A word to the masses, get a couple of spare used O.E. (MOPAR) 2 wire V.S.S.' to keep on hand, because, apparently the aftermarket is discontinuing making them and Mopar has spiked the prices for new ones to about double what they were just a year ago.
We all know how much patience wiring and electronic controls take to troubleshoot.
Wiring with splices and twist connectors are an absolute pet peeve of mine, if you have to fix a broken wire, solder and shrink wrap it.
It not only makes a positive connection, but it pretty much seals out any moisture and corrosion.
Speedo and fuel guage issues will normally be connective (chafed or broken wiring) or hardware (bad sender unit connection, or VSS) related.
Speedos operate the same as any other signal-reliant device, in that they have a CONSTANT power supply and ground, but are designed to read by a varying (pulsating)signal-ground.
Once you realize that 90% (or so) of ANY guage or controller issue is usually based on the the signal end of the wiring, troubleshooting it is really very simple.
Most of the "cluster issues" I have ran across nearly ALWAYS end up being repaired without even removing the cluster bezal!
The exception, of course, is the consistantly troublesome ODOMETER self-destruct issue.
The most basic way to start trouble shooting is to verify there is guage voltage.
Usually if another guage in the cluster reads (correctly) there is a usable power supply, this will also indicate that the guage cluster has an adequate ground.
The cluster (and dash related) ground is usually located on a steel bracket, directly to the right of the brake pedal, on the dash frame, to the left of the ashtray and is normally a gold irridite in color.
There are actuallly 2 ground wires to this plate and they are held in place with a stud and 5/16" (8mm) nut.
This plate is in turn bolted to the main (steel understructure) dash frame.
There are very basic tools that are of immense value, even if you don't know what the readings on a VOLT/OHM meter mean, it is an invaluable aid, as is a set of test leads with alligator clips or small-tipped ends.
A test light goes a very long way, as well.
I cannot stress enough how important GOOD GROUND CONNECTIONS are!
A good majority of the most hair pulling and baffling problems have been narrowed down to a simple corroded, loose or un-hooked ground.
Running a jumper wire to the voltage regulator on pre-'92 trucks is NOT the correct fix for the regulator grounding problem, making sure the cab has the grounds behind the head hooked up IS!!
There is also NORMALLY a woven "strap" ground on the right rear cab corner that attaches to the nearby bed corner, as well.
Many times these are gone, especially if the bed has been removed for any purpose, and they are seldom ever re-installed.
On Jamie's truck, apparently the NAPA (yuk) VSS has done what I have seen a couple others do, they get crap and crud in them from normal use and USUALLY, they just short out, but HIS, at idle, did something that was just a bit unique.
It acted like a "microphone" and telegraphed the vibration, via the VSS, from the transfer case to the speedo, the speedo, which in reality is a signal amplifier, translated the signal into a reading.
70 MPH while idling, INDEED!!!
I have also seen chafed wiring rubbing on the cab seam do very similar, so if you're having an erratic reading, or no reading issue, look over every inch of the wiring from the VSS clear to the engine connector plug.
On 4x4s with 3.07 or 3.54 gears, there is a black plastic rectangular box, which the VSS screws to.
This box many times is bad and, in a "no speed reading" situation, should be the first piece you check on these trucks, as they are fairly fragile and a lot of times ripped off, or broken from physical contact with an item.
IF they aren't broken visibly, they many times are broken internally due to wear over 20-odd years.
On 4.10 and deeper gears, this item is a rectangular METAL box and just as suusceptible to damage and wear.
The aforementioned boxes are NOT the same unit as the TWO WHEEL DRIVE GETRAG right angle speed transducer, which, again, is susceptible to internal damage and eventual physical malfunction.
Mark.
Got all my speedo issues figured out, it was the wiring harness that runs from the firewall by the heater box back to the tranny and what not.