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Prince of Darkness has re-visited my MGB. The left front directional signal in out. Yes. I switched out the bulb and I have all marking lights and all directional signal lights EXCEPT the left front. I suspect something has gone wrong in the wire to the light bulb fixture and/or the light fixture itself. Any thoughts of wisdom on this



79 MGB
gary hansen

all wires and connections including the earth(ing) need to be clean, secure and protected

I'm not sure where the earth is on a r/b for the front indicators, on c/b the unit earths to the car body and is often a weak spot because of rust on one or both

if it's just the one side it suggests wires or connections to that one side or that one unit, otherwise any earlier supply or connection could be also be weak and it's showing up at the weakest point

forget this Prince of Darkness stuff, much of the wiring, switches and components can still be functioning after 4 or 5 decades unlike modern components ...

... which brings me on to - did you test that the new or replacement bulb actually worked before you installed it or put in a known working bulb from the other side or elsewhere on the car?

for a number of years now I've had brand new bulbs that were faulty or broken from new but you couldn't see it in them - only this week I installed a double filament bulb in my wife's car from the pack of spares I bought for the car 6 years ago and as both my testers are either faulty or not working (what happens when you buy cheap) I had to test the bulb after fitted to the car, and not as simple as fitting a bulb on a B, of course Sod's Law one of the filaments on the brand new bulb didn't work all thought it looked fine

I then found they now sell bulbs marketed as 'Heavy Duty' at a much higher price which I guess are just made to the standards the others ought to be

for cleaning connections have brushes, electric contact cleaner and electrical contact grease

ē never assume a new part, component or fixing will not be faulty, some recent new parts have been of poor quality or faulty including rubber products like hoses, electrical items like indicator flasher units, brake light switches, CB points, condensers and rotor arms
ē never assume the parts, components and fixings on your car are the original or correct type or fitted correctly or working fully or correctly

below recent faulty new bulb that again looked fine

Nigel Atkins

Thanx Nigel. I never gave much thought to the Prince of Darkness "thing". After all the car is 35 years old. One must realize items deteriorate. I like to share these issues for several reasons.

1- find out if this is a common "MG related" issue.
2- is there a "special MG" approach to solve the problem,
3- sharing with others who may be having the same or similar problem,
4- in the end, we all learn from the problem solving process

I'm sure once I get into it, the resolve will probably be a cleansing of a corroded contact or at worst, a wire and/or socket replacement due to corrosion.

So far a bulb replacement did not fix the problem. I see some corrosion on the bulb contact, so I will pursue the contact cleaning route and will post the results of my outcome when I fix the issue.

Again, thank you


79 MGB
gary hansen

I take it the hazard flasher is out that side as well? And the left rear works OK (or at least glows even if it doesn't flash)?

If so it's common, so either the 2-way bullet in the green/red on the left side of the radiator grille. Test both sides of this and if you have 12v both sides then the problem lies towards the light unit. This could be the earth connection, which although wired on a rubber bumper is very exposed to the elements on the back of the light fitting with a flimsy connection.

If you don't have 12v either side then test the 4-way bullet where two green/reds from the main harness join a green/red to the rear harness in the mass of connectors aft of the fusebox.

If it's one side but not the other of either bullet connector then the problem lies in the connector.

Not uncommon after 40 plus years, no special MG diagnostic techniques, just standard electrical using the circuit diagrams to work out a logical approach.
Paul Hunt

Hi Gary,
I wasnít moaning, well perhaps about the Prince of Darkness stuff but carry on as you are I put up my experience of bulbs to share, all posts we make are shared as theyíre seen but others many of whom never post here

I donít know how the tone of my post is picked up by you and others but I was just putting slabs of general info (as thatís all I know) and didnít mean it to seem tetchy

if you see corrosion on the bulb contact thatíd certainly be the place to start

Iím sorry if you found my post a bit, well, I donít know what, as to me it reads fairly well, by my usual low standards, but it would to me as I know what I mean and how I mean it

carry on sharing and importantly for these threads report the outcome, what worked what didnít, etc.

I usually put that most servicing on these cars boils down to cleaning and lubing - and to a lesser extent perhaps this applies to repairs and problem solving, cleaning and lubing
Nigel Atkins

OK... The Hazard switch does work ( a bit dodgy) but the left front does NOT illuminate. The other 3 lights flash fine even when in Directional Signal mode.

So now I will try the voltage at the red/green wires as soon as I get a test lamp.

I recall a few years ago I had an issue with the left headlamp high beam. It tuned out to be the wire was no longer delivering current to the headlamp for the high beam mode. Replaced the wire and it has worked ever since.

Again, thank you both Paul and Nigel.Shall post as soon as I get the next step completed


gary hansen

things like hazard switch, horns, blower and perhaps even windscreen wipers and washers tend to play up when they're not tested or used regularly

easiest way to check them is whilst using the car regularly

for the hazard (or any other dash switch) you could rock it on and off a good number of times to loosen and clean it a bit

just taking connectors off and back on a ffew times can help clean them

or better still
. take switch out of the dash
. note locations of wire connections and remove them
. clean the connectors on the back of the switch
. a slight spray of contact cleaner inside the switch
. blow out the excess
. operate the switch repeatedly
. check the spade connectors for being clean, protected, security of wire and fit to switch connector
. adjust as required
. a slight dab of electrical contact grease on each switch connector
. put wires back on switch checking they are in correct location and they are a secure fit
. carefully push switch back into dash

connections now all clean, secure and protected and switch loosened (for now at least)
Nigel Atkins

Recently my right front turn signal wouldn't work. The bulb was fine and putting a meter in the unit showed that voltage was getting through. It turned out to be corrosion in the bullet connector behind the grille. It could pass enough current to power my meter, but not emough to light a 21 watt bulb. Replaced the connector after shining up the bullets, and all is well again.
Mike Howlett

personally I prefer a test bulb with a selection of connectors over a meter anyway

(good job as my cheap multimeter is unreliable)

whatever you test with always remember to test the tester is working correctly between each test or batch of tests

i.e. if a test bulb blows you'll be on unreliable results from then on, same with believing an untested meter
Nigel Atkins

"It could pass enough current to power my meter"

That's why when using a meter it should always be a voltage check with all connections made and components in place. This will tell you exactly what voltage is reaching any point that you connect one side of your meter to, with the other side of the meter on an earth. A test-lamp will work up to a point, but especially for things like indicators there can be enough current flowing in the circuit to light the test-lamp bulb, but not enough to make the indicator bulbs flash. By using a voltmeter along the circuit you can find how much voltage you are losing at each points, and there can easily be several slightly bad connections along the way, each contributing to the problem.
Paul Hunt

what even if your test bulb is a 21W ?

>>and there can easily be several slightly bad connections along the way, each contributing to the problem.<<
that is a good point that I thought I'd put but hadn't, plus you've put it very well, a simple explanation and use of words that often escapes me
Nigel Atkins

The problem is solved. Bought a test light. Grounded to the black ground wire on the lamp housing, turned on the lights and got the point of the test light on one of the contacts inside the socket. I got light... Turned IGN on and engaged left flasher, and with a bit MORE "grinding" with the Dremel tool, the contact in the socket lit the test light. Re-installed th bulb which was resisting. After a bit of fiddling, the bulb seated properly and we now have all direcional indicators.

So once again, one must really stay up to date with electrical contact cleaning. I tend to forget this car is 35 years old as is not going to be trouble free. One also needs to do a "walk around" your car periodically to check for these items.

Still enjoying it!!

Thanks again for all of your input.



79 MGB
gary hansen

This thread was discussed between 18/03/2014 and 21/03/2014

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