So, I was on my way home from work this evening in my '93 Dakota when something new and weird happened.

It started to whine. It sounded almost like a blower whine.

Did it probably less than a minute, but, while it was making the noise, the truck got super responsive, and the tach suddenly seemed to be reading correctly. None of the other gauges showed any different.

Noise stopped, truck went back to 'normal', and the tach went back to its usual reading off in la-la land.

Whiskey Tango Foxtrot?

Any ideas? Sudden burst of juice from the alternator?

Didn't have time tonight to really look at it, but, a quick underhood glance didn't show anything out of place.

Lets hope it isn't one of those "last hurrahs" that you hear about.
I had a 3HP minibike in the 80s. I was cruisin along at WOT, doing maybe 20 mph. Suddenly it just sped up and started hauling ass. I thought...ALLRIGHT !!!
Then....it conked out. Broken connecting rod. In my instance, I think the governer failed, leading to the immediate increase in unsafe rpm and ultimate failure.

That was the first thing I thought of too as my junk usually runs its best just before it dies.

But, on a serious note, I don't understand the correlation between how the tach is (or isn't) working and how the truck is running.

As I've mentioned before, if the tach is basically flat line and not working, the truck has no 'snap'. It almost feels like the brakes are dragging. If the tach is working, even though not accurately, the truck has 'snap'.

This whining incident seemed to be an extreme example of this.

Where do I begin to trouble shoot? It seems to be electrical related, start with the harness and work my way out checking the alternator and computer too?

The tach is driven off of the negative side of the coil, right? What if the tach wire is touching some other wire and grounding out?

I had a yz 125 that was always kinda blah but then I rocketed out of a corner with the front wheel up grinning ear to ear then it dumped a rod too. That's probably not the case here.

I'm with the crowd blaming the tach for ignition problems. Disconnect and see.

R.

It made the noise again tonite. This time I was able to quickly stop and get underhood to listen to where it was coming from......

The alternator.

Same scenario as before, responsive, tach working..etc, etc.

OK, so the alternator maybe going bad, but, what exactly is happening to cause the other issues?

Low voltage does some weird stuff... I had a bad alternator cause my aftermarket tach read funny... It could be your cause period.