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MG MGB Technical - all lights out except one on each side...?

Hi all,
My '80 B has developed a problem with the running, tail, and marker lights.
Only the right rear tail light works, the left front running light works, and only the right rear marker light works.
All the fuses are new and I've wiggled them around, I don't see how it could be a fuse problem because one light on each side works(Isn't there a fuse for each side?).
Could it be the light switch failing?
The headlights and signals work fine as well as the brake lights.


Ron. Get a copy of the wiring diagram for your car. Have it expanded at a copy shop until it is large enought to read properly (about 300% in my case, and I had mine laminated so I could draw on it with erasable markers). Check the various circuits that are bad and find where they come together. Then, start checking for voltage as you work your way back to where the problem is. Time consuming, but not difficult and a simple test light will work fine for this purpose.

Also, use a test light to check for power to the end of the fuse holder, the end of each fuse and the rear fuse holder. Minor corrosion and bad connections are always a primary cause of this type of thing.

Les Bengtson

I just received some answers on a related post. The thing I used to figure out the wiring in my '73 B was a laminated, 11" x 17" diagram from "Prospero's Garage." I got my copy from E-Bay, of all places, but you can go to to check them out. Alternatively, you can use one of the 'Net search engines to find links.

I think I paid USD$20 plus shipping for my copy and so far it's been spot-on. You may have the same luck. (Assuming, of course, a P.O. hasn't done a little mix-and-match with wires and sockets.)
R. Stevens

Ron, I went out and pulled fuses on my 80 B to see which was which. The top fuse controls the drivers side lites and the second from top controls the passenger side lites.

It would appear to me that the problem is not at the fuse box if you have any lights at all on either the left or right providing that the wires are connected at the box. I would be checking for continuity at the lights that are not working. Corrosion inside the sockets require polishing out with a dremmel tool. Check your grounds to each light, make sure that they are clean and light bulb replacement should not be opted out.

M Whitt

You can download a colored wiring diagram from Advanced Auto Wiring at: Click on Stock schematics and pick the drawing for the year of your car. I beleive that you can print them in a 11 X 17 foremat. Cheers - Dave
David DuBois

I printed out the wiring diagram from in sections on four sheets of paper and taped them together. Made a really nice large diagram to work with. When it gets too dirty, I'll just print a new one.

Ron use a test light to see if you are getting power to the affected sockets. If you are getting power to the sockets then you have a bad ground and/or corroded contacts in the socket. Sometimes just taking out the bulb and rubbing the contacts on the bulb on the sidewalk can make it work. After cleaning the contacts use dielectric grease to prevent future corrosion. If you are not getting power to the sockets, then you are going to have physically check all the connections in lighting circuit making sure they are clean, secure, and properly connected. The above assumes that the bulbs are good.
The reason I thought it was a fuse problem at first was because in a previous post you stated the "front right running light working, right rear tail light working, and only the right rear marker light working".

While working on my cousins 79 midget this past winter I found the same thing you described. Finally traced it to a bad ground at the front right side marker light. You might have a look there.


There is supposed to be one fuse for the left and one for the right parking lights, but if you have four separate red wires on the back of the top two fuses there is nothing to stop the front (say) pair of red wires getting reversed. Thus each fuse control opposite corners as you have. Even if not it is quite posisble to have two bad grounds - one at each corner, or even several blown bulbs. The tail lights (and chrome bumper front parking lights) ground through their physical mountings to the body. But rubber bumper front parking lights are in the headlights and share the ground with them. If that ground is faulty it is the headlights that will not work but the parking lights will. Side marker lights also have ground wires front and rear. The fronts share the headlight ground at the bullet connectors, the rears share the one for the reverse lamps.

Don't look for a magic bullet unless it is one of the fuses that is blown. Track 12v with the parking lights on to each non-working unit, and through it onto its ground. If you do see 12v on its ground wire then it is its ground that is the problem. If not then it is the bulb or holder. If 12v doesn't reach the light unit then it is some other connector in the red circuit.
Paul Hunt 2

Thanks all, I will dig the test light out and see if I get power and will also run a wire frm the ignition key to each socket to run a better ground to see if the lights come on.
Kimberley, my original post said
"right rear tail light works, the left front running light works, and only the right rear marker light works" by marker light, I mean the side fender mounted lights.
Sorry for the confusion.

I apologize I confused you with the Ron who made this post.,407780,407812#msg-407812


I notice that Paul Hunt responded. Go to his website and look in the 'Spanners...' section. He has great wiring diagrams 'broken out' for different areas of current and trouble. I created my own doc for the lighting diagrams to work form. I cna email to you if you want. I just chased down my own lighting problems, but doubt if my problem is the same as yours. My problems were because a PO had wired the main line going under the chassis to the back lights out of the way using metal wire. Over many years, the wire rubbed through the protective sheathing and was contacting bare metal on 2 wires and causing a short.
Good luck on your troubles.

I doubt that this is the problem, but shouldn't be ruled out in the wiring. Several years ago I had a customer with no power at all to the rear lights. However, I did have power to the bullet connector at the tail harness in the eng compartment. Using a needle, I punctured the wire to the taillites about every twelve inches going back under the car. I had power to a point, and at one point I had no power, what I found was that the wiring had corroded in half inside of the insulation. Cut, solder, good as new. Sometimes these things do happen. Also had this happen on a ground cable at the battery once. Undetectable with the naked eye.

M Whitt

Had the same as Mark in a headlight tail, the bit dangling in the wheel arch, due to damaged insulation and there being no outer sheathing for the wiring bundle. I bought new, but then it is only a foot or so.

Also had a dry joint in a battery cable, where I let a mechanic solder a lug to the cable. Against my better judgement he heated up the lug and melted solder into it, then just plunged the cold, untinned cable into it. Needlesss to say it went open circuit after a few months.
Paul Hunt 2

This thread was discussed between 25/07/2006 and 27/07/2006

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