A/T - AOD No Fourth Gear

TSB 87-50 (Aug)

SUBJECT: FORD AOD

PROBLEM: No Fourth Gear

When diagnosing a no-3-4 upshift condition, it's important to know what happens in this transmission during the 3-4 upshift.

When the 3-4 shift valve upshifts1 forward clutch oil is exhausted, and the overdrive band is applied.

The direct clutches are applied in both 3rd and 4th gears. If the transmission shifts 1-2, 2-3, and never upshifts from 3rd gear, the 3-4 shift valve is not upshifting.

If the transmission makes a shift into neutral instead of 4th gear, it indicates the 3-4 shift valve upshifted, but the overdrive band did not apply properly.

The overdrive band application is controlled by several valves in the valve body, the overdrive servo and the overdrive band.

If the 3-4 shift valve is not upshifting, the possible problems are reduced considerably. These possible problems include: the 3-4 shift valve or the 3-4 modulator valve sticking, or governor oil is not rising high enough to shift the valve.

TO CHECK:




Remove the oil pan, extension housing and the governor. Air check the valve body through the governor feed hole, and try to "feel" the3-4 shift valve for free movement while looking for excessive leakage. A common source for excessive governor leakage is the round aluminum plugs in the valve body, valve body gaskets, or where the governor support mates to output shaft.