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MG MGB Technical - Indicator tell tale glows when brake applied

Hi, just replaced the flasher unit on my 76 mgb gt as the left turn indicator wasn't flashing despite much checking of connections, bulbs etc. Now when the brake is applied the left turn tell tale glows...???!! If indicating left and brake applied the light goes out and the indicator refuses to operate. Any ideas? Thanks!!

Regards

Will
W Harrison

Sounds like your left rear lamp isn't earthing very well.
Dave O'Neill 2

Will,

Sounds like one of the dual element bulbs may have shorted out internally. Try replacing the bulbs or making sure that the bulbs are making proper contact in the socket.

HTH

Fred
Fred Wright

Hi
I go with Dave's diagnosis, crook earth at the tail light assembly.

Herb
Herb Adler

Thanks for the replies!

Just to clarify, with the new flasher the turn signals work fine, the hazards also work fine, but the tell tale on the dash glows when the brake is applied. If the earth was as bad as all that would it flash at all? I did notice a difference moving the unit about for a better earth before but not enough for the old flasher unit to operate correctly. I'll do some more digging though. Thanks!!
W Harrison

Will,

Strange "cross-over" problems are usually a bad ground somewhere.

Charley
C R Huff

The indicator flasher unit certainly won't have been the sole cause of your problems as it is common to both sides of course, and the right side was and is working normally (I assume). I'm surprised a standard flasher unit is flashing at all on the left given the symptoms, they are very susceptible to reduced current and that is a safety design feature to indicate (ho ho) to the driver that one of the bulbs has failed. It doesn't take much reduction in current to stop it flashing, one or more slightly bad connections somewhere is usually enough, leaving both corners glowing at apparently normal brightness. Brand new flasher units out of the box *are* less susceptible to begin with, becoming more susceptible as they burn in, but I'm still surprised it flashes.

There is no earth connection to a 2-pin flasher unit. If wiggling it about in its clip made a difference to flashing rate then either the wire to spade connections are suspect, or the internal mechanicals were being distorted slightly.

Modern 3-wire (one of which *is* an earth connection) flasher units operate on a different principle and aren't subject to non-flashing with slightly bad connections. They have their own 'one bulb out' safety feature which is flashing at double-speed. Some people fit one of these to overcome non-flashing with the original 2-wire unit, but it must be remembered that if the problem is bad connections, which is reducing current and hence bulb brightness, then that will still be there with the 3-wire flasher, and probably get worse as time goes by. Dim flashers are not a good idea.

On the other hand you may have been supplied with a hazard flasher or so-called 'heavy duty' flasher unit and these don't have the safety feature that stops the flashing if there is reduced current flowing in the circuit. Hazard flashers are design to operate if anything from 1 to 4 bulbs are working. Hazard flashers can be marked '21W x 4' and indicator flashers '21W x 2', sometimes with '+ 5w'. You can tell if you have a hazard flasher fitted because they operate in anti-phase to indicator flashers - when you first operate the indicator stalk the two bulbs come on straight away, then start flashing off-on-off-on after a short delay. Hazard flashers don't do anything when the switch is first operated, then start flashing on-off-on-off. The only way you can detect a 'heavy duty' flasher unit is to disconnect one of the corners (do the left-rear as that obviously has another fault anyway). If the left front flashes on its own then you definitely have an incorrect flasher unit, as well as an earthing problem at the left rear.

The rear clusters (and chrome bumper fronts) earth through their physical fixings to the rear wing, but their can also be problems inside the cluster where the bulb holders attach to the cluster body. If a voltmeter connected between the holder of an illuminated bulb and a known good earth (say at the ones for the other rear lights at the number-plate bolt) shows any voltage at all when the lights are illuminated then you do indeed have a bad earth. Test on the threaded studs inside the boot as well to you known good earth, and if that shows the same voltage as before then it is the fixings to the body that are the problem. You could tackle those, or alternatively simply install an alternative path with a wire from a stud to the bullet connectors in the earthing wires for the reverse/number-plate lights.
PaulH Solihull

Hi all, thanks for the suggestions and the extras too! I had a clean up around the grounds and tah-dah! All glow like the sun and flash as they should, I will check to see how things function if there is a bulb out though, thanks for the advice on that Paul. One less problem, just got to identify and buy a new part for the rear suspension next, new thread for that one as I can't see it listed anywhere! Thanks again gents!

Will
W Harrison

This thread was discussed between 07/09/2010 and 13/09/2010

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