Erratic speedo on start up

[black94]

I have been experiencing erratic speedo activity and rough idle on start up on my 94, 5.7L silverado. Speedo will bounce between 0 and 120, accompanied by a very rough idle. Mostly, put the vehicle in gear and drive away usually settles the problem. However, on occasion, this will continue to occur while driving and I experience tranny shift problems. ( 4L60e) eg. not shifting out of low gear. Pull over, shut vehicle off, restart all may be fine.

Can anyone shed any light on this problem? I suspect VSS. As simple as they are to change, why does my Haynes manual suggest I not do the work? Thanks Tom

[budhayes3]

I'm thinking speed sensor also...haven't done one on one of these trucks, but it should be fairly easy. There might be a speed sensor in the speedo also, but I'm not sure. Look for a connector on the transmission on the driver's side? or possibly even on the transfer case near the output shaft. Check for broken or corroded wires also.

 

As far as the Haynes manual goes...they pretty much shy away from most of the diagnostic and computer controlled stuff. Very little transmission info also. Shematics are also poopy.