Temperature Guage/PCM Failure

I've got a 2001 3.4 Alero with 186,000 miles. Within the last 500 miles, I've had problems with my temperature guage. At first, the needle didn't come all the way up to its normal operating temperature. It appeared the car was putting off plenty of heat and I confirmed my coolant level was correct. About a week later, the temperature needle stuck over the C whenever the key was turned to on. I figured it was the temp. sensor, so I replaced it. I also found the radiator fan was one even if the car wasn't warmed up. I replaced the temp. sensor, but nothing changed.

I thought it may be a wiring problem, but then yesterday the needle on the guage returned to its normal position. Now today, the needle moves off the C , but doesn't go as far up as it should.

Could this still be a wiring issue, or is it more likely I've got a PCM problem?

Welcome to AF! If the temp sensor is new and the guage is now working then don't worry about that part of the problem. You may want to check the fan relay to see if it is bad causing the fan to run constantly. Also there are two temp sensors one that controls the fan one that reads engine temp. The one for the fan should be located on the radiator. If you have not done so replace it first and then check the relay. If this sensor is truly bad the fan should work normally. Your PCM is working fine it is the sensor and fan that are causing the issue.

GM changed the cooling configuration on the Alero and all late model GM "N" bodies compared to previous versions or others that have the 3.4L V6. Just thought I would clarify since I own an '02 Alero 3.4L.

The ECT (engine coolant temperature) sensor is what the PCM references off of to turn on the cooling fans. The temperature gauge uses it too. The PCM is the sole device that turns on both cooling fans via the cooling fan relays depending on the derivative temperature reading from the ECT. There is no temperature sensor on the radiator. The 5 volt reference voltage feeding the ECT comes from the PCM and it references the same signal less voltage drop since the ECT sensor is nothing but a thermal resistor.

I see the ECT has already been replaced. Therefore, it sounds like a PCM problem or possibly a wiring problem to and from ECT to PCM.

Thanks Bob! I didn't know they changed that on the Alero! Of course leave it to GM to screw things up again.

Thanks Bob! I didn't know they changed that on the Alero! Of course leave it to GM to screw things up again.

No problem Lynn. Yeah GM is good about that. They like screwing with us old timers so we have to relearn everything. :dunno:

So I have been having this same problem with my car, on and off for the last two weeks, and yes unfortunately it is an alero. Temp guage on my car however, refuses to work at all when the problem starts up, there is no middle ground of working a little bit, its all or nothing.

So when you say the temp sensor for the fan is located on the radiator, where exactly? Is this an internal temp guage, or is this literally a sensor located on the exterior of the radiator? And the fan relays are located where exactly? I have a vague idea, but I'd rather not start unplugging and replugging fuses without just cause...

Thanks all- joe

So I have been having this same problem with my car, on and off for the last two weeks, and yes unfortunately it is an alero. Temp guage on my car however, refuses to work at all when the problem starts up, there is no middle ground of working a little bit, its all or nothing.

So when you say the temp sensor for the fan is located on the radiator, where exactly? Is this an internal temp guage, or is this literally a sensor located on the exterior of the radiator? And the fan relays are located where exactly? I have a vague idea, but I'd rather not start unplugging and replugging fuses without just cause...

Thanks all- joe

Here are some Alero 101 basics:

First off there is no temperature sensor on the radiator on either the exterior or interior.

Second the cooling fans relays are located in the engine compartment fuse box which is located on the driver's side of the engine compartment. They are marked mini-relays 12, 14, and 15.

Third the powertrain control module (PCM) is the device that turns the cooling fans on or off depending on the signal (engine temperature) from the engine coolant temperature sensor (ECT). The PCM controls the cooling fan relays mentioned above. The ECT is what the temperature gauge gets it's signal from. The ECT is located on the intake manifold by the thermostat housing if you have a 3.4L.

If it is just a temperature gauge problem at the instrument panel you could have a defective gauge, wiring or ground to it.

Unless you are willing to get into to some possibly complex DIY troubleshooting and maybe get a GM Alero service manual you might consider taking it to a GM dealer service department and have then put it on a Tech II machine. The complete circuit above can be checked in a relatively short period of time by any GM dealer willing to work on an Alero. Good luck!

All of that seems simple enough to me. I mistakenly read one of your previous posts as stating there was a sensor on the radiator, when there is not.

As for the mini-relays, I will check them first, then investigate the ECT. I have the 3.4L, so it should be easy enough to swear like a sailor due to burning the snot out of myself while trying to check the ECT near the manifold. I am crossing my fingers, but not nixing the possibility of a bad guage- it is an olds lemon after all...

I have tried to get a repair manual, but none have been made- at least none that are sold at Murrays, Pep Boys, Car Quest or Auto Zone. Did I cover them all? Thats where I have checked so far...

Also, doesn't it lead a conclusion more towards the ECT since the fans kick on the instant the car starts, and the guage doesn't move what soever? Could it be a sending unit, and is the sending unit involved with the ECT?

Thanks again

All of that seems simple enough to me. I mistakenly read one of your previous posts as stating there was a sensor on the radiator, when there is not.

As for the mini-relays, I will check them first, then investigate the ECT. I have the 3.4L, so it should be easy enough to swear like a sailor due to burning the snot out of myself while trying to check the ECT near the manifold. I am crossing my fingers, but not nixing the possibility of a bad guage- it is an olds lemon after all...

I have tried to get a repair manual, but none have been made- at least none that are sold at Murrays, Pep Boys, Car Quest or Auto Zone. Did I cover them all? Thats where I have checked so far...

Also, doesn't it lead a conclusion more towards the ECT since the fans kick on the instant the car starts, and the guage doesn't move what soever? Could it be a sending unit, and is the sending unit involved with the ECT?

Thanks again

Just wanted to clarify before you got too far into it. On the service manual only Helms makes a good one which is the GM service manual but they cost a lot more than the others but well worth it since you have a Alero.

The ECT mentioned is the temperature sensor. If you are handy with a multimeter it can be ohmed out and checked for linearity-calibration. For the fans to kick on instantly the sensor would have to be giving the PCM a false reading equivalent to 220 degrees F.

I can post the resistance values later this evening after I review the service manual.

'B'- I can't thank you enough for your help. I have a multimeter, unforunately analog, but I can squirm my way to a decent reading, had to do it on my ABS sensors as two of them kept going bad...

Somehow I had a feeling the only decent repair manual was the official GM version, which of course is slightly more than I can afford on my college budget, maybe ebay/amazon...?

Thanks again/in advance- I am sure I will be bothering you a bit more:)

joe

'B'- I can't thank you enough for your help. I have a multimeter, unforunately analog, but I can squirm my way to a decent reading, had to do it on my ABS sensors as two of them kept going bad...

Somehow I had a feeling the only decent repair manual was the official GM version, which of course is slightly more than I can afford on my college budget, maybe ebay/amazon...?

Thanks again/in advance- I am sure I will be bothering you a bit more:)

joe

No problem and you're welcome. We're here to help. Sometimes the Helms manuals show up on EBay but I understand monetary restraints. The analog meter will be OK for measurements. As I mentioned earlier, for ECT to be bad and fool PCM into turning on fans it will probably read low resistance more towards zero on the meter scale. As you can see by values below as the engine warms up the resistance decreases. Cold engine resistance will be higher. Of course, resistance will vary as the temperature of the engine changes.

In the meantime, here are various resistance values for the ECT sensor based on actual temperature:

Degrees F -- Ohms

212 - 177
104 - 1459
86 - 2238
68 - 3520
41 - 7280
32 - 9420
23 - 12,300

Good luck!

So B, after I test the ECT again for the values you listed above- I have tested it warm, and the guage was working at that moment, next I want to check it cold. Anyways- after I test the ECT for a second time, I would like to know if I can test the mini-relays to see if they are working... would it be a continuity or other test on them if that is possible?

Thanks again.

So B, after I test the ECT again for the values you listed above- I have tested it warm, and the guage was working at that moment, next I want to check it cold. Anyways- after I test the ECT for a second time, I would like to know if I can test the mini-relays to see if they are working... would it be a continuity or other test on them if that is possible?

Thanks again.

The cooling fan relays are the plug-in type. Normally a relay can be removed without any adverse effect especially in the cooling fan circuit. Also, they can be troubleshot with a multimeter. One or more of the relays would have to be stuck or forced energized to have the fans turn on before the proper temperature for fan turn on. If the part numbers match you can swap them out with each other for troubleshooting purposes.

Looks like everything is ok on this side. :-)

The fans will also come on when you start car if sensor is unpluged
true or not.

The fans will also come on when you start car if sensor is unpluged
true or not.

True after a few second delay and will stay on until you turn off the engine and reconnect the ECT sensor.

Thanks for the info.

not. sencers tell the computer that the fans need to come on.