1959 GMC turn signal indicator wiring
 

So I purchased what seems to be a very well done factory replacement wiring harness for a Chevrolet, because I can't find a GMC one, and they seem to wired almost identical anyway... "almost".

Chevrolet Apache trucks have a left and right turn signal indicator in the dash cluster but GMC are different... they have only one turn signal indicator (the right one and it's green) that flashes for either the left turn or the right turn. What you might expect to see as the left turn indicator is actually the high beam indicator and is red in color. see image below. You can see in the picture below, that the new harness has doubled up the blue wires (to make 8 wires) as per Apache... but the original harness I am replacing has only 6 wires.

My question is, how on earth does two wires going to one light bulb not backfeed the the entire circuit to make all signals flash like hazard lights AND how do I wire my truck so it's like factory? Does the signal light switch manage this or perhaps I need a resistor of some kind or what??

Re: 1959 GMC turn signal indicator wiring
 

You'll have one wire coming from the turn signal flasher, usually purple, that goes into the turn signal harness plug. It feeds both the left and right turn signal wires through the turn signal switch.
What they probably do is connect the turn signal light wire to this purple wire and any time the purple wire flashes it flashes the turn signal indicator light. I'll check the wiring diagram I have and see if it shows that. I might only have the chevy diagram though.

Chevy wires the left and right turn indicators to the left and right front turn signal lights so when one of them flashes it flashes the corresponding turn indicator.

Re: 1959 GMC turn signal indicator wiring
 

Thanks VetteVet!
I am going to look into the purple wire suggestion and report back also.

Re: 1959 GMC turn signal indicator wiring
 

After thinking about this some more I realized that the purple wire is hot any time the key switch is on. This means that the signal indicator light would also be on and it would flash off when the turn signal flasher blinks.

Wiring a couple of small diodes into the yellow and dark green, L and R, rear turn signal wires, would prevent feedback and would flash the signal indicator light whenever the L or R turn was selected.

The only glitch would be that anytime the brakes were applied the signal indicator light would illuminate. To avoid this you would have to wire the signal indicator light into the front turn lights so that only one of them is on at a time.
The front signal light wires are usually light blue and dark blue.

All I have is Chevy diagrams with the two lights so I'll have to look and see if I can find one for the GM. You've peaked my curiosity now.

There is another possibility but it would mean that the turn signal switch for the GM trucks is different than the one for the Chevys. The Gm switch would need to have separate contacts for the indicator light with separate wires to the light. Checking the part numbers for the two switches should show that.
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Re: 1959 GMC turn signal indicator wiring
 

Subscribing.

Re: 1959 GMC turn signal indicator wiring
 

I recently got the signal lights working on my '55 6700 bus. It has an add on turn signal switch, the Grote 966-4 and a Buss 550 flasher. I discovered that the flasher has 3 terminals. The 12v on one side and on the opposite side that goes to the switch. The middle terminal is marked P for pilot light, it flashes opposite of the signal lights and is connected to a bulb in the signal switch. I have indicator bulbs in the gauge cluster, so I don't need the lights in the switch. If your flasher has 3 terminals, connect the middle one to your single dash light.

Re: 1959 GMC turn signal indicator wiring
 

There were a couple years (62 & 63 I think) where the Chevy trucks used a single turn signal indicator for both sides. On those, they simply wired the indicator light between the left & right front turn signal wires. They used the front turn signals so it's not affected by the brake lights.

They didn't use any diodes or anything to prevent one side from "backfeeding" into the other. Instead, this configuration simply takes advantage of the wattage difference between the little in-dash indicator bulb and the larger external turn signal bulbs.

One side of the in-dash indicator bulb receives power from the turn signal on the activated side. The other side of the in-dash indicator bulb is connected to ground through the filament of the external bulb on the non-activated side. Since the small low-wattage indicator bulb only draws a tiny amount of current, it's not enough to make the filament of the larger external bulb glow. So I guess you could say, there is some "backfeeding" going on with this arrangement but it's limited by the high resistance of the little indicator bulb so there isn't enough current flowing to light up the external bulbs on the non-active side.