How problematic is the now 50 year old wiring? Some existing threads seem to suggest they're pretty vulnerable, yet I don't see wiring harness included on most "while the engine is out" todo lists.

Are there any that are consistently so troublesome that they should get proactively replaced even if they appear good? On the 914-6 it appears there are two for the engine bay (complete engine and alternator). That's $500 USD right there.

For reference, I'm not attempting a complete restoration. Just trying to get our car safely and reliably back on the road. The engine, transmission, and suspension components are out of the car. I've found a couple previous owner wiring additions, but they have been easy to remove.

My impression has been that if the wiring, insulation and connectors look good they probably are good. It's not like the copper wears out. Perhaps this is naive.

P.S. New plug wires are already obtained. I have no intention of removing the main harness.

I have seen a lot of old wire harnesses and they all end up being very different. Some are still soft and pliable, some crack and break just moving them around. It really depends where the car was located and how the car was stored. It’s very similar to car interiors.

I would say if it looks good, feels good and works, don’t fix it. If a close inspection shows cracks and issues around the terminal to wire connections, plan on a replacement.

The good thing is, if you need a replacement, there are multiple sources. None are cheap but at least it’s as easy as checking a box and sending payment.

You might consider unwrapping the entire harness (keep it all bundled with cable ties while you work) and cleaning it all and then ringing out each wire too make sure there are no shorts. Then rewrap.

I was fortunate to find a NOS 914-6 wiring harness in the factory sealed bag right after I bought my car. That gave me some serious peace of mind as we went forward.

As for the engine harnesses, I would do the same thing to make sure you are good to go when you assemble everything. Electrical gremlins are the worst.

Duplicate

Not 100% true.

Corrosion can spread up into the stranded wire - looks good but can be high resistance.

This is usually what happens in the yellow starter solenoid wire that runs from the ignition switch to the solenoid on /4’s. Everyone thinks it’s fine but resistance is too high and it begins dropping voltage across it when cranking. Honestly, that start issue is one that plagues way too many teeners because very few take the time to address the root cause (high resistance).

The thing the /6 has going for it is carbs and a whole lot less wiring than the /4 has to/from fuel injection sensors and actuators. Likewise it doesn’t have as many ground points that mater to reliability.

You can check resistance end to end. Should generally be very near 0 ohms. Resistance per foot is published for various wire gages. If you end up with something that reads high measuring resistance, or looks suspicious look up resistance per foot and do the rough calculation.

Flexibility of the insulation is critical. If it’s getting stiff and seems prone to cracking when bent tightly, replace it.

I actually had a section of Monster Cable for loudspeakers go bad on me inside the house. It corroded internally & turned green.

As others have noted, check for wires with bad connections, missing insulation and signs of overheating.

Bad grounds are the culprit for many unexplained electrical issues. Ensure that the common ground terminals plus the ground straps for the engine and transmission are all in good shape and that the contact surfaces are clean.

Typically the alternator harness, which is inexpensive to build or buy, is one to be carefully inspected for signs of deterioration and overheating. The other wires I would be concerned with are the yellow wire from the ignition to the relay board in the back (as mentioned by others) and the two red, heavy gauge, wires that run from the battery to the ignition switch (terminal 30) and the fuse box (Fuse 11, power side).

I'll add that a normal multi-meter can't provide accurate readings at really low resistance, so don't go getting worried if your cable should be 0.5ohm but reads as 1.5ohm.
Corroded cables or connectors will bump that number by a lot more than a couple of ohms.

While this is true with a cheap $10 digital multimeter any decent meter can read fractions of an ohm.

My cheap Ames Digital Multi-Meter (DMM) from Harbor Freight reads a 0.6 ohm resistor accurately. Likewise putting two of them in parallel reads 0.3 ohms. Sometimes jumps to 0.4 ohms which is due to the 0.1 ohms of resistance when the leads are shorted. EDIT: pulled out a 0.15 ohm 10% tolerance resistor, Ames meter measures it as 0.2 ohms.

With a DMM you just need to be aware of the offset of the meter leads themselves are. Good leads should be 0.1 ohms or zero when shorted. My Ames meter bounces between 0 and 0.1 (smallest resolution it will display) - leads are probably about 0.05 - 0.07 ohm and the meter rounds that up to 0.1 ohms.

Most analog multimeters have a zeroing potentiometer that allows the resistance of the meter leads to be zero’d out. I have an old school Radio Shack Analog meter with a 1 ohm scale that easily reads to fractions of an ohm.

A 1 ohm resistance gain in the yellow starter solenoid circuit will result in a no start condition.

In fact, even a 0.5 ohm gain in the starter solenoid circuit will cause serious problems because that solenoid pulls about 25 amps when operating normally. An incremental gain of just 0.1 ohms will create a 2.5v voltage drop if it’s in a circuit pulling 25 amps. The problem is, the solenoid circuit won’t even be able to pull proper amperage due to high wire resistance, and without enough current flow, the solenoid doesn’t work.

Bottom line: if you have a DMM that can’t measure fractions of an ohm, get a decent meter.

I would replace the alternator harness as there are at least 2 sources for this at very reasonable prices. It's a very important harness.

Barefoot

Does anyone have experience in replacing the "snorkel tube" holding the wiring harness from the tunnel to the engine compartment shelf. I have ordered a replacement, but it doesn't look like an easy job. Any tricks?

You have to remove all the wires going into the 12 pin connector that goes to the rear fuze box & tape them together. Make sure you document which wire goes where !
The connector with wires attached WILL NOT fit otherwise. Useful to run a feeder wire backwards thru the snorkel to pull the wires thru. some lube very helpful (liquid soap).
Good luck, not a fun job

Barefoot

I wanted to do this once when I went through my BB. I ended up getting new harnesses from Mr. Bowlsby and couldn't be happier with the service and quality.