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Tim's Auto Repair
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Experience:  Have owned a repair shop for 25 yrs.
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1993 Chevy: the speedometer and tach gauges have become..jump

Resolved Question:

I have a 1993 Chevy Silverado K1500 - the speedometer and tach gauges have become very twitchy, even sitting still the speedometer will jump up to 20 mph. This morning when I was coming to work the gauges started twitching then all of the sudden it was like the truck shut off for just a split second, all the dash lights came on, then went off about 3 seconds later. It was fine the rest of the way to work, except for the gauges.
Submitted: 7 years ago.
Category: Chevy
Expert:  Tim's Auto Repair replied 7 years ago.


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 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


Speedometer - Registers When Vehicle is Stationary

Number: 93-50-8C

Section: 8C

Date: NOV. 1992

Corporate Bulletin No.: 268305R

ASE No.: A6



Model and Year:
1988-93 C/K AND 1989-93 S/T TRUCKS


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.

Instruments - Tachometer Accuracy

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.

Customer: replied 7 years ago.

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?

Expert:  Tim's Auto Repair replied 7 years ago.


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


Customer: replied 7 years ago.
So based on the information that I am seeing, you do not think it is a computer issue - right?
Expert:  Tim's Auto Repair replied 7 years ago.


no.i don't think it is a bad computer


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