I already posted in C4 General but probably should have come here first.

Here's the link to that thread: http://forums.corvetteforum.com/c4-g...-troubles.html

And the problem...

Alright so my car is reading a code 15 - coolant sensor circuit low. It's a 1993 LT1

Digital reads 142 degrees (I know, that's an impossible temp to be running at) continuously despite temperature changes reflected by the analogue temp. When temp does change, it shows something impossibly low like 60 degrees and the car loses power. "Service Engine Soon" light also is on.

The primary fan is always running, even with a cold start.

Replaced the temp sensor, the thermostat, and wiring is all good and grounds. Has 5 volts to the temp sensor connector. Problem still prevails.

Been trying various resistors in place of temp sensor. Tried one that made the temp continuously read at 197. Car barely starts when cold with that resistor as the oil temp and engine temp are confusing it too much - But when the car is warm and both temps seem right to the ECM, car runs great and shows no codes. And service engine light is off. With a resistor that would read a low temp (around 60 degrees), and the car warm, it does not start. Tomorrow, we'll be trying that resistor when the car is cold to see how it starts.

My dad and I are pretty stumped. What is wrong with my car?!

 

I think your vehicle has two temperature sensors, one for the instrument panel and the other for the engine/ECM which is the one creating the problem. The ECM needs to know the coolant temperature to provide the correct amount of fuel.

 

Please correct me if I'm wrong but wouldn't my experiments with the resistors show that I've already pinpointed which sensor is involved? Using the resistors I can make the constant 142 of digital read out change to whatever I want - And make the car run perfectly if I use the right resistor for whatever temp the engine happens to be at. So I've replaced that sensor and the problem continues. I'm worried that all roads lead to a problem with the ECM... aside from that, my dad and I are so stumped. Anyone out there ever even had this problem before??... my searches of code 15 don't turn up much aside from what it is.

And why is the primary fan always running despite the engine being cold and no AC on.

 

Sensor in water pump is for ECM operation and digital temp indicating. Other sensor in block is ONLY for analog temp gauge and will affect nothing.

If the resistance of the ECT matches the temperature of the chart as per the FSM, then that would be OK and sensor is good. Then if it is, it does sound like the ECM might be the problem.

 

Thanks for the input guys! Really appreciated!

Okay, I'll be double checking the ECT soon because I don't think it quite matched up.

But if it is the ECM... I have to admit - I find that on the intimidating side. My dad has an idea for a temporary band-aid but that won't even begin to fix any ECM issues.

 

This is also a sign of a bad ECM.

 

15 means that the ECM isn't seeing a voltage drop from the Sensor. Your display, may be a separate issue - particularly if the signal passes through the CCM. Anyway, the Sensor is a thermistor which means it's resistance drops as it warms up. The reference is 5 volts (which you verified) on the Yellow wire from the ECM. The resistance in the sensor makes that voltage drop which the ECM uses to calculate temperature. With no drop; ie, a full 5 volts at the ECM, the temperature is calculated at -38 degrees and you get the 15. With O volts; ie, a complete path to ground, the temperature is calculated at 300 degrees and it spits out a 14. Use a scanner to see what the ECM is seeing and with the Sensor disconnected, that should be -38. Then jumper Yellow to Black and your scan should show 300. If the former works, but the latter doesn't, the ground is open. It's spliced to the TPS and Manifold Air Temp grounds (and if there are no codes for those sensors) troubleshoot the black wire from the connector back to the splice. If it all checks out, you're going to want to check the ECM pinout with the Sensor connected - should be less than 5 volts at any temperature - if not the signal is shorted to some other voltage source in the harness. If that checks out and your scan continues to show and open (-38) or completed circuit (300); it's the ECM.

The main fan is driven whenever there's a current troublecode. With this one, the ECM has no idea what the Coolant Temperature is and the engineers that designed it didn't want to pay for a blown engine on top of a sensor failure especially when the Sensor costs only pennies to replace.