Speed Sensor ?.

[blazerguy 90]

I have a 700R4 transmission in my 90 blazer, the problem im having is when I take off from a stop and the transmission shifts into second gear it goes into lock up and falls on its face. is lock up controled by the speed sensor at all ?, does the computer use the speed sensor in any way to know when to lock up ?.

 

[dyeager535]

I *believe* the '87-91 TBI trucks did use the VSS/ECM to lock the trans up.

On a '90-91 thought, the VSS also drives cruise control, ABS, and the speedometer. You'd notice pretty quickly if the VSS was flaking out, the speedo won't work right.

 

[blazerguy 90]

From what you discribed I think my speed sensor is bad, the ABS/Brake light is on, the speedometer bounces up and down in park if you rev the engine and lock up not working right, don`t know about the cruise control. were is the speed sensor located ?, is it screwed into the transfer case ?.

 

[dyeager535]

Yep, into the rear of the t-case, measures output shaft revolutions.

I'd recommend a testing procedure for the VSS, but I've never seen one! In your case with both ABS and Speedo acting up, it's gotta be either wiring to the DRAC, the DRAC, or the VSS itself. DRAC is up under the dash near the ash tray apparently, small white plastic box, but it has no moving parts, so components hooked to it still working, just not right, is probably not because the DRAC is bad.