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MG MGB Technical - Rear Stop Lamps / Reversing Lamps / Screen washer

Hi all,
thought I would ask before delving into the workshop manuals, does any know off hand if the Stop Lamps/Reversing lamps/ Windscreen washer are on the same fuse ? All mine are not working and MOT tomorrow !

Thx.
Jon

Hi Jon,

You do not say what year your car is and this affects the answer. You are best to look on the net at www.advanceautowire.com/mgb.pdf

Basically on most vehicles those 3 components normally come off the same fuse; the one with green wires. However a number of other components such a turn signals, heater motor and fuel guage also normally come off that fuse as well. However, on some vehicles the wipers come from the ignition switch with their own in line fuse. You need to identify all the systems that are not working and then trace them back using the appropriate wiring diagrams.

John
John Tampkins

Jon,

I have just had another look at the circuit diagrams and I would guess that your car is around 1977 or maybe later? At that time the 3 items you mention still got their feed from the same fuse as other items (green wires). However MG started connecting certain component groups together within the circuit using 3 or 4 way connectors. The 3 items you mention all share the same 3 way connector on their own which is located near the washer motor on the diagram. I would start by finding that connector and seeing (A) is it OK with all 3 wires connected and (B) is power getting to it. It appears from the diagram that the wire bringing power in from the fuse is slightly thicker than the other 2? If power is not getting to the connector and the other end is well connected to the fuse holder then there must be a break in the wire inside the loom. As a test, you could try connecting a jumper wire externally from the fuse to the connector and see if the problem goes away.

John
John Tampkins

Thx John,
forgot to say, car is 1972 BGT.
Jon

Jon,

I cannot see a 1972 circuit that could give precisely your symptoms. However, be careful, my 75B roadster has, at some time been rebuilt with a much later electrical system. If you have a 1972 system the feeds to the brake and reversing lights come separately straight from the fuse and the washer is part of the windscreen wiper circuit from the ignition switch. If it is a later system they join up at the 3 way connector along with the washers.

John
John Tampkins

Those three components, when they existed, always did come off the same fuse. However so do the instruments and indicators so if those are working it isn't the fuse itself.

If it's a 72 with electric screen washer then either it is a North American spec (UK spec didn't get electric screen washers until 77) or the electric screen washers have been added by a Previous Owner, in which case it's not possible to say how they have been wired.

On a UK 72 There are three green wires but only two spade terminals at the fusebox, so one of the spade connectors will have two wires it it and the other only one. It's possible that the feed to the brake lights and reversing lights share one conenctor, in which case it could be a bad connction between this and the spade on the fusebox.

On a North American 72 the electric screen washers share a separately fused 12v ignition supply with the wipers and fan heater, so again it isn't going to be a common fault with non-working reversing and brake lights.

It could be three (or more) separate faults, just pick on one and investigate that, and you make find it fixes all three. Start with the brake lights as they are easiest to get at. With the ignition on, bridging the green and green/purple wires at the switch (on the hydraulic conenction) should cause the lights to glow.

If it does, but not when the brakes are applied, then the switch is faulty. Be advised that current supply of these switches is poor quality and new ones frequently fail soon after fitting, and need a relay to be added as well.

If bridging the contacts doesn't light the lamps use a meter or test-lamp to check for 12v on the green wire at the switch. If not there trace back through the single bullet connector ti the fusebox. If you have 12v on that, then bridge the contacts again, and check for 12v where the green/purple from the switch joins the rear harness at another single bullet connector. If you have 12v on both sides of that check the 4-way bullet connector in thew green/purple wires by the right-hand rear light cluster.

That's enough to be going on with.
Paul Hunt

Thx folks for all the help on this one ! Before I delved into the wiring a simple clean up and reseating of the fuses and connections sorted the lights out and a new £1.60 accessory switch from Halfords sorted the screen washers :)

PS Car passed the MOT :))
Jon

This thread was discussed between 30/05/2008 and 02/06/2008

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