Ok, following very closely, and I'm still confused. I want to prevent any electrical failures on my 70 Roadrunner, which Ive owned since 1996. I still dont see a method to accomplish this. For reference, this car is a Superstorm Sandy survivor, and has had the dash cluster, harness, column electricals, and alternator replaced with new aftermarket. So, the firewall connections are not toast,.... I added a Holley EFI and electronic distributor, and no other electrical additions. I may add the headlight relay mod soon. All this considered, what is the best way to insure my electrical system is properly protected?
One of the biggest issues is when using an alternator that can put out over 40 Amps. I don't have a 1970 factory Service manual, but on my 1969 Coronet, the alternator output wire, and the large wiring to the ammeter is only 12 AWG wires size. The Factory fusible link is on the battery side of the wiring which protects the dash wiring from a short from the battery supply side, but if the engine is running, the alternator can still provide current to the dash. When the alternators output is limited to around 40 Amps or less the 12 AWG wire is at it's limit but should handle it for awhile, but the bulkhead connector won't like that much current. A high output alternator puts out too much current for the stock 12 AWG wiring size.
You can add a larger alternator output wire to the battery and add a fuse or fusible link to protect the new alternator output wire. When you do this, DO NOT use the factory alternator output wire as it will create a parallel circuit bypassing the original fusible link.
This creates an issue where all the dash power is coming in on the battery side of the ammeter, so it will always show a discharge.
An easy fix, is to use the old unused alternator output bulkhead connector in parallel with the battery in bulkhead connector with both protected by the single battery side fusible link.
This brings fused battery power into the dash on both sides of the ammeter, so the ammeter won't function, it should just stay centered.
Because all the dash power has to come through the single fusible link you have much improved circuit protection, but are more likely to blow the fusible link with heavy loads being powered through the dash. Powering the headlights through relays is one way to reduce the load through the dash and fusible link.
Another is to power the ignition through a relay. This also helps to reduce voltage loss through the ignition key and bulkhead connector for better alternator voltage regulation when using an external voltage regulator. The rest of the normal dash circuits don't pull much power except the heater blower motor. If you have power windows or convertible top then you might want to re-rout those through fused relays powered from the battery and just triggered through the dash/body sort of like the headlight relay
On my 1969 Coronet 500 wiring harness, I didn't want to cut the original harnesses, so I just removed the necessary engine side terminal connectors from the bulkhead, and the new wires to trigger the relays have the mal Packard connectors that just plug into the bulkhead connector. The relay outputs have the female Packard connectors and using the two connection connector shells, the original wires with their connectors into the other side of the connector shell so no wire cutting, and easy to put back to original.
