Howdy! "Long time listener, first time caller" here. A couple years ago I was lucky enough to get one of my dream cars, a 300zx, and all was well until it wasn't. A long story made not so long..

A friend of mine received a 1992 300zx NA 2+2 from his dad. He already had a good running car and doesn't know how to use a manual transmission, so he sold it to me. When I got the car it ran good, not great, but well enough to travel about 1500 miles back to my house. I've done a bunch of minor stuff like spark plugs, ignition coils, air filter, fuel filter and so on. The issue I am attempting to tackle now seems to be centered around the speedometer. Last year the cruise control stopped working and the speedometer became intermittent, at the time I didn't notice any issues with the power steering. Eventually the speedometer stopped displaying anything completely, but power steering continued to work until about 6 weeks ago.

I want to go back to using to using the car as a semi-daily driver, but its difficult without power steering (health issues). I've been trying to understand this wiring diagram I had found, unfortunately I don't recall exactly where I found it, maybe here? Anyways, before I go way off-topic, is anyone aware of which pin actuates the power steering solenoid or is it completely isolated from the ECU? I don't really trust the "Online AI Search Tools" which are telling me that it is Pin 4 on the ECU connector since that should be part of the Idle Air Control system.

Yes, I did find the ECU wiring diagram here! It had been so long ago since I downloaded it, I had forgotten.
https://www.300zx-owners.club/forums/topic/160949-anyone-got-a-300zx-legible-wiring-diagram/#comment-1759989

If what I've found is accurate, the speedometer itself takes the VSS AC signal and converts it to a DC output that is then sent to 3 other components. The receiving components are ECCS (ECU), Power Steering and Cruise Control. So the ECU itself does not tell the Power Steering Control Unit to adjust the amount of assist.

At the moment I am attempting to confirm what type of output the speedometer is sending. Some search results state its a frequency output of 2Hz - 120Hz, others listed it as a PWM ranging from 0v to 5v, and all of them agree its DC.

I'm still digging through related posts on the forum here as well, though a lot of links on older posts are dead.

While I am still confirming the signal coming out of the speedometer, I took a closer look at the gauge itself.

After removing the gauge from the cluster housing I spotted some discoloration.
This happens to be near a fairly sized electrolytic capacitor.

I already spotted a trace that is potentially broken, so I began cleaning the area with a soft toothbrush. Sadly, even the softest bristles caused the solder mask to flake away. Since it didn't matter any more I switched to 2 jewelers cleaning brushes, one fiberglass and one nylon. Sure enough, the damage to the trace is more visible, but it passes under a resistor and I don't know where it leads.

Removed the resistor, and unfortunately lost it, but thanks to the photos I know what its value was. Anyway, with the resistor out of the way we can see that the trace is definitely broken, and simply passes under where the resistor was.

Now that its confirmed, it has been patched using some thin wire and the resistor replaced with a 10k through-hole that I had. Also attempted to tin the exposed copper traces while I was at it. Admittedly, this will need a little cleanup before I re-seal the board, but it should help at least slow down any more deterioration of that section of traces.

I've already reattached the board to the gauge and put it back into the cluster. I'll be testing it later when the sun is up.