This is rather mind boggling....

[9pec]

Why don't you check the thermostat? It could be stucked opened when it's cold, and stucked closed when it's hot? Radiator clogged? Give the whole system a good flush? It may not be the sensor or fan afterall?

[1 Lucky Texan]

Why don't you check the thermostat? It could be stucked opened when it's cold, and stucked closed when it's hot? Radiator clogged? Give the whole system a good flush? It may not be the sensor or fan afterall?

 

At this point, that's a reasonable suggestion. I'd do that before assuming the new Temp Sensor was also bad.

Anyone have any idea of voltage readings he could take at the temp sensor? maybe a voltmeter could at least confirm the sensor is good. I guess the 21 code comes back after the ECU is cleared? If so, it may be time to suspect the ECU itself in addition to a wiring issue.

 

weird

 

 

Carl

[johnceggleston]

i'm not real sure how you would go about doing it , but if you could trick the computer into thinking the sensor was ok by wiring in the correct size resistor, then you could systematicly eliminate each component in question. you'd need to know the correct resistance of each sensor, it may be that many are the same. resistors are cheap.

 

At this point, that's a reasonable suggestion. I'd do that before assuming the new Temp Sensor was also bad.

Anyone have any idea of voltage readings he could take at the temp sensor? maybe a voltmeter could at least confirm the sensor is good. I guess the 21 code comes back after the ECU is cleared? If so, it may be time to suspect the ECU itself in addition to a wiring issue.

 

weird

 

 

Carl

[1 Lucky Texan]

this thread (for a '97 I think) has some ohmmeter tests.

 

http://www.ultimatesubaru.org/forum/showthread.php?t=40929&highlight=temp+sensor+voltage

 

there are quite few threads on temp sensor , engine temp sensor and coolant temp sensor. You may need to read a lot to find more troubleshooting approaches.

 

Carl

[BigMattyD]

Note to all:

 

To diagnose trouble codes, I have been using the online service found at alldatadiy.com. You have to pay for a yearly subscription, but the info is specific to your car, and they have excellent troubleshooting charts for the trouble codes. They also have correct wiring diagrams, not the generic ones in the Haynes manual. They even have diagrams of connector locations, pinouts, and fuse/relay locations, sensor locations...

 

I have used it to diagnose and repair problems on my Buick Skylark and my Dodge Caravan, and it has proven indispensable. I believe alldata is actually providing scans of the factory service manual for the vehicle.

 

Remember that Haynes, Chilton's, etc. try to provide good information, but for a range of model years, and engine types, option packages, etc. That said, the first thing I buy when I get a new car is a Haynes manual.

 

I have never used alldata for my Subaru, because I haven't needed it yet...

 

Matt D

[bart2546]

Ok, went and replaced my thermostat and cleaned out my radiator and all that good stuff, no luck yet again, really frustrated at this point. Also with car running, I pulled the relays (from front to back) for the a/c one at a time, when I removed relay number 1 or 3, the fans seemed to kicked down a notch for speed but still continued, when I removed 1-4 all at once the fans did stop completely but the CEL remained on. I'm at a loss for words or where to turn...

 

Regards,

Brandon Bartolomucci

[nipper]

Ok, went and replaced my thermostat and cleaned out my radiator and all that good stuff, no luck yet again, really frustrated at this point. Also with car running, I pulled the relays (from front to back) for the a/c one at a time, when I removed relay number 1 or 3, the fans seemed to kicked down a notch for speed but still continued, when I removed 1-4 all at once the fans did stop completely but the CEL remained on. I'm at a loss for words or where to turn...

 

Regards,

Brandon Bartolomucci

 

which temperiture sensor did you replace?

 

nipper

[bart2546]

The one on the upper radiator hose connection to the block. The one with two leads off of it, it's NOT the one that controls the actual temperature gauge, the other one.

 

Regards,

Brandon Bartolomucci

[nipper]

The one on the upper radiator hose connection to the block. The one with two leads off of it, it's NOT the one that controls the actual temperature gauge, the other one.

 

Regards,

Brandon Bartolomucci

 

Just confirming as that is a common mistake. Like i said give me your email, ill send you a scan of the wiring diagram (i have a 1200 noon flight tomorrow so tonight would be better) and seeif you can find out whats happening.

Did you ppull the main relay under the dash?

[bart2546]

Didn't try the one under dash yet, I try to locate it tonight when I get home from work.

 

Brandon

[bart2546]

Pulled fan relay under dash, it kills one of the fans, the other one keeps going. I just don't know, so far I have not been able to locate a decent car to buy, nor have I been able to get financed, and mine isn't working, so generally I'm about to loose my job if this keeps up.

 

Brandon

[1 Lucky Texan]

Pulled fan relay under dash, it kills one of the fans, the other one keeps going. I just don't know, so far I have not been able to locate a decent car to buy, nor have I been able to get financed, and mine isn't working, so generally I'm about to loose my job if this keeps up.

 

Brandon

 

Well, does it still work well enough to get you to work? maybe unplugging the MAF or something would at least force it into 'limp' mode such that you might be able to drive it until you can afford to take it to a mechanic/dealership.

 

Do you still have the old Temp Sensor? Can you measure its resistance and compare it to the new one?(unplugged of course). Also confirm that the leads are not shorted to its body or, the same lead is, on each one,etc.

 

Buying another used car does guarantee you won't have similar or worse problems - they just may be different problems. Plus, your present car would not be very attractive to someone as is.

[9pec]

Don't give up. My experience with "weird" issue like this is that usually it'll turn out to be something very basic and simple....that's why it is difficult to find it in the first place.

 

(Guys, if I say something wrong, please correct me. Not trying to be a smart boy here, but I really think he needs a structured approach to resolve this rather than hitting a moving target randomly. It is obviously driving him nuts:eek: )

 

Ok, let's trust the ECU for what it did do. The code tells us it's a faulty temp sensor or circuit. The aircon is functioning well, no overheating actually takes place, so radiator, cap, hose, pump, thermostat must be working. Hence, I am guessing it's electrical.

 

Temp sensor is usually a thermister, resistance drops when temp goes up. ECU sends current through the sensor via 2 wires, and takes the voltage across the sensor as signal. Somehow the ECU has been getting voltage readings that are out-of-range, hence it stores an error code, and tries to runs a "safe" mode, ratarding timing, runs higher injector duty cycle etc. coz it thinks the engine may be overheating, which is actually fine.

 

We'll need:-

 

1. Multimeter/Ohmmeter: a Taiwan-made al-cheapo one will do. To meassure resistance of the circuit and components involved.

2. An accurate circuit diagram of your car. (sorry I do not have one) You'll need to know exactly how the whole circuit is wired up in order to zero-in.

 

Let's walk through how the electrical part of a cooling system usually work.

Key swithed on. ECU checks signal from all sensos. Status check ok, it starts. Runs close loop warm up map, rich and advance, to warm things up. Waiting for temp signal to come into the prescribed range, meanwhile engine is really warming up. ECU now ready to cease warm up mode and go open loop. ECU receives signal from Temp sensor and............ohoh....

 

From this point onwards, I am guessing a few possible scenarios or a combinition of all of them:-

 

1) Voltage too high (resistance too low). That is, ECU thinks coolant is toooooo hot. Ground and run the fan circuit. Throw a CEL. Stop open loop. Goes into "save" mode. This may explain the low rpm (max duty cycle and full timing retard) If temp sensor gives good reading after a while, ECU goes open loop but stores the error code.

 

OR

 

2) No voltage, i.e. no connection (open circuit). ECU thinks either the wires has snapped, or the sensor is dead as hell. Stop open loop. Throw CEL. Run "save" mode. Fan will not run in this case.

 

OR

 

3) Voltage too low, current draw too high (the circuit is grounded somewhere). ECU cant keep up with the current draw and something will overheat and possibly a breaker circuit steps-in before something start burning. Throw CEL. Kill ignition and injection kill everything. Engine stop running.

 

So this is what I propose:-

 

a) Meassure the Temp Sensor's resistance (ohms) alone, out of the car. Record the reading down and remember this.

Dip the sensor in hot water. meassure again. It should have a lower resistance now. If not, sensor is screwed.

c) Find the 2 pins that link the sensor wires to ECU, on the ECU's side. Unplug that plug from ECU. This is where you need the ECU diagrame. One of the pin is supposed to be grounded in the ECU itself, or somewhere along the harness, or near the sensor on the block, etc. You'll need the diagram to idetify where to meassure. The diagram will tell you this.

d) Plug the sensor into the circuit (without installing it to the engine). Take ohm readings between the 2 pins on the ECU plug's side. If the wires are fine, it should give you the same resistance reading of the sensor. Very,very slight increase of ohmage is ok (like 0.1ohm more) ...it's the wires' resistance adding up. If no readings (open circuit), the wires are faulty. If the resistance shoots very high....connectors are dead. Clean them, or replace them. If the resistance drops through the floor, the wires' skin has probably shreaded somewhere and is touching the chassis, i.e. the circuit is grounded, which is not supposed to.

e) Repeat this with the sensor installed to the engine. Note if there is any difference after the sensor is installed. If the ohm drops, the sensor body is grounded to engine which is not supposed to. Sensor is at fault.

e) if all things are within spec, suspect the ECU itself. Open the casing. If visually you can see anything burned, snapped etc....well you'll probably should hunt for a replacement component or replace the ECU unit.

 

I am guessing there is nothing wrong with the fan system because the fan runs once the ECU decides to ground it and complete the circuit. If the fan system is at fault, should it throw a "temp sensor" code or a seperate code?

 

Good luck.

[Olnick]

Wow! Now that's some thinking, 9pec. Great logic and analysis.

 

Sure hope you solve the problem bart so we cam see if he nailed it.

 

Good luck.

[1 Lucky Texan]

Don't give up. My experience with "weird" issue like this is that usually it'll turn out to be something very basic and simple....that's why it is difficult to find it in the first place.

 

(Guys, if I say something wrong, please correct me. Not trying to be a smart boy here, but I really think he needs a structured approach to resolve this rather than hitting a moving target randomly. It is obviously driving him nuts:eek: )

 

Ok, let's trust the ECU for what it did do. The code tells us it's a faulty temp sensor or circuit. The aircon is functioning well, no overheating actually takes place, so radiator, cap, hose, pump, thermostat must be working. Hence, I am guessing it's electrical.

 

Temp sensor is usually a thermister, resistance drops when temp goes up. ECU sends current through the sensor via 2 wires, and takes the voltage across the sensor as signal. Somehow the ECU has been getting voltage readings that are out-of-range, hence it stores an error code, and tries to runs a "safe" mode, ratarding timing, runs higher injector duty cycle etc. coz it thinks the engine may be overheating, which is actually fine.

 

We'll need:-

 

1. Multimeter/Ohmmeter: a Taiwan-made al-cheapo one will do. To meassure resistance of the circuit and components involved.

2. An accurate circuit diagram of your car. (sorry I do not have one) You'll need to know exactly how the whole circuit is wired up in order to zero-in.

 

Let's walk through how the electrical part of a cooling system usually work.

Key swithed on. ECU checks signal from all sensos. Status check ok, it starts. Runs close loop warm up map, rich and advance, to warm things up. Waiting for temp signal to come into the prescribed range, meanwhile engine is really warming up. ECU now ready to cease warm up mode and go open loop. ECU receives signal from Temp sensor and............ohoh....

 

From this point onwards, I am guessing a few possible scenarios or a combinition of all of them:-

 

1) Voltage too high (resistance too low). That is, ECU thinks coolant is toooooo hot. Ground and run the fan circuit. Throw a CEL. Stop open loop. Goes into "save" mode. This may explain the low rpm (max duty cycle and full timing retard) If temp sensor gives good reading after a while, ECU goes open loop but stores the error code.

 

OR

 

2) No voltage, i.e. no connection (open circuit). ECU thinks either the wires has snapped, or the sensor is dead as hell. Stop open loop. Throw CEL. Run "save" mode. Fan will not run in this case.

 

OR

 

3) Voltage too low, current draw too high (the circuit is grounded somewhere). ECU cant keep up with the current draw and something will overheat and possibly a breaker circuit steps-in before something start burning. Throw CEL. Kill ignition and injection kill everything. Engine stop running.

 

So this is what I propose:-

 

a) Meassure the Temp Sensor's resistance (ohms) alone, out of the car. Record the reading down and remember this.

Dip the sensor in hot water. meassure again. It should have a lower resistance now. If not, sensor is screwed.

c) Find the 2 pins that link the sensor wires to ECU, on the ECU's side. Unplug that plug from ECU. This is where you need the ECU diagrame. One of the pin is supposed to be grounded in the ECU itself, or somewhere along the harness, or near the sensor on the block, etc. You'll need the diagram to idetify where to meassure. The diagram will tell you this.

d) Plug the sensor into the circuit (without installing it to the engine). Take ohm readings between the 2 pins on the ECU plug's side. If the wires are fine, it should give you the same resistance reading of the sensor. Very,very slight increase of ohmage is ok (like 0.1ohm more) ...it's the wires' resistance adding up. If no readings (open circuit), the wires are faulty. If the resistance shoots very high....connectors are dead. Clean them, or replace them. If the resistance drops through the floor, the wires' skin has probably shreaded somewhere and is touching the chassis, i.e. the circuit is grounded, which is not supposed to.

e) Repeat this with the sensor installed to the engine. Note if there is any difference after the sensor is installed. If the ohm drops, the sensor body is grounded to engine which is not supposed to. Sensor is at fault.

e) if all things are within spec, suspect the ECU itself. Open the casing. If visually you can see anything burned, snapped etc....well you'll probably should hunt for a replacement component or replace the ECU unit.

 

I am guessing there is nothing wrong with the fan system because the fan runs once the ECU decides to ground it and complete the circuit. If the fan system is at fault, should it throw a "temp sensor" code or a seperate code?

 

Good luck.

 

superb post.

 

Carl