Dirk de Boer asks >
> I have a question for the electronically inclined: Can you test a turn
signal
> flasher unit (say a Lucas three prong for an early MGB) on a bench using
> a 12 V power supply and a multimeter?
No, you test it using a 12 V power supply and a light bulb. Or better
yet, two light bulbs. A multimeter is a convenience.
First, how these little beggers work:
The flasher has a bi-metal strip in it. The switch in the can is normally
closed. Current flows through the flasher, through the light bulb and
back to ground. As the current flows through the bi-metal part of the
switch, the bi-metal switch bends because the thermal expansion
coefficients of the two metals are different. This causes the switch to
open and the light bulb to go off.
While the bulb is off, there is no current flow. The bi-metal switch
cools and relaxes until it closes the switch and the bulb lights up again.
You don't have to know that electrons go from negative to positive, or how
the car is wired, to test a flasher, but there are some tricks.
The MGB flasher (guessing from other cars, here) probably has three prongs
because the third is used for the single indicator light on the dash.
This is how my Europa works. The hot lead goes to the flasher bi-metal
arm. The flasher normally-on prong goes to the control lever which goes
to the front and rear lights (in parallel) which go back to ground. The
flasher normally-off prong goes to the dash indicator which goes back to
ground. Park a working car of this type in a dark garage and turn on a
signal. The dash light alternates with the outside lights.
A car having two dashboard indicators just has them hooked up in parallel
with the outside lights. Each dashboard light flashes in sync with the
outside lights.
What does all this all mean to you? You have a multimeter, or so you
implied. Use the ohm meter to find the two of the three prongs that are
connected together at room temperature. The other (unconnected) one is
the one for the dashboard indicator, and can be ignored for now. Hook up
the two you have identified as normally-closed to a circuit. Polarity is
unimportant for now. Battery goes to flasher goes to bulb (or better, two
bulbs of the sort used in the car, hooked in parallel, the way they are on
the car) goes to battery. The bulbs should light up right away, then
blink off and on. You will discover that the process is slower if you use
one bulb. If you hook up the bulbs in series, it may not work at all.
OK, if the thing flashes at an acceptable rate with two bulbs in parallel,
it should do so in your car.
If you want to test the dash-indicator prong, I suggest you use your
multimeter as a voltmeter or a low-wattage bulb like the one used on the
dash. A bright bulb like the ones you have been using will confuse the
flasher by drawing enough current to keep the bi-metal strip hot and keep
the dash light glowing and never run the signal lights.
Hook one side of your little bulb (or voltmeter) to the battery terminal
shared by the two big bulbs. Hook the other side to the "odd" prong we
have determined is the dashboard indicator prong (and ignored up to now).
If you are lucky, it will flash (indicate) alternately with the big bulbs.
If it does not work, exchange the leads on the normally-closed pair. Now
it should work.
When it is all working at once, you can draw a picture of the prongs and
label where each one is supposed to go:
1) Battery.
2) control lever (and thence to outside bulbs).
3) dash indicator.
Don't skip this step. You may have the wrong flasher for the car. Even
though they look alike, there are different wiring layouts. If the
flasher tests OK on the bench, but not in the car, check the wiring
diagram colors and the flasher socket wires to make sure you have the
correct flasher.
If the flasher tests good on the bench and bad in the car, and the color
code tells you the base and the flasher agree, you will have to start
testing the circuits on the car. I hope that your car resembles my Midget
(wiring miraculously unchanged from Lucas standard color code as shown in
my manual) and not my Europa (rewiring so bizarre that I ripped it all out
and wired the car from scratch).
Phil Ethier, THE RIGHT LINE, 672 Orleans St, Saint Paul, MN 55107-2676 USA
h (612) 224-3105 w (612) 266-6244 phile@stpaul.gov
"That's why I'm here." - James Taylor
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