845RE Transmission Oil Heater for Jeep Grand Cherokee

If your Grand Cherokee, Wrangler JL, Gladiator or Durango eight-speed is wet around the side of the transmission case, is losing coolant with no puddle under the radiator, or has pink milky fluid in the pan, the transmission oil heater is one of the first parts to look at. This is part number 68485460AB, the heater used on the 845RE and 850RE eight-speed automatics. Listed fitment is Jeep Grand Cherokee 2016-2022, Jeep Wrangler 2018-2025, Jeep Gladiator 2020-2025 and Dodge Durango 2016-2025.

What the transmission oil heater does in an 845RE or 850RE

The 845RE and 850RE are the Chrysler-built versions of the ZF 8HP. Their shift timing, clutch fill and converter lockup are calibrated around fluid that is warm, so FCA puts a heat exchanger on the side of the case, low and next to the driveshaft. Engine coolant runs through one side of a plate stack and transmission fluid runs through the other. On a cold morning the coolant warms up faster than the gearbox, so the heater pushes heat into the fluid and gets the transmission into its normal operating range sooner. Once everything is hot and the truck is working, the same exchanger lets the cooling system pull heat back out. FCA parts bulletins call it the transmission heater or TMU, and the cooler lines and transfer tubes meet the case through it, so it sits right in the path of both fluids.

Symptoms of a failing transmission oil heater

• Transmission fluid weeping at the side of the case where the heater bolts on, often mistaken for a pan leak because it runs down onto the pan rail
• Coolant level dropping with no external coolant leak, especially after a long drive
• Pink, milky or foamy transmission fluid, which means coolant and ATF are mixing inside the heater
• Harsh, flared or erratic shifts and torque converter shudder after coolant has reached the fluid
• Fluid temperature running higher than normal under load, which shows up as overheat warnings while towing

How to confirm it before you buy

Do not condemn the heater on a wet spot alone. FCA's own parts review for 8-speed leaks tells technicians to identify exactly where the leak starts, and names the transmission case at the oil heater connection and the valve body at the TCM harness connection as places to check before replacing a pan. Clean the area, add dye or use a black light, drive it to temperature and trace the wet trail to its highest point. A separate FCA review notes that a leak at the joint between the heater and the case may only need the transfer tube and O-rings rather than the heater itself. Our ZF 8HP fluid leak guide walks through the order to check things. If you suspect an internal breach, pull a fluid sample from the pan: ATF should be clear red, not pink and cloudy, and the coolant reservoir should not be dropping between drives.

Fitment

This heater is listed for the 845RE and 850RE in 2016-2022 Grand Cherokee, 2018-2025 Wrangler, 2020-2025 Gladiator and 2016-2025 Durango. Chrysler revised these heaters over the years and fitting styles differ between engines and chassis, so compare the casting, the hose and line fittings and the part number on your old unit before ordering. If you are unsure, send us the year and VIN and we will check it. Other parts for this family are on our ZF 8HP parts page.

Install notes

• Work with the engine cold. Drain the coolant below the level of the heater and cap the transmission line connections so the case does not drain onto the floor.
• Replace every O-ring and sealing ring at the coolant ports and the transmission transfer tubes. Check that the quick-connect retaining clips are present; FCA specifically notes they go missing during line removal.
• Pull the mounting bolts down evenly so the flange seats flat against the case, then refill and bleed the cooling system until the heater hoses are hot.
• Top up and set the transmission level with a scan tool at the specified temperature. The 850RE uses Mopar 68218925AB fluid, and FCA states this fluid is not compatible with ATF+4. See our Grand Cherokee 8-speed fluid change guide for the level procedure.

If coolant got into the transmission, the heater is only half the repair. The fluid has to be changed more than once, and a unit that ran on contaminated fluid for long needs the clutches and converter inspected. Plan on a pan, filter and fluid service kit at minimum, and look at an 8HP70 master rebuild kit if the shift quality does not come back after the fluid is clean. Our ZF 8HP fluid type and capacity guide explains which fluid matches which version of the transmission.

Will this oil heater fit my 2019 Wrangler JL with the 8-speed?

The 2018-2025 Wrangler with the 850RE is on the listed fitment. Chrysler revised these heaters and fittings over the years, so compare the part number and fittings on your old unit, or send us the year and VIN to confirm.

How do I know if the heater is leaking internally?

Coolant loss with no outside leak, plus pink or milky transmission fluid, points to an internal breach where coolant and ATF are mixing. Pull a fluid sample from the pan and pressure-test the cooling system.

My transmission is wet near the heater. Do I need the whole heater?

Not always. FCA parts guidance says a leak at the heater-to-case joint may only need the transfer tube and O-rings. Trace the leak with dye or a black light first; replace the heater if the body or the plate stack is leaking.

What fluid do I use after replacing it?

Use the fluid your transmission calls for. FCA specifies Mopar 68218925AB for the 850RE and states it is not compatible with ATF+4. Set the level with a scan tool at the specified temperature.

Does coolant in the fluid ruin the transmission?

It can. Coolant attacks clutch friction material and the converter clutch. Replace the heater, change the fluid more than once, and have the clutches and converter inspected if shifting stays harsh or the converter shudders.

• ZF8HP Fluid Leak: Common Spots and Fixes
• ZF8HP Jeep Grand Cherokee Fluid Change: Type, Capacity and Cost
• ZF8HP Fluid Type, Capacity and Service Interval
• ZF8HP Ram 1500 Fluid Change: Type, Capacity and Cost