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I have a 2006 Freightliner with cat c7 engine. just start…

I have a 2006 Freightliner...
I have a 2006 Freightliner with cat c7 engine. just start experiencing an unusual behavior. In mornings when ignition is switched on all clocks go through the usual pre-check movements. When engine is started it revs up and down on its own and temp guage goes dead.
Up and down rev goes away when engine warms up but temp guage does not move. Also radiator fan will not come in.
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Answered in 27 minutes by:
8/17/2017
Pete
Pete, Automotive Mechanic
Category: Medium and Heavy Trucks
Satisfied Customers: 32,629
Experience: Performed various automotive repairs on several light duty and medium duty vehicles over 15 years of service.
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Hi my name is ***** ***** I will be happy to assist with this today.In some cases I may need more info to assist better.I apologize for any delay.

From what you have described here with the engine revving up and down on its own and the temp gauge not registering on start up or when running this sounds like a fault with the engine coolant temp sensor itself.If the temp sensor is not sending a signal or is sending a false signal the computer will try and compensate or will try and run with the way the gauge reads and can cause the engine to rev up and down like this.

I would have the engine coolant temp sensor tested to see if it is faulty and if found to be at fault it will need replaced to resolve this issue here.

If more help is needed, use the reply tab to continue our conversation. If no further assistance is needed, kindly rate my service,your 5 star rating is appreciated ( should be a link to click on here or above).You can rate at any time and we can continue to work on your question as this will not close out your question.Keep in mind in some cases it can be difficult to fully diagnose or help repair your vehicle without seeing it and I work on delivering the best possible answer from what I see to work with.

Thanks Pete

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Customer reply replied 10 months ago
Thanks...will check to see if that's the issue.

Your welcome

Let me know if you need anything else and feel free to rate my help here or above ( should be a link to click on ) as this is how I receive credit for my time,this can be done at any time and you can still come back here and reply if needed.Your 5 star rating is appreciated

Thanks Pete

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Customer reply replied 10 months ago
Pete...checked engine coolant temp sensor connected at coolant reservoir and its ok. Also switched sensor from and similar truck and the problem remains. Would appreciate any other suggestions to try and remedy this issue.Thanks.

O.k.,if the sensor is o.k. and the temp is reading cold all of the time when running and with this running issue and the cooling fan not coming on this indicates that either the wiring leading from this sensor the computer has damage at some point and the wiring needs traced and inspected for any issues or there is a fault with the engine computer itself with it needing replaced.I have seen both cause this type of fault here.

The wiring needs checked first for any issues and if found to be o.k. it can be the engine computer that is not seeing the coolant temp signal properly causing this fault needing replaced.

Let me know if you need anything else and feel free to rate my help here or above ( should be a link to click on ) as this is how I receive credit for my time,this can be done at any time and you can still come back here and reply if needed.Your 5 star rating is appreciated

Thanks Pete

Pete
Pete, Automotive Mechanic
Category: Medium and Heavy Trucks
Satisfied Customers: 32,629
Experience: Performed various automotive repairs on several light duty and medium duty vehicles over 15 years of service.
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Customer reply replied 10 months ago
Hi Pete....sorry for the delay in updating you. I'm happy to report that the problem has been resolved and that your first suggestion of the coolant temp sensor was in fact the problem. We've replaced sensor and all is well with our unit again.
If it was possible I would give you a 6 star rating ******. Thanks

That is great to hear.If there is anything else I can help with here on this let me know.

Thanks Pete

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