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MG MGB Technical - A friend in Need - Indicator Issue

A friend I know through a Bike forum has posted this on there.... can anyone suggest a solution please.




Iíve been trying over the weekend to fix the in-laws MGB GT (1974) indicators, but was unsuccessful, help please.


The passenger side indicators work, just about, when you initially turn them on, they stay on for a while, then start flashing. When the ignition is on, but the engine is not running, they do not flash, just stay on.

The driver side indicators do not work at all, the little green light on the dash stays constantly on, but nothing else.

Things Iíve already done:

1. Replaced the flasher unit, works fine for the passenger side
2. Swapped the bulbs from left to right
3. Checked supply to the flasher unit, itís fine
4. Checked supply at the switch, itís fine
5. Checked supply out of the switch, fine for left and right
6. Checked supply at the block that splits the supply to front and back, fine.
7. Checked supply at the bulbs, fine.
8. Ran a new earth from the battery to the bulbs, still didnít work
9. Ran a new earth from the battery to the outside of the flasher unit, didnít work.

The wiring diagram in the back of the Haynes manual shows the supply going through the hazard warning switch, and a second flasher unit, I could not find a hazard switch anywhere, so Iím guessing it was not fitted to the car, but!, I did find another flasher unit, double the size of the one in the indicator circuit, I removed this, it made no difference, I replaced it, it made no difference.

Please help.


Andrew McGee

The flasher unit depends on a certain amount of current to flash at the correct speed, so that when one bulb fails it gives a clear indication by *not* flashing, but lighting the remaining bulb permanently. When the wiring and connections get old and resistance starts building up it makes the flashers slow.

Modern flasher units work on a slightly different principle of flashing at *double* speed when one bulb has failed, which makes them less dependant on slightly reduced current with older connections.

As long as they flash when the engine is running you are OK, but if they don't flash with just the ignition on (they will definitely take longer to start flashing) then it can be an early warning to start checking connections. So your passenger side is just about OK. But if you mean your drivers side isn't lighting the bulbs at either the front or the rear, just the tell-tale on the dash, then there is a full disconnection somewhere. The most likely place for this is in the mass of connectors by the fusebox where the rear harness connects to the main harness. If by 'drivers' you mean the right-hand side then you are looking for the green/white wires (green/red is the left-hand side). The wire from the indicator switch comes out of the main harness into a 4-way connector, another wire goes back in towards the front of the car, plus there is a third wire from the rear harness. The most likely is that the first wire is making a poor connection to the connector, and hence the other two wires. Remove all the bullets from the connectors (both sides), polish them up with fine wet and dry or emery, and if the connector looks cruddy replace it (Halfords). Put a dab of Vaseline (or any old grease) on each bullet and inside each socket of the connector and reassemble. make sure the bullets are pushed fully home, they go in so far then meet resistance, they have to be pushed past that to click fully into place. When fully seated the end of the bullet will be about flush with the end of the *metal* part of the connector, not sticking out at all.

Lighting but not flashing both sides (or not lighting anything at all) can be the flasher unit or its wiring, which on a car equipped with hazards can be caused by bad connections inside the hazard switch, which only allows the indicators to work when the hazards are *off*.

Lighting but not flashing just one side, or slow flashing both sides, can be caused by bad connections anywhere in the circuit from the solenoid up through the ignition switch, fusebox, hazard switch, indicator flasher, indicator switch, bulb holders, connections to the body, and any of the myriad connections along that circuit, as well as incorrect or tired bulbs. Bad connections diagnosed by turning them on and measuring the voltage at each point along the circuit. A drop from one point to the next indicates a bad connection between them.
Paul Hunt 2010

Thanks Paul.

I'll pass that back to him and let you know how it goes.

Andrew McGee

'Zackly what Paul said. The problem is not "someplace" rather "every place" ie, connection. Determinations of "fine" don't cut it, unless you measure actual loaded voltage drop across each connection or circuit section; a test light or static (unloaded) voltage reading will not tell you the truth. This is typical of cars that have sat a long time, but common on anything that has not been treated to a proper electrical going-over.

I just did a Spitfire that behaved exactly as you say. Each fix helped a bit, until lo, it all works blinkety-blink, engine not running. It can suck you in, since just poking around will frequently improve things temporarily, but the problems will return - and there are many associated things that go with it, like inaccurate gauge readings, dim lights, and poor charging.

You need to replace all the sleeve connectors, as they will be weak and probably broken, especially the doubles. They crack from age and heat, which is why doubles fail more often; correctly replaced they will last another 30 years. It takes about 20 doubles and 10 singles to do a B. On the Spit, one double in the whole car was not broken, and I know for fact that I replaced that one a couple of years ago.

The flasher gets power from the IGN Green circuit, so everything up to there is involved, along with all IGN controlled functions. (bat leads, browN wires, White wires, fusebox. Green wires) The power then goes through the Haz switch and back out to the indicator flasher. The Haz switch just turns the Ind power OFF when Haz is ON, but the switch contacts are frequently dicey, since they rarely get used. On the Spit, the Haz switch was good, but both ends of the power lead from Haz switch to Ind flasher had bad connections
(c1V drop each, fixed by R&R plus OxGard). Flashers are commonly replaced to fix this sort of trouble, but it is often the R&R that fixes it, not the new flasher.

Then follow through the whole system, fixing all connections in the area where you are, The Spit got all new connectors in the boot, as again every one was corroded and cracked. Lamp grounds are the final point. These can drive you nuts, because you get bad contact through the various multimetal joints in the lamps and mountings. Again, a good voltmeter and measuring under load will tell you where the trouble is, but since all these things are likely bad and easier to fix than measure, you may as well fix it and use the detailed diagnostics for remaining problems if any.

Fletcher R Millmore

Hi Andrew,

Check that the correct wattage bulbs are fitted.

W A Nixson

Does the car have hazards ? as my 1977 had several problems with indicators then I tried the hazards and had problems with them also. Replaced the hazard switch and all came good.
a goldup

This thread was discussed on 05/07/2010

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