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MG MGB Technical - 79 California MGB indicators

79 California MGB indicators

Hi guys, before I go mad trying to fix the problem, can you help.

When I have the hazard lights on they all work and blink quickly, turned off the left one works ok but the right nothing !!! where should I start ? the indicator stalk on the steering column maybe ?

Thanks Matt
Matthew Mason

Are any of your bulbs burned out? Does the dash indicator blink quickly also?

Matt. The first thing to check are all of the bulbs and their sockets. Make sure they are clean and making good contact. Actually, they are the second thing to check. The first thing to do is to turn on the hazzards and see if all of them are blinking. If not, do as Shook suggests and examine the bulb for a broken filament. If not broken, then, follow my suggestion and check for corrosion on the base of the bulbs and in their holders.

If you do not find the problem there, next look for good grounds--a common problem. Clean the terminals and where they connect to the chassis and see what happens.

If that does not correct the problem, get a wiring diagram and trace the circuit for the right side, using a test light to see where the problem exists. A somewhat time consuming problem and one I find frustrating (I do not do electrics well), but fairly easy to find and correct the problem.

Les Bengtson


How many grounding points/straps does a "B" have? My '79 has one obvious strap going to the tranny/engine area. Do we have similar straps hidden in the undercarriage that supports the back end?

I have one of those nifty laminated color electrical charts but it doesn't show the physical locations of the ground(s). The chart helped me trace my horn and turn indicator this past summer.


The hazard and turn flashers are very different animals. Whilst the hazard unit will flash with bad connections in the circuit the turn flasher in the same circuit may not flash.

If neither of the bulbs even glow on one side with the indicators, but all four do with the hazards, then you have an open-circuit near the indicator switch as you suggest. Check the connection between the main harness and the indicator switch sub-harness, and the switch itself. In the former case check the two halves of the connector are fully pushed together, and none of the pins has been pushed out the back which can happen. With the hazards on you should see 12v coming from the green/red and green/white wires in the main harness into the indicator harness. With the ignition on and the hazards off there should be 12v coming from the light-green/brown in the main harness to the indicator harness. With the indicator switch operated one side or the other there should be 12v coming from the green/red or green/white of the indicator harness into the main harness.

If the faulty side glows but doesn't flash then you have bad (high resistance) connections somewhere. With the exception of rubber bumper front indicator units there are no ground wires in the flasher circuitry. All rear lamp clusters and CB front clusters get their ground through the physical mounting to the wing, and of course this can corrode. Other possibilities are the bullet connectors in the cluster by the fusebox where the main harness joins the rear harness, by the headlights, and in the boot. Also the bulb in its holder - either the casing or the live contact - and I've even known of a bad connection between the pressed bulb holder and the cast base plate of a rear light unit. Could also be in the indicator harness to main harness connector, or the switch itself, compare the working side with the faulty side in this case.

You should be able to track the problem(s) down with a voltmeter, working along the circuit until you get a sudden drop in voltage between two points. When testing the bulb holders you should see 0v on the casing of the bulb, the pressed bulb holder, and the base plate. Anything above 0v is a bad connection to ground.
Paul Hunt 2

Woow thanks guys, I'll keep you posted. All the bulbs do work with the hazards on - so its not a bulb, I'll start at the wheel and move around

Many thanks
Matthew Mason

Shook. The main ground strap is located from the front of the engine to the chassis side of the motor mount on chrome bumper cars. On the rubber bumper cars it is the one you mention--going from the right transmission mount, front bolt, to the transmission cross member. However, there are quite a number of other ground points on the car and if you follow your various black wires you will see that they tend to come together at a common screw which is attached to the body. Look in the trunk for one example and at the ignition and starter relays for another. These are also ground points and any associated with a malfunctioning system need to be inspected, cleaned and re-attached.

Matt. Paul seems to indicate that the turn signals/indicators ground through the body of the signal housing. (I have not had to pull any of mine apart, so do not know from personal experience. Also, Paul knows a lot more about electrics than I do.) So, I would check the screws holding on the various light holders for corrosion and make sure there is not rust in the holes the screws fit into. Copper-Ese (SP?) is a commonly used anti-seize when reassembling the light bodies to the body and has been recommended for both preventing corrosion and making a better electrical connection.

I have had problems with corrosion in the sockets, on the contacts in the base of the socket and on the bulbs. I picked up a round, stainless steel brush having a wire handle (used by welders for cleaning) and use one of these to clean out the socket. For the contacts at the bottom, use the eraser on the back end of a wooden pencil. Mildly abrasive and easy to get down into the bottom of the socket. As Paul also noted, any connection is suspect. Clean and inspect all of them and see if that does not correct the problem.

Les Bengtson


I'll take your advice on the grounding points. Sounds like a good winter afternoon project.

Paul, what would be the multi-meter readings (Ohm) for the grounding points to confirm a good connection? Are all solid black wires only grounds and are all grounding wires solid black?

Thanks again for the Lucas education


Shook - don't use resistance readings, they can be deceptive. When tracing a bad connection these can vary with the current and voltage applied to them. A typical ohmeter applies a microscopic current and low voltage, very different to the 12v and tens of milli-amps in the real world. The only true test of a circuit is what the voltage readings with respect to a good ground are along the circuit when in use as I have described.

From the factory all grounds were black and all blacks are gounds, but you could well have to contend with PO wiring which could be anything. As Les says there is only one engine/transmission to body connection for the starter which was on the left-hand engine mount on earlier cars and the back of the gearbox extension on later cars. Other than that in simple terms there were lower current ground connections for the front and engine bay electrics by the fusebox, the the cabin electrics under the dash, and for the rear electrics to a number plate mounting bolt on the rear panel. However a number of components throughout the car, like late horns, voltage stabiliser, rear light clusters (and there were lots more) picked up a ground from their physical mounting.
Paul Hunt 2


Excellent reading and easy to understand. You either have an extensive Lucas education or years of trial and error.

Thanks for taking the time with us who are Lucas challenged.

My pleasure to help. Must be osmosis, my grandfather was born next to the Lucas King Street factory. I've been fiddling with Lucas electrics for 40 years, and electrics in general for longer than I can remember. My parents told me they had to buy me a set of plugs and sockets to stop me playing with the ones on the wall before i could walk. My 10 month old grandson is showing a similar interest.
Paul Hunt 2

This thread was discussed between 18/01/2006 and 22/01/2006

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