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Jim Grant's Tech Tips
‘98 Ford Expedition, Blinking O/D Light
Q: I recently purchased a '98 Ford Expedition with 72,000 miles on it. While on a long trip, the O/D light on the automatic shifter started blinking. It doesn't blink all the time. The transmission seems to shift a little harder when the light's blinking. The fluid is very clean and the level is good. Is there any info on what causes the blinking O/D light?
- Don Hall
A:
The switch on the end of your vehicle's shifting lever is called a Transmission Control Switch {TCS). When the Over Drive (O/D) indicator lamp is off, the vehicle's computer allows the transmission to shift automatically from 1st through to 4th gear. When the O/D light is on the vehicle's computer allows the transmission to shift from 1st through only to 3rd preventing the shift to 4th gear. When the O/D indicator lamp is blinking, it is telling you that there is an electronic pressure control problem or that there is a sensor problem. If there is a pressure or sensor problem, the computer activates an alternative program. This program is often called a limp mode program. When a limp mode is activated, the shifting and internal transmission operating pressures are changed, which explains the change in shifting that you refer to. An easy way to view this mode/program is as a fail safe to get the vehicle home without leaving you stuck on the side of the road, thus limp mode. You should take the vehicle to a repair facility and have the computer checked for codes. The computer codes will tell the technician what system or sensor is being affected. From that point the diagnosis and repairs can be performed. Don't ignore the limp mode warning! Get it checked out early and repaired before limp mode becomes walk home mode.
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