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MG MGA - Turn signals

I have been wanting to convert my front turn signals to LEDs to make them brighter, and also because it seems no-one recognises a white flashing bulb as a turn signal any more. I have found a LED bulb that lights white, and flashes orange, so thought it was time to get the project going.

First attempt was just to exchange the bulb. This results in a white light, a fluttering flash on the direction indicators.

Second attempt was too add a 100 Ohm 25W resistor at the feed to the turn signal switch. Light OK, no facing at all. Same result with 68 Ohm.

Third attempt was to rebuild a spare 8 point flasher relay with electronic relays as per Barney's guide here
This produces a satisfactory flash. If I switch on the sidelights, it works too, but as soon as I switch on the headlights , the flash diminishes to the point of being only marginally visible on one side, and nothing on the other.

Fourth attempt was to give up and replace the original bulbs, and this makes everything work as it should again.

Any ideas from the crew?

Once I have this working, I will be upgrading from H4 to Xenon headlights as well. I have a kit, I just need to figure out how to position all the extra little boxes.
dominic clancy

Im not sure on the led units you are using ?

it seems a new led flasher unit is the main component as well as the lamps is needed, maybe a new bulb fitting?

I have had LED rear lights for years, so have already fitted an LED compatible flasher. Everything works until I switch on the headlights, so it can't be related to the LED flasher unit, it must be related to load on the relays somehow.
dominic clancy

The original flasher unit is (was) looking for a 42-watt load. That would be current of 3.5 amps at 12 volts, or resistance of (12/3.5)= 3.43 ohms

If you have original type flasher unit, and all LED bulbs, then try a 25-watt resistor in the range of 3 to 3.4 ohms (3.0 and 3.3 ohms are standard values). Install it on the output side of the flasher unit. For the 1500 that would be between flasher "L" and T.S. relay "1". For the 1600 that would be between flasher "L" and T.S. switch "F".

If you previously had LED lamps in the rear signals and also an electronic flasher unit, aid it worked like that, then it should work as well with LEDs in the front lamps.

You wrote: "as soon as I switch on the headlights, the flash diminishes to the point of being only marginally visible on one side, and nothing on the other".

This sounds like a high resistance ground connection shared by headlamps and corner lamps in the front wiring harness. When headlights are switched on, the high resistance ground forces voltage (higher than 0-volts) onto the black ground wire. The defeats the ground connection for the high resistance LED corner lamp. Try a separate ground wire from corner lamp direct to chassis ground.
Barney Gaylord

I just put in LED's in the rear lights of my '59 MGA. The front turn signals I used #1157 A (amber) bulbs. Their is another bulb that is higher output.. #2057A? that works as well in the front.

The LED's are pos. ground replacements for a #1157 bulb(Red), so no re-wiring to do. What I found was that the light in the rear was brighter, and the flash was a little faster( I am using original Lucas flasher). Since the LED turn off and on so quickly, they are easier to see. Especially in daylight.

The fronts work as normal and provide the resistance for the flasher to work. I use the amber bulb cause no one recognizes a flashing white light here either. I would try the 2057 bulb next time just for a bit more brightness in the front.

LED's were a direct replacement bulb so no modifications were needed.
C.R. Tyrell

Bulbs were from Superbrightleds. 1157-R18-T:red bulbs @ $17.95 USD each.

Originally bought for my TD, but because of socket design in the rear tail lights the LED lights were of no benefit. So I used them in the MGA. Good result there.
C.R. Tyrell

Earthing the lights directly ( with a wire soldered to the bulb and connected to earth) made no difference.
dominic clancy


Reading your diagram on the link, I should be able to mount the extra resistors between the relays and terminals 4 and 8 on the turn signal box and hide them in the casing.

Is this correct?


dominic clancy

No, not terminals 4 and 8. BIG EDIT HERE, as my prior message is wrong. Do not connect the resistors in series with the bulbs. The resistor needs to be in parallel with the bulb(s) and connecting to chassis ground (earth).

For the 1500 the load bulbs are connected to relay terminals 2,3,6,7. Resistors should be connected to the downstream side of the relay (terminals 2,3,6,7), then grounded. For each incandescent bulb changed to LED, you need to add about 20 watts load for the flasher unit to work properly.

If you have LED bulbs only in the rear, then install 3-ohm 20-watt resistors from 3 to ground and from 7 to ground. These need to be 20-watt resistors, because the brake lights could be applied continuously on.

If you have LED bulbs only in the front, then install 6-ohm 10-watt resistors from 2 to ground and from 6 to ground. These can be only 10-watts because the bulbs flash with 50% duty cycle.

If you have LED bulbs in front and rear, then you can install 3-ohm 20-watt resistors from 2 to ground and from 6 to ground. These supply the resistive load for two bulbs, serving with 50% duty cycle while flashing. This will provide the proper load or the (original type) flasher unit, and will reduce the load seen by the brake light switch.

The load resistors could (theoretically) be installed inside the relay box (if they can fit there), but they would be adding the waste heat inside the box, which might lead to earlier failure of the relays (especially if you were using 4-way flashers continuously).

Always keep in mind that the load resistors generate heat, about 10-watts or heat for each bub flashing or 20-watts of heat for each bulb continuously on. They can get hot enough to burn your fingers when operating.

Barney Gaylord

Maybe worth an addendum to the relay link....
dominic clancy

Since the addition of load resistors makes the car non-concours, it is much easier to change the flasher unit to heavy duty or electronic type, and omit the resistors.
Barney Gaylord

the flasher unit IS already electronic. This works as long as normal bulbs are used up front, but not with LEDs up front
dominic clancy

Not to confuse flasher unit with relay unit.

Flasher unit and bulb are in series. Adding a resistor in series with the same circuit would reduce current draw and defeat the original type flasher unit (similar to having a bulb burned out or disconnected). If you use a resistor it needs to go in parallel with the LED to increase current draw to something near the original current.

If you use an electronic flasher unit, you do not need any resistor(s).

The type of relays in the relay unit should make no difference, as long as they have clean contacts.

Using electronic flasher unit and LED bulbs front and rear, failure to flash properly might be due to high resistance joint in a connector, possibly a snap connector in the harness. Switching on headlights reduces system voltage slightly. This can screw up a circuit that has a high resistance connection.

Suggest you bench test two LEDs in parallel, connected in series with electronic flasher unit. If it works on the bench with no extra circuit parts in between, then the problem will be found in the wiring harness.
Barney Gaylord

This thread was discussed between 04/10/2014 and 10/10/2014

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