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Cat Man
Cat Man, Cat Engine Expert
Category: Medium and Heavy Trucks
Satisfied Customers: 8566
Experience:  I am ASE Master Truck Certified.
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I have an 08 SDP c15 kenworth t800 that the engine fan that

Customer Question

I have an 08 SDP c15 kenworth t800 that the engine fan that won't shut off and fan light is always on. I have replaced all the a/c pressure sensors and fan hub switch on the firewall and did not solve the problem. Don't know where to go from here
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Submitted: 9 months ago.
Category: Medium and Heavy Trucks
Expert:  Cat Man replied 9 months ago.
Thanks for asking your question. I will try and help you out.The ECM will turn on the cooling fan, if any of the following conditions are detected by the Engine ECM:
The engine coolant temperature is greater than 102 °C (216 °F).
There is an active diagnostic code for the coolant temperature sensor.
There is an active diagnostic code for the intake manifold air temperature sensor and boost pressure is greater than 70 kPa (10 psi).
The intake manifold air temperature is greater than 87 °C (189 °F).
The intake manifold air temperature is greater than 72 °C (162 °F) while the boost pressure is greater than 70 kPa (10 psi).
The "Fan with Engine Retarder in High Mode" is programmed to "YES" and the retarder has been on at least two seconds.
The "A/C Switch Fan On-Time" is programmed to a value above 0 and the A/C high pressure switch is Open.
The manual fan override switch is on.
If you do not notice any of these conditions occurring, one of the items above may be faulty and causing the fan to stay on. For example, you could have a faulty AC high pressure switch, manual over ride switch, or the wiring to one of these items could also be faulty. The tough part is if one of these is faulty, there will not be a check engine light or fault code.The best way to determine what is causing the engine fan to stay on is to connect a service tool like Cat ET software and monitor the fan and all of the switches that are connected to it. If one of the above is keeping the fan on, it will be easy to see. For example, if you have a bad AC high pressure switch, it will say that the AC switch is wanting the fan on.If nothing can be seen with the service tool, you most likely have a faulty fan control solenoid, fan hub, or broken fan control wire that runs from the engine ECM. It is on pin 11. Basically, the ECM sends out 12 volts on pin 11 whenever the ECM wants the fan off. When the ECM wants the fan on, it removes the 12 volts from pin 11.If your fan is on and you have NO POWER coming out of pin 11, one of the items I first mentioned is making the ECM want to keep the fan ON.If your fan is on and you have POWER coming out of pin 11, you have a faulty fan control solenoid, fan hub, or broken fan control wire.Please reply back if you need more help or have more questions. I am happy to assist.
Customer: replied 9 months ago.
I have not checked power at pin 11 yet. But on cat et it says a/c pressure high pressure under status group monitoring.
Expert:  Cat Man replied 9 months ago.
Yeah if there is power on pin 11. The ECM does not want the fan on. If there is no power. The ECM wants the fan on.
The AC switch normally says on or off. Not sure why it says monitoring.
Please reply back if you need more help or have more questions. I am happy to assist.
Customer: replied 9 months ago.
Which pin is number 11 can you send me a diagram?
Expert:  Cat Man replied 9 months ago.
I can get you a diagram. It should be the 11th pin counting from the left on the top row. Be back in a few minutes with a diagram.
Expert:  Cat Man replied 9 months ago.

My mistake. On an SDP engine. It uses Pin 21.

Here is the image.

https://www.dropbox.com/s/h7tv4piftow9p40/pin%2021.jpg?dl=0

Let me know if you need more help.

Customer: replied 9 months ago.
Their is no power on pin 21. I'm at a loss on where to go next.
Customer: replied 9 months ago.
Checked all the things you said to check on Cat et also.
Expert:  Cat Man replied 9 months ago.
If there is no power. The ECM is seeing something making it want to turn the fan on.
Look in the status screen and see what the fan output says. Also see what the fan status is.
Customer: replied 9 months ago.
Fan output says 0%. Cooling fan says on. Requested percent fan speed is 0%. Override switch is off. A/c high pressure fan switch says high pressure
Expert:  Cat Man replied 9 months ago.

Thanks for the information.

That sounds like the AC switch may be keeping the fan on. It could be a normally open or normally closed AC pressure switch. I would try disconnecting it and also jumping it with a paper clip or wire and see if the status changes. You could have a bad AC switch or damaged wiring going to it.

Customer: replied 9 months ago.
I tried jumping it last night and didn't turn off the fan
Expert:  Cat Man replied 9 months ago.

Thanks for the information. When you jumped it, did the status of the AC switch change on the ET status screen? IF it did not, you could have a broken wire to the AC switch.

Expert:  Cat Man replied 9 months ago.

One more thing. On these engines, its possible for the Transmission to turn on the fan. I've never run into this but its in the trouble shooting guide.

Customer: replied 9 months ago.
I will dig more into the wiring tomorrow I guess nothing changed when I jumped it. How would the transmission turn the fan on?
Expert:  Cat Man replied 9 months ago.

Ok. Not sure on the trans turning it on I would assume if it had a cooler and was running hot.

Here is the complete test for troubleshooting the cooling fan. I hope this helps.

https://www.dropbox.com/s/dykhq9iarcpr7jm/Cooling%20Fan%20Troubleshooting.PDF?dl=0

Let me know if you need more help. I hope you can find the problem.

Expert:  Cat Man replied 9 months ago.
Hey. I just wanted to see how things were going. Please let me know if you need more help. I am happy to assist. If my information was helpful, please leave a good or higher rating so that I receive credit. Thanks.