Jeep WK / WH Hydraulic Cooling Fan OM642

Mopar MS-10838 spec power steering fluid available from Chrysler Jeep dealers.

 Chrysler dealers may use the wrong fluid when servicing the vehicle, or selling you fluid. It is important you double check this. I have had a dealer tell me they had to order it by the case when I said I needed two quarts. They did not want to order it to stock. I went somewhere else. Dodge Vipers used the same system for a couple years, and maybe some of the Ram trucks. It is not a widely used or understood system. If you have a hydraulic fan insist on getting the correct Mopar MS-10838 fluid, and if your fluid is red, flush and change for correct if you have hydraulic fan.

Some of the Jeep WK / WH models have hydraulic cooling fans similar to Ford Lincoln and a few other makes and models.

On the Jeep the hydraulically driven fan is powered by the power steering pump and shares the same fluid as the power steering system. Mopar spec: MS-10838 fluid is added through the power steering fluid reservoir, and drains when your badly designed Jeep return fluid lines pop off and drain your fluid on the ground.

Only use MS-10838 spec fluid or it will damage hydraulic fan. ATF+4 will not work. It may work for a short term fix, but runs risk of damaging fan unit.

This is a very complex system controlled by the computer, and a number of different sensors including air con, transmission temp, engine temp et al. More about this can be seen in these threads.

Note if you are having issues and unsure if your solenoid is bad, you can remove the thermostat wire (See image labelled below) mid center right of the motor, just under the fuel rial, and the fan should ramp up to a fairly high speed almost immediately.

Here is a layout and sensor labelling of an OM642 CRD in a Chrysler 300.

Above photo can be found at:

Here is a video I shot of how the fan reacts when disconnecting the wire and reattaching.

This is a fairly complete thread about troubleshooting this system. It includes testing pulse width modulation signal strength with a home-made oscilloscope.

This thread is parallel, and also has a lot of good information.

Above: is the three wire hydraulic fan solenoid removed. I have one for sale or loan if anybody needs one for replacement, or testing.

6 Replies to “Jeep WK / WH Hydraulic Cooling Fan OM642”

  1. Have you ever had to disconnect the high pressure line from the HEC (Fan)? Is there an O-ring there that can be replaced? If so, would you have a part number or an idea where to get a replacement?

    1. Hi. HEC? Hydraulic Engine Cooling Fan? If you are referring to the hydraulic cooling fan yes I have discoed it numerous times and swapped the whole unit back and forth a fe times. Sadly I have thrown away the old one, or else I could see exactly what you referring to. I will take a look but if it is an O ring, than any auto parts store should have an O ring set you can just match up and buy one or a few.

      1. Thanks for your reply.
        Turns out I had to buy the new high-pressure hose, a new O-ring by itself did not fix the leak. Lost the power steering pump in the process cause I took too long to notice the problem.
        Darn expensive hobby these CRDs…

        1. Sorry to hear that. Yes they require quick troubleshooting and accurate repairs. How much you paid for pump? Link to where you bought it? Did you install pump yourself? Good news is you shouldnt have to change pump again. Did you use proper MS-10838 Mopar ps fluid?

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