MGB Lights not working

My '66 MGB, converted to negative earth, is slowly coming together. It has a fresh coat of paint on it just to keep it from rusting, and I've been taking it for spins the last couple of days. Today I tried the headlights.

They don't work. I just turned the key on and flipped the switch. No headlight, no tail lights, no front corner marker lights. I do have brake lights and turn signals. The map light switch is immovable. Here's the thing: I pulled back on the turn signal stalk, and I got high beams. That's weird, since I'm sure my first '66 MGB (mom bought it new, I got it, and drove it in the '80s) didn't have that feature.

I am not a mechanic. My first guess is that the toggle switch is bad. But I don't know; it's only a guess. How do I diagnose the fault to find out where it is?

Mornin', 66 -

Attached is a link to Advance Auto wiring diagrams, in case you don't have a readable one. If you look closely at diagram 2, for '64-'67 Bs, you'll see that there is a "flash-to-pass" switch (it's incorporated in the turn signal switch) in addition to the floor-mounted headlight dipper switch. As for the lights not working, check all ground wire connections, as those are usual suspects. Also, do your gauge lights work? If you'll check the diagram again, you'll see they are on the same switch terminal as the tail and parking lights. That should confirm that the switch is OK, at least at that terminal. Check to be sure the brown wires are fully connected at the switch and the fuse box. And check continuity of that wire when disconnected to make sure it's not broken somewhere. There's a separate brown wire feed to the flash to pass switch, so you know that circuit is good.

Good luck, and let us know what you find.

https://www.advanceautowire.com/mgb.pdf


Mickey

I suspect your clue is in the first sentence - "fresh coat of paint" - if it worked before and it doesn't now it is almost certainly grounds. (black wires) you can scrape a bit of paint off, move them to bare metal, add new one - you can also test pretty easily if you have wires with alligator clips - (or even hold them in place)

If I recall correctly the headlights don't have a separate ground wire but ground the unit against the body of the car - you might have to add one.

Mickey Richaud said:

you'll see that there is a "flash-to-pass" switch (it's incorporated in the turn signal switch) in addition to the floor-mounted headlight dipper switch.

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I'll take your word for it. It's too early to try to figure out a wiring diagram, and I haven't even had my first pot of coffee yet. (Heck, I'm just starting my first cup!) It's funny that I drove my first B for years, and never knew there was a 'flash-to-pass' switch.

Mickey Richaud said:

As for the lights not working, check all ground wire connections, as those are usual suspects. Also, do your gauge lights work? If you'll check the diagram again, you'll see they are on the same switch terminal as the tail and parking lights.

Click to expand...

I didn't think to check the gauge lights. I don't have anyplace dark, so I'll have to wait until tonight to see if they come on.

JPSmit said:

I suspect your clue is in the first sentence - "fresh coat of paint" - if it worked before and it doesn't now it is almost certainly grounds. (black wires) you can scrape a bit of paint off, move them to bare metal, add new one - you can also test pretty easily if you have wires with alligator clips - (or even hold them in place)

If I recall correctly the headlights don't have a separate ground wire but ground the unit against the body of the car - you might have to add one.

Click to expand...

The car had been sitting for three years, so I don't know if the lights worked before it was painted.

All about th' grounds. If the car sat static, pull all the Lucar and spade connectors apart and clean 'em before you even start trying to diagnose more (imagined) complex issues!

Would think earths are good since they operate on flash mode. Would check the switch on the fascia and go from there.

Grantura_MKI said:

Would think earths are good since they operate on flash mode. Would check the switch on the fascia and go from there.

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It occurred to me that I didn't have a tool to remove the bezel. I've ordered a set from Moss. I've also ordered a new headlight switch, since I was paying postage anyway. Maybe it's not the switch, but I might get lucky.

Grounding issues in light circuits do not resign themselves just to British cars. I bought a new 73 Chevy Elcamino and about a year later the tail lights quit working! I didn't have time to check it out, working 12 hrs a day, driving 75 miles to a job, so I dropped it off at the dealer. The next day they said they checked everything and they couldn't find the problem?? I asked them if the checked the ground, which was grounded to the bumper, they said sure, but they still don't work! When I got home from work the following day, my wife said the dealer called said they fixed it. They told her the ground was loose, DUH! Never took it back to that dealer again, didn't want stupidity to hit me for the second time!

Trailers (especially boat trailers that intentionally get sunk in water) often have severe ground problems. I usually just bring a ground wire from each fixture back up to the connector and don't even count on the trailer frame for grounding. On my British cars I often solder an actual ground wire to each lighting fixture and take them back to a convenient nut or screw that goes into solid metal. Solved a lot of strange problems that way - my personal favorite being when the taillights work by themselves, the brake lights work by themselves, but as soon as you try to have both at the same time they all go dark. That drove me crazy for weeks.

However the fact that you get NO lights at all, but the flash to pass still works makes me agree that the switch is likely at fault. These cars hot-switch the lights (no relays) so all the current is passing through the switch itself and that can be hard on the contacts.

The early B's (pre-'68) light circuit is not fused. That the flash-to-pass circuit works and the switched lighting is totally unpowered does indeed put the switch in doubt. Also the power lead brown wire to it. That wire should have 12V whether the ignition is on or off.

Rule #1: Be cautious, don't allow any of the brown wires to touch any ground points. DON'T LET THE SMOKE OUT OF THE WIRING HARNESS!

A disconnected battery and a V.O.M. would be the preferred method of checking continuity in the harness, IMHO.

*sigh*

Got the new switch. Looked at the panel. I'll either have to remove the speaker console, or see about removing the radio blanking plate. I don't know how to remove the speaker console, and ISTR the blanking plate is bolted in from the back. I'm much too old and un-limber to do any contortions. I'll probably just take it to the mechanic and let him do the contortions. He said the first place he'd look is the turn indicator/flash-to-pass stalk. I told him I disagree, since my turn indicators and flash-to-pass work. But if he insists on looking at it anyway, I may as well replace the assembly. Right turn doesn't lock on, and the stalk has some rust on it. New ones are cheap enough, so I've ordered one. I'll have him replace the frozen map light switch too (I have a new one already, along with the headlights switch).

Pretty simple. Pull the tunnel rug, take out the fasteners on the speaker box...two per side and slide out the plastic bit. Next remove the blanking with two 5/16 nuts and you are in.
i would disconnect the battery first.