# Taillight Wire identification



## BBSoup32 (Mar 28, 2019)

Hey guys - not sure if I'm in the right section but I was wondering if anyone can help me identify the wires on the outside, larger tail light assembly that houses the turn signal. The plug has 3 wires, black, red and gray. I'm assuming black is negative. The taillight has 3 functions - night time lamp, brake and turn signal. I'm trying to identify which wire carries what signal. Can anyone find or have any information in this? 

Thanks!!


----------



## Blasirl (Mar 31, 2015)

Take a meter, pick up a free one from Harbor Freight, and ground one lead and backpin one of the three wires while someone operates them. The turn signal will be the easiest.










:moved: from Appearance to Audio & Electronics


----------



## BBSoup32 (Mar 28, 2019)

Blasirl said:


> Take a meter, pick up a free one from Harbor Freight, and ground one lead and backpin one of the three wires while someone operates them. The turn signal will be the easiest.
> 
> 
> 
> ...


Lol, sorry a dumb question, but What do you mean by backpin?


----------



## Blasirl (Mar 31, 2015)

This is just an example of an actual back pin, *Electronic Specialties 142-5 Flexible Silicon Back Probe Pin, *but it may not be compatible with the HF meter. You can rig something else carefully with a paper clip. You stick it in the back of the plug of make contact.


----------



## Ma v e n (Oct 8, 2018)

The other side of the connector should have a black wire which is ground, and a blue/brown wire which is the "night" lamp. And the 3rd brake/turn should be grey/yellow.

I don't recall off top of my head how they line up to the tail light colors, but should be easy to match.


GM doesn't recommend backprobing connectors for the most part. And neither would I if you aren't familiar with the term or procedure.

If you can't figure it out by color, just unplug the tail lamp and make your test at the connector face. Turn on the hazards and put one probe on the black wires terminal, and then test to see which other wire has the power switching on and off.


----------



## Blasirl (Mar 31, 2015)

Ma v e n said:


> GM doesn't recommend backprobing connectors for the most part.


Why?


----------



## Ma v e n (Oct 8, 2018)

Blasirl said:


> Ma v e n said:
> 
> 
> > GM doesn't recommend backprobing connectors for the most part.
> ...


Basically because it's easy to compromise a weather resistant connector.

You need to disassemble high capacity connectors to do so.

It's easy to damage small diameter terminals when backprobing(the probe you would stick through the back of connector is larger than many terminals...And the terminals are complex and delicate) 

Backprobing also requires a different diagnostic approach and a different set of values for what an electrically good or bad result is and doesn't allow one to fully test the complete circuit. 

By not disconnecting the connector and face probing you don't get to visually inspect terminal and connector condition, nor do you get to assess or measure terminal retention.

The new CSS 4cylinder engines(2.0 and 2.7 turbos from XT4 and full size trucks) sliding valve actuation system are the first components I'm aware of that GM has any backprobing diagnostics for, as they offer an "additional" diagnostic approach for these components utilizing a 4channel oscilloscope.


----------

