Tachometer and Temp Gauge Twitching: Sanity Check

So at this past weekends track day in the driving rain my car had a horrendous misfire at higher rpm with my Datsun

At the very same time the temp gauge needle would start twitching and then peg full. The teach also bounces wildly.

As soon as I upshifted the temp needle went back to normal and the misfire went away.

I thought it might be becuase the track was so wet bit once things were mostly dried the problem peristed.

Back Story: a few weeks ago the toggle switch for the ignition came apart and grounded on the rollcage. It melted a.short run of wire which I fixed.

So here's the sanity check; I assume I have a short somewhere in the harness that is effecting both gauges as well as cutting the ignition.

Note I will start digging into the harness this weekend.

Thoughts??

It seems like the majority of electrical issues can be traced to a poor or loose ground, make sure all connections are clean and tight particularly in the dash and gauge cluster.

I was thinking ground as well.

However, I have had an issue where the battery cable came away from its normal routing and was touching the header in right hand turns.  Very similar symptoms to what you describe.

Had something similar with my Miata...but the engine gave before I could solve it.

however, upon removing engine and trans I noticed the speedo cable that plugged into the trans was loose, and there were some other wires near it too, so I'm thinking it could be that. My tach and temp gauges would go crazy too, but never fixed itself.

Check ALL your grounds - bright shiny metal to bright shiny metal and tight.

However, once they are all good you will still be crazy  

It could be a short, an open or a ground issue. Based on the "what did you touch last" rule I'd start with the connections to the new switch and the switch itself.

The last time I had an issue with gauges freaking out the answer ended up being the alternator. Maybe the ignition short fried the voltage regulator?

The bad grounds and "last touched" make sense to me, but the apparent RPM-dependency make me wonder about an electrical noise issue, something like dps214 mentions.

I suppose you could also have high-RPM vibes that affect a connection or switch.

In reply to Jesse Ransom :

There is a huge harmonic in the rpm range that the misfire and gauge twitching occurs.

Note the harmonic has been there for 30 years so it in and off itself is not new.

In reply to Tom1200 :

I suppose that's one in the column for "vibration is making a dodgy connection misbehave" as opposed to "bonus electrical noise."