[Custom Jim]

I'm wanting to upgrade the alternator on my 66 to a 100 amp unit. I'm currently running an older 63 amp GM alternator with built in regulator. It's works fine until I turn the A/C on and with the blower fan running, the volts drop to 12.5 and if I kick it up on high I'm down to 12. I don't run it that much but when I do, I don't want the battery going dead. I've read conflicting information regarding wire size for the charging wire. Some say 8 ga. while others say 10 ga. is fine if you're not running a lot of accessories which I'm not. I currently have a good 10 ga. wire running from the alternator to the horn relay, then from the relay to the battery. I would rather not have to pull the harness out and unwrap it to replace wires. Also, I have a set of the old style reproduction battery cables and the charging wire is made into the end. I don't want to change them either. Will the 10 ga. wire be enough or should I try an 80 or 90 amp alternator instead?

Let me ask you this, when you turn on the AC and then kick on the fan to high, does the voltage drop down to 12 and stay there after a while of driving or after let's say a half hour of driving the voltage keeps getting lower and lower and dropping down to something like 11.5V, then down to 11V, then 10.5V and so on with the AC still on and the fan still on high ?.

To me if that is happening with the voltage continuing to drop the longer it's driven with the same load, then the alternator is not outputting enough and the battery is supplying what is needed but cannot continue to keep the voltage up since the battery is a storage device and the more you pull out of it, the lower the voltage will become.

If the voltage drops down to 12V and never drops below this then I have to think the alternator is putting out enough and where the voltage meter is wired to may be affected by sharing the same circuit as the AC and blower. If the wiring is marginal and you have the AC and blower off, this circuit and connections are just fine as they only have to supply minimal power but then when the AC is kicked on with the fan on high, now this circuit could become limited with voltage due to some poor connections and/or wire sizes.

On your measurements, are these with the car just sitting there in park and at idle or is this while driving down the road ?.

I know how it is with the spring ring terminals as I put them on my 68 and how it limits any additional connections to it (or being able to change out the fusible link end that is molded into it).


Some charts I use for wire sizing as well as fusing:

Jim

 

[Custom Jim]

I'm on the fence whether an upgrade in alternator output will do anything as I would probably do some more testing.

I would maybe see what you have across the battery terminals and at the alternator output and then see what you have at the volt gauge (which I'm guessing is inside the car and wired to existing wiring under the dash ?) using the same meter or gauge. To be consistent you need to use one meter or gauge for all tests as each one will probably not read identically to each other across the same test points.

If at the battery and the alternator you are not seeing the drop but only at the interior gauge, then it may very well be connection issues.

I've seen a lot of firewall bulkhead main feed connectors become overheated and then have increased resistance through them. Also connections that were fine 40+ years ago might not be as good today.

Jim