ECT sensor

[smdk2500]

Im having some cold starting issues with my 95 2500 5.7. When the ambient temp is low roughly 20 f or lower it has to run for roughly 3-5 minutes before it will go into high idle. Every other EFI engine Ive had when you start it when its cold it will idle around 1200ish rpm. This one runs about 800 rpm then picks up to around 1200 after 3-5 mon. Also when first starting it before it idles up its idles like a high horse power drag car. It is very rough. Once in high idle it runs smooth. I have checked plugs and all seem ok. Cap and rotor are in good shape and show no signs of carbon build up on the contacts and i don't see any cracks. Wires look ok. It has roughly 4-5000 on tune up.

What all does the ECT sensor control. I know it sends a temp signal to the ecm for timing adjusments, how much fuel to add. Does it also control when the engine goes into and comes out of high idle? Also I cannot read that sensors reading with a scanner. I don't know if its a none readable sensor on obd1 or not. I had to replace it when I did my intake gaskets due to breaking it when i was torqueing the bolt by in down. I had to replace it with a cheap parts store brand and now for the life of me cannot remember if it idled correctly before i replaced it. Any one have a idea if it could cause any of the problems im having?

 

[delta_p]

If you want to check the ECT you can do it without the scanner. The ECT is just a type resistor called a thermistor who's resistance changes with calibrated accuracy as temperature changes. when the temperature is cold the resistance will be high, and when the temperature gets hotter the resistance will go down. The type circuit the ECM uses to read this resistance is called a voltage divider. It is a common technique can be looked up all over google. The net effect is that the ECM voltage reading off the ECT will be higher when the resistance is high when temperature is cold, and will go down as temperature increases because the ECT resistance is dropping.

To check it you can go out with a multimeter and un plug ECT measure the resistance across the two pins when cold. Then idle up the temperature and turn off the car and go unplug it measure the resistance when hot. if it is changing and going down is a good sign it's working.

When plugged in, one of pins is goin to ground, the other is receiving the 5V signal from the ECM. if you could back probe the connector when plugged in and measure the voltage across them you see about what the ECM is seeing in volts. This is going to be towards 5V (something less) when cold, and towards low volts when hot (about 1.5 - 2 volts @ 185F to 203F).

A voltage divider is shown here.

You must be registered for see images attach

If you want to know what the resistance of the ECT should be when cold, you can take the multimeter up to O'reilly's and have the guy let you measure it. Just make sure to note the temperature and check yours cold about the same.

 

[smdk2500]

If you want to check the ECT you can do it without the scanner. The ECT is just a type resistor called a thermistor who's resistance changes with calibrated accuracy as temperature changes. when the temperature is cold the resistance will be high, and when the temperature gets hotter the resistance will go down. The type circuit the ECM uses to read this resistance is called a voltage divider. It is a common technique can be looked up all over google. The net effect is that the ECM voltage reading off the ECT will be higher when the resistance is high when temperature is cold, and will go down as temperature increases because the ECT resistance is dropping.

To check it you can go out with a multimeter and un plug ECT measure the resistance across the two pins when cold. Then idle up the temperature and turn off the car and go unplug it measure the resistance when hot. if it is changing and going down is a good sign it's working.

When plugged in, one of pins is goin to ground, the other is receiving the 5V signal from the ECM. if you could back probe the connector when plugged in and measure the voltage across them you see about what the ECM is seeing in volts. This is going to be towards 5V (something less) when cold, and towards low volts when hot (about 1.5 - 2 volts @ 185F to 203F).

A voltage divider is shown here.

You must be registered for see images attach

If you want to know what the resistance of the ECT should be when cold, you can take the multimeter up to O'reilly's and have the guy let you measure it. Just make sure to note the temperature and check yours cold about the same.

Yes i know how it works and how to check it with a DVOM. It is the same type of sensor that the reefers I work on use. Thank you for for your input but it doesn't completely answer my question. What all functions does this sensor control?

 

[c1500deathtrap]

there are 2 ect's on tbi. a single wire style that feeds the gauge, and a two wire that sends coolant temp in the manifold to the ecm. The 2 wire one has a direct effect on fuel mixture and it is readable data on obd1. unsure if it controls high idle. But to see if the computer is commanding the high idle close the IAC when it is idling high. If there is no change, it is likely a vacuum leak.

 

[c1500deathtrap]

If you want to check the ECT you can do it without the scanner. The ECT is just a type resistor called a thermistor who's resistance changes with calibrated accuracy as temperature changes. when the temperature is cold the resistance will be high, and when the temperature gets hotter the resistance will go down. The type circuit the ECM uses to read this resistance is called a voltage divider. It is a common technique can be looked up all over google. The net effect is that the ECM voltage reading off the ECT will be higher when the resistance is high when temperature is cold, and will go down as temperature increases because the ECT resistance is dropping.

To check it you can go out with a multimeter and un plug ECT measure the resistance across the two pins when cold. Then idle up the temperature and turn off the car and go unplug it measure the resistance when hot. if it is changing and going down is a good sign it's working.

When plugged in, one of pins is goin to ground, the other is receiving the 5V signal from the ECM. if you could back probe the connector when plugged in and measure the voltage across them you see about what the ECM is seeing in volts. This is going to be towards 5V (something less) when cold, and towards low volts when hot (about 1.5 - 2 volts @ 185F to 203F).

A voltage divider is shown here.

You must be registered for see images attach

If you want to know what the resistance of the ECT should be when cold, you can take the multimeter up to O'reilly's and have the guy let you measure it. Just make sure to note the temperature and check yours cold about the same.

Or you can just unplug it and read ohms across the 2 pins.

You must be registered for see images attach

 

[delta_p]

Yes i know how it works and how to check it with a DVOM. It is the same type of sensor that the reefers I work on use. Thank you for for your input but it doesn't completely answer my question. What all functions does this sensor control?

Ok, I apologize didn't answer your Q. Yes, as C1500deathtrap mentioned the ECT has a direct affect on fuel delivery at idle and else. when your start up the engine the computer is using the ECT, the MAP, the distributor module RPM, and the reference O2 voltage to run in open loop. Since the O2 haven't started varying yet, that reference is what is sensed and the computer use it also for the base fuel setting with those other sensors. initially, even just before you crank up, the computer looks at the ECT, the throttle position, the map, and the ignition signal and decides the air/fuel just to get it started. To go into closed loop, the computer wants to see ECT above a certain temperature, varying voltage on the O2, and a specific amount of time passed after starting. It can go in and out of loop status too. But yeah, the ECT is a pretty important sensor for the truck to run well from idle to warm.

BTW specific details of the operations can be read in the '95 TBI C/K driveability manual. I believe Hatzie put it here for us https://www.gmt400.com/threads/88-95-service-manuals.43575/

 

[smdk2500]

Ok, I apologize didn't answer your Q. Yes, as C1500deathtrap mentioned the ECT has a direct affect on fuel delivery at idle and else. when your start up the engine the computer is using the ECT, the MAP, the distributor module RPM, and the reference O2 voltage to run in open loop. Since the O2 haven't started varying yet, that reference is what is sensed and the computer use it also for the base fuel setting with those other sensors. initially, even just before you crank up, the computer looks at the ECT, the throttle position, the map, and the ignition signal and decides the air/fuel just to get it started. To go into closed loop, the computer wants to see ECT above a certain temperature, varying voltage on the O2, and a specific amount of time passed after starting. It can go in and out of loop status too. But yeah, the ECT is a pretty important sensor for the truck to run well from idle to warm.

BTW specific details of the operations can be read in the '95 TBI C/K driveability manual. I believe Hatzie put it here for us https://www.gmt400.com/threads/88-95-service-manuals.43575/

Sorry if i came across sounding like a dick that wasn't my intention. I just like knowing what how things work and what they do with the information that they come up with. I wonder why i cannot see the ect on my scanner. I can see every other sensor with it. I will check for a vacuum leak. I wish i could remember if it did this before i did intake gaskets.

 

[Schurkey]

I wonder why i cannot see the ect on my scanner. I can see every other sensor with it.

Faulty sensor
Faulty wire harness
Faulty ECM
Faulty scan tool
Scan tool not configured properly to match the vehicle
Info is displayed as "Coolant Temperature Sensor" (CTS) not "Engine Coolant Temperature" (ECT)

 

[delta_p]

No problem, understand.

it is strange the scanner won't display the temperature. At least there aren't codes coming up either.

To see if the PCM is good on this sensor, run the engine for 8 minutes and unplug the temp sensor and make sure a DTC 15 throws (temp too low). That manual says it should it should affect the idle too because the PCM is going to assume 64 degF. If you can short the signal to ground on a cold engine for 2 seconds or longer, it should throw a DTC 14 (temp too high). After that if the sensor itself follows the temp/resistance table above and doesn't skew at all along the way, then it probably is a vacuum leak somewhere. if you do unplug it use the DVOM across the harness pin to make sure the computer is providing ~5V.

Maybe another vehicle can be checked with scanner to see if it reads the ECT?

 

[c1500deathtrap]

some scanners will falsely display ect as intake air temperture.