Coolant Temperature Sensor

[KING_3]

Hi Guys,

Just need your technical advice. My ride is a Espero 1.5DOHC and I'm having a problem with my CT sensor. It's newly replaced but the reading is off the true temperature ( it shows higher temperature than actual). So to get rid of this offset, i thought of putting a potentiometer in series with the CTS then i adjust the total resistance based on what i think is the actual temperature.
Here's what i did:
1. i turn on the engine and wait until the thermostat opens fully (this should place the needle on the middle of the temp gauge right?)
2. while i'm doing this, i have a voltmeter connected to the ECU terminals of the CTS.
3. then i turn the potentiometer to adjust the voltage to the voltage set for normal operating temperature which is (from what i read on the manual) 1.5 to 2 volts.

another method that i do (which i'm not sure if correct) is
1. i set the the potentio to its lowest setting (0 resistance, which would also contribute 0 voltage drop)
2 then i wait again for temp gauge to be at middle (at this point, even when the temp gauge is on the first line or lower portion, the fans would turn on, and the compressor would not switch on, meaning the sensor is already registering an overheat)
3. when the temp gauge reaches middle, i then turn the potentio to a higher resistance (making the "measured" temp colder) until the A/C turns on and the idle sets at its lowest - 800rpm- which i think are the conditions for normal temperature operation)

the problem is when another day comes and i have to start the engine again, it would not have the same setting i made. the rpm would be higher when the temp gauge is at the middle.

so do you think there's something wrong with my procedure? can you give advice on an alternative solution?
for now, i refuse taking it to a shop with a scanner to see all data at once since they charge a lot per scan (O.T. is there a work around here, i mean can i have an improvised equipment and software to have the scanner functions )

need your badly as this has been my problem for months now. Many thanks!

regards
King

[Daniel]

Try to determine the origine of defect. Sensor, gauge or ECU supply.

-Check the 5.0 volt from ECU with sensor terminal disconnected. This will eliminate ECU problem.
-Since you have a meter set your poty at 300 ohms and connect it in place of sensor. You should read approx 82°C. This will eliminate gauge problem.
-Your can also compare resistance values of IAT sensor and CTS providing there are both at same temp. They should be similar at +/- 10%. This will eliminate sensor problem.

BTW is your engine running fine with temp needle on top?

Daniel

[KING_3]

Hi Daniel,

Thanks for the reply

Check the 5.0 volt from ECU with sensor terminal disconnected. This will eliminate ECU problem
i'll try this, but do you know where else i can connect the voltmeter probes to measure the ECU reference voltage?

Since you have a meter set your poty at 300 ohms and connect it in place of sensor. You should read approx 82°C. This will eliminate gauge problem.
i have two separate sensors: one for the dash board temp gauge, and the other for the ECU. I just assume that the gauge sensor is saying the correct temp since if i set the fans to be always on, the temp remains in the middle and is being regulated by the thermostat to stay that way. by the way where must the needle stay if the temp is 82degC? also what must be the temp for best engine performance and fuel efficiency?

Your can also compare resistance values of IAT sensor and CTS providing there are both at same temp. They should be similar at +/- 10%. This will eliminate sensor problem.
I'll also try this, during the morning


BTW is your engine running fine with temp needle on top?
I can't say. May i ask what conditions should i check for me to say that the engine is running fine (e.g. correct idle rpm, temp, speed of rpm rise and decline)?


thanks again

[Daniel]

With two separate sensors for gauge and ECU it's quite different.
The CTS/gauge and CTS/ECU are two independent circuits. And since you only untrust your gauge reading there is no reason to add a adjustable resistor on CTS/ECU. Also forget what I've said about setting 300 ohms to read 82°C since gauge and ECU thermistors seems to be of different values and variation laws. And also 5V reference is not concerned here.

I would do the following:
-check the temperature embossed on the thermostat element and verify it's the good one for your engine.
-put a reference thermometer in the surge tank and allow engine coolant temp to climb.
-compare thermometer readings and gauge temp positions while coolant is heating.

FYI the temp gauge on my Nubira (1.6DOHC) is just above the second graduation (out of 5) at ~82°C.

Daniel

[KING_3]

do you mean 82deg C is the optimal engine temp?

what rpm should i expect at this temp?

[Daniel]

Well I think Daewoo's engineers made a good job by setting temp at 82°C for this 1.6DOHC engine. In comparision of other makes it's a "cold" engine but mine has over 100.000km w/o any problem.

Idle speed is 850 rpm when temp is stabilized.

I'll never think that customer's cars are rocket science and I deal with some tolerances on specified values.

Daniel

[KING_3]

Ok, i'll buy a thermometer

by the way, i also observed one thing when i was adjusting the potentio:

while the A/C is on, i turn the potentio to decrease resistance (higher temp "sensed" by the sensor) until the compressor switches off. then i adjust it again to increase resistance (to lower the temp) until the compressor switches back on. then after this the rpm "normalizes" down 850-900. now my question is, does the engine need to reach a certain temp threshold (when coming from a cold start) for it to normalize operation?

[Daniel]

>
>
does the engine need to reach a certain temp threshold (when coming from a cold start) for it to normalize operation?

The ECM reads values from sensors to switch from "open loop" to "closed loop". While engine is cold the correct air/fuel ratio is set by CTS and MAP. When O² sensor give a correct signal it makes the loop closed. This take some time.
That's what I've understood by reading "engine controls", "general description and system operation" section of our service manuals. .
Now I believe that there are as many "thresholds" than ECU p/n.

Daniel

[REY04]

what i know is that the ct sensor is responsible for the cooling system. the engine coolant temp sensor is responsible for the temp gauge.pls correct me if im wrong.


during cold starting the engine will rev up to 1000rpm for a few minutes until it reaches the operating temp of 87-90 deg celcius. it will be steady at 750rpm as its normal idling.