Welcome to our resource for MG Car Information.



MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Electrical problems

I am about to sell my current GT, but I hate selling cars with problems, so I am after some advice to help me sort it.

In the rain it has an issue where if I turn on the heated rear screen the indicators slow, if I have the heater and lights on too then they stop (in the on position with the bulb lit).

Where is the best place to start looking for a fault like this? Its not like a short where things don't work, it seems to me (with my very basic understanding) that there is some kind of resistance?

Any hints would be great as I am going to try and fix it this weekend.


Graham L

Graham, it sounds like a poor connection causing increased voltage drop with increased current. I'd suspect the fuse block myself based on prior experience. I don't have the wiring diagram here at work, but if you trace the source of all those items mentioned I think you will find a common point at the fuse block for all. Lucas fuse blocks have a nasty habit of loosening and corroding around the rivets which hold the spade connectors to the fuse clip. I recommend removing the block and soldering all the rivets to the spade connectors using a lot of flux to ensure good solder flow and connections. That's my best guess, hope it helps.
Bill Young

There are a whole lot of connections that could be causing the problem, which is why so many people opt to replace the standard flasher unit with a modern electronic rather than fix the real problem.

The 74 1/2 to 76 model used a relay which powered the heated rear screen from the purple circuit which doesn't drag down the voltage for the indicators anywhere near as much. Before and after that the manual switch takes power from the green circuit, and it is these that suffer the greatest problem. It depends on year, but in some cases it picks up power from one of the several 4-way bullet connectors behind the dash, and there could be more than one on the way back to the green fuse. So bad connections in each of those 4-way bullet connectors, the spade on the fusebox, the rivetted connection on the back, the green end of the fuse holder, the fuse itself, the white end of the fuse holder, *its* rivetted connection on the back, the white spades, a 4-way bullet connector for white wires in the main harness by the fusebox, the white connectors for the ignition switch, the switch itself, the *brown* connectors for the ignition switch, and finally where the brown wires terminate on the solenoid can all contribute to low voltage. Those around the fusebox and the white bullets close by are the most likely, followed by the browns on the solenoid. However any bad connections in the green circuit feeding the indicators, out to the bulbs and to the body will also contribute.

With the heated screen on plot the voltage through all those points and look for those with the greatest drop. For myself I installed a relay (even though the voltage drop wasn't enough to stop the indicators), taking power off the brown circuit via a fuse, which not only avoids dragging down the voltage for the indicators but also results in significantly improved rear screen demisting.
Paul Hunt 2

Thanks for the tips, there is a lot to be looking at there. I have been scared of electrics for ages, now is the time to jump in!

I notice that my fuse box has no cover so I'll check this first and work from there. Does anyone know if its the same fuse box as on my 76 midget (its a 77 B GT)?


Graham L

Probably. If it is a four-fuse box, and if two of the fuses feed the parking lights one fuse per side, then almost certainly, this Califonian 78-80 diagram is

Get a cover, they hold two spare fuses!
Paul Hunt 2

Thanks for that link Paul.

Do you know how/where the indicator is earthed? Can't remove them yet to look as the screws are rusted solid. I have soaked them in WD40 ready for a go later.

I realpaced a bunch of bullet connectors yesterday which lead to the headlights, although working they were about to fall off.

Also managed to break my heated screen switch as I took it out to check conections. One job leads to another eh!

Thanks all

Graham L

Ok, found my test meter. I have 12v to both indicator bulbs. I have 12v to the flasher unit on either left of right turn selected at the indicator stalk.

Left turn works but its quite slow. Right turn the light stays on (both front and back).

I am not really sure what to do next, I'd be grateful for any pointers.

Graham L

Hi Graham - the front indicator light units in an RB car have an earth wire which goes to a bullet connector by the headlights just behind the grill. This is shared with the headlights and electrical cooling fans. I think it only gets back to a body ground either somewhere near the fusebox, or possibly to the firewall high up behind the dash on the right-hand side.

The rear indicators earth through the physical fixings to the wing.

You need to measure the voltage between the metal body of the bulbs and a known good earthing point on the body as well on the 12v side of the circuit, for anything *above* zero volts. I have known bad connections between bulb body and the holder, between the light cluster and the body, and even between the bulb holder and the light cluster casting, so you need to measure these others as well if any bulb shows more than zero volts on its metal body.

You are looking for differences of tenths of a volt so a digital instrument is pretty-much essential. As well as bad connections you can have tired bulbs (swap first one then the other side to side and see if the problem moves with it) and a tired flasher unit. In the case of the flasher unit just replacing that may seem to do the trick, only for them to slow down and stop a few weeks later. This is not always because that flasher unit has gone faulty as well, but new units always work better until they have 'bedded in'.

But first you do need to plot the voltages through from the brown at the alternator, brown white and green at the fusebox, and green at the flasher unit to see if the problem is more a case of low voltage than low current from bad connections. The hazard switch is a common cause as it is rarely used, sometimes flipping it back and fore a few times can improve things, or even stop them working together! In either case dismantle the switch in a poly bag to catch all the bits, dig out the old hardened grease, and put in a bit more.
Paul Hunt 2

This thread was discussed between 08/12/2006 and 10/12/2006

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.