Short answer: the transmission “coolant” sensor you mean is the transmission fluid temperature (TFT) sensor. It’s a threaded thermistor in the 42RLE case that tells the powertrain computer how hot the transmission fluid is. Replace it by isolating the area, unplugging the wiring connector, removing the old threaded sensor (catch the small fluid leak), install a new sensor with a fresh O‑ring, torque snugly, reconnect, top up/check fluid and scan for codes. Below are the detailed explanations, components, why it matters, step‑by‑step procedure, testing, and what can go wrong — written for a beginner mechanic.

What the sensor is and what each component is
- Temperature sensor (TFT / thermistor)
- A small threaded sender that screws into the transmission case. Inside is a thermistor (a resistor whose resistance changes with temperature). The computer (PCM/TCM) reads that resistance and converts it to temperature.
- Usually a two‑ or three‑pin electrical connector on top. Two‑wire is common; some models use three pins for shared circuits or different sensor designs.
- O‑ring or sealing washer
- A soft rubber or rubber‑coated ring that seals the sensor to the transmission case to prevent fluid leaks. Analogy: it’s the sensor’s “stopper” like a cork in a bottle.
- Electrical connector and wiring harness
- The mating plastic plug with pins that connects the sensor to the vehicle wiring. Includes clips/retainers that hold it in place and then route the wires to the PCM.
- Transmission housing / mounting boss
- The threaded hole in the transmission case where the sensor installs. It’s metal and contains the fluid behind it.
- Engine/PCM/TCM (powertrain control module / transmission control module)
- The computer that reads the sensor signal, uses it to manage shift timing, torque converter lockup, cooling control, and to detect overtemperature or faults.
- Transmission fluid and cooler circuit
- The fluid whose temperature is being measured. The PCM can use that info to control engine cooling fans and shift logic; the cooler (radiator/aux cooler) removes heat.

Why this repair is needed (the theory, in plain terms)
- The thermistor is the sensor that tells the computer how hot the trans fluid is. Computers use that info to:
- Adjust shift pressures and shift timing (cold fluid is thicker; the computer may shift differently when cold).
- Control torque converter lockup (lockup tends to be delayed until fluid is warm for comfort/efficiency).
- Turn on fans or request extra cooling if fluid is hot.
- Set diagnostic limits and trigger warnings/limp mode if overtemp.
- If the TFT sensor is bad, the PCM may get incorrect temperature readings. That causes incorrect shifts, getting stuck in limp mode, failed emissions tests, poor fuel economy, or transmission overheating because the computer doesn’t call for extra cooling.
- Analogy: The PCM is the driver’s brain; the TFT sensor is the thermometer on the dashboard. If the thermometer lies, the brain makes wrong decisions.

Symptoms of a bad TFT sensor
- Hard or abnormal shifting (late/hard shifts, harsh torque converter lock/unlock).
- Transmission stuck in default gear or limp‑in mode.
- Transmission temperature warning lamp or MIL/check engine light.
- Incorrect temperature reading via scanner (e.g., stuck at a single value or no change).
- Fluid leak at the sensor location (perished O‑ring).
- Intermittent faults — connector corrosion or wiring damage can cause intermittent behavior.

Tools and supplies you’ll need
- Basic hand tools: ratchet, extensions, appropriate deep socket or open‑end wrench for the sensor hex (often small, e.g., 19 mm / 3/4 in or sensor‑specific socket). If you don’t know exact size, a small open wrench set and a thin socket set are useful.
- Pick or small screwdriver to release retaining clips.
- Multimeter (for resistance testing).
- OBD‑II scanner (to read codes and current fluid temperature).
- Clean drain pan and rags.
- New replacement TFT sensor (make sure it’s correct for your year/model and includes new O‑ring).
- Transmission fluid for top‑up (same type spec as vehicle).
- Gloves, safety glasses, jack stands or ramps (never rely only on a jack).
- Torque wrench (helpful to avoid over‑torque).

Step‑by‑step replacement procedure (beginner friendly)
1. Safety first
- Park on level ground, set emergency brake, chock wheels.
- Disconnect negative battery terminal (optional but reduces risk of shorts while unplugging connectors).
- Raise and support vehicle safely on jack stands if sensor is under vehicle. Use ramps if preferred. Wear eye protection and gloves.

2. Locate the sensor
- The TFT on the 42RLE is threaded into the transmission case — follow the wiring harness from the transmission or look for a small plastic electrical connector on the side/top of the case near the cooler lines. It’s easier to find with a scan tool showing current transmission temp — pick the harness and follow it to the housing.

3. Prepare for fluid loss
- Place a drain pan under the sensor location. Expect a small drip of transmission fluid when the sensor is removed. You don’t normally need to drain the whole fluid; removing the sensor only lets out a small amount.

4. Disconnect connector and wiring
- Release the connector clip (use a pick if needed), depress tab, and unplug. Inspect pins for corrosion or damage. Free any retained clip that holds the harness to the case.

5. Remove old sensor
- Using the proper socket or wrench on the hex body of the sensor, turn counterclockwise to unscrew it. Catch the fluid and keep rags handy. Remove the sensor and the old O‑ring or washer.

6. Prepare new sensor
- Compare the new sensor to the old one. Lubricate the new O‑ring lightly with clean transmission fluid (this helps prevent pinching and gives a good seal). If your new sensor has no O‑ring installed, fit the supplied O‑ring.

7. Install new sensor
- Thread the sensor in by hand to avoid cross‑threading. Turn until hand‑tight. Then snug with wrench. Tighten to manufacturer spec if known; if not, snug to seat and then about 1/4–1/2 turn with a wrench. Typical small thread sensors torque is low — over‑torque can strip the transmission case.

8. Reconnect wiring
- Plug in the electrical connector until it clicks. Re‑secure any clips.

9. Reconnect battery (if disconnected) and lower vehicle
- Lower the vehicle off stands or ramps.

10. Check and top‑up fluid
- Because you only lost a small amount, top off to the correct level if needed. Some vehicles require fluid to be checked at operating temperature and with engine running (park or neutral). For the 42RLE, consult service procedure: usually check level with engine running at idle and at operating temp, with selector in Park. If you don’t have the exact procedure, add only the small amount you removed and plan to check final level after test drive (avoid overfilling).

11. Clear codes and test
- Use your scanner to clear any stored codes, then start engine and watch for transmission temp reading and for leaks. Do a short test drive to warm the transmission, then re‑check for leaks and sensor reading.

How to test the sensor (bench and in‑vehicle)
- In‑vehicle: use OBD‑II scanner to read transmission fluid temp. Warm the vehicle and observe that the temp rises smoothly. If the scanner shows no change or implausible values, suspect sensor/wiring.
- Resistance test (bench or on car with sensor unplugged):
- Measure resistance across the sensor pins with a multimeter. Put the sensor in hot water and watch resistance change — thermistors change value as temperature changes. Compare to factory resistance vs temp chart if available. Generic behavior: an NTC thermistor decreases resistance as temperature increases.
- Wiring test:
- Check connector voltage/ground per service manual. Wiggle wiring while watching scanner for intermittent changes.

Common things that go wrong and how to avoid them
- Cross‑threading or over‑torqueing the sensor
- Consequence: damaged transmission case threads — expensive repair. Avoid by hand‑threading first; use proper torque or small final wrench turns.
- Damaging or reusing old O‑ring
- Consequence: slow leak and contamination. Always install a new O‑ring and lubricate it with clean fluid before installing.
- Contaminated replacement sensor
- Keep new sensor clean; don’t let dirt fall into the port.
- Broken connector/wiring
- Wiggles, corrosion, or chafing can cause intermittent or failed readings. Inspect pins and harness; repair wiring with solder and heat shrink or proper crimps.
- Not checking fluid level properly
- Overfill or underfill causes poor operation or damage. Follow fill/check procedure for the vehicle; check after warming up.
- Using wrong sensor
- Make sure part number matches year/engine/transmission. Install the correct sensor or the PCM reading will be wrong.
- Ignoring stored codes
- Clear then recheck codes; if codes return, address wiring or sensor.

Possible fault codes and diagnostic hints
- The car may store transmission temperature sensor open/short codes or general transmission fault codes. Use an OBD‑II scanner to read codes and live data. If code shows open/short, start by checking for corrosion or broken wiring before changing the sensor.
- If the scanner shows a temperature that is constant at some value (e.g., always 0°F or a single number), that often indicates an open circuit or failed sensor.

Final checks after repair
- Start engine, let idle and monitor for leaks and scanner temp reading.
- Test drive to operating temp and confirm shifting behavior is normal.
- Recheck fluid level if the correct procedure requires checking hot and running.

Safety and final notes
- Work safely under the vehicle on stands — never under a car supported only by a jack.
- If you’re unsure about fluid level procedure for your exact Jeep model/year, look up the factory service manual or an accurate repair manual for the 42RLE — the fill/check method matters.
- If you encounter stripped threads or a sensor that won’t seat, stop and get advice before forcing anything.

That’s the full picture: what the TFT sensor is, why it matters, the pieces you’ll touch, how to test and replace it, and what commonly goes wrong. Follow the steps carefully, take your time hand‑threading the new sensor, and confirm proper fluid level and no leaks before concluding the job.
rteeqp73

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