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MG MGB Technical - More electrical gremlins

A very odd problem started yesterday with my directionals indicators on the dash. This problem seems (at the moment) to be restricted to the passenger side(right turn). Without the directionals on, I noticed that at times that the right turn indicator on the dash glows a faint green and then shuts off. Sometimes when I turn the right directionals on it flashes while driving, but stops when I come to a stop at a junction and then comes on again as soon as I start to move.. It doesn't happen all the time but very noticeable. I have not noticed the same problem with the left hand

Thanks

Gerry
GG Ginty

Gerry,

My first guess would be a weak ground somewhere. Depending upon what kind of MGB you have, it could be tied up with the 4-way flashers. For someone who really knows this stuff, they would probably want to know what car you have.

Charley
C R Huff

Sounds like a poor earth (ground) Try putting temporary earths from the securing screw on each light unit to a good ground on the chassis or block. See(sorry) if that sorts it, then you can make a more permanant job. It's usually the front side/turn lights, as they are earthed by their fixing to the wings, rust is a good insulator! The rear units SHOULD have ground wires attached to one of the fixing studs, but these often get missed on re-builds.
Allan Reeling

Thanks My B is a 74
GG Ginty

Gerry. It would be good to know what year your car is. Do not know if it would make a difference, but it might and would, at least, allow us to examine the same wiring diagram.

"Sometimes when I turn the right directionals on it flashes while driving, but stops when I come to a stop at a junction and then comes on again as soon as I start to move". This does sound like a "grounding problem" which might be the ground of the bulb holder or may be the build up of corrosion inside the bulb holder. The rear tail light, left hand side, showed a similar problem recently. The tail light holder does not have a ground wire, but grounds through the unit. Problem turned out to be corrosion on the brass of the bulb and inside the base where the bulb fits. This resulted in the tail light going out when the brake was applied and neither the brake light nor tail light (they share a common socket) was, then, illuminated. The tail light came back on when the brake pedal was released. But, on this 79 model, the turn signal bulb and the brake light bulb are different and it is hard to see how they could influence each other. That is why it would be useful to know the year of your car.

As to the turn signal indicator dash light glowing sometimes when the turn signals are not in use, this would have to be some form of bleed over from some system that is being powered at the time the incident takes place. I am told that a bad common ground point could cause such an incident.

Thus, removing the bulbs and cleaning and inspecting the brass section of the bulbs and the sockets they fit into would be a good place to start. Then, check the tightness of the rear tail light assemblies to the body and the tightness of the front light assemblies. Any assembly that has a ground wire (as shown in the wiring diagram) should have the ground wire inspected for tightness and cleanliness at both ends. Common ground points, as in the trunk near the latch mechanism, should be taken apart, cleaned, and reassembled.

Les
Les Bengtson

Hi Gerry,

I agree with the faulty ground and would go so far as to predict it is one of the rear lights, since the problem occurs when you stop, ie when the brake light is also "on". A quick way to find which light it is would be to turn on the driving lights and then have someone apply the brakes. This would also "kill" the riding light, as they would share a common earth through the fitting.

The dash light glowing can be some backwards leakage ( I can't think of a better way to describe it) from a high resistance ground.

Herb
H J Adler

I'd agree with Herb - right rear light unit.

As I'm assuming the indicators and the brakes use different bulbs, even if the running/parking lights are with the indicators instead of the brake lights as in the UK, then it is almost certainly a bad connection where the light unit bolts to the body. When using the brakes or running lights some of the current will come back through the indicator wiring and earth through the dash repeaters, which is why they glow at various times, and also why the indicators may stop working when the brakes are applied.

Rear light units *never* had earth wires from the factory, nor chrome bumper front park/indicator units. Only headlights and reversing lights had them, and rubber bumper front indicators and later number plate lights. Probably easier to add earth wires, using the existing bullet connectors, than fiddle about removing the light units and trying to clean up the inside surface of the panels.

P Hunt

Just goes to show how mean BMC/Leyland were. Since there is a xmas tree of earths bolted to the boot/hatch lock striker plate, i'd always assumed the light cluster would have had a connection there. Anyway adding an earth in the boot is easy and clean and worth the 10 minutes effort.
Allan Reeling

It's not just BMC / BLMC but in Australia our "own" car the Holden, by GM, had very similar issues. My Dad's utility had a panel at the back wherein the spare was housed. On it was the number plate and the number plate illuminating light. You probably guessed it, that the ground return was left to the vagarities of the light fitting mounting and the hinge. Needless to say it didn't work for very long, cured with a bit of wire to the body.

Herb

H J Adler

Thanks.. I'll take a look once I can get the car running again.. Hope I haven't fried another starter motor. Had problems starting yesterday. Cranked the car several times and it sounded like a weak battery. The car had been standing for a couple of days in the garage in cold weather. The battery read about 12.6V but still sounded very weak when I cranked. Stopped cranking for a few minutes and then I tried again and all I got was the dreaded clunk from the solenoid :-(
GG Ginty

Is Rhode Island very cold? Car batteries should give 2.2volts per cell, i.e., 6 x 2.2 = 13.2volts. But that's under ideal conditions. A cold battery will give considerably less than that, the colder the more reduced will be the output. Add to that having to stir up thick (cold) oil, the losses of the long battery cable, maybe compromised (corroded) connections, short journeys where the alternater doesn't get chance to replace the energy used to start the car in the first place. Under conditions like this, even if the battery does manage to turn the engine over, there is little energy left to give the coil a meaningful output. Especially if you're still on points ignition.
Check battery acid levels and S.G, top up and charge if necessary, clean connections all the way through the system, warm the battery and engine with a fan heater if possible and try again.
Allan Reeling

Yes, I remember 2.2v per cell from school as well. But in practise the standing (i.e. some minutes after charging has stopped) voltage of a battery is more like 12.8v. You need to measure the voltage of the battery *under load* to check whether any cranking problems are down to it or its connections. With a meter directly on the *battery posts*, not the clamps, a good battery should show about 10v when cranking, weak batteries will be progressively lower than this. If you connect your meter to the brown at the fusebox and the engine block and crank again, the difference in volts there to the battery is what is being lost in the connections and cables. You should be able to get this down to about 0.2v each leg, i.e. no more than half a volt less at the starter solenoid than the battery. Excessive cranking with low voltage at the starter, from either cause, *will* smoke the starter.

When I restored my CB roadster - no earth wires on the number-plate lights - I had to provide some. The bulb to holder to plinth to overrider to painted bumper iron to painted body was simply no go. Didn't have a problem with the light clusters.
P Hunt

Ay I,
the trouble (????) with well restored cars, is everything has plenty of paint on it, unlike factory originals! I have everything properly grounded, to circumvent possible problems caused by all that paint, waxoyle etc..This not only provides proper return paths, but I can still remember the days when my medium/long wave radios used to suffer interference because the boot lid and bonnet weren't earthed properly! Force of habit you see!!!! I even seem to remember a suggestion that the hubs also caused a problem! But that is going back a bit to memories of Car Mechanic's problem pages! Oh and another memory of lying under my Hillan Imp while my dad turned the key and seeing the throttle cable glow because the engine earth strap had corroded! Happy days.
Allan Reeling

This thread was discussed between 11/12/2009 and 13/12/2009

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