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Old 05-25-2021, 09:31 AM   #1
lrbraner
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2002 L-Series 3.0L Sedan
2003 L-Series 3.0L Wagon
Default Temperature Gauge Issue.

2008 2.4 . 160K miles. Temperature gauge will start to climb normally. When it reaches around 140 F the gauge drops to zero. Will not show any temp thereafter. Even after letting the car sit and then restarting it. Disconnecting/reconnecting the battery results in the gauge initially working then dropping to zero. Changed the ECT, still does the same thing.

Any ideas ?
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Old 05-25-2021, 10:40 AM   #2
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2002 L-Series 3.0L Sedan
2003 L-Series 3.0L Wagon
Default Re: Temperature Gauge Issue.

I should also note that when the gauge stops reading. The fans turn on and the A/C wont run
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Old 05-25-2021, 11:10 AM   #3
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2003 L-Series 3.0L Sedan
Default Re: Temperature Gauge Issue.

Plug in a reader to see coolant temps. Since the OBD II gets info from the bcm, the ecm sends coolant signals to the bcm. The bcm drives the temperature gauge and sends the same info to the reader. This may help diagnose which way to go.

In (presumed) electrical logic, the coolant sensor connects to the ecm directly and sends temperature signals to the bcm. The bcm drives all the gauges, interior and exterior lights, processes security, and is the gateway for OBD II signals to a reader or scantool.
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Old 05-25-2021, 01:09 PM   #4
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Default Re: Temperature Gauge Issue.

Should have noted that we did that. The reader shows coolant temp even when the gauge doesnt.
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Old 05-25-2021, 02:20 PM   #5
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Default Re: Temperature Gauge Issue.

Without service manual info, I'm guessing based on presumed info of electrical wiring. With a reader always displaying coolant temps while the gauge dies, the bcm, I believe, sends signals to the gauge to move the needle. Temperature signals should flow from ecm to bcm to instrument panel gauge but as serial data in a pipeline along with other signals. If I'm not mistaken, a chip in the instrument panel cluster (IPC) receives bcm signals then disseminates them to each gauge and indicator. If the chip fails, the ipc is replaced. Swapping ipc is one way to see if the ipc has the failure. All signals are sent as digital data in a serial pipeline where the ipc chip separates then sends data to each gauge and indicator. My guess is partial failure of this ipc chip. This presumes all indicators and gauges are operating normally with the exception of the intermittent temperature gauge. There's a possibility of the temperature gauge circuit being intermittent. Isolating ipc issues from bcm issues, I believe, is on an electronic level of serial communications thru chip circuitry.
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Old 05-25-2021, 04:42 PM   #6
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Default Re: Temperature Gauge Issue.

Thanks for your insight. I will report back
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Old 05-25-2021, 05:49 PM   #7
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Default Re: Temperature Gauge Issue.

The fail safe mode, to protect the engine, when the ECM deems the temperature measurement to be unreliable is to run the fans and stop the AC from operating. This is sometimes caused by faulty sensor, sensor wiring or thermostat. Someone in a thread here found that the rubber gasket was bad and was allowing coolant to bypass the thermostat even though the thermostat was functional. This caused the engine to never reach operating temperature.
There would normally be a code set for this condition, usually P0128, you didn't mention if you read any codes. You also didn't say what the temperature values that you read were. You should get something around 200 degrees f when the engine is warmed up and thermostat and sensor are working properly. The thermostat being mechanical is independent of everything else. If you measure a lower temp on a reader, suspect the thermostat. The temp gauge in the IPC likely doesn't even start to move until the temp is over 120 maybe 140 degrees.
https://www.700r4transmissionhq.com/p0128-saturn-aura/
The IPC is not capable of controlling the fans or AC. As fdryer says, it translates the digital temperature reading to the analog gauge display. If you were to swap out the IPC with another...the odometer will no longer count up and it cannot be programmed, even by a dealer. This I know from my own experience.
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