Fan Relay Troubleshooting

Sputter

TJ Enthusiast
Supporting Member
Joined
Aug 13, 2020
Messages
326
Location
Jacksonville
Need some help troubleshooting why my fan won't turn on. 2003 with a 2.4L engine. Getting error code P1491. A/C was removed by PO. I run the engine until 210F and the fan does not kick in. Here is what I have done so far.

  1. I have replaced both the coolant temperature sensor and the fan relay.
  2. If I jumper pins 1 & 2 on the relay connector, the fan comes on. So I know the fan is working and the circuit is good downstream from the relay.
  3. If I jumper the coolant temperature sensor, the fan does not come on.
  4. I am getting 12 volts at run position on in 1 of the fan relay connector.
Thoughts?

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Do you have a good ground at pin 3 of the relay?
If that’s ok it’s either the wire between the pcm and pin 4 or the pcm.

does the coolant temp gauge read normally?
 
Ok I cleaned and tightened the grounding point of G191 and got .01 for ground check so I think I am good there.

checking pin 4.

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Do you have a test light? put the test light clamp to 12v and touch the tip of test light to pin 3 of the relay. Checking/cleaning the ground location doesnt tell you if that wire is good back to the relay.
 
I’m getting 5 volts at the engine coolant sensor so when I jumper the coolant sensor connector, I should be expecting 5 volts back to C1 pin 16 - correct? It doesn’t look like that is happening so I am suspecting an issue with the wire from the sensor to c1.
 
Do you have a test light? put the test light clamp to 12v and touch the tip of test light to pin 3 of the relay. Checking/cleaning the ground location doesnt tell you if that wire is good back to the relay.
Yep it is lit.

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No when you unplug the temp sensor the PCM reads 5v. When you jump the two pins together the PCM reads 0v.
 
The temp sensor/wiring would only be of concern to me if your coolant temp gauge isnt working right. Typically cheap scan tools can read engine coolant temp in real time.
 
Im about 99% sure if you jump the 2 pins together at the coolant sensor the PCM would kick the fan on since the PCM would think its whatever the max temp of the sensor is. I havent worked on a 2.4 in a long time. Based on everything else you have checked I am pretty sure the PCM is bad:/
 
I hope you’re wrong about the PCM. I’ve been having the problem since I bought this thing. I recently had the PCM replaced with a refurb but this issue has not gone away. PO did an aftermarket pigtail on the coolant sensor. When I run a continuity test on it back to C1 pin it shows bad. I clipped the pigtail off and ran the test on the wire and it shows good continuity so I’m going to replace the pigtail before sending back the pcm.

Testing Pigtail


Testing Wire


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Touch the 2 wires together at the temp sensor that you cut and see if the fan comes on.
 
It looks like when you probed the connector you are on the black wire but when you are checking the bare wire you are on the tan wire?
 
Do you happen to have a scan tool? To monitor actual coolant temp? Or if you key on with the wires together does the gauge max out?

Was your coolant gauge functional?
 
Coolant gauge was functional but it's not helpful when troubleshooting. I attached a scan tool and watched the temperature go up to 210 without the fan kicking in. With the two wires touching, I get a check gauges error. Now getting a P0117 Coolant sensor - low input voltage error.
 
I’m going to be completely honest and say I don’t know what temp the fan is supposed to come on at. Not sure if that’s published in the FSM.
 
Well I got some other funky things going on with this and a few residual issues. Tomorrow I will be replacing the thermostat, getting the air out of the coolant system, replacing the pigtail and the battery so we'll see where we go from there. I will report back once I have all that completed.
 
I will say this, you jumped the relay and the fans work. That eliminates 50% of the circuit. You replaced the relay. you have a good ground to the relay coil. You have ohmed out the wire between the relay coil and the PCM that supplies 12v to the relay coil. Your coolant sensor seems to be working properly(given that it got up to 210...).

The PCM interprets coolant temp and AC on/off to determine if it wants to turn the fan on.