P0117. "Low Engine coolant Temp Sensor Circuit."

[shihabafi]

So, I've got my 850 that's throwing a P0117, and as with everything else, it's ripping me apart on the inside. The car hesitates for a few seconds when I try to start it, and the fan runs from the moment I put the key in, until a good 5 minutes after the car is off.

We got a "new" sensor from the junkyard, but I think it's also bad. I'm getting 5v from the wire going into the sensor with the car on II

Any ideas?

[misha]

You shoud check for resistance not volts.
Resistance is temperature dependant and at operating temperature should be around 300ohms.

[RickHaleParker]

5V is the feed voltage, the voltage is supplied by the Engine Control Module (ECM). The same voltage source supplies the throttle position sensor. If you want to do this by voltage, you should be looking at the voltage drop across the sensor.

Leave the sensor disconnected, see if the symptoms change.
Pick a resistance from the table, place the resistance across the harness (disconnected from the sensor), see if the symptoms change.

Test the sensor:
1. Stick it in a glass of ice drip water, the resistance should be ~7300Ω
2. Bring sensor up to room temperature 68°F, the resistance should be ~2800Ω
3. With the engine at operating temperature, the resistance should be ~300Ω
4. Stick the sensor in boiling water,the resistance should be ~150Ω

Or you could put a the sensor in a pan of ice cold water then monitor the water temperature and resistance as you heat the water.

Engine coolant temperature sensor resistance table.
32°F (0°C) .............................................. 7300Ω
68°F (20°C) ............................................. 2800Ω
176°F (80°C) ............................................. 300Ω
212°F (100°C) ............................................ 150Ω

Possible sources for EFI-123 (P0117)

• Signal too high:
  short-circuit to supply voltage in signal cable
  open-circuit in signal cable or ground lead
  contact resistance in terminals
  defective engine coolant temperature (ECT) sensor.

Signal too low:

• short-circuit to ground in signal cable
  defective engine coolant temperature (ECT) sensor.

Faulty signal:

• defective engine coolant temperature (ECT) sensor
  defective coolant thermostat
  contact resistance in terminals
  short-circuit between signal cable and another cable.

[mrbrian200]

Careful Rick,
Voltage drop across the sensor: you would need to know the internal resistance of the ECU between the signal input to ground to be able to calculate what the voltage drop across the sensor should be at a given temperature to make a meaningful assessment. If there are any diodes/transistors in the internal ECU circuit path (likely) it might get, well, complicated. Without an internal schematic for the ECU, or a table of voltage drops at various temperatures from a known good sensor its a moot issue.

Probing the isolated/disconnected sensor with an ohmmeter at various temperatures is the way to do it. That table of values is known, as you listed them.

[RickHaleParker]

The statement is carefully worded. Read what is there and nothing more.

One could calculate the internal resistance of the ECU, using 1. measured sensor resistance 2. voltage drop across the sensor 3. the feed voltage. From them three measurements, the remaining resistance in the loop can be calculated.

Measuring the voltage drop across the sensor without knowing the internal resistance of the ECU is not necessarily useless.
Assuming shorts or opens in the harness have been eliminated ...
If the voltage drop across the sensor = the feed voltage, the sensor is open.
If the voltage drop across the sensor = Zero, the sensor is shorted.
If the voltage drop across the sensor does not change with temperature, the sensor is completely dysfunctional.