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MG MGA - Turn signal puzzle

Hi everyone,

Just noticed today that both my brake lights and all four turn signals are out.

Shorting the brake switch (near the starter switch) activated the brake lights, so it appears I need a new hydraulic switch.

The turn signals are a different story however. Shorting the green-to-green/brown wires confirmed that all four bulbs are working and that the flasher unit is getting power. Shorting out the green-to-light-green wires showed that the panel indicator light is working.

This leaves either a faulty flasher unit or an improper load on the unit (e.g. wrong bulbs). I did a few more tests, but I can't determine what it could be. Here is my data:

Measuring resistance from green/brown wire to ground (wire disconnected from the flasher unit):

w/both bulbs in: 1.0 ohms
w/only front bulb in - increases to 2.0 ohms
w/only rear bulb in - 2.5 ohms
both bulbs removed - infinite (e.g. nothing shorted out)

Same measurements, but with wire connected to flasher unit:

key off - 25 ohms (assume this is the resistance of the flasher unit)
w/both bulbs , signal on - .9 ohm
w/rear bulb removed, signal on - 1.6 ohm
w/front bulb removed, signal on - 1.8 ohm
ignition key on - 0 ohms - shorted (even w/no bulbs) - 11.5 V

I'm confused by this. I thought the turn indicator bulbs have about 7 ohms resistance (21 W), but this data seems to indicate that 4-6 W bulbs are in place.

So is it the bulbs?

R Stokes

Er, did my math wrong at the end there. My calculations show that these are probably 50W bulbs...
R Stokes

The resistance of filament bulbs is dependent on their temperature. 21 W is the power taken at the working temperature. I doubt your meter is passing enough current through the bulb to light it up to full power. You cannot easily tell the power rating of the bulb by a simple ohm reading.

Is your car a 1500 or 1600 by the way.
Malcolm Asquith

I was afraid of that. 1600 MGA.
R Stokes

I've had this problem a couple of times. There's a couple of bullet connectors in the engine bay that have worked loose that bring them all the flashers down together. IIRC it's above the brake switch on my car (1500).

It may be completely different on a 1600 ...
Dan Smithers

Here is the diagram for the turn signals on a 1600.
Start at the flasher.
Is there power at the green wire, if not that wire goes directly to the fuse,check backwards.
The LG (light Green) wire should not show power untill the lights are flashing.
The GN (green with brown) should show power, if not the flasher is bad.
With power on the GN you can check power at the two output wires from the switch the GW (green white) and GR (green red). If power comes in but not out of the switch it is bad.
All the other wires and connectors are on one side or the other, not common to both, and any faiure with not cause both to fail.
The best tool to check this is a simple test light. No need for a multi meter.
Hope this helps



R J Brown

This thread was discussed between 27/04/2008 and 28/04/2008

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