Having problems with your 1998 GMC Sierra ?
SHIFT IS UP AND DOWN BETWEEN 40 AND 50 MPH.98 gmc sierra 4x4
You may have a worn throttle position sensor that is erratic at those speeds. The engine computer may or may not pick up on this defect--take the truck to Autozone or similar business for a free computer scan. You need an oscilloscope to prove the part to be defective as it shows up on the scope as a series of noise pulses on what is normally a smooth rise and fall of voltage from the sensor. Hope this fixes your problem!
That is the torque converter lock up. they do that on those trucks the best thing to do is to shut off the over drive
Repair Help & Product Troubleshooting for 1998 GMC Sierra
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