AW55-50SN / AF33 TCC Shudder, RPM Surge and No Lockup: Diagnosis and the Real Fix

AW55-50SN / AF33 TCC Shudder, RPM Surge and No Lockup: Diagnosis and the Real Fix

AW55-50SN / AF33 TCC Shudder, RPM Surge and No Lockup: Diagnosis and the Real Fix

You are cruising at 45 mph in a Volvo XC70, a Saturn Vue, a Chevy Equinox or a Saab 9-3, and the tach starts breathing. Up fifty rpm, back down, over and over. Lift off to coast and the rpm surges. Roll into the throttle lightly and it surges again. The fluid comes out hot and dark, and sooner or later a TCC code sets.

Most people attack this with a converter and a solenoid. On the AW55-50SN family that buys a few thousand miles at best, because the converter is the victim, not the cause. The cause is nearly always bore wear in the lockup circuit of the valve body.

Which transmission you actually have

This is one unit wearing a lot of badges. Sonnax notes that the same problem unit shows up dressed as a Nissan, Volvo, Saab, GM, Saturn, Renault or Opel, and that while the automakers share the transmission there are variations in the shafts, drums, converters, valve bodies and TCM calibration (Sonnax, "AW 55-50SN: Join the Club"). Sonnax lists the same repair parts for the 55-50SN and 55-51SN as fitting the AF23/33 and Nissan RE5F22A versions of the design.

Transmission Digest adds that a similar valve body will bolt to a GM, Volvo, Saturn, Nissan, Saab, Opel or Renault, and failing to recognize the differences can create hours of extra effort (Bob Warnke, Transmission Digest).

Diagnose it before you pull anything

This complaint can be measured with the transmission still in the car. Per Sonnax, all the pressure plugs use 12 mm heads, and the 27 mm band anchor is not a fill plug or pressure tap; if it comes out, the band has to be recaptured, which means pulling the servo at minimum.

1. Check fluid level first

Sonnax is blunt: low fluid or excessive overfill creates erratic pressure and poor linear solenoid control, and eventually forces the TCM into a high-pressure strategy. A shudder on a unit that is a quart low is a fluid complaint until proven otherwise.

2. Read lube pressure

The lube tap sits between the secondary regulator valve and the main shaft bushings. Sonnax's numbers: lube can reach 30 psi at -10 F, and at 150 F normal lube is 5 psi in Drive, 8 psi in Reverse. Pressure that starts low and stays at zero means low pump output, a worn regulator bore, or bad bushings.

3. Run a cooler flow test

The single most useful test for a TCC complaint here. Sonnax's figures: normal flow in Drive with TCC released is 1.3 gpm, dropping to 0.7 gpm during TCC apply. On a Saab you need an adapter at the cooler element.

4. Watch the SLU react

The SLU is the linear lockup solenoid, pulsed during the 1-2, 2-1, 2-3 and 3-2 shifts, on engagement into Drive or Reverse, and during TCC apply. Sonnax describes the correct flow-meter signature as roughly 1.0 gpm in Park, a sharp drop to about 0.06 on Drive engagement, then a quick return. Their rule: harsh up and down shifts with no cooler flow toggle means the SLU is not adjusted or not working. All solenoids in this unit measure 5.0 to 5.6 ohms and operate at 300 Hz.

If lube and flow are in range and the SLU toggles correctly, look at the converter and the TCM calibration. Otherwise you are going into the valve body, and that is the common outcome.

The causes, ranked by how often they are the answer

Lockup control valve and sleeve wear

The most common single cause. Sonnax states that bore wear at the TCC control valve and/or sleeve leaves the torque converter clutch applied and transferring shift shock, and that loss of TCC control also causes rpm fluctuation. Their symptom list reads like the complaint sheet: harsh downshifts, TCC apply and release concerns, rpm surge on coast, rpm surge on light acceleration, TCC codes and overheated converter fluid (Sonnax 59947-29K).

A second flavor of the same failure: Sonnax describes oscillation of the steel lockup control plunger valve wearing the cast-aluminum sleeve bore, letting converter apply pressure exhaust so the converter never fully applies. Their drop-in kit 59947-03K addresses that sleeve without reaming.

Lockup relay valve bore wear

Sonnax attributes no lockup, delayed lockup, burnt converter and inadequate lubrication to worn or sticking lockup relay and control valves, caused by oscillation of the relay valve and stroking of the control relay valve in its sleeve. The correction is reaming the bore and installing oversized kit 59947-77K. If the complaint is "it never locks up" rather than "it shudders," start here.

Secondary regulator valve bore wear

This explains the cases where the converter, the bushings and the fluid all cook together. Sonnax says the 55-50SN secondary regulator bore is very active and wears the casting: wear at the spring end lets the SLT circuit and regulated converter pressure cross leak, and wear at the balance end reduces converter and lube flow. Since this valve regulates converter feed by SLT pressure and is the primary distributor of lube flow, wear here produces TCC slip, TCC surge, overheated fluid, bushings and converter, harsh Reverse, low lube flow and bushing failure. The fix is oversized kit 59947-16K. A harsh Reverse riding along with your shudder is a strong tell.

Contaminated or misadjusted linear solenoids

Sonnax lists contamination, spring sag and wear as the problems with the SLU, SLS and SLT linear solenoids, and warns that swapping SLT and SLS solenoids between cores without adjusting to the valves is generally not acceptable, because they are calibrated to their own valve body. They are fed by a modulator valve limiting maximum pressure; when that bore wears, the symptoms resemble a bad GM actuator feed circuit. Sonnax describes excess bore wear as a discoloration or polished area shaped like a moon phase, at the ends of the bore or opposite the fluid entry, and notes wear shows up at the bores more often than on the anodized aluminum valves.

The fix

The honest version: the valve body comes to the bench, worn bores get reamed and fitted with oversized valve kits, the solenoids get cleaned and returned to their measured adjustment, and the converter gets replaced if the linings are cooked. Sonnax also lists an AW55-50SN Zip Kit covering burnt clutches, delayed engagement, inadequate lubrication, low pressure, slips and flares, soft shifts and TCC codes.

If the pan came out with friction material in it, stop pricing valve body kits. Overheated fluid and starved lube take clutches with them, and the unit needs a teardown, not a hydraulic repair.

AW55-50SN shift solenoid kit for Volvo, Saab, GM and Nissan applications
AW55-50SN Shift Solenoid Kit - replace the on/off shift solenoids while the valve body is out.

Replace while you are in there

  • Shift solenoids. Sonnax notes six different shift solenoids in this family, four of which can be installed in the wrong location or swapped normally open for normally closed. A matched AW55-50SN shift solenoid kit removes the guesswork, and the transmission solenoid catalog covers the other models.
  • Filter and pan gasket. Do not put a valve body back on a unit that just cooked its fluid without a fresh AW55-50SN transmission filter. Sonnax's lube diagnostic assumes the filter and pump are clean.
  • End plug seals. Sonnax's o-ringed end plug kit 59947-21K covers delayed forward, poor shift quality, TCC slip, low SLT pressure and overheated fluid; their notes call for sealing the end plugs at C-1 control, B-4 release and B-1 control.
  • Bushings. If the secondary regulator bore was worn, assume lube flow has been marginal and inspect every bushing. Sonnax ties bushing failure directly to that bore.
  • Full rebuild parts. If the pan says the clutches are gone, an AW55-50SN master rebuild kit is the starting point, and the rebuild kit collection covers the rest of the models.

Effort and difficulty

Cooler flow and lube pressure testing is about an hour on a lift and requires removing nothing. Solenoid replacement plus a filter service means dropping the pan and valve body, a solid half day on most of these transverse applications. Reaming worn valve bores is a machining operation needing the correct Sonnax tool kit and a fixture, so if you are not equipped, plan on a remanufactured valve body or a shop that is. A teardown on a burnt unit is transmission-out, multi-day work. One planning note from Sonnax: tool kit F-59947-TL29, required for the 59947-29K lockup control valve repair, is no longer in production, so check availability with a distributor before committing to that path.

FAQ

Will a new torque converter fix AW55-50SN shudder?

Usually not by itself. Sonnax attributes burnt converters here to worn lockup relay and lockup control valve bores and to a worn secondary regulator bore starving converter and lube flow. A converter installed behind an unrepaired valve body sees the same uncontrolled apply pressure.

What is the difference between rpm surge on coast and normal TCC cycling?

Sonnax lists "RPM surge on coast" and "RPM surge on light acceleration" as symptoms of lockup control valve and sleeve bore wear. Normal TCC modulation shows as a clean flow toggle on a cooler flow test; if the surge is there and the toggle is not, the circuit has lost control.

Can I just swap in another used valve body?

Be careful. Sonnax notes a GM Equinox valve body has been installed successfully onto a Volvo, but the harness must match the SLT linear connector position, and the #2 solenoid is normally open on GM while the Volvo requires normally closed. Because the linear solenoids are calibrated to their own valve body, a core swap without adjustment is generally not acceptable. As Transmission Digest puts it, a clean core does not mean the valve body is functioning correctly.

Related reading

If the complaint is a flare on the 2-3 upshift or a hard 3-2 coming down, that is a different circuit in the same unit: AW55-50SN / AF33 2-3 Flare and Harsh 3-2 Downshift.

Sources