I just changed my radiator and took the truck for a drive, but now it’s shifting pretty hard. The transmission fluid levels are good. Where should I start in troubleshooting this issue?
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I have overdrive but didn’t notice it flashing while I was driving.
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The harsh shifts are likely caused by limp mode. Is your speedometer working properly? Any burnt-out brake lights? If the speedometer is acting up, there is a speed sensor on top of the differential, remove and clean it, as well as the one on the transmission. Sometimes, the main harness near the back on the passenger side of the transmission fills with oil or comes loose. Remove the heat shield and clean that plug, as it controls the shift solenoid pack. There isn’t much wiring up front near the radiator that would affect transmission shifting, so that’s likely a separate issue. Forums provide more detailed discussions on these problems. If you have a tuner installed, try removing it and taking the truck for a drive to see if that changes anything.
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I just fixed a similar problem by reseating and cleaning the connection of the trans harness underneath that heat shield.
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Is your OD still present? Is there a flashing OD light?
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I just went for a drive and discovered that the overdrive is always flashing.
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Since you have already checked the radiator, take a look at the sensor on top of the cover where the injection pump is bolted. It’s a two-wire sensor that sends RPM data to the computer. Often, the wires get exposed near the sensor, and if they touch each other, it can cause the transmission to go into limp mode, leading to hard shifts and a flashing overdrive/no overdrive light.