Weird issue

[bd]

Okay bd, I jumped the flasher terminals and here are the results:

With test light on B- no visible light on either blades.

Was the left turn signal energized with the ignition switched ON during this test?

[JohnnyPopper]

Affirmative sir

[bd]

It sounds like there may be two issues. 

First, if zero turn signal power reaches the bulb, then there is an open in the left turn circuit to the bulb.  Is there any change in power at the bulb contact when you energize the 4-way flashers with the turn signals cancelled?

Second, voltage measured on the turn signal contact when the park lights are on implies either a less-than-perfect ground to the lamp socket or cross-feeding between the turn and park lamp circuits.  Was the park/turn bulb installed when you measured the extraneous voltage on the 'turn signal' contact?

[JohnnyPopper]

It sounds like there may be two issues. 

First, if zero turn signal power reaches the bulb, then there is an open in the left turn circuit to the bulb.  Is there any change in power at the bulb contact when you energize the 4-way flashers with the turn signals cancelled?

Haven't tried that yet, will do.

Second, voltage measured on the turn signal contact when the park lights are on implies either a less-than-perfect ground to the lamp socket or cross-feeding between the turn and park lamp circuits.  Was the park/turn bulb installed when you measured the extraneous voltage on the 'turn signal' contact?

No, the bulb was not installed, measured at the blade of the socket.

[bd]

Bear with me.  Remove the right front park/turn bulb then perform your voltage checks at the left front just as before.  Any differences? 

And, what were the results with the 4-ways energized and turn signals canceled?

[JohnnyPopper]

Sans Bulb, no difference.

Haven't tried the 4-way yet.

[JohnnyPopper]

Well, just tried the 4-way, all four corners work, even the front side marker. 

[bd]

It sounds like there may be two issues. 

First, if zero turn signal power reaches the bulb, then there is an open in the left turn circuit to the bulb...

Second, voltage measured on the turn signal contact when the park lights are on implies either a less-than-perfect ground to the lamp socket...

You may have a compromised turn signal switch.  The front and rear turn signals are isolated through the switch, so there could be a connection problem in the switch that affects only the left front turn and left dash indicator without impacting any of the other lamps.  The four-ways are switched separately.  Since the four-ways work, the wiring to the left front turn lamps is intact.  You can test this by unplugging the turn signal switch at the column and jumping the cab harness plug purple wire to light blue wire with the ignition switched ON.  The left front and left dash indicator should blink with the flasher if there is sufficient lamp load with only those lamps.

I still think you have an imperfect ground to the left front park/turn lamp.

[JohnnyPopper]

Even weirder- Started working again today...

[Shifty]

 

[Chevygold]

I had a similar problem but with one of the rear lights, brake lights and 4 ways but no flasher on one side and it was the indicator switch, fitted a replacement and all good again, the left dash indicator light has never worked guess that is the dash pcb at fault?
Graham