Speedometer started to fluctuate when I go over 30 mph. It will go up to somewhere around 50 mph then drop down to almost 0 mph. The rpm guageeedle will not go over 4000 rpm while in park is this normal? All of this started to happen after midas r&r my serpentine belt. Which still has me burned up about. Never knew the brakes and the belt had anything in common. :>(

SOURCE: MY 1998 RAM 2500 (5.9

The problem is caused by electrical noise making the torque convertor go in and out of lock up. The dodge dealer has a computer flash for it. The fash only works about 30% of the time. This is a major problem to diag. Usually requires ground straps installed on pcm or pcm replacement could also be the altenator, wiring, or anything that runs to the pcm or tranny.

SOURCE: speedometer intermittent and ABS/Brake light stays on

Vehicle speed sensor located on top of the rear diff at the brake line clamp a single bolt with a wiring harness. replacement from NAPA comes with a new extension harness. use grease on the terminals before assembly.

SOURCE: ABS light & brake light on, problems with speedo working properly

The rear wheel speed sensor is the most common problem on those trucks and will do what you are talking about. It is located at the top of the rear axle in the center. It picks up it's signal from a tone ring on the diff inside! If a tranny input or output sensor is bad then the tranny won't shift properly!

SOURCE: speedometer does not work until

There is a sending unit in the transmission that has gone bad. They don't have a cable like back in the old days. It is all electronic now. The sensor in the transmission has gone bad. You can get a new one at any auto parts store.