66RFE Won't Shift Into Overdrive: Causes, Diagnosis and Fix

The 66RFE is the six-speed Ram put behind the gas HEMI in its 2500 and 3500 pickups and chassis cabs. A HEMI truck already lives at higher rpm than a Cummins, so when it stops taking 5th and 6th the driver notices right away: the engine is screaming at 70 mph, fuel economy falls off a cliff, and the truck feels like it is towing even when the bed is empty.

"No overdrive" is not one problem. On this unit it is usually one of five things: a gear limit the driver set and forgot, 4th-gear limp home, an OD pressure switch code, a worn solenoid switch valve bore that is starving the overdrive clutch, or an overdrive clutch or hub that has physically failed. They cost anywhere from nothing to a full overhaul, and you can sort them in roughly that order without guessing. The 66RFE shares its electronics, solenoid pack and most of its valve body with the 545RFE and 68RFE, so the factory RFE procedures below carry straight across.

Recommended 66RFE parts

The parts that fix a 66RFE that will not hold overdrive: a solenoid pack for a failed OD pressure switch or solenoid, a remanufactured valve body when the switch valve bore is worn, and a clutch module when the OD pack has burned. Ships from the USA. Secure checkout.

66RFE Solenoid Kit 1999-2018

$180.58View part

66RFE Valve Body 2010-2018 - Remanufactured 5-Ball, Standard Duty

$405.00View part

66RFE Clutch Module

$302.38View part

Which Gears Need the Overdrive Clutch

Start with the apply chart, because "overdrive" on a 66RFE means more than 5th and 6th. Per the 65/66/68RFE apply chart Sonnax prints in its CHR145/CHR146 valve body instructions:

• 3rd (1.41:1): Underdrive (UD) plus 4th clutch (4C)
• 4th (1.00:1): UD plus Overdrive (OD)
• 5th (0.81:1): OD plus 4C
• 6th (0.62:1): OD plus 2nd clutch (2C)

The OD clutch is in every gear from 4th up. That is the single most useful fact for this complaint. If the truck tops out in 3rd, flares on the 3-4, or takes 4th but will not hold 5th and 6th under load, the same clutch and the same hydraulic circuit sit in the middle of all of it. A truck that never completes a clean 3-4 is not going to reach 6th no matter what you do to the 5-6 side.

Before you plug in anything, find out what the truck is actually doing. Watch the gear display in the cluster or log commanded gear and actual gear on the scan tool. "Won't shift into overdrive" from the owner can mean stuck in 3rd, stuck in 4th, or shifting into 5th and immediately falling back out.

Step 1: The No-Cost Checks

ERS gear limit

The six-speed in the 2500/3500 uses Electronic Range Select. Per Ram's 2018 user guide, tapping the ERS (-) switch on the column shifter activates ERS mode, shows the current gear in the cluster, and makes that gear the top available gear. Set the limit to 4 and the transmission will not shift above 4th. It only exits ERS when you push and hold the ERS (+) switch until the gear limit display disappears. Owners bump that switch backing a trailer and drive for a week thinking the transmission has failed. Check the cluster for a gear-limit number first.

Fluid temperature: too cold or too hot

The second free check is the temperature on the scan tool. ATSG's technical manual for the 68RFE, which shares its gearset and clutch layout with the 66RFE, states that the shift into 5th and 6th happens only after the unit has completed the shift into 4th, and that upshifts into 5th and 6th are delayed when fluid temperature is below 40 F (4.5 C) or above 240 F (115 C). A truck that holds 4th for the first mile on a cold morning is doing what it was designed to do. A truck that drops out of 6th on a long hot pull with a trailer may be protecting itself from heat, not failing. Log transmission temperature during the road test so you know which one you are looking at. A bad temperature sensor reading can cause the same lockout, so compare the reading to the real temperature of the pan with an infrared thermometer. The full gear and clutch chart is in our 66RFE gear ratios and specs reference.

Fluid level and the cheap stuff

Chrysler's RFE pre-diagnostic procedure, which every OD code test points back to, starts with checking the pinion factor for the tire size, checking fluid level, and charging the battery. Chrysler states plainly that many transmission symptoms are caused by low fluid, and that testing should only be done with a fully charged battery to avoid a false diagnosis. Low fluid on an RFE drops line pressure, and the OD pressure switch logic is the first thing to complain.

Record DTC event data before anything else. Chrysler warns that a battery disconnect clears all DTC event data and resets learned transmission values, which can give you erratic shifting on the road test and destroy the evidence you need.

Step 2: Stuck in 4th Is Limp Home, Not "No Overdrive"

If the truck takes off in 4th and stays there in Drive, the controller has not refused overdrive. It has shut the transmission down. Ram's 2018 user guide for the 2500/3500 six-speed is specific: when the transmission controller detects a condition that could cause damage, Limp Home Mode activates and the transmission remains in fourth gear regardless of which forward gear is selected. Park, Reverse and Neutral still work, and the MIL may be on.

The same guide gives the owner reset: stop, shift to Park, ignition off, wait about 10 seconds, restart. If the fault is no longer present, the truck gets all its gears back. That is why so many of these trucks come in with "it does it sometimes." A momentary fault resets on the next key cycle and comes back on the next hot drive. Do not clear codes and road test as a diagnosis. Pull the stored codes and event data, because the code that put it in limp is the code you chase.

Step 3: The OD Pressure Switch Codes

The two codes I see most on a 66RFE that will not hold overdrive are P0871 and P0870. They look alike and mean different things.

P0871: OD pressure switch rationality

Per Chrysler's service information for the RFE family, the transmission uses five pressure switches to monitor the L/R, 2C, 4C, UD and OD clutch circuits, and the controller continuously checks each switch against the state it should be in for the commanded gear. P0871 sets when the OD switch is open or closed at the wrong time. On a 66RFE that means the OD switch should read closed in 4th, 5th and 6th and open in Park, Reverse, Neutral and 1st through 3rd.

Chrysler's P0871 cause list is worth reading in full, because it is mostly hydraulic:

• Low line pressure, or line pressure codes present
• No. 4 or No. 6 check ball cut or damaged
• OD pressure switch sense circuit open, shorted to ground, or shorted to another circuit
• Cut OD/Reverse piston seal, or a broken or missing OD/Reverse piston bleed orifice
• Broken reaction shaft support seal ring, or a broken or missing dribbler orifice in the reaction shaft support
• Poor machining on the pump valve body face
• Cut OD accumulator piston seal, cracked OD accumulator piston, or a broken or missing OD bleed orifice in the main valve body
• The solenoid/TRS assembly, and last, the controller

The Chrysler test flow also branches on how many switch codes are present. If two or more other pressure switch rationality codes set along with P0871, it has you look at whether all the switches read closed in the event data. That points to a common electrical or power problem, not the OD circuit.

Split electrical from hydraulic in one test

GEARS magazine's RFE pressure switch article gives the fastest way to split it. The controller sees system voltage, 12.6 to 14.4 volts, on a switch that is open, and only 25 to 50 millivolts on one that is closed; each switch circuit runs through a 300-ohm resistor that limits current to 50 mA or less. Run the engine, put it in Reverse, and look at all five switches. In Reverse every switch should read open. If the OD switch reads closed in Reverse, you have a wiring, connector or switch fault. If all five read open in Reverse and the OD switch still misbehaves in 4th through 6th, the problem is oil, not electricity.

P0870: OD hydraulic pressure test

P0870 is an active test, not a passive one. Per Chrysler, after a shift into a forward gear with engine speed above 1000 rpm, the controller momentarily turns on element pressure to clutch circuits that are not applied and checks that the matching pressure switch closes. If the OD switch fails to close twice, the DTC sets, the MIL comes on, and the transmission goes into its orderly shutdown routine. That is your 4th-gear limp home.

Chrysler's flow sends you to any loss-of-prime, TCM power or line pressure sensor code first, then to P0760 (OD solenoid circuit) if present, and it groups P0870 with P0733, P0734, P0735 and P0871. During the test, line pressure should hold steady between 85 and 95 psi in each position. The cheap check is the same one the slipping guide uses: with the scan tool on line pressure, push in on the line pressure sensor connector. If the reading jumps to about 30 psi, clean and reseat the terminals before you go any further.

Step 4: The Usual Hydraulic Cause

GEARS reports that worn valve body bores and outer solenoid switch valve plugs cause P0871 about 90 percent of the time, from erosion and porosity in the aluminum casting. Sonnax's 66RFE unit listings tie a burned OD clutch and a burned 4th clutch to the same wear and list oversized switch valve kits as the cure. The switch valve cycles constantly. As the bore wears, OD apply oil crossleaks, so the OD switch sees pressure when it should not or misses it when it should, and the OD clutch gets less apply force than the controller thinks it has. Leave it long enough and the clutch burns.

A new solenoid pack will not fix a worn bore. Pull the valve body and vacuum test the switch valve bore and plugs. If it does not hold, ream it and fit an oversized valve and plugs, or replace the valve body. Count the check balls while it is apart. GEARS lists the classic rebuild mistakes: a No. 6 ball in a five-ball valve body sets P0871, and a missing No. 4 ball brings the converter clutch on in Drive. Chrysler's own P0871 list calls out the No. 4 and No. 6 balls for the same reason.

Step 5: When the Hardware Has Failed

Burned OD clutch

If the codes are ratio codes, not switch codes, and the scan data shows slip in 4th, 5th and 6th together, the OD pack is gone. Chrysler's 4th-gear ratio code (P0734) lists a burned OD or 4C clutch alongside worn switch valve plugs, a stuck main regulator, cut OD or 4C piston and accumulator seals, and missing bleed orifices. Fix the reason it burned (usually the switch valve bore or line pressure) before you put new frictions in, or the new pack goes the same way.

Cracked OD hub and shaft

This one gets missed. Sonnax states that overdrive clutch hub and shaft failure is common in both the 66RFE and 68RFE, in stock trucks as well as built ones. The failure starts as small cracks at the lubrication cross holes and grows through the shaft until the gear train fails. On teardown of any 66RFE with an OD complaint, clean the hub and shaft and look hard at those cross holes. Sonnax's 72570-01 is the OE 52119776AA assembly cryogenically treated with shot-peened cross holes, and it drops into any 66RFE or 68RFE.

Quick Diagnostic Order

1. Look at the cluster. Is an ERS gear limit showing? Check fluid temperature too: 5th and 6th are held off below 40 F and above 240 F.
2. Check fluid level hot, charge the battery, record codes and event data before clearing anything.
3. Stuck in 4th from a stop is limp home. Chase the stored code.
4. P0871 or P0870: do the engine-running Reverse test on all five switches, check the line pressure sensor connector, then vacuum test the switch valve bore.
5. Ratio codes in 4th through 6th: the OD clutch is slipping. Fix the bore and pressure, then the pack, and inspect the OD hub and shaft.
6. After any repair, perform a quick learn and road test so the controller relearns clutch volumes on the new hardware.

FAQ

Why won't my 66RFE shift into 5th or 6th gear?

Check the cluster for an ERS gear limit first, because a limit of 4 keeps the truck out of 5th and 6th until you push and hold ERS (+). Then check fluid level and pull codes. An OD pressure switch code such as P0871 or P0870, or ratio codes in 4th through 6th, point to the overdrive clutch circuit, most often a worn solenoid switch valve bore.

Why is my 66RFE stuck in 4th gear?

Stuck in 4th in every forward position is Limp Home Mode. Ram's user guide says the six-speed in the 2500/3500 stays in fourth gear when the controller detects a condition that could damage the transmission. A key-off reset may bring the gears back, but the stored code tells you what actually caused it.

What does P0871 mean on a 66RFE?

P0871 is the OD pressure switch rationality code. The controller found the overdrive pressure switch open or closed at the wrong time for the commanded gear. GEARS magazine reports worn valve body bores and switch valve plugs cause it about 90 percent of the time, with wiring, check balls, seals and the solenoid pack behind that.

What is the difference between P0870 and P0871?

P0871 is a passive check: the OD switch was in the wrong state for the gear. P0870 is an active test: above 1000 rpm the controller briefly pressurizes the unapplied OD circuit, and if the switch fails to close twice it sets the code and shuts the transmission down into limp home.

Can a 66RFE OD clutch hub crack?

Yes. Sonnax states that overdrive clutch hub and shaft failure is common on both the 66RFE and 68RFE, including stock trucks. Cracks start at the lubrication cross holes and spread through the shaft, so inspect those holes on any teardown with an overdrive complaint.