46RE / 47RE / 48RE Shift Flare
Causes and Fix

A shift flare is the RPM jumping during an upshift instead of dropping cleanly. Behind a Cummins it usually shows up towing: the tach climbs on the 2-3 or 3-4, then the shift lands with a bang.

These are a three-speed hydraulic box with bands and a bolt-on overdrive unit. Unlike the later RFE units, much of what controls shift quality is mechanical and adjustable: the throttle valve cable and the band adjusters. Adjust before you buy. We fix more flares with ten minutes on the TV cable than with any solenoid.

Which Shift Flares Tells You Where to Look

• 1-2 flare: the kickdown (front) band applying through the kickdown servo, cushioned by the accumulator. A loose band, leaking servo or worn accumulator seal shows up here.
• 2-3 flare: the front clutch applying while the kickdown band releases. If the band lets go before the clutch grabs, the engine flares.
• 3-4 flare: the overdrive clutch in the rear unit applying, controlled by the overdrive solenoid.

Step One: Fluid Level and ATF+4

These units take ATF+4. Check it hot, engine running, in Park on level ground. Low fluid aerates on hard stops and softens every apply. Burnt fluid or friction material in the pan means it is already a rebuild.

If it just looks tired, service it. The 46RE/47RE/48RE pan with drain plug and filter kit ($92.99) adds a drain plug.

Step Two: Throttle Valve Cable Adjustment

This is the big one. The TV cable tells the valve body how hard you are pushing the pedal, and it raises line pressure and delays shifts to match. Gas trucks run it to the throttle body, mechanical-pump Cummins trucks off the injection pump linkage, and the 48RE uses one too.

Too loose and the transmission shifts early on low pressure, flaring and slipping until it burns the front clutch and kickdown band. Too tight gives late, harsh shifts. Set it with the factory procedure for your year: engine off, adjuster unlocked, transmission throttle lever held fully forward against its stop, then relock. No slack at idle. On a Cummins with a turned-up pump or changed linkage, recheck it.

Some later 48RE trucks use an electronic throttle valve actuator ($143.99) instead of a cable. A failing actuator does the same damage as a loose cable.

Step Three: Codes and Governor Pressure

Since the mid-1990s these units use a governor pressure solenoid and sensor in the pan instead of a mechanical governor. Pull codes:

• P1763 / P1764: governor pressure sensor voltage high or low.
• Generic P07xx codes: solenoid circuits and ratios.

Governor faults change when it shifts more than how firmly, but an upshift at the wrong moment can feel like a flare. Replace the solenoid and sensor as a pair. The A518/46RE/47RE governor pressure solenoid and sensor kit ($111.99) covers both; or use the 46RE governor pressure solenoid ($152.59) and 47RE governor pressure solenoid ($152.59). The 46RE solenoid kit with sensor and filter ($148.99) does the whole pan job.

Step Four: Band Adjustments

Kickdown (Front) Band

The adjuster is on the outside of the case, driver side. Loosen the locknut, tighten the screw to 72 in-lb, back off the turns your service manual lists (it differs by model and year), and lock it. A never-adjusted band is a classic cause of a slurred 1-2. If the anchor or strut is worn, it will not hold; the band anchor and heavy-duty strut set ($57.99) is cheap insurance on a Cummins.

Low-Reverse (Rear) Band

Inside the pan, same procedure. It only works in manual low and Reverse, so it will not cure a Drive upshift flare.

Step Five: The Overdrive Unit and 3-4 Flare

In first through third, a heavy spring holds the overdrive unit's direct clutch applied. For fourth, the OD piston compresses that spring and applies the overdrive clutch. A 3-4 flare with good fluid usually means worn OD frictions or leaking piston seals. The overdrive and converter clutch solenoids live on the valve body; a sticky OD solenoid gives a lazy 3-4. The 47RE solenoid kit ($244.99) replaces them together. For the OD unit, the 47RE overdrive clutch spring compressor T-1302 ($76.99) makes spring removal safe.

Cummins Trucks: Torque and Tunes

The 47RE and 48RE were built for stock Cummins power. Add injectors or a tune and the front clutch, kickdown band and OD clutch give up first, as a flare under load that worsens monthly. A shift kit raises line pressure and buys margin. The TransGo 48RE HD2 reprogramming shift kit ($267.99) is our pick for a working 48RE. On slipping frictions it only delays the rebuild.

Valve Body and Shift Kit

Worn valve bores soften everything.A shift kit like the 47RE shift kit ($156.94) or 46RE shift kit ($156.94) firms a healthy transmission. If the casting is worn, swap the valve body: the 2003-2007 48RE TransGo valve body ($199) or the 1996-1999 47RE Sonnax-updated valve body ($425).

Symptom to Fix: Quick Reference

Repair vs Rebuild

Clean fluid and pan? Fix it in the truck: fluid, TV cable, kickdown band, governor solenoid and sensor, then shift kit or valve body.

Burnt fluid, clutch material, or a flare that survives every adjustment means the unit is coming out. On the bench, the 1998-2002 46RE master rebuild kit with filter ($278.99), 1998-2002 47RE master rebuild kit ($307.98) or 2003-2007 48RE master rebuild kit ($265.99) covers seals, gaskets and frictions. Towing with a Cummins? Upgrade the front clutch and band while it is apart.

FAQ

What is the most common cause of a 46RE or 47RE shift flare?

A misadjusted throttle valve cable and a kickdown band that has never been adjusted. Both are cheap to check and both cause soft, flaring shifts. Check fluid level, set the TV cable, adjust the front band, then road test before buying anything.

Can a bad governor pressure solenoid cause a flare?

It changes when the transmission shifts more than how firmly it shifts. A failing governor pressure solenoid or sensor gives late, early or erratic upshifts, and an upshift that happens at the wrong moment can feel like a flare. P1763 and P1764 point at the governor pressure sensor circuit. Replace the solenoid and sensor together.

Will a band adjustment fix a 2-3 flare?

Sometimes. The 2-3 is the front clutch applying while the kickdown band releases, so band clearance affects the overlap. A band backed off too far flares the 1-2 and changes the 2-3 timing; one set too tight binds on the 2-3. If the band is set to spec and the 2-3 still flares, look at the front clutch and line pressure.

What fluid do the 46RE, 47RE and 48RE take?

ATF+4. Check it hot, engine running, in Park on level ground. Do not top off with a generic multi-vehicle fluid if you can avoid it, and never run it low; these units aerate and flare on hard stops when the level is down.

Does a Cummins tune cause 47RE or 48RE shift flare?

Yes, often. Added torque overwhelms the stock front clutch, kickdown band and overdrive clutch long before it hurts the engine. If the flare started after injectors, a pump turn-up or a tune, a valve body or shift kit with more line pressure helps, but a hard-working Cummins eventually needs upgraded frictions.

How much does it cost to fix a 46RE, 47RE or 48RE flare?

In-truck fixes run from a TV cable adjustment that costs nothing, to about $93 for a pan and filter kit, $112 for a governor solenoid and sensor kit, $157 for a 46RE or 47RE shift kit and $199 for a 2003-2007 48RE TransGo valve body. If the frictions are burned, a master rebuild kit is about $266 to $308 in parts before labor.