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Solving Electronic Tranny Troubles


A reader (iwgbtl@yahoo.com) writes: I have an early 1999 GMC Sierra w/5.71 & Raptor Jr. aftermarket auto transmission. The unit has approximately 10,000 miles on it. The problem I'm having is a very hard shift from first to second gear. It seems to shift softer after it has been driven for a couple of miles. I changed the oil and filter, but it didn't help. Is there a simple fix without going through the tranny for hundreds of dollars?

I am not a big fan of modifications for performance or otherwise, because the symptom or diagnostic charts do not necessarily apply. Unlike older transmissions that shifted by a combination of throttle pressure (a cable between the throttle and the trans) or a vacuum modulator (a vacuum line from the engine to the trans) and governor (centrifugal weights on the trans output shaft), a combination which was easily modified and adjusted, your transmission shifts entirely electronically. The computer assesses critical data (engine temperature, road speed, rpms, throttle position, etc.) and shifts by electronic solenoids.

Was your computer program modified with the new transmission? Should it have been? I don't know. Perhaps the company you bought it from has a helpline (they usually do). The one thing I can tell you is that high line pressure usually causes a hard shift. As the transmission fluid warms and thins out, pressure drops, and the shift becomes less obnoxious. You could try the newer Dexron 3 fluid, which is thinner. Mobil 1 ATF might have the same effect.


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