Most common problems with the General Motors 4L60e transmission are diagnosed through the power control module (PCM) and its diagnostics readout. There are, however, a few things that can happen that won't show up as trouble codes on the PCM.
The first check should always be to look at the transmission's fluid. If levels are low, if fluid is contaminated, or if the fluid is dirty, that is the most likely cause of your trouble. The 4L60e, like most modern tranny's, is sensitive to overheated, mixed, or dirty fluid. Engine oil, coolant, and other fluids mixing with the trans fluid can cause the transmission to fail and require a full rebuild to repair.
If the PCM is not giving a code and the fluid is OK, then you may need to replace the PCM or have it tested. Any GM shop will have replacement PCMs for testing that can be added to the vehicle temporarily. If the new PCM does not give a code either and the problem persists, then proceed with other checks. Also check wiring to and from the PCM for scraping and possible short hazards.
When something goes seriously wrong with the electronics on the 4L60e, it defaults to "limp home mode." This allows the driver to manually use 2nd, 3rd, and reverse gears. In this mode, it's safe to drive the vehicle short distances to get to a garage or driveway. Excessive speeds, a lot of stop-start driving, etc. should be avoided as in this case, the transmission is operating at maximum compression and slippage when starting in 2nd gear causes a lot of heat.
Some other common failures and easy fixes:
- Slow, slipping, or no reverse: could be several things, but adding a quality high-viscosity transmission additive may re-seat a broken seal, repairing the issue.
- Transmission will not upshift, speedometer does not move from 0: is a sign that the VSS has failed. Remove and replace (it's on the rear of the trans).
- Goes into limp home mode, PCM gives lot of warnings/errors: a good sign that there is an electrical short. Check the underhood PCM/transmission fuses and replace if needed. If it continues to pop, trace wiring for a short and fix.
- Cannot manually shift to 1st or 4th, but trans is auto-shifting otherwise: the vehicle is not in "limp home" so the problem is in the ShiftA solenoid which may or may not give a PCM code. Remove pan, replace solenoid, and test wiring from trans to PCM.
- Vehicle is sluggish starting out, rolls easily, starts fine if manually shifted into 2nd: means the ShiftB solenoid is likely out. Replace, as above.
- Soft shifting and performance degradation over time: is a likely sign that there is a boost valve leak or clogged EPC filter screen. Use a Transgo HD2-C kit (OEM) to repair (no trans. removal required). If that does not work, add seal restorer to the fluid. If that fails, full rebuild may be required.
- Vehicle only starts in manual 3rd gear, allows manual shifting through all gears when moving, works fine when cooled (having been shut down for a while): means the VSS is dropping off on vehicles with a Hi-stall converter. Usually due to "hot dogging" or tire squealing (holding brakes while gunning the motor). A good GM shop should be able to reprogram the PCM to compensate for the dropoff.