How low should the water pump coolant sensor read on a cold engine for a 95 lt1.
Bought a sensor from O'reilly auto parts and it never reads below 100 degrees.

 

Are you talking about for the dash cluster?

I have an 85 and anything below 100 just reads "LO", so I assume it's probably normal, but then again, I don't have an LT1 to compare to.

 

The one that sends a digital signal to the computer and digital dash unit.

 

Should read actual temp ...(I think)...... as I recall (I'm old) my temp is always below 100 on the digital read-out when starting out.
There are two coolant sensors..... digital (water pump), analog (pass. side head). They use different connectors, so there should not be a mix up.

This may be more than you care to know, but (FSM):
"The ECM supplies a 5 volt signal to the CTS through a resistor in the ECM and measures the voltage. The voltage will be high when the engine is cold, and low when the engine is hot. By measuring the voltage, the ECM knows the engine coolant temp. Engine coolant temps affects most systems the ECM controls.

A failure in the CTS circuit should set a code 14 or 15."

 

Had 5 volts at the connection for the cts and had a 15 in history.

 

The possible causes of a code 15 are bad sensor, dirty sensor connector contacts or problem in the wiring between the sensor and the ECM.

Measure the resistance of the sensor. This is the CTS chart out of the FSM:

 

http://www.chevythunder.com/cts_diagnostics.htm

http://www.chevythunder.com/cts_diagnostic_pg2.htm

 

Sensor checked ok according to the chart.
I have 5 volts to the sensor and voltage did not drop when I moved the wires around.
Cleaned the contacts on the sensor and the plug and put some dielectric grease on the plug and the sensor reading on the dash is close to ambient temperature now. I traced the wires back to the passenger side of the engine but I don't know what it's pluged into or how to get it off.

 

Not true on his 95, I have seen 30's on my 94's digital readout.

 

Did the new sensor change anything from the original indication (problem).

What does the digital temperature read after the engine is at normal operating temperature. That should be from 20 or 30 minutes of driving?

When starting out with a cold car (morning), the digital reading for engine temp should read close to the same reading as the temperature readout on your HVAC control head. My car is garaged so things are pretty much equalized but read closer than 5* in the morning.

In the un-plugged open circuit condition, you should have 5 volts on one pin (yellow wire) and close to '0' ohms on the other pin (Black wire). If it is a high reading, it will cause the digital temperature to read to low. However your problem is the other side of the coin where the PCM thinks there is a lower resistance which would indicate a higher temperature.

When connected there should be a voltage drop on the yellow wire (pin B). The FSM says at around 200*, the voltage across the sensor is 1.50 - 2.0 volts. Sensor resistance at that point should be about 240 ohms.

No way to calculate the voltage drop at lower temps across the sensor since there is an additional (pull up) resistor (in PCM) in the circuit of an unknown factor so all values are not known.

However, at 70* F with about 3500 ohms for the sensor, the voltage drop across it should be higher.
*** I measured the voltage at 65* F and the yellow wire indicated 2.40 volts. At 100*F, I measured 1.43 volts.***

Additional info from my 94 FSM. The pins on the PCM are B9 and B16 (ground) which connects back to the sensor. I guess there is always the possibility the PCM is the problem but I would not run with that as of yet.