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MG MGB Technical - Brake lights ain't...

Greetings Gents,

My 75 B Roadster has recently lost power to the rear stop lights. Running and turn signal lights ok.
Bulbs and fuses look fine, and I replaced the 'Stop Light Switch' (found under hood by brake lever pivot)

Where might I look next?


Steve in Santa Fe

Check all your fuses, then jumper across the switch terminals to check the switch, I know it's new but. Check the wiring connectors located in the trunk and under the bonnet on the right side where all the harnesses come together for a lose or dirty connection.
John H


If the jumper test John mentioned above doesn't make the brake lights come on (with the key on, of course), then look for voltage on the green wire. If you don't have voltage there, then you need to find where the break or bad conection in that green line is. Go to and click on the "stock schematics" button to download a color schematic that will be easier to follow than those in the manuals.

if you do have power on the green wire, then you will need to trace the green/purple wireing to the rear. Somwhere in that line there will be a break or a loose connector.
Dan Masters

Are the bulbs good? Use a test light to see if you have power at the brake light switch. Check the grounds in the boot.

Dan missed a couple of letters in his link, I think.

Seems like it should be advanceautowire.

If you *have* replaced the brake light switch with a new one then you are quite likely to experience failure in that as well, going by popular experience. It seems normal to have to fit a relay (with a diode quench) with the replacement switches commonly and currently available.

Common sources of sudden failure of both brake lights (after you have bridged the contacts on the switch) are where the green from the switch tail joins the green from the washer pump (is this working?) at a 2-way bullet connector, where the green/purple from the switch tail joins the green/purple in the rear harness at a 2-way bullet connector in the mass of connectors by the fusebox, and where the green/purple in the rear harness joins the tail from the right-hand light cluster and the extension to the left-hand cluster at a 4-way bullet connector by the rear light.
Paul Hunt 2

All the brake light problems I've ever had; in fact just about all the electrical problems I've ever had traced to a double bullet , as Paul suggests. I think it may be because the double ones are exposed to more air/moisture (through the unsealed area between the wires as they enter the connector, and because the center "bridge" part of the connector is much more vulnerable.

I'm inclined toward shotgun approaches to fixing these things, even though it may obscure the original cause. If you don't know the age/condition of your double bullet connectors, it's not a bad idea to systemmatically replace all of them. I've had the ones Paul mentions fail, and I've had the one on the white wire (from the ignition switch to coil and fuel pump) fail. I think the doubles serving just three wires are even worse as air gets in through the empty "socket" as well. I can't remember when I've ever had a single bullet connector fail.

Also, did somebody suggest checking the gournds? In particular, the one under the rear registration plate - although it seems unlikely that both brakelamp gournds would fail without any of the others failing.
Allen Bachelder

I should explain: "gournd" is a little-known American term for the equally little-known British term: "erath".

- Allen
Allen Bachelder

Erath? ;->

Rick Stevens

Yeah sure! Gournds and eraths are a nightmaer in the restoartion porcess. 8^)

- Allen
Allen Bachelder

Grounds, eraths or earths the rear light clusters on all models (AFAIK) ground through their physical fixings to the wings, therefore have independant grounds. Also one of the symptoms of a bad ground to a light cluster is the lights seem to work OK when only one filamant at a time is used, but when you try to use two i.e. brake with the lights on the running light goes out and the brake light doesn't come on.

The number plate bolt is only used for the reversing lights, American side-marker lights and fuel pump, and earlier fuel tank sending unit.
Paul Hunt 2

Hey men, been awhile, but I have high speed at home now so I can be more involved in these learning's without skipping lunck at work to use the internet.
Glad to be back.
Anyways, came across tis discussion. Was dealing with last few lighting issues myself. Came across some of the double bullet connectors in the from head light wiring. When you replace doubles with singles, I am assuming you just cut off the doubles and re-crimp 2 wires in a single bullet connector? Anyone replace these with just covered spade connectors or best to stick with bullets? Best regards,
DT Toms

Not original, but the best solution I've found is to replace with sealed connectors. These can be cannibalized at the junkyard but make sure you can get the pins for them. I like the Ford connectors pretty well but weatherpack should be OK too or for that matter any that you can get pins for. That way you keep all the oxygen out and they last a long time, plus they are easier to connect and disconnect. Not for purists, but good technology all the same. If you keep the bullet connectors just get new connector bodies (splices) and squirt a little protective gel in them. You can get it from the electrical supply house. May not prevent embrittlement but it should maintain the contact.

Jim Blackwood

Why replace doubles with singles? I doubt you'd get two wires in a single OE bullet anyway. Also these have to be soldered, not crimped. In the UK at least the crimp-type bullets don't fit the OE connectors. If you want to protect new bullet connectors against corrosion and bad connectivity push some Vaseline into the connector as well as on the bullet before assembly. Aids assembly as well as keeping moisture out. The more work you do like *replacing* otherwsie sound bullets the more problems you are likely to create for yourself in the future. Present Owner fitted connectors to existing wiring are never going to last as long as the factory originals.
Paul Hunt 2

Was commenting based on Allen Bachelder's thread above:
I thought Allen was saying he didn't like the double bullet connectors and replaces them with singles, especially the doubles with only three input wires.
Sounds like bullets are fine, just put the dielectric gel in there. Best regards, Doug

DT Toms

No I didn't mean to replace doubles with singles. I simply meant to replace them with new doubles. I respect Paul's opinion but I still recommend replacing the doubles (with new doubles!). The Vaseline sounds like a good idea. I don't see any need to replace bullet connectors with anything else - if they're clean when assembled, they'll last.

Like Paul, I hate crimp connectors. I admit to using them a few times, never to replace an original bullet or bullet connector, but in adding electrical accessories. And sooner or later, I have lived to regret even that. New bullets need to be soldered on to the wires. I have frequently stolen them from old wiring harnesses, going through the tedious process of unsoldering them, and if I couldn't get enough of the old solder out, I would drill them out with a small bit, in order to solder the new wire in.

DT - I didn't mean to imply that I don't like double bullets. But unlike single bullet connectors that will last 40+ years, the doubles will fail in half that. The next time you see a failed one, take it apart and you'll see what I mean: it's the metal "bridge" from one side of the connector to the other that simply disintegrates over 20 years or so. The easiest solution is simply to replace them with new double bullets. Mark it down in your service log so you'll remember to replace them again in another 20 years.

Allen Bachelder

Thanks for the clarification.
I seen bullet listed in a parts book.
They're on my list for next order.

Best regards, Doug
DT Toms

Thanks for all the tips gang! I have been out of town and haven't gotten to work on it, but I'll report pregress.


Allen Bachelder-

New bullet connectors and snap sleeves are available at
They have both the solder and crimp types.


Thank you for the tip. I am aware of available replacements. ' Think Moss also has them. The problem occurs when one breaks off or one has been buggered by a DPO, and I don't have any new ones in my shop.

You bring up another point that needs clarifying. I'm used to using the term "bullet connector" to mean what you're calling the "snap sleeve".

Steve and DT, if you're still tuned to this thread allow me to correct myself: when I said to replace double bullet connectors, I believe Kimberly would more accurately say to replace the double snap sleeves. The actual bullets soldered to the ends of the wires rarely need any more service than cleaning with steel wool,a wire brush or emery cloth.

Allen Bachelder

Like Allen I'm all for replacing old doubles with new doubles when necessary, it was replacing doubles with singles I was questioning.
Paul Hunt 2

This thread was discussed between 26/10/2006 and 02/11/2006

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