Cooling Fan troubles
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Joined: Oct 2007
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From: Abbottstown Pa.
86vetted, ECM is rarely the cause. What you really need to check is your Coolant Temp Sending Unit.
It's located on the front of the intake baseplate and has a plug with a plastic clip. (Not to be confused with the cold start sensor switch next to it with a metal clip.)
The ECM turns on the main fan when this sensor reads 228F. It also uses it to control fuel mixture, and some other things.
If this sensor is reading low the main fan will come on late or not at all. You would have to use an ohm meter to read what it's telling the ECM, and find the chart that says what it's supposed to read. The reading on the dash is from a different sensor.
You could also try unplugging it and seeing if the ECM throws a code. It might even turn on the fan if it can't read the sensor, I don't know.
The ECM also uses vehicle speed to control the main fan, but I assume your speedo works.
It's located on the front of the intake baseplate and has a plug with a plastic clip. (Not to be confused with the cold start sensor switch next to it with a metal clip.)
The ECM turns on the main fan when this sensor reads 228F. It also uses it to control fuel mixture, and some other things.
If this sensor is reading low the main fan will come on late or not at all. You would have to use an ohm meter to read what it's telling the ECM, and find the chart that says what it's supposed to read. The reading on the dash is from a different sensor.
You could also try unplugging it and seeing if the ECM throws a code. It might even turn on the fan if it can't read the sensor, I don't know.
The ECM also uses vehicle speed to control the main fan, but I assume your speedo works.
86VETTED
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From: The Top of Utah
Originally Posted by 86vetted
Thanks for the reply. I guess it is cheaper and easier to just replace the sensor that feeds the ECM than replacing the ECM.
So I guess the cost of a sensor is a good place to start. Is the ECM temperature a thermocouple or some other kind of sensor?
86VETTED
So I guess the cost of a sensor is a good place to start. Is the ECM temperature a thermocouple or some other kind of sensor?
86VETTED
I am still having a problem reconciling your opening post statement:
Originally Posted by 86vetted
It only runs when the temperature reached around 2180F - 2220F.
Originally Posted by 86vetted
Well the problem is that the main fan never comes on. The car overheats when the engine is on. The auxiliary fan comes on at about 235F but the main fan doesn’t.
Team Owner
Joined: May 2002
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From: San Diego , CA Double Yellow DirtBags 1985..Z51..6-speed
Originally Posted by CFI-EFI
There is a lot of [misinformation] flying around these days.
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Team Owner
Joined: May 2002
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From: San Diego , CA Double Yellow DirtBags 1985..Z51..6-speed
It's on the high pressure line (the small one) feeding into your evaporator core. There should be two on there, I think the one with the larger plug is the high pressure switch. My guess is that it works fine and just your assumption about the A/C control over the fans is wrong.
And pat yourself on the back as you have just discovered the poor man's fan override switch.
Last edited by CentralCoaster; Oct 30, 2007 at 10:19 PM.
Thread Starter
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From: Abbottstown Pa.
86VETTED
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From: Cincinnati, Oh USA
Did you read my first post? Did you try any of the diagnostic tests I recommended? What did you find? Here is an easy list of things to try:
1. Test your c. fan fuse in your fuse block with an ohmeter, it supplies 12v to the fan relay coil.
2. Do you have 12v on the red wire on the fan relay? This will prove the fusible link is not open circuit.
3. With the ign in run (engine need not run), ground the green/white wire on the fan relay, you should hear it click and the fan motor should run. When the relay closes, you should have 12v on the black/red wire on the fan relay showing the relay closed and its contacts are ok.
4. Use your ohmeter to see if the black wire on the motor connector goes to ground, ground connections do corrode and go open circuit.
5. Ign in run, engine not running, short terminal B to ground on the ALDL connector and the fan relay should close and the fan run showing that the ECM is ok.
6. Use a scan tool to see what the engine coolant temp sender (under the MAF) is telling the ECM. If this sender never reports 228 F or higher, then the ECM won't turn on the fan.
1. Test your c. fan fuse in your fuse block with an ohmeter, it supplies 12v to the fan relay coil.
2. Do you have 12v on the red wire on the fan relay? This will prove the fusible link is not open circuit.
3. With the ign in run (engine need not run), ground the green/white wire on the fan relay, you should hear it click and the fan motor should run. When the relay closes, you should have 12v on the black/red wire on the fan relay showing the relay closed and its contacts are ok.
4. Use your ohmeter to see if the black wire on the motor connector goes to ground, ground connections do corrode and go open circuit.
5. Ign in run, engine not running, short terminal B to ground on the ALDL connector and the fan relay should close and the fan run showing that the ECM is ok.
6. Use a scan tool to see what the engine coolant temp sender (under the MAF) is telling the ECM. If this sender never reports 228 F or higher, then the ECM won't turn on the fan.
Thread Starter
Intermediate
Joined: Oct 2007
Posts: 40
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From: Abbottstown Pa.
Did you read my first post? Did you try any of the diagnostic tests I recommended? What did you find? Here is an easy list of things to try:
1. Test your c. fan fuse in your fuse block with an ohmeter, it supplies 12v to the fan relay coil.
2. Do you have 12v on the red wire on the fan relay? This will prove the fusible link is not open circuit.
3. With the ign in run (engine need not run), ground the green/white wire on the fan relay, you should hear it click and the fan motor should run. When the relay closes, you should have 12v on the black/red wire on the fan relay showing the relay closed and its contacts are ok.
4. Use your ohmeter to see if the black wire on the motor connector goes to ground, ground connections do corrode and go open circuit.
5. Ign in run, engine not running, short terminal B to ground on the ALDL connector and the fan relay should close and the fan run showing that the ECM is ok.
6. Use a scan tool to see what the engine coolant temp sender (under the MAF) is telling the ECM. If this sender never reports 228 F or higher, then the ECM won't turn on the fan.
1. Test your c. fan fuse in your fuse block with an ohmeter, it supplies 12v to the fan relay coil.
2. Do you have 12v on the red wire on the fan relay? This will prove the fusible link is not open circuit.
3. With the ign in run (engine need not run), ground the green/white wire on the fan relay, you should hear it click and the fan motor should run. When the relay closes, you should have 12v on the black/red wire on the fan relay showing the relay closed and its contacts are ok.
4. Use your ohmeter to see if the black wire on the motor connector goes to ground, ground connections do corrode and go open circuit.
5. Ign in run, engine not running, short terminal B to ground on the ALDL connector and the fan relay should close and the fan run showing that the ECM is ok.
6. Use a scan tool to see what the engine coolant temp sender (under the MAF) is telling the ECM. If this sender never reports 228 F or higher, then the ECM won't turn on the fan.
I wired a switch into the cooling fan relay to ground the “B” terminal in the cooling fan relay connector. It works but you have to keep an eye on the temperature gauge. On one of the other posted replies is there a chart to cross reference using an ohmmeter to see if the sensor is reading accurately? I con verify the readings when I meter the sensor wires. I appreciate all the great advice I have receiver on this forum and am glad a friend told me about it. Again thanks.
86VETTED
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From: San Diego , CA Double Yellow DirtBags 1985..Z51..6-speed
From 85 service manual: (I'm sure the sensor is the same part in 86)
Check resistance across the coolant temp sensor in front of the intake.
210F = 185 ohms
160F = 450 ohms
100F = 1800 ohms
70F = 3400 ohms
40F = 7500 ohms
At normal operating temps (200-225F?) the sensor will output about 1.5 to 2.0 volts.
Low resistance / low voltage caused by Temp > 275F for 4 seconds, or a shorted circuit, will set Code 14.
High resistance / high voltage, caused by Temp < -31F for 4 seconds, or an open circuit will set Code 15.
If disconnecting it sets a code 15 after a minute or so, the ECM is reading it fine.
The ECM uses it to control:
Fuel delivery
Spark Timing
Knock Control
Idle
Torque Converter Lockup
Vapor Canister Purge
A.I.R. System (smog)
EGR
Cooling Fan
Check resistance across the coolant temp sensor in front of the intake.
210F = 185 ohms
160F = 450 ohms
100F = 1800 ohms
70F = 3400 ohms
40F = 7500 ohms
At normal operating temps (200-225F?) the sensor will output about 1.5 to 2.0 volts.
Low resistance / low voltage caused by Temp > 275F for 4 seconds, or a shorted circuit, will set Code 14.
High resistance / high voltage, caused by Temp < -31F for 4 seconds, or an open circuit will set Code 15.
If disconnecting it sets a code 15 after a minute or so, the ECM is reading it fine.
The ECM uses it to control:
Fuel delivery
Spark Timing
Knock Control
Idle
Torque Converter Lockup
Vapor Canister Purge
A.I.R. System (smog)
EGR
Cooling Fan
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From: The Top of Utah
Originally Posted by CentralCoaster
Wrong. In 85 the main fan goes off at 35mph. I recall later years go off at 40 mph. I can't speak for 84 as it has very crude fan controls.
Originally Posted by CFI-EFI
There is a lot of [misinformation] flying around these days.
RACE ON!!!
Apparently so.
RACE ON!!!
Apparently so.
Originally Posted by jfb
You are seeing things! Your 88 vette's stock ECM turns on the main fan when the coolant temp sensor up by the MAF tells the ECM that the coolant temp is at or above 228 F and has no instructions or means to turn the fan off above 35 mph!
Originally Posted by tequilaboy
The 35 mph speed threshold that exists in the bin and in the editors is not used to turn off the fan above the set speed in 86-89 bins.
It is only used to determine the fan duty cycle for AC on and AC pressure high below this speed to serve as an alternative to the coolant temperature dependent duty cycle table.
Below 35 mph with AC on and AC pressure high, the fan duty cycle is set to a fixed constant value which is equal to 100%. Otherwise the fan duty cycle value is pulled from the coolant temperature dependent table. This is intended to ensure maximum cooling at low speed with AC on and AC high pressure if a lower duty cycle value would be obtained from the table.
It is only used to determine the fan duty cycle for AC on and AC pressure high below this speed to serve as an alternative to the coolant temperature dependent duty cycle table.
Below 35 mph with AC on and AC pressure high, the fan duty cycle is set to a fixed constant value which is equal to 100%. Otherwise the fan duty cycle value is pulled from the coolant temperature dependent table. This is intended to ensure maximum cooling at low speed with AC on and AC high pressure if a lower duty cycle value would be obtained from the table.
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Team Owner
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From: San Diego , CA Double Yellow DirtBags 1985..Z51..6-speed
Next time I head out I'll run a test light to the fan ground input and see if it turns off at 35 mph and how the A/C effects it. Maybe 86-up is different, but I don't have those books, and I don't have electronic a/c either if that means anything.
I don't know anything about the fan duty cycles, I just know what the factory manual says. But it's been wrong before.
I don't know anything about the fan duty cycles, I just know what the factory manual says. But it's been wrong before.
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From: The Top of Utah
I don't have any of those books either, and I don't have your (1985 FSM?). For sure the '84 has no ECM involvement, at all. From all the posts on this forum, until now, none of the cars before 1990 had a speed shut off for the fan. I know things changed for the fans in 1990. All 1990 and up have two fans, no auxiliary fan. They are side by side, not fore and aft as the 1985 to 1989s, and both those fans 1990 and up are computer controlled. I don't know what year the speed shut off was introduced. My guess was 1990, but I am quite sure the 1996 has it. I'd like to know for sure.
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Melting Slicks
Joined: Aug 2003
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From: Lakeville MI
It appears to me that the 85s (and only the 85s) do indeed have a 35 mph fan shut off feature. The 86-89s do not as previously described.
I've seen this in 85 $1F bin files with the TunerPro $1F xdf file that I have. I have no actual experience with an 85 to confirm.
The item is located at hex address 300, and is labelled as "Fan, Maximum Speed to Enable". It is set to 35 mph.
I've seen this in 85 $1F bin files with the TunerPro $1F xdf file that I have. I have no actual experience with an 85 to confirm.
The item is located at hex address 300, and is labelled as "Fan, Maximum Speed to Enable". It is set to 35 mph.
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From: The Top of Utah
That would make the '85 have the most unique fan set up of any C4. It is the only year, that without an aux fan, it still has a fan switch that acts as a back up to the ECM, and now the only pre-1990 with a speed shut off. CentralCoaster, I for one, anxiously await the results of your test. Can anyone say for sure when the speed shut off feature resumed? 1990?
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Thread Starter
Intermediate
Joined: Oct 2007
Posts: 40
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From: Abbottstown Pa.
Thanks for the help
Hello All:
A finial reports on the “Cooling Fan Troubles”. I tried to test the temperature sensor on the front of the intake manifold but couldn’t get the test leads on it, so I decided to remove it and test it. I noticed the “Cold Start Sensor was broken. I replaced both and after resetting the ECM by disconnecting the negative terminal on the battery the fan now starts at 223F and turns off at 208F. The fan still doesn’t run when the AC comes on but that can wait until summer. Thanks for all the advice and knowledge all of you freely share. Again thanks.
86vetted
A finial reports on the “Cooling Fan Troubles”. I tried to test the temperature sensor on the front of the intake manifold but couldn’t get the test leads on it, so I decided to remove it and test it. I noticed the “Cold Start Sensor was broken. I replaced both and after resetting the ECM by disconnecting the negative terminal on the battery the fan now starts at 223F and turns off at 208F. The fan still doesn’t run when the AC comes on but that can wait until summer. Thanks for all the advice and knowledge all of you freely share. Again thanks.
86vetted
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From: The Top of Utah
Originally Posted by CentralCoaster
Thanks for responding back, it helps! Most people take the tips and run off never to be seen again, so nobody finds out what the culprit was.
Indeed. Thank you for the report in the outcome.
Speaking of reports, CC, do you have any report on, "Next time I head out I'll run a test light to the fan ground input and see if it turns off at 35 mph and how the A/C effects it."?
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6th Gear
Joined: Dec 2006
Posts: 6
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From: Oakdale California
Hey Guys, I had the same prob. Grounding the relay (both main and aux) would only turn on the aux fan. Checked voltage. All voltage readings were good. Swapped relays. BAM! The main fan now kicks on... looks like a bad relay.
