Unidentified Wire in Dash Harness 1978

[mikebrow74]

I acquired a full instrument cluster and harness from a recycle yard donor. I cut the engine harness about 12-inches on the engine side. It was a 1978 Monza with V6, Manual Trans, A/C, same as my car. I refurbished and re-wrapped the harness and it's ready to go back in. I have one unidentified wire. There is not a terminal on the dash end. It is brown w/white stripe and is a single strand wire. It goes through the firewall connector and the engine side is a brown multi-strand wire. In the photos the dash end is marked #1 and the engine side firewall connector is the Brown wire on the left connector 3rd down (Blue, Green, Brown). I have looked at wiring schematics and can not find this wire. Anybody have an idea what it is?

[spencerforhire]

It could possibly be for tach, which is not shown on most diagrams.

[mikebrow74]

I thought about that but I have traced the tach lead from the circuit board to the engine harness. It is brown tach to distributor. This wire leaves the harness near the HVAC controls.

[spencerforhire]

I have never owned a Monza with A/C, and the diagrams I have do not show any of the A/C wires. It could be a trigger wire going out under the hood to activate a relay for the heater blower high speed. Or it could be for some obscure option that you do not have.

[mikebrow74]

What is very odd about this wire is that it is a single strand solid wire up to the firewall connector. The vehicle I pulled it from did not have the rear window defogger or intermittent wipers either. I was looking at the 1979 wiring diagram and it does shows a brown w/white stripe wire going from the circuit board connector to the alternator. I'll have to look at the dash harness I'm going to pull out a little closer and see what it has.

[mikebrow74]

I think I figured it out. I found a diagram that specifically calls out the BRN W/WHT single wire on circuit 25. It runs between the alternator connector and the ORG wire at the ignition switch on the column. I acquired the full gauge cluster and harness from a recycle yard donor. The tilt column had already been removed and instead of unplugging the harness at the switch, Bubba cut the bundle about 8-inches back. I didn't want to cut my original harness so I located a 78 Buick Skyhawk donor w/o gauges and cut the needed pigtails out. I butt spliced all the wires back together and re-wrapped the harness. Now that I'm looking at that single lead, it ends right about where I spliced in the pigtails. I must be missing the continuation of that wire up to the ignition switch connector. Maybe there is a difference between the Gauge/No Gauge wiring on that circuit. Glad I found that now. That would have been a beach to locate and fix that when it is back in the car. I seem to recall that these single strand wires are resistor wires. I hope that adding 8-inches of stranded wire here isn't going to make a difference.

[Monza Harry]

Mike I didn't look at my diagrams [GM 1976] but '74 should have been HEI no? So no resistor wire there. I am wondering if your car had a high idle solenoid, for when the A/C is on that would go from HVAC area, to the under hood, those were very common on that era of 4 bangers [No V6/V8 Vega's from the factory]. Just some thoughts. Spencer the GM wiring diagrams does have the A/C ones, ok one at the back(?) as I recall. Harry

[spencerforhire]

Not sure what diagram you're looking at, but hooking the alternator directly to the ignition switch will cause some mayhem. Specifically, it will start and charge fine, but when you turn off the key the engine will still run. The ign. terminal at the alternator needs the indicator bulb in the circuit to function properly.

[mikebrow74]

Spencer,

viewtopic.php?t=21140

Take a look at 1175 and 1176, circuit 25 off the bulkhead connector. It runs from the L terminal on the alternator to the ORG terminal on one to the ignition switch connectors Circuit 300. That ORG runs up to the oil pressure switch on the L4. It does not appear to be used on the V6 or V8.

[spencerforhire]

According the diagram you linked for me, the brown/white is connected into the 12g orange at the ignition switch, and then spliced to the brown alternator "exciter" wire that comes from the charge indicator bulb in the cluster. Doesn't really make any sense. Are you sure this is the same wire you have? I'd just ignore it.

[mikebrow74]

I looked at the one now in the car (non-gauge) and it does not have that wire in the dash harness. I couldn't determine if the matching brown wire was at that same exit location on the engine side is. I'll need to take a better look with photo in hand. I am leaning heavy at ignoring it, but that is hard for me to do without a clear understanding. Both non-gauge cars I have harnesses from do not have anything attached that 12 ga Org.

[zeke]

On non gauge cars, the alternator gets excited through the generator indicator lamp. On gauge cars, the alternator gets excited directly from the ignition switch on I3 terminal (300). The switch has two separate ignition terminals, I1 and I3. Distributor is fed from I1 so alternator can't feedback to the ignition circuit because it is isolated from it when the switch is off.

[zeke]

In a voltmeter car, the alternator L terminal would be fed from I3 on ign switch, through a short piece of single strand nichrome resistor wire to the bulkhead connector. This added bit of resistance ensures that the ignition circuit doesn't try to draw power from the alternator through the L terminal. The 10SI alternator doesn't need very much voltage on the L terminal to energise the field circuit.

[zeke]

Gauge dash cover with no alternator / generator light

That other diagram the op linked above is drawn wrong. It's missing the labels W O GA and W/GA. I find mistakes in lots of these old H schems.