AS68RC Overheated Fluid, Low Cooler Flow and Converter Damage: The Cooler Blow-Off Valve Fix

AS68RC Overheated Fluid, Low Cooler Flow and Converter Damage: The Cooler Blow-Off Valve Fix

AS68RC Overheated Fluid, Low Cooler Flow and Converter Damage: The Cooler Blow-Off Valve Fix

Your AS68RC-equipped truck is running hot. Trans temp climbs on grades and refuses to come back down, the converter starts shuddering, and eventually it comes apart internally. You replaced the cooler, flushed the lines, dropped the pan for a fresh filter, and the temperature still goes where it should not.

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On the Aisin Seiki AS68RC one wear point produces exactly this pattern and almost nobody checks it: the cooler blow-off valve bore in the upper valve body casting.

What the AS68RC is, and what it does when it gets hot

The AS68RC is a six-speed, computer-controlled automatic with a lockup torque converter, rated up to 26,000 GVW, available with PTO capability in both 2WD and 4WD configurations, according to the Aisin AS68RC technical information sheet. It was fitted to Ram 3500 cab and chassis, Ram 4500 and 5500 trucks, and to Isuzu and Mitsubishi box trucks, with production running from the 2007.5 model year through 2012 before the AS69RC took over, per NextGen Diesel's AS68RC overview.

Two behaviors get misread as electrical faults when you are chasing heat. The same Aisin sheet documents limp-in: a fault in 4th, 5th or 6th fails the unit to 5th until it is safe to downshift to 3rd, and a fault in 1st, 2nd or 3rd fails it to 3rd. You will always have Reverse. Separately, the tow/haul or O/D off switch omits 6th on the first press, omits both 5th and 6th on the second, and restores all six gears on the third. Make sure the missing top gear is not simply that switch.

Diagnosis: prove the flow before you buy parts

Work the cheap causes first. Gears magazine's overheating writeup for the 48RE, 68RFE and AS68RC lists the leading causes as plugged cooler lines, a plugged cooler, a plugged internal transmission filter, vehicle overload, engine overheating, and a malfunctioning engine fan clutch. Inspect the lines for pinches, kinks, dents and crush marks.

  1. Measure cooler flow. Per Gears, flow should be at least a gallon per minute, measured with a flow meter or by collecting into a bucket. Anything less is a restriction or a hydraulic leak, not a cooler sizing problem.
  2. Know your temperature targets. Gears puts the ideal maximum at 200°F and states that for every 20 degrees above 200 you cut expected transmission life by a factor of two. That makes 240°F roughly a quarter of the life you paid for.
  3. Check the thermostatic bypass valve. Its job is to warm the transmission faster in cold weather and sit wide open in hot weather. Gears notes that when it sticks open, flush machines cannot clean the cooler, and the only remedy is replacing the cooler assembly.
  4. Vacuum test the valve body. This separates a plumbing problem from a valve body problem. Sonnax's guidance for the cooler blow-off valve is that you need the repair if vacuum testing at the indicated port fails to hold the recommended minimum in-Hg, or if wear is visually detected. Use the Sonnax AS68RC valve body layout to find the port.

Why a worn bore cooks the converter

Sonnax describes the failure on its Oversized Cooler Blow-Off Valve 99740-06 page: AS68RC units often develop converter-related issues caused by excessive wear in the cooler blow-off valve bore in the upper valve body casting. Excess wear allows valuable converter pressure to exhaust, and the leakage causes low cooler flow, overheated converter fluid and eventually converter damage.

That is the whole chain. Pressure that should be pushing fluid through the converter and out to the cooler is instead dumping to exhaust past a worn valve. Flow at the cooler drops, so heat is not carried away, and a converter that is already a known weak point on this unit fails. The NextGen Diesel teardown notes the AS68RC converter's aluminum cage can snap under load, so there is not much margin for running it starved and hot. A new cooler and new lines cannot fix this, which is exactly why the complaint survives the obvious repairs.

The fix

The correction Sonnax specifies is to restore hydraulic control by reconditioning the bore and installing the oversized cooler blow-off valve, part number 99740-06. Two details from the part page matter before you order: the valve is hardened steel to resist the wear that killed the original, and it is recalibrated to meet original OE pressures using the OE springs, so you are not changing the unit's calibration. It ships in a package quantity of three, and the required tooling is Sonnax reamer kit 99740-TL4.

That tooling is not optional — reconditioning the bore is what makes the oversized valve work, and the same kit also services the linear solenoid dampener valves. We stock it here:

AS68RC Sonnax 99740-TL4 reamer tool kit for reconditioning the cooler blow-off valve bore
AS68RC Reamer Tool Kit 99740-TL4 — tooling for the 99740-06 bore repair.

Replace while you are in there

The valve body has to come off the unit either way, so address the other documented wear points on the same teardown. All of the following are listed on the Sonnax AS68RC unit page:

  • Oversized K3/B2 valve kit 99740-09K — listed for 2-3 flare, delayed Reverse and pressure switch codes. Requires reamer kit 99740-TL9. See our AS68RC Oversized K3/B2 Valve Kit and the rest of the shift kits and valve body repair kits.
  • Linear solenoid dampener valves 99740-05K and 99740-04K — listed for gear ratio codes, solenoid performance codes, low solenoid feed pressure, delayed and harsh engagement, soft and flare shifts, and delayed Reverse. Both use 99740-TL4.
  • Oversized solenoid modulating pressure blow-off valve 99740-03 — soft shifts, flare shifts, ratio and solenoid performance codes.
  • Clutch control valves — plunger valve kit 99740-01K for Dodge units, or clutch control valve spring 99740B-01 for Mitsubishi and Isuzu units, both listed for downshift clunk, flare shifts and harsh shifts.
  • Fresh valve body gaskets. Do not reuse them. Our AS68RC main valve body gasket – lower and the matching upper are in the valve body parts collection.
  • Heavy duty K2 hardware if the unit is already apart: HD K2 clutch hub 99572-01, high capacity K2 piston kit 99582-01K and K2 backing plate kit 99582-02K, listed against burned K2 clutch and no 4th, 5th or 6th.

One ordering caution from NextGen Diesel: the internal wiring harness changed from 10-pin on 2007.5 through 2010 units to 12-pin on 2011 and 2012 units, and Mitsubishi/Isuzu variants differ in valve body calibration, identified by the numbers cast on the valve body. Sonnax splits several pieces by application as well, so confirm which unit you have before ordering.

Cost and effort

This is a valve-body-out repair, not a pan-gasket job: drop the pan, remove the valve body, split it to reach the upper casting, ream the bore, install the oversized valve, reassemble with new gaskets, then re-verify cooler flow. As a planning figure rather than a published spec, treat that as a half to full day of bench and install time; the bore work needs the correct reamer and a steady hand, not muscle. Sonnax frames the payoff as salvaging these very expensive and hard-to-find valve bodies — the real comparison is against a replacement valve body plus a converter, which is where this complaint ends up if you keep driving it hot.

FAQ

I already installed a bigger cooler and it still overheats. Why?

Because a cooler cannot cool fluid that is not reaching it. If the cooler blow-off valve bore is worn, converter pressure exhausts internally and flow to the cooler drops regardless of cooler size. Measure flow first; Gears calls for at least a gallon per minute. If flow is low and the lines and cooler are clear, vacuum test the valve body.

Does the oversized valve change how my truck shifts?

Sonnax states that the oversized 99740-06 valve is recalibrated to meet original OE pressures using the OE springs. It restores the factory hydraulic function rather than adding line pressure.

My truck lost 6th gear and runs hot. Is that the same problem?

Not necessarily, and the order of checks matters. First confirm the tow/haul or O/D off switch is not simply commanding 6th off. Then scan for codes, because the Aisin technical sheet documents that a fault in 4th, 5th or 6th fails the unit to 5th until it can safely downshift to 3rd. Overheating and a missing top gear can be two separate faults.

Related reading

For a shift-quality complaint rather than a heat complaint, start with AS68RC 2-3 flare and delayed Reverse: K3/B2 valve bore wear or AS68RC P0746 and P0776. For temperature targets across units, see transmission temperature: what is normal, what is hot, and when to worry.

Sources