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Hammer Time
Hammer Time, L1 Master Technician
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Experience:  ASE Mastertech with Adv level Engine performance cert.
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99 Jeep Grand Cherokee 4.0 The cooling fan does not come ...

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99 Jeep Grand Cherokee 4.0
The cooling fan does not come on.How do I tell if its the relay or the computer?
Useing a scan tool the cooling temp sensor is reading 240 degrees. The fan has
been tested and it is good. Can I activate the relay to test it manually ?

Alex hagar [email protected]

Optional Information:
1999 Jeep Grand Cherokee 4.0

Already Tried:
Test fan -tok
locate lead from computer
check temp with scan tool -240
degrees,fuse ok and voltage present at fuse under hood
and ground to fan ok

You can pull out the relay and check the outside of it for a diagram for which pins are the control side and which is the load side. Most relays have a pictures on the side or an indicator showing the way the pins are. If you know which pin is which you can check for voltage/ground coming in to the relay from the ECU. You need to know if it's a ground side switching circuit or a positive side switching circuit. If you're having problems finding which pin is which inspect the wired coming in and out of the back of the fuse cavity. Bigger wires are the load side and smaller wires are the control side. This is a "general" type description since I'm not a jeep tech but i see you know your way around a meter so I posted.

If this helps please accept my answer, thanks

The cooling fan relay in this vehicle doesn't even have any pins. It uses a "solid state" relay with transisters and they do have a lot of problems with them.

You need to check a few things first.

Make sure the fan motor itself is good by jumping power to the Dk Grn wire and jump ground to the Bk/pk wire. The relay is responsible for applying both.

Next check the #10 fuse which is 40 A

If all this checks out, your only leaving the wiring, the relay or the PCM.

The relay is the most likely cause of that problem if the motor is good.

The next problem is finding the relay. they hide it well. here's the location. It's hidden behind the right headlamp.

The Pulse Width Modulated (PWM) cooling fan relay is located below the right headlamp behind the bumper fascia.


  1. Remove front bumper and grill assembly.
  2. Remove 1 support bolt near front of reservoir.
  3. Remove 2 reservoir mounting bolts.

  4. Remove reservoir from vehicle to gain access to vacuum hose. Disconnect vacuum hose from reservoir fitting at rear of reservoir.

  1. Disconnect electrical connector at relay.
  2. Remove 2 relay mounting bolts and remove relay.


  1. Position relay to body and install 2 bolts. Tighten bolts to 3 N.m (25 in. lbs.) torque.
  2. Connect electrical connector to relay.
  3. Connect vacuum hose to reservoir.
  4. Install reservoir and tighten 2 bolts to 3 N.m (25 in. lbs.) torque.
  5. Install front bumper and grill assembly.

I figured it would be something like this, that's why i stated my answer was general in nature. Sorry if I confused you.

Customer: replied 11 years ago.
99 Jeep Grand Cherokee 4.0 What type of signal is coming from the control module to activate the cooling fan relay? voltage +( how much) ,ground or
digital ?
It's Pulse Width Modulation. It's not something that you can measure. That's why I gave you the instructions that I did.
Hammer Time, L1 Master Technician
Category: Car
Satisfied Customers: 4942
Experience: ASE Mastertech with Adv level Engine performance cert.
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