first,load test the charging system,to make sure you aren't getting a power surge from the alternator..if you have no way of doing a load test,auto zone and most parts stores will do this for free..you may also need a new vss[vehicle speed sensor...this is located on the tail shaft of the transmission.[if it is 4wd,it will be on the side of the transfer case
Date: NOV. 1992
Corporate Bulletin No.: 268305R
ASE No.: A6
Some 1988-93 C/K, and 1989-93 S/T vehicles will register a speed, often as high as 12 MPH, when engine speed is increased with the vehicle stationary and the transmission in neutral.
This condition is due to the sensitivity of the vehicle speed sensor and is not an indication of a malfunction. When the engine is "reved up" normal engine vibration is transmitted through the transmission, causing the reluctor wheel used for speed sensing to also vibrate. Although the vibration is minute the sensitivity of the speed sensor is such that a speed signal is induced.
As previously stated, the vibration is normal and the level of sensitivity of the speed sensor must be maintained to accurately support vehicle systems such as cruise control and antilock brakes that require vehicle speed input.
When the vehicle is moving, the spinning reluctor wheel overshadows any vibration that may be present and an accurate speed reading is maintained.
Since the condition is normal, no attempt to eliminate it should be made. Replacing parts will not be effective.
Group Ref.: Chassis/Body Electrical
Bulletin No.: 368305
Date: February, 1994
The tachometer used in C/K trucks with RPO U16 is calibrated so that the first mark shown on the tachometer display is 500 RPM. At idle, this may leave the impression that engine speed is too low or that the tachometer is inaccurate when neither is the case.
If engine idle speed is normal and the tachometer appears to be correct at engine speeds above idle, the tachometer is operating as designed and no attempt should be made to alter the vehicle tachometer reading at idle.
Maybe I need to provide a little more information. My trip to work is about 30 miles - when I first start out - let's say over half way to work - everything is normal. Then all of the sudden the speedometer and tachometer start jumping around - typically about 5-7 mph back and forth - the tach moves about 3000 rpm back and forth. It only seems to do this after you have driven for a while, everything is OK until then.
This morning was the first time that the vehicle actually "shut off" - it was actually like someone flipped the switch off then immediatley back on - the vehicle actually lost power for a split second, no steering, nothing. I can have a load test done, but would either one of your suggestions relate to the vehicle shutting off and then back on?
yes,an alternator shorting out or a loose ground wire,may cause this to happen,,you apparently have a power disruption..a load test on the charging system may give you more information
no.i don't think it is a bad computer