Speed sensor questions

[FLJoe]

I hope this is in the right location. Truck is a 96 Tahoe 4x4.

Today when I got in the truck for lunch, as I was leaving the parking lot I noticed that it wasn't upshifting. The speedometer was also dead on zero. I tried manually shifting to second and third and the transmission held normally in those gears- it just didn't upshift when it was in O/D. Suddenly the speedo started working and the transmission started behaving normally. Then when driving around I noticed the antilock brakes engage briefly once or twice.

I have a feeling It's a bad speed sensor. Is there one on the transfer case? Would this be it, or would it be one on the hubs? Not sure where to start.

Thanks very much!

 

[kennythewelder]

Yes, the speed sensor will make the trans shift impropriety if it's acting up. It is located on the transmission tail shaft.

 

[FLJoe]

Thanks very much.

 

[Schurkey]

It is located on the transmission tail shaft.

Trans tail shaft housing on 2WD. Transfer case tailshaft housing on 4WD.

I would TEST the sensor before replacing it. My experience is that it's more often the DRAC or the wire harness, than the sensor itself. And when it is the "sensor", it's sometimes a matter of metal filings interfering with the gap between the sensor and the reluctor on the tailshaft. Clean the filings, and the sensor goes back to being reliable.

On my '88, the DRAC is in the instrument cluster, the gauges plug into it. On later vehicles--I guess--it's somewhere under/behind the dash. The DRAC gets the signal from the speed sensor, then converts/modifies/processes the signal into something that the ECM, speedo, odometer, ABS, and cruise control can work with.

 

[FLJoe]

Thanks for the replies.
Just a follow up... the truck had only acted up that once during that short trip out for lunch, after which it had returned to normal. Regardless, I ordered a new sensor off Amazon and swapped it in the next day. I've put about 500 miles on the truck since with no issues. The old sensor did have some metal filings stuck to the end, so it's very possible that was actually the cause, and not a truly bad sensor itself. In any case, I cleaned up the old one and tossed it in my tool box I carry in the truck, just in case the Chinese one I got on Amazon fails on me and I need a replacement on the road. It seems that all the replacement sensors, including AC Delco, are now Chinese. My original sensor says Made in USA on it.