Transmission not shifting back from third

1996 ISUZU BIGHORN
401,000 MILES • 3.1L • 4 CYL • TURBO • 4WD • AUTOMATIC
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THOMAS9000
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Hi all, I'm wondering if anyone knows what is making my transmission move from first to second. Second to third. Third to overdrive and all the way back to first perfectly when the vehicle is cold witch is great. The problem comes once the vehicle is warm the transmission will move from first to second. Second to third and into overdrive perfectly but when coming back down the gears it will drop out of overdrive into third but will not go any further down the gears. Any idea's why?
Sep 19, 2019 at 10:59 PM
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MASTERWRENCH
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Welcome to 2CarPros!

The transmission in your vehicle is electronically controlled by a Transmission Control Module(TCM). It uses various "input" sensors to determine things like vehicle speed, throttle pedal position, engine speed, fluid temp and a few others. It then takes this data and uses solenoids to control gear changes in the transmission. If your transmission is shifting fine going up through the gears but not back down as the vehicle slows down, there's a good possibility the transmission output speed sensor is failing. The TCM uses this sensor to determine what gear to shift into based on vehicle speed. If the sensor is failing and causing the TCM to think you're still moving at a faster speed than you really are, it may cause it to become "stuck" in a higher gear.

Below is the speed sensor replacement procedure. I've also attached a pictures of the speed sensor location(at the rear of the transmission).

REMOVAL & INSTALLATION > SPEED SENSOR > REMOVAL & INSTALLATION
1. Disconnect electrical connector at speed sensor located on rear of transmission. See Figure . Remove speed sensor bolt, speed sensor and "O" ring.

2. To install, reverse removal procedure using a NEW "O" ring. Install and tighten speed sensor bolt to specification. See TORQUE SPECIFICATIONS .
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Sep 21, 2019 at 6:24 AM