Technical 66 Caddy charging issue

I am helping a buddy with his 66 Caddy. It has a 500 with a 400 trans. It has a dual battery setup with a solenoid style battery isolator.

It wasn't charging and the alternator looked really old and crusty, so we put on a new Powermaster 160 amp 1 wire. I ran new 4 gauge wire from the alt to battery, & grounded the alternator to the battery with an 8 gauge wire. With only 1 battery connected, it was charging at a little over 13 volts, but when you gave it some gas the voltage dropped. With both batteries hooked up its about 12.8 volts, but its going down as you drive. It got down to about 11 volts when we parked it.

Any thoughts on why the alternator isn't charging? I did try to excite it with a jumper wire from pin 2 to the battery terminal, that did nothing.

Also, why would the voltage drop with RPMs? Maybe that has to do with it not charging
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Voltage drop when revving the engine is a classic sign of belt slip.

 

I don’t think the belt is slipping. Even when I add RPMSs slowly, the voltage drops.

 

I would personally get real battery terminals, there are a lot of connections there to lose voltage...

Have you checked for output at the alternator or going off a gauge? You need to check the actual alternator output, if it’s good then go looking for a poor connection, voltage drop testing is your friend here.


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Thanks.

I checked the voltage at the alternator stud, and it’s the same. I may need to get another alternator to try. Just seems odd to have the same problem with the old one and now this one.

 

What the hell is this Micky Mouse setup and why is it even on there? That is one of the most gawd awful wiring setups I have seen outside of the extension cord for tail light wiring under my flatbed when I bought it. I'm assuming that all of those cables feed the sound system and pump (s) for the bags but nothing looks right with that mess. One battery should power the car and it's normal accessories, the auxillary battery should run the compressor and maybe the sound system The auxiliary battery should be connected only though the relay / continuous duty solenoid.
I'm thinking that the passenger side battery is the main and only battery on a 66 Cad as you see in the correctly connected battery in the bottom photo as what should be the primary battery in the top photo isn't even connected.

 

This is the correct way to hook up the dual batteries. If it were me I'd run all of the extra cables that are hooked to the auxillary battery off a bus bar on the fender that has a short cable to the + post and not have that nest of wires on the battery post.

 

Thanks for the replies. Mr48chev that drawing you posted I basically what’s going on here, but obviously much cleaner. The previous owner had a crazy stereo and 4 compressors for the airbags. He pulled the radio, and left a lot of the wiring. The 4 compressors are still there. Cleaning up the wiring will be a project in itself, but first I want to try to get it charging. I suppose my next step is to unhook the second battery and see if it charges. I will grab a voltage reading right at the alternator.

 

I thought the year break was '65.