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Transmission TECH/TALK

Fixing 4L60-E 1-2 shift issues

The complaint

A 1993 Chevrolet 1500 with the 5.0-liter engine and 4L60E transmission has a complaint of second gear starts. The truck seemed to start off in first gear on an initial cold start but would stay in second gear at a stop or even when shifted out of Drive to Park or Reverse and then back into Drive again. The scanner showed the computer commanded a first gear start and both shift solenoids were confirmed to be on with a voltage check. First the shift solenoids were replaced to no avail. Then the valve body was replaced with another complete assembly and shift solenoid feed orifices in the spacer plate were enlarged to .035 in., the same problem remained.

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Cause 1

The 4L60E valve body is in second gear by default when the engine is off. Once the engine is started and line and AFL pressure have generated 48 lbs. or more, the 1-2 shift valve will be stroked into first gear. It requires 48 lbs. to overcome the tension of the 1-2 shift valve spring. A deficiency in the line or AFL pressure that allows line pressure to drop below 48 lbs. is going to create second gear starts. The line pressure was checked and it was reading at the correct value of 62 lbs. in the drive range, this ruled out any line pressure, AFL valve or forward accumulator piston problems.

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Cause 2

The technician discovered on a subsequent road test that in the manual low range the transmission would bind up. In checking the 1-2 accumulator piston seal and the piston pin bore, the technician observed the 1-2 accumulator piston was missing the rubber seal; refer to Figure 1.

Tech-Talk-Figure-1-600
Figure 1.

When the piston seal is leaking or missing accumulator pressure is going to feed past the second accumulator piston into the band apply circuit and apply the band; refer to the green circled arrows in Figure 2. 

Tech-Talk-Figure-3-1400
Figure 2.

The correction

With a seal installed on the 1-2 accumulator piston (Figure 3), a short road test confirmed the fact that the complaint had been eliminated.

Tech-Talk-Figure-2-600
Figure 3.

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