Hi, it seems my cooling fans are running from start to stop now (with or without heat on.. it's winter) which has brought the running temp down to about 170°F according to the digital gauge.

Any help is much appreciated (daily driver) it's a 94 LT1 automatic, 52k miles.

 

Standard equipment for all LT1 F-cars with A/C is a dual electric fan setup with primary (LH) and secondary (RH) fans. There are two different wiring schemes used for these fans, an early design that was used in '93-'94 and a late design that has been used from mid-'94 up. Note that non-A/C F-cars have a single primary fan which operates at a fixed high speed.

In '93 and early '94 models with A/C, the two cooling fans are independently operated by the PCM (Powertrain Control Module) at a high fixed speed by using a single relay for each fan. Late '94 and newer F-car models operate both fans simultaneously in either a low or a high speed mode by using 3 relays. In low speed mode, the fans are powered in series. In high speed mode, the relays operate to power both fans in parallel, resulting in a higher speed of operation.

One way to tell which setup you have is by looking at the alternator. If an F-car is equipped with the 124 amp alternator (KG7), then the vehicle has the early design setup and the fans are operated independently. If the vehicle has the 140 amp alternator (KG9), then it also has the newer design configuration which operates the fans simultaneously in low or high speed modes.

The PCM operates the coolant fans based on input from the Engine Coolant Temperature (ECT) sensor, A/C Pressure Sensor, Vehicle Speed Sensor (VSS), and various other inputs. The F-car coolant fans operate at the following temperatures and pressures:

Fan Mode
Temperature A/C Pressure
Primary (LH) or Dual Low-speed Fan(s) ON: 108 C 226 F 248 psi*
Primary (LH) or Dual Low-speed Fan(s) OFF: 105 C 221 F 208 psi*
Secondary (RH) or Dual High-speed Fan(s) ON 113 C 235 F 248 psi
Secondary (RH) or Dual High-speed Fan(s) OFF: 110 C 230 F 208 ps

*Note - this information is probably incorrect, although it is quoted from the service manual.

Additionally, the PCM will turn off the fans at higher vehicle speeds (above 70 MPH I believe) since running fans can actually impede airflow through the radiator at high speed. Each fan or fan mode has a minimum running time. Once activated, the primary fan or dual low-speed fans will run for a minimum of 50 seconds, and the secondary or dual high-speed fans for a minimum of 30 seconds. Finally, certain Diagnostic Trouble Codes (DTCs) may cause the PCM to turn on one or both fans.

SOURCE: http://www.theherd.com/articles/lt1_cool.html

 

Hmm, so it could be caused by the ECT, A/C Pressure Sensor, VSS, or fault codes from another issue.

Well atleast I've got something to look at now, thx Idk if they shut off at higher speeds due to road noise.

I rly need to find one of those 14 pin GM code readers too!

 

Nice write up by "seabright".

But in reality you probably have the two independent fan operation as I do. The primary fan will come on with the A/C or at 229*. And the secondary fan comes on at 239*. The fans do shutdown at expressway speeds but it is lower than 70 mph as I have seen on the tech1.

Trying to find a off the shelf inexpensive scanner from a parts store is going to be tough in today’s market as your car is OBD1 with a OBD2, 16 pin connector. You will need to find a used tech1 with the proper software and adaptors (not easy). Or buy an expensive scanner, or get a older laptop with a serial port and some good software to interface with the car.

The bright side is the on board diagnostics for extracting the codes works very well. Each computer module has its own set of codes which can be cleared. This will give you the same basic information as a external scanner plus even more. Learn how to use this as it is a great feature. Basically you are connecting pins 4 & 12 and turning the key on. There is a bit more but need to get you the link and additional information. The more expensive units will enable you to do some advanced functions, but the codes will be the same.

I would at least start our disconnecting the battery and try to reset the PCM and see if the problem goes away. The secondary fan is strictly controlled by the PCM and driven by the digital temperature readout by the sensor in the water pump. The primary fan is also driven by the PCM but has the A/C system as well as the digital temp as drivers as to when it comes on.
(Of course the fan relays turn on the fans which is energized by the PCM).

If both fans have the problem and continue to run, then I would initially look at two things. One is the PCM and its connections. And the other is to make sure the digital readout on the speedometer is measuring correctly.

 

ECT will cause the fans to come on but will throw a check engine light.

A/C pressure switch shouldn't cause the fans to come on. Turning off the A/C, should turn off the fans (if that was what was causing them to run)

VSS might.

Other codes might.

ECT is most likely if it is related to a code. Start with pulling codes from your car. You don't need any fancy scanner for that. Any autozone will do it for free

 

I don’t believe that AutoZone has the equipment to pull the codes on a 94 (-95). They don’t know the car and will bring out a OBD 2 code reader because it has the OBD2 connector and won’t understand why they get a communication link error. It won’t work because it is OBD1. Then they will tell you that there is a computer problem in the car.

That is why the for the current situation the best thing to do is to use the on board diagnostics and get it done properly and do it yourself.

*Edit*
On the 94 it is a OBD1 system but it has the OBD 2 connector. You need the short pins 4 and 12, pin 4 is the ground pin. After that turn key to on but don't start (crank) engine. It will display the codes if there are any. Watch for 1, 4 and 9 on the spedometer area. Write down the corrosponding codes for the related modules. Mod 1 is the CCM which is where the SYS error comes from. Mod 2 is the PCM errors and Mod 9 is the ABS system. After it runs thru, turn key off and do it again if need be. You will see a C12 in mod 1 which is not a error but a happy code when you see it, it says the CCM is working and is a self test function. ** Remove the jumper when finished!!

http://www.corvettebuyers.com/c4vettes/codes.htm

And here are the PCM codes

1994-1995: Diagnostic Trouble Codes
DTC #11: Malfunction Indicator Lamp Circuit.
DTC #13: Bank #1 Heated Oxygen Sensor #1 Circuit:
DTC #14: Engine Coolant Temperature Sensor Circuit Voltage Low.
DTC #15: Engine Coolant Temperature Sensor Circuit Voltage High.
DTC #16: Distributor Ignition System Low Pulse.
DTC #18: Injector Circuit.
DTC #21: Throttle Position Sensor Circuit Voltage High.
DTC #22: Throttle Position Sensor Circuit Voltage Low.
DTC #23: Intake Temperature Sensor Circuit Voltage High.
DTC #24: Vehicle Speed Sensor Circuit.
DTC #25: Intake Air Temperature Sensor Circuit Voltage Low.
DTC #26: Evaporative Emission Canister Purge Solenoid Valve Circuit.
DTC #27: EGR Vacuum Control Signal Solenoid Valve Circuit.
DTC #28: Transmission Range Pressure Switch Assembly Fault.
DTC #29: Secondary Air Injection Pump Circuit.
DTC #32: Exhaust Gas Recalculation.
DTC #33: Manifold Absolute Pressure Sensor Circuit High.
DTC #34: Manifold Absolute Pressure Sensor Circuit Low.
DTC #36: Distributor Ignition System High Pulse.
DTC #37: Brake Switch Stuck On.
DTC #38: Brake Switch Stuck Off.
DTC #41: Ignition Control Circuit Open.
DTC #42: Ignition Control Circuit Shorted.
DTC #43: Knock Sensor Circuit.
DTC #44: Bank 1 LF Heated Oxygen Sensor #1 Circuit Lean.
DTC #45: Bank 1 LF Heated Oxygen Sensor #1 Circuit Rich.
DTC #47: Knock Sensor Circuit Or Module Missing.
DTC #48: Mass Air Flow Sensor Circuit.
DTC #50: System Voltage Low.
DTC #51: EPROM Programming Error.
DTC #52: Engine Oil Temperature Sensor Circuit Voltage Low.
DTC #53: System Voltage Low.
DTC #55: Fuel Lean Monitor.
DTC #58: Transmission Fluid Temperature Sensor Circuit Low.
DTC #59: Transmission Fluid Temperature Sensor Circuit High.
DTC #62: Engine Oil Temperature Sensor Circuit Voltage Low.
DTC #63: Bank 2 RF Heated Oxygen Sensor #1 Circuit Open.
DTC #64: Bank 2 RF Heated Oxygen Sensor #1 Circuit Lean.
DTC #65: Bank 2 RF Heated Oxygen Sensor #1 Circuit Rich.
DTC #66: A/C Refrigerant Pressure Sensor Circuit Open.
DTC #67: A/C Pressure Sensor Circuit Sensor or A/C Clutch.
DTC #68: A/C Relay Circuit.
DTC #69: A /C Clutch Circuit.
DTC #70: A/C Clutch Relay Driver Circuit.
DTC #72: Vehicle Speed Sensor Loss.
DTC #73: Pressure Control Solenoid Circuit Current Error.
DTC #74: Traction Control System Circuit Low.
DTC #75: Transmission System Voltage Low
DTC #77: Primary Cooling Fan Relay Control Circuit.
DTC #78: Secondary Cooling Fan Relay Control Circuit.
DTC #79: Transmission Fluid Overtemp.
DTC #80: Transmission Component Slipping.
DTC #81: Transmission 2-3 Shift Solenoid Circuit.
DTC #82: Transmission 1-2 Shift Solenoid Circuit.
DTC #83: Torque Converter Solenoid Voltage High.
DTC #84: 3-2 Control Solenoid Circuit.(Auto Only).
DTC #84: 2nd And 3rd Gear Blockout Relay Control Circuit.
DTC #85: Transmission TCC Stock On.
DTC #90: Transmission TCC Solenoid Circuit.
DTC #91: One To Four Upshift Lamp(Manual Only).
DTC #97: VSS Output Circuit.
DTC #98: Tachometer Output Signal Voltage Wrong.

 

Great posts guys!

No, the parts stores cant scan it for me cause it's OBD1... already tried

and the A/C is NOT on (it's 20° out here) so I'll see if I can get some codes out of it

 

Yeah, I forgot about that silly OBDII connector on there. Why the heck did GM do that anyway. You could just jumper the data wire from the OBDII to the OBDI code reader and add a ground but that is pointless busywork. Use the paperclip trick and report back!.

 

Ok.. thx to trusty ol' paperclip I got these codes:

1 = C12
4 = H36, H42
9 = H62
A = (nothing)

and then it says 1.0... which im guessing is a version # perhaps?

 

H=Historical, C=Current

1.0 indicates manual mode and the CCM is awaiting technician instructions. (according to alldata)

12 means you have no codes. The historical codes indicated you either recently changed the opti-spark or you probably will have to soon.

So, it would appear your fan issue isn't related to a sensor or PCM input.

 

Is the defrost on?? When you turn on the defrost, it kicks the A/C compressor on...

 

No, no defrost

Do those code #'s relate to pcolt's post? Just nevermind the DTC part? I noticed code 12 wasnt in that post; is there more then 1 bank of O2 sensors btw?

 

and these guys say "---" means no codes.... I'm confused! lol \o/

http://www.corvettebuyers.com/c4vettes/codes.htm

 

Yes, the code numbers relate to PColts post. --- technically means no codes, but 12 is the delimiter intended to show where the ECM code display starts and stops. If you didn't have the historic codes, you would have no codes. I guess technically, I should have said "you have no current codes".

Not sure if you have more than one bank of 02 or not. On my 94 LT1 f-body, I only had one. Since your car is mostly OBD1 I'm fairly certain you only have 1 bank. If there is an 02 on each side of the motor, you have 2 banks. If it's on the y-pipe or only on one side, you have 1 bank.

 

Alright, cool

Is it possible that the AC compressor is stuck open? I know I dont have anything turned on.

and would changing the temp sensor switch do anything you think?

If there's only 1 bank of o2 sensors then GREAT... I'd rather not deal with anymore of those then is absolutely neccesary!

 

Doubt the A/C is stuck on, but you can check. Open the hood and start it up. See if the clutch is engaged and turning the A/C pump.

You could try a temp sensor. It could be shorted closed so the car thinks is always running hot. It will cost you a whopping $12 to buy one so it might be worth it.

 

Hmm, i wonder if my uber sucky fuel mileage (13) could be caused by the ECM thinking the engine is really hot and running the car too rich...?

Bought the fan temp switch today, $13, I'll post the r2 results asap; looks like it's going to be hard to get to though, of coarse :P

 

It isn't too bad. Should be near on the front of the waterpump. Make sure you don't give your optispark a drink. Last one I changed....I had the new one ready to go wrapped with teflon tape. Loosened the old one then quick yanked it out, quickly popped the new one in the hole and threaded it in to stop the coolant loss. I think I only lost about 1 cup of coolant and it saved me from having to drain the system.

 

Haha, well I'm thinking of changing the thermostat as well since the car's running very cool; so I'd have to drain it atleast a little anyway.

If it's the one by the water neck what's the sensor back by the #6 cylinder?? The Haynes book said it was back there for the fan, but it does include a few different year models.

 

When I purchased my '95 LT1 auto with C60 A/C (manual control), the compressor always ran and the fans were on low speed. Lots of diagnostic red herrings but in the end the problem was traced to a shorted transistor in the Heater-A/C control head. The Service Manual shows a rotary switch but that's not what's in there. There's a bank of transistor logic that feeds to the computer. The shorted transistor was saying "turn on the A/C" to the computer and it did. 79 cents at Radio Shack fixed the problem and now the compressor and fans cycle as they should and only when the control buttons are set to A/C or defrost.
BTW, the shorted transistor was easy to spot. It looked like a half peeled banana and ohming it proved the short. Getting to it was a pita.
Further, if the correct diagnostic reader is connected, the fans will run with the key on. And if the ECM or PCM (year dependent) has a poor ground, the fans will run.
Hope this helps, recognizing that you may have manual or auto temp control.