Wire/Connector ID help

[eric8]

Have a factory solid blue wire with a single pin connector that is unused on my truck. Very curious what it is for. It exits the main engine harness back by the bellhousing mating area on the driver's side. This is on a virgin 77 C10, 350 engine, 3 speed manual transmission.
Attached is a photo

[hatzie]

Solid blue. Possibly your truck was converted to gauges and that's the oil pressure lamp switch wire.

[eric8]

Good thought but 100% sure this has factory gauges with the OE mechanical oil pressure gauge copper line exiting the block. Maybe the trucks without gauges had an electric oil pressure switch that this wire went to? For the dummy light? And all trucks had the same engine wiring harness regardless of how they were optioned?

[Dead Parrot]

77 may be too early but on trucks with electric choke carbs, there was often a connection to an oil pressure switch that kept the choke heater from getting power until there was nominal oil pressure.

[hatzie]

Open up the 1977 GM Light Truck wiring diagrams.
In 1977 the V8 OIL warning lamp was Circuit 31. It's a Dark Blue single wire from the instrument cluster, through the bulkhead, to an overmolded Female Packard 56 connector.

The 1977 wiring diagrams show that there was a choke heater oil pressure switch on some models but it was a two terminal switch with Light and Dark Blue wires in one terminal and two Brown/White wires in the other terminal.

The four wires on the oil pressure switch connection, for the trucks so equipped, are circuits 901/931 & 141A/141B.

1. Circuit 931 is Dark Blue and appears to run back through the bulkhead plug to the instrument cluster using the same slot as Circuit 31.
2. Circuit 901 is a Light Blue wire to the Choke heater.
3. Circuit 141B is Brown/White to the idle stop solenoid from the wye on the oil pressure switch.
4. Circuit 141A is Brown/White to the bulkhead plug and connects to two circuits 141 & 43 on the bulkhead socket inside the cab. 43 is the Yellow radio power wire. 141 is ignition switched power ACC & ON.

If your truck has the manifold stove choke coil then I doubt your truck is equipped with this electric choke wiring.

The only other Dark Blue wires in the engine bay are the RH Turn and the Wiper motor.

[eric8]

Well I wanted to give a shout out to Hatzie (almost 4 years late) because I’m just getting around to replacing that original engine wiring harness with one from American Autowire. The only engine harness they list for a ‘77 C10 equipped with a SBC is part #38136: “ENGINE HARNESS, CK, V8, 305, 350, 400 c.i., w/o electric choke Note: With oil pressure switch behind distributor on block”

So, after finding pics of their engine harness, I discover it looks exactly what my stock (factory gauges) one looks like. Has the lone, unused blue plug I referenced behind the distributor. So mystery solved I guess.

The plug was there in the harness for the oil pressure dummy light switch, as you noted. I can only assume this plug is present in the harness regardless if your truck had gauges or not (at least in ‘77). If it had gauges, the (wireless) mechanical oil pressure gauge was used. Same green wire used regardless if you had temp gauge or dummy light. And I guess the engine side wiring is the same going to the alternator regardless if you had voltmeter or dummy battery light.

So long story short, if you have factory gauges with the unused oil pressure wire dangling behind your distributor, I guess it’s normal for it to be there!

[projo198]

Not sure, but do your reverse lights work? These are prompted by your transmission via a single terminal on the side.

EDIT: sorry, I see you figured it out!