50mph+= Brake/ABS light "service brake system" Speedo goes up and down roughly +7 -7 mph erratically

[Erickk120]

As the title reads, lately I noticed after passing 50+ mph my speedo jumps up and down +7-7 mph and it triggers my abs and brake light as soon as it does it, along the message "service brake system." Speedo acts normal under that speed, but as soon as I go a touch over 50 it goes up and down, and eventually goes back to normal, it repeats from time to time after "sbs" message stays on though. I know I should check for codes etc.. but I wont be having time to work on it. So I wanted to ask for ideas, or common problems. I'm going to investigate my speed sensor in the Tcase and connection/wires going to it. I will check front bearing and ABS sensors too even though they are fairly 10k+ new timkens. It happens every time after it gets warmed up.

My theory is check grounds and for chewed wires, mainly because it was acting fine before, and it started happening 500+ miles after I had the tranny out, so I might have pinched or rubbed some wires idk yet. What are the odds the speed sensor on the np241 tcase is bad. After all this bad boy is from 1995.

Or maybe it can actually bee the ABS or brake system, I will need to inspect everything any ideas are appreciated it.

 

[Mudsport96]

Id say bad vss.
But if it is a 95, may get better answers in that section of the forum than you will get here.

 

[MassHoe04]

I did not have any issue with the speedometer, but I did have a very similar issue with the ABS light coming on and the chimes ringing 3-4 times REALLY loud, every time the ABS light came on. Same thing... Acted normal around town. Then, at 50 mph or higher... The light and chimes go on and off until I slowed down below 50.

I didn't have a Tech 2 scanner, but I did have a Zurich scanner with ABS code capabilities from Harbor Freight. Plug it in and it would say no stored codes. Then, I stumbled into a special menu function that was called "GM OEM Enhanced" (or something close to that...). once in there, I was able to find the specific code, indicating a bad left front ABS speed sensor.

I changed that one sensor and things have been perfect ever since.

If you have a Tech 2 scanner or something else, that can get a little deeper into those ABS sensors and codes that don't come up as a DTC at the top level scan... You might be able to pinpoint your problem to a specific sensor without throwing parts darts.

You may find your problem to be a bad sensor and not the worst-case scenario of bad ABS control module...

Good luck!

Checking grounds is always a good idea. Just because.