Pressure Control Solenoid “B” Electrical

Part of the generic P0778 overview — this page covers the Toyota meaning specifically.

What P0778 means

The 3.5L V6 Avalon (2013-2018, XG40 generation) drives through Toyota's U660E automatic transaxle, where six forward ratios are produced by the TCM switching a bank of linear shift solenoids — SL, SL1, SL2, SL3 and SL4 — on and off in different combinations. SL2 is the pressure-control solenoid that P0778 refers to as solenoid "B"; in Drive it stays off for 1st through 3rd and energizes for 4th, 5th and 6th, so it is central to the higher-gear apply logic. The TCM continuously watches the current flowing through the SL2 winding while the car is moving and shifting. Normally the commanded output duty stays below 100%, meaning the circuit accepts and regulates current as expected. If the module has to drive the output to a 100%-or-more duty cycle to try to reach its current target — the signature of an open or shorted SL2 winding or wiring — it flags P0778 on a single trip and immediately turns on the check-engine light. Because it is a one-trip, continuously monitored electrical fault, the code sets fast and Toyota's fail-safe strategy holds the box in a reduced set of gears to protect it.

Symptoms

• Check-engine light comes on quickly, often on the same drive the fault occurs, since P0778 uses one-trip detection
• Transmission drops into fail-safe (limp) mode, feeling locked in a single gear or a narrow gear range
• Harsh, delayed, or missing upshifts into 4th, 5th and 6th, because SL2 is the solenoid that engages those top gears
• Higher-than-normal engine RPM at highway speed when the box will not reach its overdrive ratios
• Poor fuel economy and sluggish acceleration while the Avalon runs in its protected, limited-gear state

Common causes

• A failed SL2 shift solenoid with an open or shorted winding (measured resistance outside the factory 5.0-5.6 ohm window)
• Damaged, chafed, or shorted wiring in the internal transmission harness (transmission wire) feeding the SL2 circuit
• A corroded, contaminated, or loose transaxle case connector where the internal harness meets the TCM or external plug
• ATF intrusion, debris, or wear inside the solenoid causing an intermittent electrical fault under heat
• A faulty TCM (transmission control module) or its internal driver circuit for SL2, though this is the least common cause

Diagnostic approach

1. Read codes and freeze-frame first — Scan the Avalon's TCM and record every stored transmission DTC plus the freeze-frame data before clearing anything. Note whether companion SL1/SL3 solenoid codes (P0748, P0798) or speed-sensor codes are also present, because a shared harness or connector fault can set several at once. The freeze-frame speed and gear tell you whether the fault appeared during a specific upshift into the SL2-controlled range.
2. Inspect the internal transmission (SL2) wire resistance — With the TCM removed from the transaxle, back-probe the SL2 terminals of the transmission wire and measure winding resistance at about 20°C (68°F). The factory standard is roughly 5.0 to 5.6 ohms across the SL2+ and SL2- terminals; an open (infinite) or near-zero reading condemns the internal wire. Also confirm each SL2 terminal reads 10 kilohms or higher to body ground, which proves there is no short to case.
3. Bench-test the SL2 shift solenoid itself — If the wire is bad or suspect, remove shift solenoid valve SL2 and measure across its two terminals; it should again read about 5.0 to 5.6 ohms at 20°C (68°F). Then apply battery voltage through a 21-watt bulb to the solenoid terminals and listen for the valve to click and move. A solenoid that is out of spec electrically or does not stroke audibly should be replaced.
4. Check the case connector and external harness — Examine the transaxle case connector and the harness run to the TCM for spread or corroded pins, ATF seepage, and chafed insulation. Do a wiggle test with the meter connected to catch an intermittent open or short that only shows up hot. Clean, reseat, or repair any marginal connection before condemning a component, since a bad connector mimics a failed solenoid.
5. Confirm the repair and rule out the TCM — If the SL2 wire and solenoid both pass their resistance and function checks, the remaining suspect is the TCM driver for that circuit. After any repair, clear the code, road-test through all six gears, and verify the Avalon completes clean 4th, 5th and 6th upshifts with SL2 duty back in its normal operating band (above 3% and below 100%) and no return to fail-safe.

Make & model notes

Toyota: On the 2013-2018 Avalon the U660E transaxle is fitted only to the non-hybrid 3.5L V6 (2GR-FE). Toyota's own P0778 detection is one-trip: it flags the code when the SL2 output duty is forced to 100% or more (normal duty stays below 100%), monitors continuously for a one-second duration, and lights the MIL immediately while dropping the box into a limited-gear fail-safe. The Avalon Hybrid uses a different eCVT-style hybrid transaxle and does not apply this U660E-specific procedure.

Toyota: The same U660E and its SL2 solenoid are shared across Toyota's V6 front-drive lineup — the V6 Camry, Highlander, Sienna and RAV4, plus the Lexus ES 350 — so the SL2 resistance spec near 5.0-5.6 ohms at 20°C and the identical fail-safe logic carry over to those siblings. Note that four-cylinder Camry and similar models use the U760E, a related but separate unit, so confirm you are working on the U660E before applying these Avalon figures.

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