Welcome to our resource for MG Car Information.



MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Electrical issue, where to start

Out with my newly purchased 80 Roadster. I went over some speed bumps and at each one the lights flicked, the engine cut out, came back on but eventually just died. Turning on the ignition does absolutely nothing. I do have horn and the trunk light does on. It would seem to be a loose connection somewhere but where to start looking? I have the schematics
Hope to have it resolved for MG 2008.

Ed Emery

Check the bullit connectors for a loose connection and check that the fuses are tight in their holders and not corroded. Also check the ground connections.

If all else fails start at a component that has lost power and work your way thru the wiring connections towards the battery using a voltmeter untill you get a good 12 volt reading on the meter.
J Heisenfeldt

The double bullet sleeves are noted for splitting and losing their effectiveness. If I had to guess, I would tend to lean towards one of the brown doubles under the dash that supplies current to the headlamp switch AND the ignition switch is having problems. Buy yourself a can or tube of dielectric grease at your local autoparts store. Buy yourself a little wire brush intended for cleaning the inside of the barrel of a .177 pellet gun or else a .22 rifle at your local sporting goods store. Buy yourself a handful of new bullet sleeves from (NFI) or another Lucas supplier. You might also want to order yourself a wiring diagram from (NFI)to supplement the ones in your manuals.

Start looking under the dash for brown connectors and check them for lack of springiness, clean them with the rifle brush, squirt some dielectric grease in and put them back together. Move on to the next one.

Learn the Lucas wiring colours. They are NOT random.
Brown is power that is not fused or switched.
Purple is power that is fused but not switched like the horn and cigar lighter.
White is power that is switched by the ignition, but not fused like the coil.
Green is power that is switched by the ignition and fused.
Red is parking lights
Blue tends to be headlights
Tracer colours generally indicate that it has passed through a switch.
Lots of details at places like

David "be glad you are not trying to reach under the dash of a midget" Lieb
David Lieb

David, this wiring colour info could have come in very handy for me earlier on! ;)

Ed, you need to be methodical and work backwards. Dont just assume a connection is alright cause its connected. Keep notes on what youve checked and using the printout of your wiring plan highlight the lines that are interconnected and connected to non fuctional systems, then see where they link and check there.
Ross Kelly

Being that you are not the original owner, be aware that some (if not a lot) of the wiring on this car has probably been altered by the previous owners. Pay particular attention to the ignition circuit. There's a good chance that the distributor and accompanying attachments have been changed out and rewired.

Flickering lights on speed bumps can sometimes be traced to loose and/or oxidized connections at the fuse block. Thorough cleaning of this assembly along with all bullet connectors and ground connections can often suppress those ugly gremlins that often haunt these LBC's.
Steve Buchina

I gave away all my MGB manuals that had wiring diagrams, but my Midget wiring diagrams show a double bullet sleeve in the brown wire just before it gets to the ignition switch. In fact, there are three wires in it... one coming from the starter solenoid, one going to the ignition, and one going to the headlights. Locate that sucker and I will almost guarantee that you will find that one of the sleeves is split.
David "going out on a limb here" Lieb
David Lieb

Check the Brown lead wire at the starter solenoid.
Daniel Wong

Ah, but if that were bad, he would not have a boot light nor a horn.
David Lieb

BTW, don't really trust a voltmeter or a test light for this, because you can draw enough current to drive the light or the meter even through a connection that won't drive anything else. Lots of potential for false positives.
David Lieb

check the condition of the battery leads at the battery, they may just need cleaning
Peter Murray

I agree with Peter. Check your battery terminals. If either is warm it is a bad connection.
D M Tetlow

When you're messing around, cleaning these connections, disconnect the battery. And if you're checking for warm spots, do it right after there's a load - start the car (does it turn over quickly?), turn it off, then see if the cables are hot. Since you haven't complained of starting, I'd bet it's not battery cables. Bullet connectors are a common source for this kind of trouble.

If "turning on the ignition does absolutely nothing", but "I do have horn and the trunk light does on", I really doubt that it is the battery connectors. I kinda doubt that the engine is "turning over quickly", too, unless Ed has a different definition of "absolutely nothing" than most of the rest of us ;-)

Unless Ed is making up symptoms to confuse us or deliberately misleading us (generally more of a winter pastime), it almost has to be a bullet connector in the brown circuit between the solenoid and the ignition switch or else a break in that wire. In a Midget, there are two of the 4-wire bullet sleeves in between there; either one could cause this, although it is more likely to be the one under the dash, especially if he were to test his hazard lights and they work.
David "where is that online wiring diagram?" Lieb
David Lieb

My definition of "Absolutely nothing " is that when I tuun on the ignition I have no ignition or brake light indicators on the dash. Turning on the key to the start position I get no starter activity.
Hopefully I'll get some time to look at this over the weekend.
Thanks for the info

Ed Emery

Good, Ed, that is what I was assuming that you meant. But hurry up, willya? The suspense is killing us ;-) I really think that if you follow the brown wire from the headlight switch to the first bullet sleeve, you will find your problem.
David "but I have been wrong before" Lieb
David Lieb

Hey guys, you can get free wiring schematics at Download and blow it up and print it out.

I think David is right on. Check out the brown wires from the starter up to the fuse box.

I would also suggest you check the ignition relay.

Pat Harrison

NOT to the fuse box!!! Brown does not go through the fuse box (it is green when it comes out the other side. The circuits that are failing on him are NOT fused.
David Lieb

Provided there has not been any DPO electrical work done -

One brown goes to the headlight switch and also to the ignition switch.

Other brown goes to left side of #4 fuse and then on to the ignition relay. This is the fused connection to the Purple wires (horn, trunk, etc.) so that says you are getting current on that circuit.

Start at the battery ... be sure that positive and negative terminals are clean and connections are tight.
Check the negative to the body and be sure it is good as well.

Move to the starter solenoid ... make sure all of the connections are clean and tight here as well.

As the brown wires go up from the starter toward the main loom, there is/was a plastic splice box. I believe this is where the original ammeter was hooked in. Check the connections at this box. One of these connections heads to the #4 fuse ... and we know it is providing current.

Since headlights are failing and engine is dying, I would take a look at the brown wire headed for the light switch and then on to the ignition switch. Check for good connections at each place and verify you have 12V at each place. I suspect that you will find your fault somewhere along this path.

Just to cover everything, check the #3 fuse to be sure it is good.

Let us know what you find.

Pat Harrison

This thread was discussed between 11/06/2008 and 14/06/2008

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.