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MG MGB Technical - 3rd Brake Light (LED)

I'm trying to add a LED brake light to my 72 roadster. I wired it all in but can't get it to light off the brake light circuit. If I supply power from an external source it lights up. If I supply power from the tailight circuit it works. When I supply power from the brake lights though I get nothing! The electrical system is stock for a 72 MGB. Frankly I'm stumped- I added a similar LED unit to my 73 BGT and it works fine wired the same way.
william fox

The key to LED is light emitting DIODE. Like any diode, current can only flow 1 way, kinda like a check valve. Reverse the polarity and all should be well.

Mike
Mike Phillips

I am sure that you are aware that LEDs will only light when the power is connected to the proper leads. All that I can suggest is to reverse the wires and see if that makes a difference. Also check to make sure that you have ample power to the circuit and make sure that there is no corrosion at any of the fuseboxes or wire junctions that could provide resistance to the circuit. I had a similar problem with a headlamp, it would light from external power, had power to the lamp bucket, but the lamp in the bucket would not light. After several hours of frustration it decided to work. The problem is that I don't know what I did to fix the problem.
Robert

Even though it is LED the power to the brake lights is the same polarity as to the tail lights. Also if the brake lights light with the LED unit connected then there is no point in checking things back towards the fusebox or switch, unless the brake lights don't work either when the LED unit is connected. Double-check your connections to ground and to the brake light connector, which is the green/purple.
Paul Hunt

Thats why this is so aggravating. All the connections are good. Its wired to the correct polarity. There is power to the brake lights and they light up nicely. The ground is apparently good as the LEDs light up when I connect the positive lead to the taillight supply. When the supply comes from the brake light supply they no workee.
wiliam fox

Do you have them wired in series or parallel? LEDs need something like a .8 difference in potential to forward bias and thus conduct electricity.

Mike
(formerly from belleville)

Mike Phillips

Check and check again, with a voltmeter both on the cars connectors and the additional light. If they light up when connected to a red (tail light) connector but not to the green/purple connector and the brake lights are still lit, you are not making a good connection.

These LED arrays are wired in series internally. At the external connections they are simply a 12v device like any other. Putting them in series with an incandescent lamp will likely cause the LEDs to glow but not the incandescents (connections permitting!) because of their different current requirements and internal resistances.
Paul Hunt

You can triger it with an SCR.
mickeybme

It seems as though you are doing everything OK, and the fact that you already performed
a successful install on a B/GT tells me that you already know the drill.

My 3rd light LED array poofed-out only after a week of duty.

A careful autopsy revealed a loosely soldered power wire on the LED circuit board.

I had to use a magnifier loupe to spot it.

It was probably loose to begin with, but opening & closing the rear hatch exacerbated the situation.
Daniel Wong

I connected the positive lead to an open spot on the bullet connector that the two brake lights branch off from. The negative is grounded to the trunk lid. What is an SCR?
william fox

silcon controlled rectifier, i think. another name for a diode
andy heston

Humm. Are you sure your trunk lid provides a good ground? There could be paint between the hinges and lid or body. Worth a check, anyway.

Jon
Jon Wright

How would I connect a diode?
william f ox

Hi

An SCR (also known as a thyristor) is similar to a diode, but has a third terminal (the 'gate') which must be 'triggered' by a small current before it will conduct in the forward biased direction.

It will not conduct in the reverse biased direction whatever the gate current.

Once triggered by a gate current, it will continue to conduct until the supply current is removed.

Don
Don

BTW, LEDs don't like having reverse polarity applied to them, but I guess most commercial third brakelights include a protection diode.

A typical red LED needs about 2V at 20mA to illuminate, so if you are making your own third brakelight I would suggest connecting 5 in series, with a series current limiting resistor of around 180 ohms. You could always double this up by making a second set.

Some LEDs are much more efficient than others (ie produce more light for the same current).

It is worth noting that LEDs are availible with different viewing angles (ie the viewing angle from which the light can be seen), if the vehicle behind yours is your 'target', LEDs with a small viewing angle will appear brighter to the driver.

Don
Don

If you remove the diode pack and replace with a bulb (rest of your wiring the same) does the bulb light?

Mike
Mike Phillips

I didnt try an incadescent bulb, but I did tinker with it today. I was able to get the LEDs to light connecting straight to the power lead for the brake lights- without the brakes lights connected. When I connected one the LED's still worked, but when the second was added the LEDs went off. I guess those halogen brake bulbs are drawing too much current. I'm tempted to use a relay for the stock brake lights and power it and the LED's off the brake light circuit.
willam fox

Using a relay probably work, but ...
Still sounds more like bad connections.
Use the method of probing the connections for voltage drops as it goes towards ground. Start at the power source and clean connectors as you proceed. Repack the bullets with dielectric grease.
You can whip up copper cleaner using salt & vinegar 1/2 teaspoon per cup or so. Cut 2 soda bottles in half, 1 for cleaner 1 for rinse. For sh*ts, put an old crusty penny in there for 15 seconds and watch it turn new.
You can use a push needle to safely pierce the plastic sleeve on wires to probe for voltage while the circuit is loaded. You're looking for voltage drops across connections.

Mike
Mike Phillips

What do you need an SCR for? That device 'operates' given a trigger voltage then remains operated even though the trigger voltage may have been removed, until some other device in the output circuit opens. If the LEDs won't light now then adding more devices in series is just going to make things worse.

If the LEDs won't light with two lamps connected but do with one then you are dropping too much voltage in bad connections between the source (battery) and the load) lamps. Have you tried it with the engine running or just the ignition on? With the engine running you get a couple more volts throughout the system, which is why the turn signals flash faster. The brake lights are powered off the green circuit which supplies a lot of things including the turn signals, you may find your LEDs flashing on and off in anti-phase with them! You could use the brake lights to operate a relay and the relay to power the LEDs off the battery via an in-line fuse, or off the purple circuit which is already fused and in the boot.
Paul Hunt

This thread was discussed between 28/04/2003 and 01/05/2003

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