68 XR7 charging system problems

Working on a failure to charge the battery and I don’t know much about electrical stuff. The wiring diagrams are making my head hurt.

Voltage at battery while running is only just above 12V. The battery will go dead if I start/drive it around for a while. I have had to use a trickle charger to recharge the battery

My alternator is brand new from Deadnutson. Same for voltage regulator. Alternator harness is a new one from Cougarsunlimited (big block XR7 harness) because the old one was a butchered mess. Under hood harness is the original.

From looking at the diagrams, I think I should have continuity from the center black wire at the alternator harness connector (38 in yellow circle) to the black wire on the under hood harness that connects to the battery side of the starter solenoid (38A in red circle) because they are spliced together inside the harness (blue circle). Am I reading that correctly?

I do not have continuity between 38 and 38A right now. Is that my problem?



Okay, I’m pretty sure I got this figured out. I was stumped as to why that splice would suddenly go bad when I installed the new alternator, alternator harness, and voltage regulator. So I went back and looked at the “before” photos of my alternator and connections at the solenoid:







I hadn’t really noticed it when I disconnected that mess of wires, but there are two heavy gauge wires connected to the battery terminal of the alternator and the extra one connects directly to the battery side of the solenoid. When I “fixed” the harness by putting in the nice one, I removed the wire that was providing power to the battery:







Again, I’m not good with electricity, but I think this will also account for my ammeter not functioning, since no current was traveling on that circuit.

Also, I think this means that my whole car was always running off the battery, with the alternator only charging the battery? This may account for some other issues I was having, like wonky sequential signals? I think I read that the relays don’t like it when they aren’t getting full alternator power.

Anyway, I think it’s time to back my main harness out and unwrap it to that splice. Fingers crossed.

Can somebody tell me what’s the best way to access the plugs at the dash end of the under hood harness? Can I work them out into the engine bay to unplug them or do I need to reach up from the dash side and unplug before I pull them through?

Edit: I hadn’t realized that the harness went so far in past the firewall before it connected all the way over behind the heater controls.

You’ll have to access those plugs under the dash area. I don’t think there’s enough length/room to pull them out through the firewall grommet.

The harness from the firewall to headlights feeds from under the dash through the firewall. Not exactly easy to change if you have power brakes. That harness is reproduced and IMO one should consider changing it if repairing an old one is questionable.I redid my 68 XR7 wiring over the winter and that part of job was likely the hardest. Take your time with it.

'68 XR-7 should not have a wire connecting the alternator directly to the starter solenoid. It has a “shunt” built into the main harness so that connection is made in parallel with the amp gage. If you install a wire like that it bypasses the ammeter. Sounds like someone has dorked up your main wiring harness in the past, probably due to letting the smoke out of the wires at some point. No telling what you are going to find when you unwrap it.

Thanks very much for all the responses. I pulled my gauge cluster today and was able to access the harness plugs after dropping the heater controls out of the way to give me some room.

I tested all the wires at the solenoid and had good continuity to the end of the harness on every wire except for the large black wire in the center of the plug that connects to the alternator harness, “38” in the yellow circle of the diagram. When I jabbed the multimeter probe through the insulation behind the plug it had good continuity. I’m guessing that somebody yanked on that plug while holding the wire and pulled it out of the crimp.

All the wires inside the harness, at least from the solenoid to the voltage regulator, look good and unburnt so I have a pigtail on the way from WCCC and we’ll see if that takes care of it.