I have a 2001 Dodge Ram with a 3.9L engine. The temperature gauge will only read the engine temp at idle. Once going down the road, the needle on the gauge will bottom out completely until the next time the truck is stopped. I've replaced the temp sensor twice-no help.
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diognostics say its a sensor ?????? No diagnostic's ever tell's you it's a sensor . You as a technician would need to prove that by testing electrical circuits , viewing coolant temp sensor . Hooking scan tool and trying to command the gauge with by directional control . The temp gauge not showing any temp. ? No movement ?
Engine Coolant Temperature Gage Inaccurate or InoperativeStep
Schematic Reference: Instrument Cluster Schematics
Did you perform the Instrument Cluster Diagnostic System Check?
Go to Step 2
Go to Diagnostic System Check - Instrument Cluster
Install a scan tool.
Turn the ignition ON, with the engine OFF.
With the scan tool, perform the Lamp and Gauge Check in the IPC Special Functions list.
Does the engine coolant temperature gage move up and down when commanded?
Go to Testing for Intermittent Conditions and Poor Connections in Wiring Systems
Go to Step 3
Replace the IPC. Refer to Instrument Cluster Replacement .
Did you complete the replacement?
Go to Step 4
Operate the system in order to verify the repair.
Did you correct the condition?
Go to Step 2
Would hate for you to replace the coolant temp sensor an find out that's not the problem ! Does the vehicle start an run good ,trans mission shift good ? If so it's not the coolant temp sensor .
Engine Coolant Temperature Gage
The ECM sends coolant temperature as determined by the coolant temperature sensor to the BCM via CAN serial data. The IPC displays the engine coolant temperature with the gage when the BCM sends a class 2 message to the IPC indicating coolant temperature percentage. The engine coolant temperature gage defaults to cold (0°) or below if:
• The ECM detects a malfunction in the engine coolant temperature sensor circuit.
• The IPC detects a loss of class 2 communications with the BCM.
• The BCM detects a loss of CAN communications with the ECM.
First hook up a mechanical pressure gauge to get a true reading--you may just have a bad sender. If pressure is less than 10 to 15 psi at hot dead idle, tell us how many miles on engine--what oil you are using--how often you change it. I have sometimes gone up one notch on thickness of oil on worn engines and run them many more miles.
If your temperature gauge is reading too high, it can only be one of two things: The engine is overheating or the gauge is "lying" to you. I would recommend first taking the temperature of the cylinder head with an infrared thermometer to determine which one it is. If the engine is actually as hot as the guge says it is, then you need to check your cooling system. If the engine is not as hot as the gauge says it is then you need to diagnose and repair your gauge circuit. (most likely an inaccurate temperature sensor)
I would suspect either the temp guage or the coolant temperature sensor being inaccurate. The new thermostat is probably working good, or else you wouldn't have a very warm heater. If you have electric radiator fans, you might check to see if they are running all th time. Electric cooling fans should only come on when the temperature sensor reaches a predetermined coolant temp, or when the a/c or defroster is turned on. If the fan runs all the time, the engine would not reach normal operating temperature.
Check the gauge by holding in the trip odometer reset button and turn key to run,hold button for 10 seconds this will iniate a diagnostics on all the gauges,and lights if gauge doesn't move during this test replace gauge,if it does move then check the wires on the plug at the engine coolant temp sensor (ects) you can check the sensor by using a ohms meter by unpplugging it and check ohm's or resistance when cold and then start engine as the engine warms up the ohm's reading should gradually lower as the sensor is a thermister and as it warms up the resistance will drop.