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I have a 99 gmc 2500. It’s shiftig very hard and the speedometer will stop working. It doesn’t do it every time I drive it and it doesn’t do it the whole time I drive it.. it’s just on and off. The check engine light is NOT on. Please help! Also has a new alternator and battery. 

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Not sure if these trucks have a speed sensor on the transmission but it seems like that could be a culprit.  Maybe the sensors on the wheels also?

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if it is 4 wheel drive the speed sensor is on the back of the transfer case it has a 2 wire plug . remove the sensor and see if it is covered with metal filings if it is clean it and change oil in transfer case

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Yeah, my K3500 did that...  there is a sensor on the side of the 4L80 transmission, which interfaces with the computer about road speed, gear, etc.  And the actual speed sensor (which is what your truck sounds like it needs checking/replacing) is as Silveradosid says, on 4x4 trucks, is on the transfer case.   If either of these sensors is not working right, you'll get hard shifting (limp mode, default)... but only the speed sensor will also cause the speedometer to glitch.

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