Couple wiring questions

[BCCHowdy]

Good day!

I'm adding some gauges to my instrument panel and trying to do some troubleshooting on some gremlins and had a few 'hmmmm' moments, so I'm reaching out for help.

1. The upper right most instrument light has a pig tail on it that went farther in to the dash somewhere. I have no idea where it went or really what purpose it serves. There's no continuity between either side of the bulb socket so I'm pretty sure it's not a ground. If anything it seems more a tether but I really don't know its purpose. What's it for?

2. Truck has factory cruise control which I'm not interested in and have already removed most of the components. I'm converting to TPI and I'm wondering if I can use the output from the speedo as an input to the TPI computer. Is this possible?

3. When I turn on the dash lights, both signal indicators illuminate and the light is very dim. Almost seems like the bulbs are grounding through the indicator lights. Not sure where to start troubleshooting here - any advice greatly appreciated.

4. Picture of the gauges I've put in place of the warning lights:

Thank you!

 

[nvrenuf]

I’m not good with wiring but that first pic is a fiber optic cable that goes to the ashtray.

 

[BCCHowdy]

Well that makes sense! Mystery solved. Won't be needing that. Thank you!

 

[dyeager535]

Been a long time, I want to say it's pin A10 on the ECM connector. Yes, you can use the VSS buffer (piece in your hand) as the input for the ECM.

Depending on how far you want to get into it, and what you have for parts, may be an idea to swap to the electronic speedo. Not huge advantages, but the mechanical speedo is more problematic IME, and going with the digital speedometer allows you to correct the speedometer (and input to the ECM) with some soldering, instead of having to play around with drive/driven gears. Also nice to eliminate the speedometer cable.

 

[BCCHowdy]

Thanks dyeager535. Do you know which is the signal wire?

Speedo to VSS buffer I presume is:
Red: +
Black: -
Green: Signal

The wires on the output are:
Black: -
Pink: ?
Yellow: ?

I wasn't able to find these wires on the schematics I have. Once I get the harness work done I could probably determine which is + with key on then whatever is left is signal.

 

[dyeager535]

Kind of frustrating, I don't see where it is in a cursory online search.

Following GM's wiring scheme, black should be ground, pink should be ignition switched 12V, leaving yellow as the output. I'd still verify two of the three though, and you are correct, with a multimeter you could easily tell. But yellow sounds correct.

A trick here, is to look at the ECM schematic for the vehicle you are dealing with. This era GM was pretty standard with the VSS input across the car/truck lines, I recall it always being A10 for TBI/TPI. Just find the ECM connector pinout in the electrical manual (probably found here http://67-72chevytrucks.com/vboard/showpost.php?p=5621043&postcount=1 ) for your truck, and verify ECM input is whatever color. It MIGHT also indicate the terminal on the VSS buffer it goes to. The terminal labels may be on the VSS buffer connector (usually a letter). As I recall, those VSS buffers used different pinouts, basically just more terminals, depending on vintage and what options the vehicle had, such as cruise, VSS to an ECM, etc.