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MG MGB Technical - reversing lights, et al

During a pre MOT inspection I noticed that the o/s reversing light and the rear number plate lights are not working. The bulbs are good (confirmed by swapping with the n/s). It's a '75 BGT.

Does any one know what connection/component may cause this. It may also effect the brake lights but I need to get a new bulb to confirm .

Thanks in advance.

Dave G
Dave G

I forgot to mention that all the fuses in the main box are fine and I can now confirm that the brake lights are also not working (new bulb fitted)
I'm baffled!
Rgds
Dave G
Dave G

Sounds like earthing around the boot area, you will be able to measure 12v both sides of all the bulbs if this is the case.
Stan Best

Dave. No, I do not know what causes this type of thing. People like Paul Hunt, Dave DuBois, and a few others read something like this and immediately know exactly what is wrong. Very hard to like people such as that. But, they have been so kind over the years that I tend to forgive this type of understanding and still enjoy their friendship.

For us "normal" people, things take a little longer, can seem more frustrating, and feel so very good when we get the problem sorted.

The electrical circuit needs three things--power, a good light bulb, and a good ground. So, we test the bulbs, as you have done. I have found some bad bulbs, even when brand new. Hence, a couple of long wires, one held to the base/side of the bulb and one at the electrical connection at the tip of the base, then connected to the battery. That way I know I have a good, working bulb.

Any time there are light bulbs, cleaning the sockets they fit into is a good idea. Some small amount of corrosion can build up in there. preventing the "electron-thingies" from flowing properly and causing the bulb to illuminate.

If the bulb is good, and the socket clean, the only problem that can be present is the electron-thingies cannot flow thorough the circuit. A circuit diagram, preferably a copy of the one for you car which has been expanded about 300%, will allow you to see the current flow. I have my copies laminated so I can draw on them with a dry-erase marker. I do that a lot as it allows me to trace out all of the affected circuits and see where their common points are. When one has problems with several circuits, either their common ground point, or their common power input point, is a good place to start.

I prefer to start at the power supply side, using either a test light or a volt meter to see where power is present. When you have power up until some point, then no power, the problem lies between the point where you have power and do not have power--bad fuse, bad switch, loose connection, or bad wire.

Done is a reasonable, logical manner, most of us can figure out what the problem is and how to correct it. An expanded wiring diagram and a test light will be of use to you during your entire MG ownership period. A volt meter, or a multi-meter, will pay for itself quickly.

Les
Les Bengtson

This is really not that difficult to do an initial analysis on. All you need is the aforementioned electrical meter....about $10 most anywhere tools or electrical supplies are sold.

Set the meter to 50v dc. Turn on the appropriate car lights. Apply the positive (red) probe to the center contact in the bulb socket. Then apply the negative (black) probe to any clean ground such as an unpainted bolt head or scratch in the paint inside the trunk, or the inner sides of the bulb socket.

If the meter reads 12v, then you have power to the bulb socket and have either a bad bulb or bad ground.

If the meter reads nothing, you are either not making good electrical contact with the probes (so try a few times) or you have no power to the circuit.

Whichever the result, you will find out from which side of the bulb socket you need to track the problem.....to the grounding side or back towards the fuse block.

BH
BH Davis

Thanks guys
I did look at a wiring diagram ....and gave up, but the idea of enlarging by 300% sounds helpful and you are right, I should add a volt meter to my tool box.
This site is, as always, enlightening.
Cheers
DG
Dave G

Dave - As the others have suggested, the problem is either no power to the bulbs, a burned out bulbs, or no ground to the bulbs. Since you have checked the bulbs, your next step is to look at the power and ground. The power is supplied via the harness that comes up into the boot area and is then routed to the individual bulbs on each side of the car. since the lights on the near side work but not on the off side, I would suspect the ground wire for that side. The ground for all the rear lights (and the fuel pump) is from under one of the nuts that hold the license plate (registration plate) in the boot. Since your fuel pump and the lights on one side are working, look at the bullet connectors on all the wires in the boot, going to the offside. I would recommend pulling the individual bullets out of both sides of hte sleeves and cleaning the bullets and the sleeves, then smearing some dielectric grease on the bullets before reassembling the connectors. Let us know what you find. Cheers - Dave
David DuBois

A quick update:
Ironically all three problems are unrelated !
Reg plate lights : bad ground
Reversing light : faulty fitting (cured with a small piece of wire wedged in the right place)
Brake lights : confirmed as supply problem - haven't isolated it yet, could be the 2" cubed electrical box thing next to the fuse box with the green wire that matches the supply to the bulb.... I need to get that volt meter.
Rgds
DG
Dave G

"2" cubed electrical box thing next to the fuse box "

They are usually the relays, which aren't involved in the brake lights, and don't have green wires anyway. However a PO could have fitted a brake light relay (making 3 in that area on a 75) as current (ho ho) supply of brake light switches (mounted on the pedal box on a UK 75) seem to be very poor quality and the contacts can't carry the load of the lamps. However original Lucas relays are about 2*1*1, modern replacements about 1" cubed. Whatever, green should go to one side of the brake light switch and one side of the relay contacts, a new wire from the other brake light switch contact to one side of the relay winding, the other side of the relay winding to earth, and the green/purple going to the brake lights comes off the other relay contact.
Paul Hunt 2

Hi Paul
Thanks for that, I think it is a relay but as there are only two (one each side of the fuse box) it therefore cannot have been a PO fitting (what is PO ? I guess its 'post ....')Any way it definately has a live lilac wire, an earth (black) ,a white with black line and a green wire. Ive checked the wiring diagram and Haynes have not listed this relay.
You say the switch is on the pedal box, is that the bulk head above the pedal.. sort of above ones knee ?
Kind rgds
Dave G
Dave G

Just realised where the B/L switch is (in the engine bay) ! what is the small switch/box above ones knees with G & GB wires ?
Dave G

Dave - PO = Previous Owner (often proceded by a D which can stand for any adjective that seems appropriate). The brake light problem is probably a bad brake light switch. Unfoutunately, the replacement switches, as Paul stated are junk and don't last very long (I had one fail in only two weeks). If you have to replace the switch, include a relay with to make the switch last a reasonable amount of time. You can see instructions for making up and installing a relay/arc suppression circuit at: http://www.mgexperience.net/article/brake-light-relay.html
Cheers - Dave
David DuBois

Thanks Dave D
It is the switch at fault, I by passed it and the lights came on. I was thinking of getting an original salvaged switch, I notice there are a few on e-bay from your neck of the woods.
By the way you gave me some advice on fuel pumps a while back ... and you were right. My salvaged SU didn't perform under pressure. I'm about to fit a new electronic pump.
We live and learn.
Kind rgds
Dave G
Dave G

My 72B brake light switch went out. Replaced it with a NAPA aftermarket over 1.5 years ago, still working.
James Huggins

Ah, just remembered that the first 12 months or so of rubber bumper GT (except V8) production had an extra relay for the heated rear window, which is what those colours indicate.

Green/black was only used for the fuel gauge and went direct from the tank to the gauge. Other little boxes on the bulkhead by the (RH) drivers knees could be instrument voltage stabiliser (green and light-green/green), indicator flasher (green and light/green brown), and hazard flasher (brown and light-green/brown).
Paul Hunt 2

Dave,

To avoid having to take a chance on a replacement switch, I rebuilt mine. Details are posted somewhere in the archives. I think that was about 3 years ago, and it still works great.

The memory is spotty, but the plunger that made contact was able to cock itself to an unnatural angle, avoiding contact across the circuit. I considered it a design flaw, and modified the switch.
Fred Doyen

Fred,

Following your lead, I took the switch apart (had to be brutal with the metal casing though) the plunger was corroded and sticking in it's sheath but the contacts were still good. I cleaned the plunger, put it back together and it now works properly.

Thanks again to every one who has helped.

Next it goes for the MOT....

Dave G
Dave G

This thread was discussed between 22/09/2007 and 26/09/2007

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