Question about Chevrolet Silverado 3500

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I have a 1989 Silverado 3500 3 speed auto with overdrive. I replaces transmission filter, fluid and modulator when I bought it, but now it won't shift into overdrive. I think this may be why the dummy light comes on around 50 MPH. could my modulator be out of whack, or might it be there are bubbles in the tranny fluid? or do I have a more serious issue?

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Maby the overdrive disk is burnt out

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It is not the vacuum modulator, it might be the vehicle speed (VSS) sensor, this is where the powertrain control module get it's signal to shift into overdrive.

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I had a similar problem with my 98 mystique when i bought it used. I had no overdrive and was lucky to find a honest transmission shop that found the problem. In my case the overdrive was unplugged. They plugged it back in,cleared the computer and changed my tranny fluid and i was good to go. You should be revving about 2400rpm at 65mph. The other thing you can check is to make sure that the wires that go from the overdrive button are not broken off or coroded.

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