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GM Bob
GM Bob, ASE Certified Technician
Category: Pontiac
Satisfied Customers: 962
Experience:  30 plus years. experience as GM technician, ASE Master Technician and GM Certified.
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Customer Question

Submitted: 14 days ago.
Category: Pontiac
Expert:  john aka eaut replied 14 days ago.

Hi . Thank you for asking your question on JustAnswer.
whats your question

Thank you again for trusting us with your problem. John aka eauto

Customer: replied 14 days ago.
Hello, my 1997 Grand Prix GTP (Supercharged 3800 Series II) is having a problem activating the electric cooling fans. I think the problem appeared slowly over time.The water pump is said to be close to new. The serpentine belt is almost new (5,000 miles). The tensioner is almost new. New thermostat and gaskets. New coolant flush. New ECT Sensor. Cooling system was properly bled. ECT appears to read true on the dash temperature gauge, and a resistance check shows the resistance varies up or down depending on temperature.When the ECT is unplugged, both cooling fans turn on a few seconds later and stay on until the sensor is plugged back in or until the car is shut off. When the sensor is plugged in the temperature climbs to dangerously high levels, into the red zone, and still won't turn the fans on. The fans can also be turned on via the obd2 port connection.The only thing I can think of from here is if I'm missing something in the PCM schematic, or if the PCM itself is bad. 98% of problems diagnosed as a PCM are incorrect diagnoses according to an ASE supported book, so I am hesitant to consider that as the problem.Look foreword to hearing from you, thank you.
Expert:  john aka eaut replied 14 days ago.

i have no diagrams today i will open this up for another expert

Customer: replied 14 days ago.
Ok, thank you.
Expert:  kalamykid replied 14 days ago.

Have you replaced the ect yet?

Customer: replied 14 days ago.
Diagnostic information done so far is posted above. Yes that is one of the things that has been done.
Expert:  kalamykid replied 14 days ago.

What color is the coolant in the radiator?

Customer: replied 14 days ago.
I believe it was the factory recommended coolant colored green. When the radiator was flushed for diagnostics it was refilled with pure water.
Expert:  kalamykid replied 14 days ago.

First off the car should have an orange coolant in it called dex cool. Green antifreeze is to acidic for the aluminum in this engine, as well as the plastic. If you can unplug the ect and the fans will come on, then the ect has to be bad. The ecm is seeing that when it is unplugged, there is a problem and it is turn ng the fans on, so we know the rest of the system is still good. The ect does not run the temp gauge. The ect only tells the ecm what the temp of the engine is. The coolant temp sensor tells the gauge what the temp should be for the gauge on on the dash.

Customer: replied 14 days ago.
I only know of the coolant temperature sensor under the thermostat, that's all my schematics show. This sensor was replaced three times. Is this the sensor you are referring to?
Expert:  kalamykid replied 14 days ago.

Yes that is the ect. The coolant temp sensor has a black and a dark green wire running to it. Do you have a digital volt meter?

Customer: replied 14 days ago.
I have two digital multimeters. Voltage and resistance tests have been performed and the thermistor appears to operate normally. Additionally, the temperature gauge on the dash which is one of the things that corresponds to this sensor reads accurately with the sensor plugged in and gives no reading with the sensor unplugged. The gauge will start in the off position when the sensor is plugged in and climb into the red zone as the engine over heats, but the fans don't come on unless it's unplugged.
Expert:  kalamykid replied 14 days ago.

What would you say the temp of the engine is right now?

Customer: replied 14 days ago.
The same as outside because I'm not driving it.
Expert:  kalamykid replied 14 days ago.

What I need you to do is go out and unplug the sensor, and with your meter set to ohms, see what the reading is, then warm the car up and see what it is when the temp gets up around the normal range please.

Expert:  kalamykid replied 14 days ago.

I need you to test the sensor itself not the wires and the sensor needs to be left unplugged.

Customer: replied 14 days ago.
Given that the sensor was replaced three times with quality parts and each time gave the same results, a bad sensor has been ruled out. Additionally a scan tool linked to the obd2 port shows temperature readings climb smoothly through the full spectrum. Lastly the sensor is giving the correct temperature to the temperature gauge, showing that ohms and degrees calibrate correctly to vehicle specifications.
Expert:  kalamykid replied 14 days ago.

The ect does not go to the gauge. The gauge has its own sensor. If the ect is working correctly according to a scan tool, then the drivers for the fans in the ecm must be bad. This would means that you need to replace the ecm. The ecm has three sets of drivers in it for the fans. One for when the ac is on, the low and the high. It is possible that when you unplug the ect that the ecm is using the ac drivers to turn the fans on.

Customer: replied 13 days ago.
You may be looking at the wrong schematics as this vehicle requires piggy backing off of the sensor to get its reading to the cluster. This is shown in schematics and verified by the fact that unplugging the sensor cuts the signal to the cluster off. The PCM idea is what I mention in my original post, but I hear it is increadibly rare for that to be the case, 98% of these diagnoses are technician error
Expert:  kalamykid replied 13 days ago.

If the relays where bad, the fans where bad, then the fans would not come on when you unplug the ect. You said the ect readings where good when it was connected to a scanner So the only thing left will be the ecm itself.

Customer: replied 13 days ago.
The PCM could be bad, we could be missing something of importance in the schematic which could be tied into the system, or an intermittent opening is possible. I am thinking it is the second or third...
Expert:  kalamykid replied 13 days ago.

Then please go do the test I asked you to do.

Customer: replied 13 days ago.
As stated it was already done and the sensor was replaced three times ruling it out. Thank you for the try but no new information has come from this and you were not looking at the correct schematic. Thanks anyway.
Expert:  kalamykid replied 13 days ago.

Your ect has a yellow and a black wire, the engine coolant temp sensor has a dark green and a black wire. If you have done the test I asked for, what where the readings?

Customer: replied 13 days ago.
Well firstly it is a three prong and thus has three wires, but I don't understand what you mean by "ECT" and "engine coolant temp sensor", what does ECT stand for in your acronym? Usually it means engine coolant temperature...?
Expert:  GM Bob replied 13 days ago.

Hello my name is ***** ***** I can help.

Does your scan tool show live data.

Need to know what the PCM coolant temp is. The dash gauge it self my be reading higher than the pcm temp

The fans wont come on until 230

Expert:  GM Bob replied 12 days ago.

Did you understand what I was asking for?

Please let me know if you need more help?

Customer: replied 12 days ago.
Hello, yes I have live data feed, my most recent check showed the dash cluster and data stream were synchronized reading the same very hot temperatures. I know these engines are programmed to run hot but something changed where the car used to work correctly and now won't cycle the fans on at the correct temperature. Still trying to resolve...
Expert:  GM Bob replied 12 days ago.

I know you are saying hot temperature. Can you tell the degrees?

If you turn on the A/C do the fans come on at low speed?

When you unplug the sensor can you tell if the fans are on high?

Does your scan tool control the fan speeds?

Customer: replied 12 days ago.
In degrees Fahrenheit we are talking ~260*F, may go higher but I have not wanted to let it.Unfortunately the A/C system isn't charged so I am guessing the low pressure switch won't allow the system to activate. Fan drivers are integrated on this PCM such that there is no independent "AC is on, turn fans on" type of driver. The low and high PCM drivers are both good when commanded via OBD2.I think the fans are on high when I unplug the sensor.My scan tool has full control of fan speeds and fans respond appropriately on all settings.I think I am going to test...
PCM Connector C1
Pin 56, black, Sensor Ground for ECT Sensor A
PCM Connector C2
Pin 52, yellow, ECT Sensor B input voltageMaybe the PCM is getting a lower than true Pin 52 voltage causing yet to be in range but read falsely cool temperatures...
Expert:  GM Bob replied 12 days ago.

So when the temp gauge on the dash reads 260. The PCM also shows 260.

Is that correct?

The fans do not come on?

Customer: replied 11 days ago.
That is correct, the dash reads very hot (260 for example), the scanner shows very hot (260 or so for example) and the fans do not come on. I have to command them on with the scanner to cool it down which works, or I unplug the ECT and they come on and cool it down which works. The latter causes a loss of temperature statistics but does activate the fans and cool things down.
Customer: replied 11 days ago.
Also wanted to say that when the engine over heats it does not trigger a code with the ECT plugged in, only triggers the ECT code if unplugged.
Expert:  GM Bob replied 10 days ago.

On the scan tool does it show. Do you have any live data that shows fan relay request ON/OFF

The fact you can control the fans. Tells me the PCM can turn on the relays.

Customer: replied 9 days ago.
The live data has the fan modes which correspond with the fan relay requests, im not so sure about it listing them as specific relay numbers as supposed to fan functions.Yes the PCM can turn on the relays and all 3 relays are good. I keep coming back to the thought that if the inputs are good and the outputs are good, but the right inputs don't cause the right outputs, then it must be a failure in the PCM's logic circuit for this specific channel.These computers are just so reliable and so often misdiagnosed that I am reluctant to call it the case, but I may end up having to and just repairing the PCM. If you want you can let me know if you have any final thoughts or input, otherwise, have a great holiday weekend.
Expert:  GM Bob replied 8 days ago.

I agree that the computers are reliable.

You have all the right input data and outputs. But the PCM wont turn on the relays itself.

Is this the original PCM?

Can you scan tool read the VIN# ***** the PCM? If so make sure it matches the car.

Thank you for getting back with me.

Expert:  GM Bob replied 7 days ago.

If you still need more help let me know.

If you can take a moment and rate my service.

Thank you

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