bad vehicle ground?

I have a 1998 Chevy 3/4 4X4 Diesel Suburban that is acting STRANGE!

• The rear air conditioner will intermittently turn on with the switch OFF
• It will die as if you turned the switch off, no sputtering just dies
• The speedometer will jump when you turn the key on
• The electronic 4 wheel drive has problems (engaging, & disengaging)

The only thing I can think of is a bad ground, I don't see how the air can turn on when the switch is off

Certainly the first thing to eliminate, and the +12V battery contact.

Also have the battery and altenator checked out at your local auto parts store after you check/clean the battery cables.Chadillac

Did the 98 have a BCM, also are the AC switch contacts shorting? If you can use diagnostic equipment first, it might point to a specific causing the speedo to jump though I think you have a separate gearbox management problem here, ref 4 wheel drive and speedo.          Suggesting a bad ground, for your sake I hope so but I think you're clutching at straws. I can't see what the AC switching and the other faults would have in common (such as a shared ground).

I believe that vehicle's HVAC controls had an open circuit board directly behind it.  if you remove the radio and look down in that cavity you can see the circuit board for the HVAC controls.  if you've replaced the radio or even did anything new to the factory radio you should've covered that circuit board to prevent anything from touching it and possibly causing it to short.  remove the radio and look to see if anything could've possibly touched the open circuit board.  that's the only thing i can think of for this year vehicle...and if it did touch the open board it's probably time to replace it. ...half of the truth can be worse than a lie. <----Roger Russell said that..