1999 Dodge Ram 1500, 5.9 360, 4x4, Automatic. Speedometer needle is back and forth, totally sporadic whether im idle or driving. Also, truck seems to be stuck in 2nd. I have it in drive the rpms spike way up, speedo goes crazy and and can barely get up to 30, almost like a limp mode? Just replaced the VSS (the output shaft sensor on the tranny) and checked for contuity and shorts in circuit. None found. Any other sensors that may cause this? PCM maybe? if so how do you test?

 

is the brake light and abs light comming on? if so try replacing the sensor on the rear diff, thats the speed sensor....and yes it does sound like its in limp mode...hows the fluid? it usually goes into limp mode if it overheats IIRC

 

Brake light is on, but not the abs. The abs pump is in the truck but disconnected because it makes some weird buzzing noise. I bought the truck like that, havent gotten around to fixing it, seems to be no big deal? Changing the sensor in the rear diff is next, just need to go get one haha. I had checked the fluid a few days before it started acting up, it seemed fine. Im not leaking anything. When i pulled the sensor the was a small amount of shavings on the magnet. I have changed the tranny fluid and filter 3 times since owning the truck, and thats been for about 9-10,000 miles.

 

Oh..when i scanned the truck with the MODIS the code it pulled was P0720...output shaft sensor circuit. But like i said i checked the circuit and sensor and all was good.

 

id change the sensor on the rear diff....its only ke 30 bucks....

 

ill probably go get one today

 

it takes maybe 10 minutes to change....just need a 14 mm socket, and maybe a screwdriver to pry it up out of the pumpkin

 

replaced that sensor. Didnt fix the issue. Speedo bounces back and forth, rpms rev to 4000, and wont shift. The tps doesnt have anything to do with this does it?

 

Try resetting the computer. Changing anything on the truck could mess up the computer if it isn't reset.

 

yes i didnt mention that you should disconnect the battery while doing this job.....just un-do the terminals for about 15 minutes, and while they are disconnected, pull your light switch on for 5-10 seconds (i believe this resets the pcm)