Wiring Question

[SteveGIndy]

The wiring harness to the engine compartment has a branch that comes out right next to the battery. It has three wires. I have been scratching my head looking at the wiring diagram and can't figure out what they are for.

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[Semper_Dad]

I believe they belong to to the defunct diagnostic system.

Brown checked ground at the battery (Terminal 8 Diag Plug)
Brown/White Stripe checked ground at the chassis (Terminal 17 Diag Plug)
Yellow/Red Stripe checked for electrolyte level in the special VW battery (Terminal 7 Diag plug)

Your Yellow/Red Stripe looks more like a Brown/Red on my monitor.
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[SteveGIndy]

Thanks! Makes perfect sense.

[SilverThing]

I know that the question has been answered and the wires are mostly useless based on the fact that the diagnostic machines that can plug into the diagnostic port are exceedingly rare, but I thought I would chime in on a little context of what these specific wires did back in the day.

I'll start with the special battery. I have one of the original batteries in my stash and, unfortunately, I haven't taken the time to investigate how the sensor worked. I don't know if it is a simple yes/no in terms of electrolyte level (and how it does it) or if it does something more sophisticated. I went to check a couple minutes ago and, of course, my multimeter has a dead battery... The reason I mention whether it is more sophisticated is I have a dealer diagnostic reference card from April of 1973. There is a section on running a simulation on the tester to check to make sure it is running properly. The battery electrolyte level in the simulation has a value between 000 and 002. It seems odd to me that the values aren't 0 and 1 (or yes/no) if they are just checking for the electrolyte being full enough.

Now onto the other two wires: it probably seems weird that someone would be interested in measuring the ground at the battery vs the ground at the chassis. Wouldn't they be the same? What the computer system is doing is using the ground strap as a shunt to measure current supplied by the battery during engine cranking. When there is current flowing through the strap, there will be a voltage difference between one end and the other because there is a small resistance in the strap (current multiplied by resistance is voltage). The computer would use this to check for proper starting current. It also did a clever bit where it used the voltage outputs to calculate a crude compression test on the engine since the starter has to work a little harder when a cylinder is compressing. It didn't give any absolute numbers, but would alert you to one cylinder being significantly lower than the rest.

The diagnostic system was a bit of a gimmick. Repair shops had plenty of individual tools that could do all of the same tests, but I still think it is a cool piece of technology that pre-dates modern on-board diagnostics.
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