If GMC's from then are like Chevys, turn signals and the second brake
light were dealer installed optional equipment on step sides. To
get the original wiring diagram for these lights, you need the Accessory
Installation Manual. Chevy Duty and others sell one for the 57 Chevy's
that should be close enough. This wiring is not covered in the basic
shop manual or assembly manual.
Assuming you have a step side, there were two rear lights in the
optional setup. Each had a dual filament bulb (No. 1157 for 12 volt),
like most newer cars and trucks. The low power filament is the
tail light (on when the parking lights or headlights are on.) The
high power filament is the turn/brake light, on when the turn signal
or brake lights are on.
The wiring for the brake/turn signal goes from the power source (keyed)
to the brake light switch, located on the outside of the angled part
of the floorboard next to the brake pedal shaft, to the turn signal
switch in the steering column, to each tailight independently. The
turn signal switch decides whether or not the brake light gets its
power from the flasher or from the brake light switch.
It sounds like the PO just had the extra lights wired up in parallel
with the tailights.
Bruce K
57 3200
Mt. Iron, MN
It's not difficult, but the wires to the back change color a couple
times where the accessory wiring is spliced in place of the original
wiring.
At Monday, 17 June 2002, you wrote:
>Well I just got my first "old" truck (1959 GMC 1/2 ton shortbed
stepside).
>
>I bought her a week ago and after changing oil / filter, I'm finally
finding
>time
>to look at the non-functioning break lights.
>
>I'm pretty sure the '59's only had single rear lights on each side
(correct me
>if I'm wrong), but someone
>has put a monster of a rear bumper on this truck (which will be
going to the
>scrap yard) and in addition to the original rear lights there are two
>additional lights (mounted in this monster of a bumper).
>
>Here is what I've discovered so far (wiring book soon to be ordered).
>
>Main harness:
>Black wire: turn signals (original rear lights)
>Yellow wire: parking lights (one of the additional two lights added)
>Blue wire: BRAKES??????? The blue wire is spliced into two wires
(red &
>black) one running to one
>of the additional lights and the second wire to the other added light.
>
>Basically I just want the original "single" round lights. I'm assuming
these
>would be the parking, turn and brake
>lights.......correct?
>
>I've cleaned the contacts, re-spliced the old splices and tried
different
>bulbs.....but I've not traced back to the brake pedal switch, yet.
>
>Any input will be greatly appreciated.
>
>Thanks!!
>SteveGet more from the Web. FREE MSN Explorer download :
>http://explorer.msn.com
>oletrucks is devoted to Chevy and GM trucks built between 1941 and 1959
>
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