Thoughts on brake light/speedo wonky-ness? Searched my brains out.

[Minnesota]

Ok, so put the heavy search on here and the google-box. Symptoms seem narrowed down? Vehicle always running fine, as far as I can tell. And apologies for length.

1. 1990 K5 w/350 and 700r4/NP241

2. Puttering along bumpy high-country road and brake dash light comes on solid.

3. Get to paved portion where speeds get to ~40 and notice speedo whipsawing +/- 15 mph. Appears to try to stabilize at correct speed, then takes a dive. Always down, not up.

4. Get home, unhook the battery to reset computer and get my CK5 search on.

5. Hook up battery, everything back to normal, go for a in-town cruise. Problem re-occurs relatively quickly.

6. Say screw it and go buy a new VSS. Vehicle is new to me and has 260K on body. Also check what VSS wiring I can readily see. Reset computer. Everything cool for a few in-town days. Problem re-occurs.

7. Here's where I'm scratching my head: Unhook battery, find DRAC, pull connections on and off for whatever that's worth. Also notice that DRAC is kinda resting on back of stereo connections. Move DRAC below radio wiring.

8. Reconnect battery and put well over a week's worth of in-town and highway/Interstate miles on. A rare week, as I'm driving all over hell's creation. Also get brake fluid flush as was long overdue. No Problems.

9. Still thinking about it, and pondering if somehow proximity of DRAC to stereo wiring was somehow tweaking it out...

10. Finally go for an ultra-light offroad scenery loop with the parents. Never even in 4, and mid-way through a 2.5 hr loop, problem finally re-occurs.

11. Get home, no computer reset, and straight-up unplug the DRAC while running. Brake light then starts flashing as of now unspecific code.

12. Computer reset and all good in the driveway as of this moment.

Any thoughts would be much appreciated.

 

[Pookster]

Ok, so put the heavy search on here and the google-box. Symptoms seem narrowed down? Vehicle always running fine, as far as I can tell. And apologies for length.

1. 1990 K5 w/350 and 700r4/NP241

2. Puttering along bumpy high-country road and brake dash light comes on solid.

3. Get to paved portion where speeds get to ~40 and notice speedo whipsawing +/- 15 mph. Appears to try to stabilize at correct speed, then takes a dive. Always down, not up.

4. Get home, unhook the battery to reset computer and get my CK5 search on.

5. Hook up battery, everything back to normal, go for a in-town cruise. Problem re-occurs relatively quickly.

6. Say screw it and go buy a new VSS. Vehicle is new to me and has 260K on body. Also check what VSS wiring I can readily see. Reset computer. Everything cool for a few in-town days. Problem re-occurs.

7. Here's where I'm scratching my head: Unhook battery, find DRAC, pull connections on and off for whatever that's worth. Also notice that DRAC is kinda resting on back of stereo connections. Move DRAC below radio wiring.

8. Reconnect battery and put well over a week's worth of in-town and highway/Interstate miles on. A rare week, as I'm driving all over hell's creation. Also get brake fluid flush as was long overdue. No Problems.

9. Still thinking about it, and pondering if somehow proximity of DRAC to stereo wiring was somehow tweaking it out...

10. Finally go for an ultra-light offroad scenery loop with the parents. Never even in 4, and mid-way through a 2.5 hr loop, problem finally re-occurs.

11. Get home, no computer reset, and straight-up unplug the DRAC while running. Brake light then starts flashing as of now unspecific code.

12. Computer reset and all good in the driveway as of this moment.

Any thoughts would be much appreciated.

You have a bad ground. It doesn't affect drivability , but as it gets hotter and hotter, the connection gets worse. In fact, Im surprised you didnt mention that some of your gauges may have also been a little lower than usual. I'd get it fixed. you got something melting/shorting/bad connection somewhere.

 

[Minnesota]

I hear what your saying. I'll keep investigating.

Thanks for the thoughts.