A question about why turn signals sometimes won't flash but just stay on
steady (usually when a bulb is burned out) has come up before here, and
yesterday one of the IML members asked me privately again about this. If
anyone is interested, here is my reply on the subject:
First: the question - - -
> To: "Dick Benjamin" <DickB@xxxxxxxxx>
> Sent: Sunday, April 06, 2003 10:08 AM
> Subject: turn signal
> Dear Dick,
>
> I'm sure you've forgotten this, but several months ago we had a
> discussion on the IML regarding a non-functioning turn signal on a '68.
> I told the person posting the question to check & make sure that all his
> taillight bulbs were working, because my turn signals will not function
> (blink) if one bulb is out.
Well, last night I noticed
> that one of my bulbs was out (the furthest left one) and I just replaced
> it. My turn signal now blinks again. (It used to just come on and stay
> on, but wouldn't blink.) I knew I wasn't crazy!
>
> Do you have any idea why this is? You mentioned something about needing
> some sort of "interrupter"--? Not sure about that.
and my comment:
There are two types of flashers utilized in turn signals. One type is
independent of current drain (meaning number of bulbs on the circuit) and
one is dependent on the number of bulbs to set the flashing rate. The
former type will flash no matter what load is attached to it; this is
required if the number of bulbs on the circuit will change from time to
time, as would occur if one occasionally towed a trailer. If you go to the
auto parts store and look through the store display, you'll see both types
mentioned.
The one with which most cars come equipped from the factory is the type
which is sensitive to the load, because it gives a sort of poor man's
warning of a burned out bulb by ceasing to blink. I haven't checked on a
68, but I'd guess this is noted in your owner's manual as a symptom of a
burned out turn signal bulb. If a car had ever been used to tow a trailer,
the flasher would probably have been upgraded to the more expensive but no
longer load sensitive type, or else the flashers would have blinked super
rapidly when the trailer was connected.
I don't know what would have elicited a mention of "interrupter" by me -
unless perhaps I described the flasher operation in excruciating detail (see
below).
The way the flasher sets it's flash rate is by allowing current to flow
through a coil of wire which is wrapped around a bi-metallic switch arm - as
the current heats the arm, it bends and breaks the circuit momentarily, thus
"interrupting" the current momentarily. As the arm cools, contact is
re-established, repeating the cycle. The repetition rate is thus controlled
by the amount of current flowing. If the current flowing is insufficient to
heat the arm to the contact breaking point, the turn signal bulb will just
light and not flash at all. This is what happened with your car - the loss
of one bulb reduces the current drain more on a 68, because there are only
two rear turn signal bulbs per side. The drop in current from the loss of
one bulb is nearly 30% on a 68, apparently enough to totally stop the
flashing. (The little bulbs in the indicators hardly count, as they use so
little current, and the cornering light is on a different circuit.)
I note in passing that those of us with the more luxurious and elegant 67's
may not have this worry, as we have 3, yes that is three per side on the
rear,
thus the loss of a paltry single bulb reduces the current by only around
20%!
In the case of flashers of the type which are not load sensitive, the
heating
current is provided by a separate internal circuit path which is not
affected by the number of bulbs - these are usually referred to as "heavy
duty" flashers.
Dick Benjamin
Back to the Home of the Forward Look Network