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Old 06-19-2021, 04:03 AM   #1
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Temp gauge fluctuates

I have a newly purchased 2009 Tiffin Phaeton with a 360 Cummins and 70,000 miles.
When driving the temp gauge slowly but constantly fluctuates between straight up and increasing to 3/4 on the dash gauge.
I took it to a qualified Freightliner/Cummins repair shop,they confirmed the temp fluctuates between 180-210, they changed the thermostat, costing $2100 and made no improvement.
Has anyone had this problem and what was the fix?
They thought the next step would be checking coolant pressure which might indicate a bad water pump or radiator
The scary part is the cost of finding the problem,not the actual cost of repairing!
Any info would be greatly appreciated.
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Old 06-19-2021, 08:29 AM   #2
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Might check to see if radiator is dirty and needs cleaned.
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Old 06-19-2021, 11:27 AM   #3
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180 -210 degrees sounds normal. We have a 13 Phaeton and it also fluctuates between those temps. Our temp gauge goes from the 1/4 mark to 1/2 way. I believe the cooling fan kicks up to a higher speed around 210 degrees which provides additional cooling and the temp drops. The fan then goes back to the lower speed and the process repeats. I did clean the radiator last year and it seems to not fluctuate as frequently but still does vary while driving.
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Old 06-19-2021, 11:41 AM   #4
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I agree with Don as being normal. Our temps will run in the low to mid 190s under normal flat driving conditions, but when climbing hills they will rise to 210 monitored with a ScangaugeD. At that point the fan clutch goes into high speed mode and the temps then drop back to the upper 180s. Your best bet is to get a ScangaugeD or other monitor where you can see exact temps and not depend on the dash gauge. As mentioned, you do need to check you radiator to see if it needs cleaning as that should be a regular maintenance check.
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Old 06-19-2021, 07:29 PM   #5
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Thanks, good to hear someone else has the same things going on, I will give the radiator a thorough cleaning.
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Old 06-19-2021, 07:33 PM   #6
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Thanks, good to hear someone else has the same things going on, I will give the radiator a thorough cleaning.

Just a caution, don't clean your radiator with a pressure washer, it may bend all of the fins over and you'll have to buy a new one if that happens. Use a garden hose but even then be careful the fins are pretty delicate. Good luck.
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Old 06-28-2021, 03:11 AM   #7
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After talking to Cummins, it is staying in the normal range so as long as no codes or alerts pop up I will run it as is( will clean the radiator thoroughly first.) Thanks for the replies!
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Old 06-28-2021, 12:22 PM   #8
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210 is when the fan kicks on and 180 is when it cycles off.

Wow, they charged you $2k to change a $100 part and couple gallons of coolant.
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Old 06-28-2021, 12:40 PM   #9
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If you have a clutched cooling fan OR a hydraulic fan with controller, you WANT temperature to vary some between thermostatic control and fan to high speed.


Were I designing it, the clutch engaged/fan to high would be 10 degrees F above thermostatic control.



That would be a good compromise between using unnecessary HP and fuel to run the fan more than needed AND at the other end, insuring that coolant temperature to rise so much that you would have little reserve if on a long grade.
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Old 06-28-2021, 03:43 PM   #10
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My 96 RAM 3500 did the same thing. It has 2 thermostats, One opens and one closes, Normal operation. There was a TSB to change the stats. I did that and it stopped fluctuating.
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