I was on my way to church this morning and my speedometer started acting up going to like 127 mph for a few seconds at a time. That is all it was doing. I went to leave church and head to the shop and more problems... As soon as I turned the key on the speedometer instantly pegged out again. The truck started fine and when I went to leave its like the convertor was stuck in lockup and the tranny is in high gear.I can put the shifter down into 1 or 2 and the tranny will shift into those gears but the speedometer remains pegged out and anytime the speedometer is pegged out and you put the shifter in D the tranny stays in high gear. I'm thinking possibly the VSS on the rear end? Any suggestions/tips would be GREATLY appreciated.

Hmmm... Did you try the cluster test to verify the gauge cluster? http://mopar.mopar1973man.com/cummins/2ndgen24v/cluster/cluster.htm Then did you check for error codes to see if it gave a clue to what failed? http://mopar.mopar1973man.com/cummins/2ndgen24v/obd2-error-codes/obd2-error-codes.htm

I forgot to mention that I checked for codes with my Scan Gauge II and it did not pull up any codes. I did not try the cluster test. I have a feeling it may be the VSS. Going to grab a new one in the morning and start there.

We have 2 vss.The one in the tranny i think has something to with shifting but i could be wrong again.

I replaced the sensor on the rear axle and it is still doing the same thing with the speedometer and when the speedometer pegs out the tranny stays in high gear.I also am not pulling ANY codes on my Scan Gauge II. Should I have the truck hooked up to a regular scanner and will that tell me what sensor needs to be replaced?

I believe the sensor on the rear axle is for the ABS. The one on the transmission/transfer case would be for the speedo. I would try unplugging it first and see if it drops to zero or remains pegged. Might put your truck into limp mode but if the sensor is the problem that should, in theory, cause the limp mode and keep the torque converter from locking up. Might also want to check the settings on the edge if you used it to correct your speedo for the bigger tires, something could have gone haywire there.

I replaced the speed sensor on the back of the transmission this morning and wouldn't you know it... same junk! I traced every bit of wires and could not find a problem there.So I drove it a few times and it was doing the same thing its been... working fine then all of a sudden pegging out. Got back to the shop and shut the truck off and started messing with unplugging the power to the Superlift TruSpeed box. All that did was remove the speed signal all together. So after plugging the power back in to that box and the speedo pegging out just turning the key on I gave the TruSpeed box a good whack and wouldn't you know it problem went away. So I guess its a call to Superlift tomorrow to see what they say.

did you clear the codes after you changed parts?