AW55-50SN / AF33 2-3 Flare and Harsh 3-2 Downshift: Diagnosis and the Real Fix

AW55-50SN / AF33 2-3 Flare and Harsh 3-2 Downshift: Diagnosis and the Real Fix

AW55-50SN / AF33 2-3 Flare and Harsh 3-2 Downshift: Diagnosis and the Real Fix

The complaint always sounds the same. On a light-throttle 2-3 upshift the engine flares for a beat before the gear catches, and coasting down to a stop sign or turning into a parking lot, the transmission delivers a hard 3-2 bang. Sometimes there are no codes, sometimes P0780, P0785 or P0745.

Sonnax calls this the AW 55-50SN "club," and their point is that almost every one of these units eventually joins it. The same transmission shows up badged as a Nissan, Volvo, Saab, GM, Saturn, Renault or Opel, and the 2-3/3-2 issue is common regardless of brand (Sonnax, "AW 55-50SN: Join the Club"). Transmission Digest adds that GM's version, the AF33, ran from 2000 to 2015 in the Chevrolet Equinox and Captiva, Pontiac Torrent and Saturn Vue.

What is actually happening during the shift

Per Sonnax, the 2-3 upshift requires correctly timed application of the B-4 band and release of the B-5 brake clutch, so the shift depends on matching a changing drum speed to a clutch release. The 3-2 downshift reverses it: B-4 releases and B-5 applies. When coasting or turning, the axles load and drive the 4-5 drum, so the band apply force needed to hold it changes with load. If TCM, solenoid and valve timing lets B-4 release but delays B-5 apply, the unit briefly neutralizes, and that neutral is the bang the customer feels.

Diagnosis: test before you tear down

This transmission has pressure ports for every circuit, so most of the work can be done with it still in the car. The plugs use a 12mm head. Sonnax is emphatic about one thing first: the large 27mm nut on top of the case is the B-4 band anchor, not a pressure tap. Remove it and the band has to be recaptured, which means pulling the servo at best and possibly the whole transmission.

  1. Compare B-4 servo pressure to C-1 clutch pressure in third. Sonnax says these should not vary by more than 10 psi. If B-4 is lower, suspect leaking servo feed-tube O-rings. If C-1 is lower, that input clutch pressure problem is your delayed engagement and flare.
  2. Check the C-1 apply curve. Sonnax gives zero psi in Park, stepping to 25 psi then 50 psi within half a second as Drive is selected, then following load to a maximum near 200 psi. If C-1 jumps straight to 160 psi on engagement, the unit is likely already in a TCM failsafe strategy and the 2-3 will be harsh no matter what you do hydraulically.
  3. Watch the B-5 to B-4 handoff. Time how long B-5 takes to release while B-4 applies. A prolonged low or zero reading points at a valve body or solenoid control problem.
  4. Check SLT. Sonnax specifies the line rise solenoid should start around 5 psi in Drive and rise with load to 80 psi maximum, never exceeding 90 psi at maximum line. If output hangs at a fixed level instead of tracking load, the solenoid or the valves it works against are worn or contaminated.
  5. Check lube and cooler flow on a converter complaint. Sonnax lists normal lube at 150 F as 5 psi in Drive and 8 psi in Reverse, with cooler flow of 1.3 gpm in Drive with TCC released, dropping to 0.7 gpm during apply.
  6. Wet air test the SLT circuit. Before the valve body comes out, Sonnax has you pressurize the SLT port with 80-100 psi shop air and watch where fluid is pushed out. The leak points they name are the steel side cover, the SLT accumulator, and the B-1 and C-1 control valves.

The causes, ranked by how often they are the answer

Valve bore wear, not valve wear. Sonnax is clear that in this unit it is more common to find wear at the bores than at the anodized aluminum valves, and that wear severe enough to cause a 2-3 flare or 3-2 clunk can look like nothing more than a slightly polished half-moon in the casting. The most overlooked bore for this complaint is the B-4 release valve near the spring spool. Buffing out a ridge does not help if a cross leak exists, because the valve will still hang.

The solenoid modulator bore. This circuit feeds all three linear solenoids, and Sonnax compares a worn modulator bore to a bad GM actuator feed circuit: it lowers pressure to the solenoids so they react slowly, and caps their maximum output.

Contaminated or mismatched solenoids. Sonnax counts six different shift solenoids for this family, four of which can be installed in the wrong location or swapped for a normally-open unit where a normally-closed one belongs. All solenoids should measure 5.0 to 5.6 ohms and operate at 300 Hz between 10 and 40 percent duty cycle.

Case and geartrain wear. Transmission Digest identifies case wear where the output gear assembly mounts as one of the most common failures in the unit, with the resulting bushing damage also costing cooler pressure. They flag a second pattern that is easy to miss: pinion shafts wearing into the planet carrier, producing a noise complaint after an otherwise good rebuild.

The fix

If the pressures point at control rather than mechanical damage, the repair is a valve body correction plus clean, correct solenoids. Sonnax notes an in-vehicle option of altering the servo pin to 0.095 to 0.110 inch of travel or updating the servo cushion spring, and that Volvo published an updated servo and cover in bulletin 43-37 dated 2-25-03, but they are candid that this route alone is generally not 100 percent successful.

AW55-50SN AF33 shift solenoid kit for 2-3 flare and harsh 3-2 downshift repair
AW55-50SN Shift Solenoid Kit - the starting point when the on/off solenoids are contaminated or slow to snap open and closed.

Sonnax's requirement for the on/off shift solenoids is that they flow more than their feed orifices supply when open, and seal when closed. A restricted or leaking solenoid parks its valve halfway, which is exactly the mid-stroke condition that produces a lazy B-4 apply. That is why a complete AW55-50SN shift solenoid kit beats reusing seasoned solenoids, and why our solenoid catalog is organized by unit.

Replace while you are in there

  • Filter and pan gasket - a new AW55-50SN transmission filter is cheap insurance against the contamination that got into the solenoids in the first place.
  • Frictions and steels if the unit is apart - the AW55-50SN friction clutch kit and matching steel plate kit.
  • Full overhaul on a high-mileage core - the AW55-50SN master rebuild kit, plus our rebuild kits and clutch pack collections.
  • Bushings, if the case shows the output gear wear Transmission Digest describes. Sleeving the case and installing all new bushings restores geartrain centerline and the cooler circuit.
  • Clean speed sensors, routed away from stray frequency. Sonnax notes both are Hall effect and that input contamination causes harsh shifts, high line pressure and TCC concerns.

Effort and difficulty

The diagnostic work above is a gauge set and an afternoon, and it is the difference between a targeted repair and a rebuild you did not need. Solenoid and valve body service is doable in the vehicle on most applications. Everything touching the B-4 band anchor, the case bore at the output gear, or the planet carrier is a full removal job on a transverse front-wheel-drive vehicle, so plan around which side of that line your readings put you on. Our guide on fixing harsh shifts without a rebuild covers the same order on other units.

FAQ

Will replacing the solenoids alone fix a 2-3 flare? Sometimes, but only when the solenoids themselves are the restriction. If the B-4 release valve bore is worn or the solenoid modulator bore is polished, new solenoids get fed the same degraded pressure and the flare comes back. Test the SLT curve and the B-4 to C-1 relationship first.

Can I use a valve body from a different brand? Sonnax reports a GM Equinox valve body has been installed successfully onto a Volvo, but only with the matching harness for the SLT linear connector orientation, and the #2 solenoid must be the Volvo normally-closed type rather than GM's normally-open.

Shifts got harsh after a rebuild, with converter symptoms too. What now? Check the SLU adjustment. Sonnax documents an in-vehicle adjustment through a pass-through hole at the top left corner of the cover using a 3mm Allen wrench passing roughly 1.60 inches: clockwise reduces SLU output for firmer 1-2 and 2-1 shifts and later TCC, counter-clockwise increases it for softer shifts and earlier TCC. Sonnax also notes lube pressure and flow on this unit are low compared to others, so the converter clutch may stay fully applied at very low speed and high load; see our guide on TCC shudder causes and fixes.

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