Radiator fan turns on as soon as car starts: please help!

Here's another one! Help me solve it, please!

As soon as I start my Riv, the radiator fan turns on. Even at a cold start with no A/C turned on.

Ive searched the net and people say its because the PCM thinks the engine is hotter than what it is.

My coolant temps look fine tho. My tune is fine. I even changed fan settings back to stock just to check that off.

I have no other symptoms (that I can detect) to point me to where the issue is.

Anyone have any ideas what is causing the radiator fan to turn on immediately after car is started?

please help!

I can use HPT scanner to turn the fan off, but that is not the right answer. And if I turn it off, it wont start on its own when it reaches the set parameters.

I am clueless as usual..

Here's the '99 Riv Cooling Fan schematic. The PCM controls the ground side of the entire circuit(s). The relays are hot at all times, the PCM just supplies ground (via ground driver inside the PCM) in order to turn the fans on. Being that the PCM isn't powered "ON" until the Ignition is turned to "RUN", it may make sense that one of the drivers inside the PCM is stuck (shorted).


You very well may just have a "stuck" relay (most probable cause).
Before anything else, I would find a known good relay and put it in the Cooling Fan Relay #1 spot to see if it cures your problem. If that doesn't work, put the original relay back in, and move on to the Series Parallel Cooling Fan Relay.
If swapping out the relays doesn't find your problem, it's time to break out the Test Light and Ohm-meter....

Thanks, Scott. I will get working on this asap.
Is cooling fan relay #1 located under the rear seat with all the other relays?

charlieRobinson wrote:

Is cooling fan relay #1 located under the rear seat with all the other relays?

I would assume so considering they made sure you know that the other two relays are in the EC Fuse block.

Abaddon wrote:

charlieRobinson wrote:

Is cooling fan relay #1 located under the rear seat with all the other relays?

I would assume so considering they made sure you know that the other two relays are in the EC Fuse block.

That's what I was thinking.

I will check it out. Thanks again!

Forgot to update. I found both the relays on the pass side under the hood. Replaced them both with known good relays to test and it made no difference.

If the computer isn't getting a reading from the temp sensor I believe it runs the fans all the time as a fail safe to stop it from getting too hot.

stydel311 wrote:

If the computer isn't getting a reading from the temp sensor I believe it runs the fans all the time as a fail safe to stop it from getting too hot.

The coolant sensor? dash gauge and scan tools seems to be giving accurate readings.

charlieRobinson wrote:

stydel311 wrote:

If the computer isn't getting a reading from the temp sensor I believe it runs the fans all the time as a fail safe to stop it from getting too hot.

The coolant sensor?   dash gauge and scan tools seems to be giving accurate readings.

How many coolant temp sensors do these cars have? I always thought there was one for the gauge and another one for the computer

I dont know. That's why I asked. I only know of the one under the thermostat on the LIM.

Where is the other one located?

Just one sensor on these engines. It sends the temperature to the PCM, then the PCM sends it through the UART data line to the gauge if I remember right.

So it's not my issue if all the readings are correct, right?

This is very long. Sorry so long, but it is what it is......remember, this is copied and pasted. I cannot save pages, I can merely copy/paste, so it looks funny. Hopefully you can decipher it.
I've bolded and underlined the beginning of each table to make it easier to find.

1999 Buick Riviera | Aurora, Riviera VIN G Service Manual | Engine | Engine Cooling | Schematic and Routing Diagrams | Document ID: 338574
--------------------------------------------------------------------------------

Electric Cooling Fan Diagnosis
Refer to Cell 20: Cooling Fans.

Circuit Description:
The cooling fans are controlled by the PCM based on various inputs. Ignition voltage is supplied to all three cooling fan relay coils. The PCM controls the low speed cooling fan relay by providing the ground path. The PCM controls the series/parallel and the high speed cooling fan relays together by providing a ground path. The left and right cooling fans are connected in series. This will enable both fans on low speed when the low speed relay is energized. When all three fan relays are energized, both fans will operate at high speed. The high speed is possible because the fan relays are wired in a parallel circuit. When the PCM detects that certain DTCs are set, the PCM will enable the cooling fans.

The PCM will enable the engine cooling fans when certain Diagnostic Trouble Codes are set. The PCM will also enable the cooling fans after the ignition is turned off. When engine coolant temperature exceeds 140°C (284°F) and the ignition is turned off, the cooling fans will be enabled on low speed.

Diagnostic Aids
If the owner complained of an overheating problem, determine if the complaint was due to an actual boil over, or the warning indicator light illuminated, or the engine coolant temperature gauge indicated overheating. The gauge accuracy can also be checked by comparing the Engine Coolant Temperature (ECT) sensor reading using a scan tool and comparing its reading with the gauge reading. If the engine is actually overheating and the gauge indicated overheating, but the cooling fan is not coming ON, the Engine Coolant Temperature (ECT) sensor has probably shifted out of calibration and should be replaced.

If the engine is overheating and the cooling fans are ON, the cooling system should be checked, refer to Engine Overheating.

The PCM will command low speed fans ON at 108°C (226°F) and OFF at 103°C (217°F) and, high speed fans ON at 113°C (235°F) and OFF at 108°C (226°F).

Test Description
The numbers below refer to numbers on the diagnostic table.

2.The PCM will enable the engine cooling fans when certain Diagnostic Trouble Codes are set. Go to applicable DTC Table first before proceeding with this table.
3.Allow engine coolant temperature to cool below 100°C (212°F) before proceeding with diagnosis.
Electric Cooling Fan Functional Check Step
Action
Value(s)
Yes
No

1
Did you perform the Powertrain On-Board Diagnostic (OBD) System Check?
—
Go to Step 2
Go to Powertrain On Board Diagnostic (OBD) System Check

2
Are any DTCs set?
—
Go to applicable DTC Table
Go to Step 3

3
 1. Install a scan tool.
 2. Engine coolant temperature must be below the specified value for all the fan diagnoses.
 3. Turn ON the ignition, with the engine and A/C OFF.
Are the cooling fans OFF?
100°C (212°F)
Go to Step 4
Go to Cooling Fan table #1

4
With a scan tool, command the low speed cooling fan relay ON.

Are both cooling fans ON?
—
Go to Step 5
Go to Cooling Fan table #2

5
With a scan tool, command the high speed cooling fan relay ON.

Do both cooling fans switch to high speed?
—
Go to Step 6
Go to Cooling Fan table #3

6
 1. With a scan tool, exit outputs.
 2. Idle the engine leaving the A/C OFF.
Are the cooling fans ON?
—
Go to Step 8
Go to Step 7

7
Turn ON the A/C.

Are the cooling fans ON?
—
System OK
Go to Step 9

8
Does the scan tool display A/C request as YES?
—
Go to Functional Check for further diagnosis of the A/C system.
Go to Step 10

9
Does the scan tool display A/C request as YES?
—
Go to Functional Check
Go to Functional Check

10
Important: Program the replacement PCM.

Refer to Powertrain Control Module Replacement/Programming.

Replace the PCM.

Is the action complete?
—
System OK
—


Electric Cooling Fan Table #1 Step
Action
Value(s)
Yes
No

1
Did you perform the Cooling Fan Functional Check?
—
Go to Step 2
Go to Cooling Fan Functional Check

2
Disconnect the low speed fan relay.

Are both fans OFF?
—
Go to Step 8
Go to Step 3

3
Disconnect series/parallel fan relay.

Are both fans OFF?
—
Go to Step 4
Go to Step 10

4
 1. Disconnect the left fan.
 2. Probe the fan feed terminal of the left fan connector using a J 35616-200 test lamp that is connected to a good ground.
Does the test lamp illuminate?
—
Go to Step 11
Go to Step 5

5
Probe the series/parallel relay switch feed terminal at the left fan connector using a test lamp that is connected to a good ground.

Does the test lamp illuminate?
—
Go to Step 12
Go to Step 6

6
Probe the series/parallel cooling fan relay control circuit at the series/parallel cooling fan relay connector using a test lamp that is connected to B+.

Does the test lamp illuminate?
—
Go to Step 7
Go to Step 13

7
 1. Turn OFF the ignition.
 2. Leave the test lamp installed.
 3. Disconnect the PCM.
 4. Turn ON the ignition, with the engine OFF.
Is the test lamp still illuminated?
—
Go to Step 14
Go to Step 16

8
Probe the low speed cooling fan relay control circuit at the low speed cooling fan relay connector using the test lamp that is connected to B+.

Does the test lamp illuminate?
—
Go to Step 9
Go to Step 13

9
 1. Turn OFF the ignition.
 2. Leave the test lamp installed.
 3. Disconnect the PCM.
 4. Turn ON the ignition, with the engine OFF.
Is the test lamp still illuminated?
—
Go to Step 14
Go to Step 16

10
Repair the RH fan feed circuit for a short to voltage. Refer to Wiring Repairs.

Did you find and correct the condition?
—
Go to Cooling Fan Functional Check
Go to Step 15

11
Repair the LH fan feed circuit for a short to voltage. Refer to Wiring Repairs.

Is the action complete?
—
Go to Cooling Fan Functional Check
—

12
Repair the short to voltage in the series/parallel relay switch feed circuit. Refer to Wiring Repairs.

Is the action complete?
—
Go to Cooling Fan Functional Check
—

13
Replace the low speed cooling fan relay.

Is the action complete?
—
Go to Cooling Fan Functional Check
—

14
Repair the high speed cooling fan relay control circuit or the low speed cooling fan relay control circuit for a short to ground. Refer to Wiring Repairs.

Is the action complete?
—
Go to Cooling Fan Functional Check
—

15
Replace the high speed cooling fan relay.

Is the action complete?
—
Go to Cooling Fan Functional Check
—

16
Important:: Program the replacement PCM.

Refer to Powertrain Control Module Replacement/Programming.

Replace the PCM.

Is the action complete?
—
Go to Cooling Fan Functional Check
—

Electric Cooling Fan Table #2 Step
Action
Value(s)
Yes
No

1
Did you perform the Cooling Fan Functional Check?
—
Go to Step 2
Go to Cooling Fan Functional Check

2
Did either fan turn ON when the low speed cooling fan relay was commanded ON?
—
Go to Step 3
Go to Step 5

3
 1. Install a scan tool.
 2. With a scan tool, command the low speed cooling fan relay ON.
Did the other fan turn OFF?
—
Go to Step 19
Go to Step 4

4
Disconnect the high speed cooling fan relay.

Did the fan turn OFF?
—
Go to Step 20
Go to Step 21

5
 1. With a scan tool, command the low speed cooling fan relay ON.
 2. Disconnect the low speed cooling fan relay.
 3. Probe the low speed cooling relay control circuit at the the low speed cooling fan relay connector using a J 35616-200 test lamp that is connected to B+.
Does the test lamp illuminate?
—
Go to Step 6
Go to Step 13

6
Probe both feed circuits at the low speed cooling fan relay connector using a test lamp that is connected to ground.

Does test lamp illuminate for both circuits?
—
Go to Step 7
Go to Step 22

7
Important: Leave jumper in place for remainder of this table.

Connect a fused jumper wire between the switch feed circuit and the LH fan feed circuit at the low speed cooling fan relay connector.

Do both fans turn ON?
—
Go to Step 23
Go to Step 8

8
 1. Disconnect the left fan.
 2. Probe left fan feed circuit at the fan harness connector terminal with a test lamp that is connected to a good ground,
Does the test lamp illuminate?
—
Go to Step 9
Go to Step 24

9
Connect a second fused jumper wire between the left fan harness connector terminals.

Is the right fan ON?
—
Go to Step 16
Go to Step 10

10
 1. Reconnect the left fan.
 2. Disconnect the C fan relay.
 3. Probe the switch feed circuit at the series/parallel fan relay connector using a test lamp that is connected to a good ground.
Does the test lamp illuminate?
—
Go to Step 11
Go to Step 25

11
Using the second jumper wire, jumper the switch ignition feed circuit and the RH fan feed circuit together at the series/parallel relay connector.

Do the fans come ON?
—
Go to Step 26
Go to Step 12

12
 1. Reconnect the series/parallel fan relay.
 2. Disconnect the right fan.
 3. Probe the fan feed circuit at the fan harness connector terminal with a test lamp that is connected to a good ground.
Is the left fan ON?
—
Go to Step 16
Go to Step 18

13
 1. Test lamp still connected.
 2. Turn OFF the ignition.
 3. Disconnect the PCM.
 4. Probe the low speed cooling fan relay control circuit at the PCM connector, with a fused jumper wire that is connected to a good ground.
Is the test lamp ON?
—
Go to Step 20
Go to Step 21

14
Inspect the PCM connections. Repair faulty connections as necessary. Refer to Intermittents and Poor Connections Diagnosis and Repairing Connector Terminals.

Did you find a problem and correct it?
—
Go to Cooling Fan Functional Check
Go to Step 27

15
Repair the low speed fan relay control circuit for an open or short to B+. Refer to Wiring Repairs.

Is the action complete?
—
Go to Cooling Fan Functional Check
—

16
Inspect the fan motor ground circuit for an open or the fan motor connections and repair as necessary. Refer to Intermittents and Poor Connections Diagnosis and Repairing Connector Terminals or Wiring Repairs.

Did you find and correct the condition?
—
Go to Cooling Fan Functional Check
Go to Step 17

17
Replace the fan motor.

Is the action complete?
—
Go to Cooling Fan Functional Check
—

18
Repair the RH fan feed circuit for an open. Refer to Wiring Repairs.

Is the action complete?
—
Go to Cooling Fan Functional Check
—

19
Replace the fan which was not operating.

Is the action complete?
—
Go to Cooling Fan Functional Check
—

20
Replace the high speed fan relay.

Is the action complete?
—
Go to Cooling Fan Functional Check
—

21
Repair the series/parallel fan relay switch feed circuit for a short to ground. Refer to Wiring Repairs.

Did you find and correct the condition?
—
Go to Cooling Fan Functional Check
Go to Step 26

22
Repair the open or the grounded circuit for the circuit that did not light. Refer to Wiring Repairs.

Is the action complete?
—
Go to Cooling Fan Functional Check
—

23
Replace the low speed fan relay.

Is the action complete?
—
Go to Cooling Fan Functional Check
—

24
Repair the LH fan feed circuit for an open. Refer to Wiring Repairs.

Is the action complete?
—
Go to Cooling Fan Functional Check
—

25
Repair the series/parallel relay switch feed circuit for an open. Refer to Wiring Repairs.

Is the action complete?
—
Go to Cooling Fan Functional Check
—

26
Replace the series/parallel fan relay.

Is the action complete?
—
Go to Cooling Fan Functional Check
—

27
Important: Program the replacement PCM.

Refer to Powertrain Control Module Replacement/Programming.

Replace the PCM.

Is the action complete?
—
Go to Cooling Fan Functional Check
—

Electric Cooling Fan Table #3 Step
Action
Value(s)
Yes
No

1
Did you perform the Cooling Fan Functional Check?
—
Go to Step 2
Go to Cooling Fan Functional Check

2
 1. With a scan tool, command the low speed cooling fan relay ON.
 2. Command the high speed cooling fan relay ON while observing the fans.
Did both fans operate with no change?
—
Go to Step 8
Go to Step 3

3
Did the left fan stop operating?
—
Go to Step 9
Go to Step 4

4
 1. Disconnect the high speed fan relay.
 2. Probe the high speed fan relay control circuit at the high speed fan relay connector using a J 35616-200 test lamp that is connected to B+.
 3. Command high speed cooling fan relay ON using a scan tool.
Does the test lamp illuminate after several seconds?
—
Go to Step 5
Go to Step 12

5
Probe the coil feed circuit at the high speed fan relay connector location using a test lamp that is connected to a good ground.

Does the test lamp illuminate?
—
Go to Step 6
Go to Step 13

6
Probe the switch feed circuit at the high speed fan relay connector using a test lamp that is connected to a good ground.

Does the test lamp illuminate?
—
Go to Step 7
Go to Step 14

7
Jumper the switch ignition feed circuit and the RH fan feed circuit together at the high speed fan relay connector.

Is the right fan ON?
—
Go to Step 15
Go to Step 16

8
 1. Turn OFF the ignition.
 2. Disconnect the PCM.
 3. Disconnect low speed fan relay.
 4. Jumper the switch feed circuit and the LH fan feed circuit together at the low speed fan relay connector.
 5. Turn the ignition ON.
 6. Probe the PCM harness connector for the high speed cooling fan relay control circuit with a fused jumper that is connected to a good ground.
Do the fans switch from low to high speed?
—
Go to Step 22
Go to Step 17

9
 1. Disconnect the series/parallel fan relay.
 2. Probe the high speed fan relay control circuit at the series/parallel fan relay connector using a test lamp that is connected to B+.
 3. Command the high speed cooling fan relay ON using a scan tool.
Does the test lamp illuminate?
—
Go to Step 10
Go to Step 18

10
Probe the ground circuit at the series/parallel fan relay connector using a test lamp that is connected to B+.

Does the test lamp illuminate?
—
Go to Step 11
Go to Step 19

11
Probe the coil feed circuit at the series /parallel relay connector using a test lamp that is connected to a good ground.

Does the test lamp illuminate?
—
Go to Step 20
Go to Step 21

12
Repair the high speed fan relay control circuit for an open between the high speed fan relay and the splice. Refer to Wiring Repairs.

Is the action complete?
—
Go to Cooling Fan Functional Check
—

13
Repair the open in the high speed fan relay coil feed circuit . Refer to Wiring Repairs.

Is the action complete?
—
Go to Cooling Fan Functional Check
—

14
Repair the open in the high speed fan relay switch feed circuit . Refer to Wiring Repairs.

Is the action complete?
—
Go to Cooling Fan Functional Check
—

15
Replace the high speed fan relay.

Is the action complete?
—
Go to Cooling Fan Functional Check
—

16
Repair the open in the RH fan feed circuit between the high speed fan relay and the splice. Refer to Wiring Repairs.

Is the action complete?
—
Go to Cooling Fan Functional Check
—

17
Repair the high speed fan relay control circuit for an open or for a short to B+. Refer to Wiring Repairs.

Is the action complete?
—
Go to Cooling Fan Functional Check
—

18
Repair the open in the high speed fan relay control circuit between the series/parallel fan relay and the splice. Refer to Wiring Repairs.

Is the action complete?
—
Go to Cooling Fan Functional Check
—

19
Repair the open in the ground circuit. Refer to Wiring Repairs.

Is the action complete?
—
Go to Cooling Fan Functional Check
—

20
Replace the series/parallel fan relay.

Is the action complete?
—
Go to Cooling Fan Functional Check
—

21
Repair the open in series/parallel fan relay coil feed circuit . Refer to Wiring Repairs.

Is the action complete?
—
Go to Cooling Fan Functional Check
—

22
 1. Inspect the PCM connections. Refer to Intermittents and Poor Connections Diagnosis.
 2. Repair faulty connections as necessary. Refer to Repairing Connector Terminals.
Did you find and correct the condition?
—
Go to Cooling Fan Functional Check
Go to Step 23

23
Important:: Program the replacement PCM.

Refer to Powertrain Control Module Replacement/Programming.

Replace the PCM.

Is the action complete?
—
Go to Cooling Fan Functional Check
—

hello sorry to bring this one back up but Im having same problem with fans staying on at high speed all the time, also coolant level light is on but its full o coolant Im gonna replace the coolant level sensor in the radiator and hope dis fixes the problemo.

Im hoping this fixes the problem as I can kinda remember that I shut the car off with the ac on and then the fans started doing this. But I dont think it is the pcm cus I can turn the ignition on and it takes awhile for the fans to come on. If the pcm was shorted they would come on soon as the ignition is turned on.

sloshua wrote:

Im hoping this fixes the problem as I can kinda remember that I shut the car off with the ac on and then the fans started doing this. But I dont think it is the pcm cus I can turn the ignition on and it takes awhile for the fans to come on. If the pcm was shorted they would come on soon as the ignition is turned on.

It's tedious but I would just follow the troubleshooting Abaddon posted.

Some of these cases turn out to be multiple sensors failed or got marginal, or one dies and shorts then takes the others on the same circuit with it.

Ok got the part in the mail yesterday but had to haul ass to work i do night shift, so during my shift we had a pretty good thunder storm after work get in the car fire it up and no coolant level light and fans no longer stay on all the time im keepin dis damn CLS in case it happens again .

Albert I have printed out the flow chart for trouble shooting this thing in the future cus I know some where down the line shes gonna do this again.....

ya the coolant level sensor fixed the problem. The tip of the sensor was all cloudy, some kinda led sensor.