66RFE P0731: Gear 1 Incorrect Ratio Causes & Fix

P0731 on a 66RFE means the module commanded 1st gear and the ratio it measured did not come back as 1st. On paper that sounds like the simplest code in the family. In the bay it is one of the more misread ones, because 1st gear on the RFE six-speed is held by a different set of parts at a stop than it is two seconds after launch. Figure out which half of that hand-off failed and you know whether you are looking at a clutch pack, a one-way clutch, a hydraulic leak or a wire.

The 66RFE is the gas-engine six-speed in the RFE family. Ram's 2013 Heavy Duty launch material put it behind the 5.7L HEMI: "The HEMI V-8 is mated to a 66RFE six-speed automatic transmission" [6]. It shares the 65RFE and 68RFE apply chart and gear ratios, and the factory gear-ratio diagnostic logic is common to the whole RFE line, so where a fact comes from a sibling unit I will say so.

Related reading: 66RFE P0733 (3rd gear ratio) and 66RFE speed sensor testing.

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Recommended 66RFE parts

For a UD clutch or sprag teardown: the 2014-18 master kit, the 2012-2013 clutch module, and a remanufactured valve body for circuit leaks. Ships from the USA. Secure checkout.

66RFE Master Rebuild Kit 2014-18

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66RFE Clutch Module 2012-2013

$202.64View part

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

$405.00View part

How the Module Decides 1st Is Wrong

There is no gear position sensor doing this job. The module divides input speed by output speed and compares the answer with the gear it commanded. Chrysler's RFE diagnostic chart for P0731 states the set condition plainly: the code sets if the ratio of input rpm to output rpm does not match the current gear ratio, monitored continuously with the engine running and the transmission in gear [1].

On the 65RFE, 66RFE and 68RFE, 1st is 3.23:1 [2]. So with 1st commanded, input speed should run about 3.23 times output speed. If the input side climbs above that, something between the input shaft and the reaction member is letting go.

Three details from the factory chart are worth knowing before you touch anything [1]:

• The MIL is slow. P0731 can take up to five minutes of problem identification before the light comes on. The customer's "it slipped pulling away" and the code setting may be minutes apart.
• The sensors read different targets. The input speed sensor reads 60 teeth on the input clutch hub and the output speed sensor reads 30 teeth on the park gear.
• A lost ground looks like a ratio problem. If the common speed sensor ground is lost, both inputs read the input sensor's pulses at a stop in neutral, which gives an apparent 1:2 ratio and can set P1794. That is a wiring repair, not a teardown.

What Actually Holds 1st Gear on a 66RFE

The RFE units make every gear by applying two elements at a time. In 1st, the underdrive (UD) clutch drives the geartrain. The reaction side is shared between two parts, and that is the whole story on this code.

Sonnax's apply chart for the 65RFE, 66RFE and 68RFE shows 1st as UD clutch on, low/reverse clutch on, and the low overrun clutch holding, with a footnote: the L/R clutch is on only with output shaft speed below 150 rpm [2]. ATSG's 68RFE service information carries the same footnote and describes the freewheel device as the low sprag [3]. Sonnax spells out what that means on the road: the L/R clutch is applied at a stop in 1st and turns off right after launch, and a neutral condition after launch in Drive points to a low roller or low sprag that is not holding [2].

So a 66RFE can set P0731 from two very different failures:

• The UD clutch is slipping. Input speed flares even with the L/R clutch applied. The ratio is wrong from a standstill, and it often shows up in 2nd, 3rd and 4th too, because UD drives all four.
• The low sprag is not holding. The truck holds fine at a stop while L/R is applied, then neutrals out or flares the moment output speed passes about 150 rpm and the module drops L/R.

The pressure switches on the solenoid pack let you watch that hand-off. ATSG notes the switches on the L/R, 2C, 4C, UD and OD circuits close at 23 psi and open at 11 psi, and the module monitors them for the correct state in each gear [3].

Step One: Clear the Codes That Cause Ratio Codes

The factory chart does not start with the clutches. Its first question is whether any solenoid, line pressure, speed sensor or loss of prime codes are present, because any of those can cause a speed ratio error. If P0944 (loss of prime) is stored, it is diagnosed first [1]. Our 66RFE P0944 guide covers that one.

Then do the basics properly:

1. Pull every module code and save the freeze frame. Note gear commanded, vehicle speed, fluid temperature and throttle when P0731 set.
2. Check the fluid type. Mopar's Ram owner's manual for the six-speed automatic with a gas engine says to use only ATF+4 [7]. A truck that has been topped off with a universal fluid can shift soft enough to slip.
3. Check the level hot. The same manual says the six-speed is best checked at normal operating temperature, 170-180 F, which normally takes at least 15 miles of driving [7]. Low fluid is on the factory P0731 cause list, and a unit that is low will lose prime on a launch first [1]. Our 66RFE fluid and level guide walks through the dipstick check.
4. Look at the fluid. Dark ATF+4 with friction material in the pan says a clutch has already slipped. Clean fluid with a 1st-only code leans toward the sprag, the sensor side or an intermittent hydraulic leak.

Step Two: Split UD From the Sprag on the Scan Tool

With related codes ruled out, the factory chart runs a 1st gear clutch test and asks one question: did input speed stay at zero [1]? With the truck held stationary in gear, the UD clutch and L/R clutch are both applied and the output is held, so the input shaft should be locked. If input speed climbs during that test, the UD side is slipping and the unit is coming apart. Keep the test short. The chart carries a caution not to overheat the transmission.

If the stall test passes, road test it and record these items together:

• Input speed, output speed and calculated ratio. In a healthy 1st the ratio sits at 3.23 under load.
• L/R pressure switch state. It should show pressure at a stop in Drive and drop out just after launch. If the ratio goes wrong at the same moment the L/R switch opens, look hard at the low sprag.
• UD pressure switch state. If the UD switch is open while 1st is commanded and line pressure matches desired, the pump made pressure and the UD circuit did not hold it.
• Line pressure, actual versus desired. Low across every gear is a supply problem, not a 1st gear problem.

Electrical checks at the case connector

Sonnax lists room-temperature solenoid resistance at the transmission case connector: UD solenoid pins 10 and 17 at about 1.3 ohms, LR/TCC solenoid pins 10 and 2 at about 1.3 ohms, and the line pressure solenoid pins 10 and 12 at about 4.3 ohms [2]. Check at the case first, then at the module end. Good at the case and bad at the module is a harness fault. The factory cause list also includes the input speed sensor and its wiring, the solenoid/TRS assembly and the control module itself [1].

The Causes Behind a 66RFE P0731

Chrysler's P0731 chart lists the internal causes in one long column [1]. Grouped by what they do, they are easier to work through:

1. Oil supply problems

Low fluid level, a cracked or misinstalled primary oil filter or filter seal, a stuck or sticking main regulator valve, low line pressure, and poor machining on the pump valve body face. All of these starve every circuit, so line pressure data will usually be low in more than one gear.

2. A leak in the underdrive circuit

A cut UD piston seal, a cut UD accumulator seal ring, a broken UD accumulator piston, a broken reaction shaft support seal ring, a broken or missing main valve body bleed orifice, and a broken or missing dribbler orifice assembly in the reaction shaft support. These let the UD clutch apply and then bleed off under load, which is exactly the slip-after-engagement pattern P0731 is built to catch. Air check the UD circuit before condemning a pack.

3. A failed overrunning clutch

The chart lists it by name [1]. If the sprag rotates both ways, the truck pulls fine while L/R is on and neutrals out just after launch. Nothing short of a teardown fixes it.

4. The UD snap ring on the 66RFE input drum

This is one to check on every 66RFE that comes apart. TransGo's RFE kit instructions describe the factory tapered underdrive snap ring as a ring that "breaks and pops out," and supply a flat .091 to .094 in. heavy-duty ring to replace it, installed on top of the UD pressure plate and seated in six places [4]. If that ring lets go, the UD pack loses its backing and the clutch cannot hold. TransGo lists UD clearance for both 2012-2013 and 2014-up 66RFE input drums as .030 to .063 in. [4]. Measure it, do not assume it.

5. The low/reverse switch valve

Sonnax describes the RFE low/reverse switch valve as a one-way valve for the L/R clutch in Reverse and Drive. When the bore wears, oil leaks past it and exhausts through the manual valve, causing low apply pressure. Listed symptoms include a burnt L/R clutch, delayed or no Reverse, and delayed or no 1st and 2nd [5]. Its vacuum test pass mark is 18 in.-Hg. Anything below that means wear. Sonnax lists repair sleeve kits 92835-41K and oversize 92835-45K for the 66RFE [5]. A worn switch valve is more likely to show up as a delayed or harsh engagement than a clean P0731, but on a high-mileage valve body it belongs on the list.

Watch the 2012-2013 vs 2014-Up Split When You Order Parts

The 66RFE changed partway through its run, and it affects the very clutches you will be replacing for a UD slip. TransGo notes that 2014-up 66RFE OD clutches are thinner with thicker steels, and the UD/OD and OD/Rev pressure plates changed at the same time from the 2012 and 2013 units. It also warns that a 2012-2013 66RFE may already have been repaired with 2014-up input drum parts, and gives Mopar service package 68321415AB as the update that brings a 2012-2013 unit to 2014-up specs [4]. Identify the drum before you order a clutch kit. Our 66RFE identification guide covers the year split.

The Fix, and Getting It Back Together Right

Match the repair to what the data showed:

• Electrical (sensor, ground, harness, solenoid pack): repair in the truck and recheck the ratio on a road test.
• Supply or valve body leak: filter and seal, valve body work, or a remanufactured valve body.
• UD slip with friction material in the pan, or a sprag that will not hold: the unit comes out. Replace the UD pack, the tapered snap ring and every input drum seal, and inspect the low sprag and its race. Then deal with whatever cause took the clutch out, or the new pack will follow the old one.

If you put a valve body in, Sonnax's install tips give 105 in-lb for the six valve body to case bolts, 53 in-lb for the solenoid pack screws and 40 in-lb for the accumulator cover screws. On a universal RFE valve body going into a 65RFE, 66RFE or 68RFE, the 2nd and OD accumulator springs are swapped for the six-speed springs before install [2].

After any repair, the module needs to relearn. Sonnax's procedure is to warm the fluid to 185 F, run a quick learn with a capable scan tool, and road test through 10-15 upshift and downshift cycles. Some downshift clunk, a slight bind on the 3-4 and a 2-3 flare are common during relearn and typically settle within those cycles [2]. Clear the codes, then confirm 1st holds 3.23:1 on a loaded launch before the truck leaves.

How to Keep It From Coming Back

• ATF+4 only, at the right level, checked hot. Low fluid and the wrong fluid are the cheapest causes on the list to prevent [1][7].
• Do not let a slip ride. A UD circuit that is bleeding down will burn the pack it feeds. The five-minute MIL delay means the driver has usually felt it before the light comes on.
• On every rebuild, replace the tapered UD snap ring with a flat heavy-duty ring, set UD clearance to spec, and vacuum test the valve body bores rather than assuming they are fine.

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FAQ

What does P0731 mean on a 66RFE?

The module commanded 1st gear and the ratio it calculated from the input and output speed sensors did not match 1st, which is 3.23:1 on the 66RFE. The code sets when the transmission slips after engagement. It can take up to five minutes of problem identification before the check engine light comes on.

Is P0731 on a 66RFE the clutch or the sprag?

It can be either. The underdrive clutch drives 1st, the low/reverse clutch holds only below about 150 rpm output speed, and the low sprag holds after that. Slip from a standstill points to the UD side, while a neutral or flare right after launch points to the sprag.

Can a speed sensor cause P0731?

Yes. The ratio is calculated from the input and output speed sensors, so a bad signal or a lost common ground can make a healthy 1st gear look wrong. Chrysler's chart has you diagnose speed sensor, solenoid, line pressure and loss of prime codes before the ratio code.

What fluid does a 66RFE use?

Mopar specifies ATF+4 only for the six-speed automatic with a gasoline engine. Check the level hot, at 170-180 F, which normally takes at least 15 miles of driving. Low fluid is on the factory cause list for P0731.

What is the UD clutch clearance on a 66RFE?

TransGo lists .030 to .063 in. for both 2012-2013 and 2014-up 66RFE input drums. Replace the factory tapered UD snap ring with a flat heavy-duty ring during the rebuild, since the tapered ring is known to break and pop out.

Related 66RFE guides

• 66RFE P0733: Gear 3 Incorrect Ratio Causes & Fix
• 66RFE Slipping: Causes, Diagnosis and Fix
• 66RFE Input/Output Speed Sensor Symptoms, Codes and Testing
• 66RFE Valve Body Symptoms and Testing
• 66RFE Delayed Engagement: Causes and Fix