Wiring and Battery Drain

[Guest]

The interior harness and engine harness has been hacked up over the years and there is a current drain on the battery that will kill it overnight.

There are areas that have multiple splices on one run with different gauge wires, there are several cut wires not going anywhere, and there are multiple splices using cheap plastic connectors with copper showing on both sides.
It’s not really bad enough to replace the entire harness and I have rewired cars before so I plan to replace the bad connections and even pull some of the runs completely from scratch.

The main question that I have is what gauge wire was used as a standard on the ’65?
I know that there are some larger gauge runs from the starter relay and altenator, but I am mainly concerned with all of the other wiring.

I have a wire gauge tool that shows that the existing wiring is bigger than 16 gauge but smaller than 14 gauge AGW.


Thanks in advance,
NightBird Ken

[simplyconnected]

The interior harness and engine harness has been hacked up over the years and there is a current drain on the battery that will kill it overnight.

There are areas that have multiple splices on one run with different gauge wires, there are several cut wires not going anywhere, and there are multiple splices using cheap plastic connectors with copper showing on both sides.
It’s not really bad enough to replace the entire harness and I have rewired cars before so I plan to replace the bad connections and even pull some of the runs completely from scratch.

The main question that I have is what gauge wire was used as a standard on the ’65?
I know that there are some larger gauge runs from the starter relay and altenator, but I am mainly concerned with all of the other wiring.

I have a wire gauge tool that shows that the existing wiring is bigger than 16 gauge but smaller than 14 gauge AGW.


Thanks in advance,
NightBird Ken

I'll answer in reverse order. Commonly, American Wire Gauge (AWG) comes in even sizes. Rarely, will you find odd wire numbers. That means, there is no size between #16 AWG and #14 AWG. Your tool will show even sizes as well.

Fuses are there to protect the wire. If you oversize the fuse, your wire becomes the fuse. It's that simple. We size each branch circuit to accommodate the load. To put another way, the supply must meet the demand.

Fuse size depends on a few factors but mostly copper diameter and the insulation's ability to shed heat. If the wire is bundled and taped or in pipe, we de-rate the amperage because heat is concentrated. Wire in 'free air' can shed heat much faster.

So, in a house (governed by the National Electrical Code), #14 wire is fused at and can carry 15-amps all day long. #16 carries 10-amps.

Cars are different. They use insulation that is gas and oil resistant and normal temp rating is 90-degrees C, instead of 60-degree C. That allows automotive wire to carry more amps using the same wire size OR the wire size may be smaller (like #18). Also, most car wiring is in free air.

The yellow wire feeding your dash should be #10 (from starter solenoid to your headlight switch). Most other wires are either #18 or #16. I commonly use #14 as a replacement for #16, especially on motor circuits. Ford used #8 from the battery to the starter, but I use #6 WITH a #6-THHN stranded copper ground wire that is connected to a bellhousing bolt. I also run a bare #6 all the way from my battery to my tail light housings, branching off with taps of #14 along the way (to the dash, electric windows, seats, convertible top motor, fuel tank, trunk light, tail lights, etc.).

The best wire harness is the one you make because your choice of wire and quality of connectors should be the best. Yes, there is a learning curve but cutting and stripping correct lengths goes fast.

Let's troubleshoot your battery drain... Thankfully, your car uses plugs and receptacles:

Disconnect either battery cable. Now, connect a 12-volt test light between the disconnected battery cable and the battery post. I like to make my own test light using long wires and a simple dashboard bulb (like a #57 bulb with ten feet of #18 'speaker' wire).

If there is a current draw, your test light will shine. If the battery is internally shorted between cells, the light will not shine. Stretch the lit test light to your fuses. Pull the fuses, one at a time, until the light goes out. This will identify the faulty circuit. Some circuits are NOT fused, like your ignition circuit. In this case, you can disconnect the wire harness plugs. Think of the guys assembling your car as each wire harness must install quickly and easily. Follow your wiring diagram as you go along because all the harnesses and end devices are shown.

Let me know how it goes and don't be afraid to ask questions. - Dave

[Ladysmith Bob]

As an explanation of wiring and troubleshooting I have seen none explained so clearly - 2 thumbs up Dave!!!

[simplyconnected]

Thanks, Bob. Many folks are deathly afraid of electricity, which is a good thing because the minute we lose respect for it is the moment when we get hurt.

Understanding electricity is a learned vocation, founded on personal injury and fire prevention. I know many who think they 'understand' electricity but in reality, they know enough to be dangerous. They don't bother to learn 'the rules' because they see rules and experienced inspectors as 'confining restrictions'. Not so. Just because 'it works', doesn't mean it is safe.

Notice, modern car electrical systems have evolved dramatically. Far more relays and fuses are found both in the engine and passenger compartments. Chassis grounds are all but eliminated as well because wire harnesses now include ground wires all the way to the tail lights.

A typical battery has enough power to start a 400-500hp engine yet Ford left some classic car circuits UNfused. Not any more.

A car's 12-volt system is a great place to learn basic electricity because low voltage will not harm anyone. If you understand that volts times amps = power in watts, a 15-amp circuit in a car only carries 180-watts. By comparison, the same household 15-amp circuit carries (115x15=) 1,725-watts. That makes car wiring ten times safer to learn on.

I urge all restorers and enthusiasts to buy a multimeter (they include instructions). Harbor Freight sometimes gives them away in their ads. Otherwise, they cost under ten bucks. The meter safely checks low voltage and household voltage. It also verifies your ignition's coil resistance, battery status and much more. I make my own test light and jumpers. With these inexpensive tools and a wiring diagram, troubleshooting even the most daunting convertible top circuit becomes much easier. - Dave

[FARONZ 66 Q CODE]

Dave, Thanks for the basics. I've printed this out and will slide it into my '66 electrical notes. I would advise others to do this . Save yourself some headaches.

Thanks again !

Faron

[Silent Thunder]

My understanding is the 64-66 Thunderbird came with a 40amp alternator. I always like to put the largest size available alternator that came that year in my cars. If that is the largest I would be reluctant to go any higher for fear the wiring can't handle it. Your thoughts?

[jopizz]

My understanding is the 64-66 Thunderbird came with a 40amp alternator. I always like to put the largest size available alternator that came that year in my cars. If that is the largest I would be reluctant to go any higher for fear the wiring can't handle it. Your thoughts?

A 55 amp alternator was available as an option so you're probably safe going to 60-65 amp but I wouldn't go any higher with the existing wiring.

John