Q: The automatic transmission in my Jeep Grand Cherokee shifts differently when the weather turns cold. Is this normal?
Answer: When the temperature drops, the fluid inside your transmission thickens. The transmission control module takes this into account and changes its shift strategy to compensate for the colder fluid.
On Chrysler 45RFE electronic automatics (which are used in Jeep Grand Cherokees and are similar to Chrysler's 41TE and 42LE transmissions), the Transmission Control Module (TCM) looks at the engine coolant temperature sensor reading when the vehicle is first started and use that as a reference point for estimating the ATF fluid temperature. Once this has been done, the TCM updates the estimated transmission oil temperature as the vehicle is being driven based on torque converter slip speed, vehicle speed, gear position and engine coolant temperature. The estimated oil temperature will be pretty close to the real oil temperature if the vehicle is driven normally and there are no other problems. But if the transmission is overfilled with fluid, the transmission oil cooler is restricted, the engine is running hot, or if the vehicle is driven aggressively in low gear, the TCM may underestimate the oil temperature causing the transmission to shift at the wrong points.
The Chrysler 45RFE automatic transmission in your Jeep uses several "shift schedules" that are based on ATF oil temperature. There is an "extreme cold" schedule when the oil temperature is below -16 degrees F that allows Park, Neutral, Reverse, 1st and 3rd gears only. If the oil temperature is between -12 degrees and 10 degrees F, it uses the "super cold" shift schedule that delays 2-3 and 3-4 upshifts, and provides an earlier 4-3 and 3-2 coastdown shifts. High speed 4-2, 3-2 and 2-1 kickdown shifts are prevented in this mode. When the ATF is between 10 and 36 degrees, the "cold" schedule takes over and the transmission shifts at higher throttle openings and high speed 4-2, 3-2 and 2-1 kickdown shifts are still prevented. Also, there is no torque converter clutch lockup in the cold, super cold or extreme cold ranges.
Once the ATF is above 40 degrees, the TCM goes to the "warm" schedule which allows normal upshifts, kickdowns and coastdowns, but still no torque converter lockup. When the ATF reaches 80 degrees, the TCM changes to the "hot/normal" mode and begins to engage the torque converter when vehicle speed is above about 22 mph.
If the ATF gets too hot (above 240 degrees F), or the engine starts to overheat (coolant above 244 degrees F), the TCM will employ an "overheat" schedule that delays 2-3 and 3-4 upshifts and changes the torque converter lockup points. If the ATF gets really hot (above 260 degrees F), the TCM goes into a "super overheat" mode which further delays 2-3 and 3-4 upshifts and prevents the torque converter clutch from unlocking above 22 mph unless the throttle angle is less than 4 degrees or a wide open throttle 2-1 kickdown is made.
The point here is that the temperature of the ATF as well as the engine coolant can have a major effect on how your transmission operates. So too can the inputs from the throttle position sensor, crankshaft position sensor, MAP sensor and other engine sensors.
The 45RFE automatic transmission can be put into an overheat or superheat shift schedule by anything that causes the engine to overheat, by aggressive driving in low gear, by towing a trailer in the "OD" (overdrive" position (using "3" is recommended if frequent gear shifts occur), by driving in heavy stop-and-go city traffic, or by an engine that is idling too fast (stuck AIS motor).
If the indicated engine coolant temperature stays too low too long (which may be due to an open thermostat, low coolant level or faulty coolant sensor), the TCM may go into a cold mode and prevent the torque converter clutch from locking up.
AUTOMATIC TRANSMISSION LIMP-IN MODES
Under certain conditions, which may include the loss of one or more vital inputs to the TCM, the transmission will go into some kind of limp-in or "default" mode. On Chrysler's 45RFE transmission, there are three different default modes:
* Immediate shutdown mode -- The TCM deenergizes the transmission control relay and locks the transmission in 3rd gear when the shift lever is in the Drive position, or 2nd gear if the gear shift is in "2" or "L" position. The causes here is usually an internal transmission solenoid, sensor or pressure switch problem.
* Orderly shutdown mode -- If the problem is less serious, the TCM will remain in whatever gear it is in but default into 3rd gear and stay there as soon as the vehicle slows down (or 2nd gear if it is in "2" or "L").
* Logical shutdown with recovery -- The TCM operates the transmission at a steady preset pressure value and uses 1st and 3rd gears only while in Drive. The transmission will resume normal operation if the problem goes away.
If your transmission goes into a limp-in mode, there is a problem that will have to be diagnosed and fixed. A scan tool will be needed to check for any transmission fault codes.
NOTE: If your Jeep has a data bus communication problem between the TCM and Powertrain Control Module (PCM), you probably won't be able to access any transmission codes until the wiring problem has been found and fixed. Possible causes include an open or short to the ground or battery in the PCI bus circuit, or an internal failure of any module or component that is attached to the bus.
The data bus is monitored any time the ignition key is on. If no messages are received from the PCM for 10 or more seconds, it tells the TCM something is wrong and it sets a data bus code. On the 45RFE automatic transmission, a data bus problem (codes P1716 or P1719) can cause the transmission to shift poorly, delay 3-4 shifts and prevent torque converter lockup.
If the transmission can find an engine speed signal from the crankshaft position sensor or PCM, the transmission can be forced into the limp-in mode. The problem should set a crank sensor circuit fault code and turn on your Check Engine light. Possible causes include an open or short in the crank sensor circuit, a TCM connector problem, an open or short in the crank sensor ground circuit, or an internal fault in the TCM or PCM.
Transmission Repair Options
If your transmission has reached the end of the road, your repair options are to replace it with a new unit or remanufactured unit from Chrysler, to have the transmission rebuilt by a transmission shop, or to have it replaced with a remanufactured or used transmission. A typical transmission rebuild or replacement can run $1500 to $2500 or more depending on where you have the work done. Labor is often the most expensive part of the job.
Replacing your old transmission with a used transmission from a salvage yard is often the least expensive alternative, but it can be risky because a high mileage transmission might not have that many miles left in it. If you do opt for a used transmission, try to find one from a low mileage wrecked vehicle. Most salvage yards will offer some kind of guarantee on a used transmission, possibly 30 days to 90 days or longer. A rebuilt or reman transmission, by comparison, will usually have a much longer warranty from 6 months up to 3 years.
Related Articles:Automatic Transmission Fluid & Filter Service
Automatic Transmission Fluid (types and applications)
Diagnosing Automatic Transmission Complaints
Automatic Transmission Slipping
Automatic Transmission Service
About Electronic Automatic Transmissions
Torque Converters (basic operation & diagnosis)
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