The battery is new, starter is new, Ignition switch is fairly new. I added an additional ground to engine a while back as the original one was loose and had no way of tighten it. The engine spins great with the starter so no issues with that. The White wire from the under dash connecter to the switch runs very warm. too hot to touch but doesn't melt the insulation. I have had that problem for awhile. No other wires at the switch, coil, fuse box, starter relay, XR box get warm. Pulled the steering cover and looked sat the switch all appears normal. I am running a newer Bits 4 Brits fuse box with 3 relays. Is this a coil issue, starter relay, Crane box issue?
Ben Stone

"The White wire from the under dash connecter to the switch runs very warm. too hot to touch but doesn't melt the insulation. "

That indicates to me there could be a partial short on that wire somewhere. At the most headlight and HRW wires might feel a bit warm after being on a while but never too hot to touch. In which case it could well be dragging the voltage to the ignition down, occasionally maybe below what is needed to run the ignition system, you might be able to see that on a voltmeter on the white at the coil.

But even if that is not the cause you really need to investigate why that white wire is getting so hot, before it burns your harness.

The white (and the brown) circuits are unfused from the factory, it depends on how your new fusebox system is wired as to whether that is still the case and how the fuses and relays are connected.

From this https://www.mgexp.com/forum/mgb-and-gt-forum.1/bits-4-brits-fuse-box-kit-installation.3425130/ it looks like the original white to the fusebox powers a relay which powers the two green circuits via separate fuses instead of both being fed by the same fuse as originally, but that still leaves the original white circuit powering the overdrive, ignition warning light, fuel pump, and ignition as well as your new fusebox relay so the problem could be on any one of those branches.

If you have an ammeter you could connect that where the white from the ignition switch joins a nearby 4-way bullet connector with three other wires, but unless you have beefy ammeter capable of handling more than 10 amps I suspect the current being drawn will be too much for the meter.

An alternative is to hook up a temporary 17A fuse in series with that white from the ignition switch, and I suspect it will blow. If so you can remove each of the other wires from that connector in turn, with a new fuse each time, until you find the wire causing the problem.

If it's the white feeding the fuel pump, ignition and your new relay then there is another 4-way with three white wires in the mass of connectors where the main, rear and gearbox harnesses join together in the angle between the bulkhead and right-hand inner wing. That will allow you to separate the fuel pump from the other two and see if either of those is causing the problem.

But having dealt with no less than five MGBs that have had shorts at the fuel pump and burnt white circuits, that would be my best guess. For that reason I always recommend inserting a permanent inline fuse where the rear harness connects to the main. And in the overdrive circuit for that matter where the yellow in the main harness joins the yellow/red in the gearbox harness.
paulh4

I should also have asked the following:

When (if) it dies as you are driving along while the momentum of the car is still spinning the engine what's the tach doing i.e. registering or dropped to zero?

Secondly under the same circumstances what is the ignition warning light doing i.e. glowing while the engine is still spinning, or not glowing until it has stopped?
paulh4

Paul thanks for your comments. I haven't had time to check out all your suggestions out . I did find the 4 way connector under the dash it seems to be fine with only the ignition white getting hot. When I had this problem before I found the 4 connector you mentioned in the Engine Bay, it did have a rusted out insert. I replaced that and cleaned connectors and thought that had solved the problem until it re-appeared . This replacement Switch seems to have slightly smaller gauge white wire compared to the old switch, would that create the resistance that's making it hot or could it be internal of the switch. The car does have overdrive which works well and I am running a Faucet fuel pump that has been on the car several years. As far as the Tach. I think it just goes to zero and the charging light goes out but I need to verify.
Ben Stone

If only one section of wire is getting hot that can happen if the connection at either end is poor i.e. switch or bullet connections. Voltage is lost there, which creates heat in the resistance, and that will travel down the wire. That would result in a lower voltage at the 'load' end of the wire, and everywhere downstream of there, than at the 'source' end.

A thinner gauge wire _will_ get warmer than a standard gauge with a given load, but modern 'thinwall' wiring can use the same conductors with thinner insulation. But even if the conductors are thinner I would never expect it to get too hot to touch without there being a fault elsewhere. And anything that results in heat in a length of wire will result in a loss of voltage to the components it is supplying.

You do need to measure voltages on the white wires at different points on the car to try and work out what is happening.
paulh4

This thread was discussed between 15/04/2024 and 18/04/2024

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