Fans working, but not coming on at temperature

[B6 Dude]

Having an odd issue... I swapped both radiator fans, fan control module (but now have the "D" version), and the temp sensor in the lower radiator hose. But my fans won't come on at temperature. They both come on when I turn on the A/C though, so I suspect the controller and fans are fine. Only OBD codes I get are for an unrelated cylinder misfire.

Anyone know what else may be wrong here?

Thanks.

[rocket1420]

I think it'll be hard to know for sure without getting VCDS out. What do you mean by "at temperature?" You can idle all day long, and depending on the weather, the fans might never kick on.

[B6 Dude]

Using an OBD gauge, fans did not come on up to 225 deg-F. I'm pretty sure the fans came on at a lower temp previously (when the car was running fine).

Need to figure out where my VCDS is.

[daught]

You should look at measuring blocks 131 for temps and 135 to see if the car is turning the fan on.

When you say the fans work under what conditions do they come on?

[B6 Dude]

If I turn on the A/C, the fans come on right away, which indicates that the fan controller and fans are working.

Will check those blocks... once I figure out where my VAG-COM is.

[B6 Dude]

Found the Vag-com, but was chasing down the cylinder misfire, which turned out to be a bad coil connector.

I never used the measuring blocks before, and it's pretty neat. With actual engine outlet temp up to 100-102 deg-C, Radiator outlet temp at 39 deg-C, Thermostat duty cycle varying from the low teens to low 30's %, Fan 1 activation duty-cycle was fixed at 11 %. Idling for a while, radiator outlet temp went up to as high as 80 deg-C, but fan 1 activation stayed at 11%. BTW, at 11%, the left fan was off. I put on the A/C and fan 1 activation DC went to the 30's %, the fan was visibly running, and radiator outlet temps dropped back to the 30's deg-C. Engine outlet temp did stay at no more than 103 deg-C though.

I originally raised this concern as the temp gauge went just past centerline, which is not normal on this car. I did not push it to that point this time until I first know where the threshold is for me to be concerned. Anyone know what that temp is? And is it based on radiator outlet temp or engine outlet temp?

BTW, what is "Engine Outlet Temp (specified)"?

Thanks.

[5ktq]

That's the temperature at the plastic water flange on the back of the head - the green sensor on there.

The gauge in the cluster runs off it too. The cluster runs modified code ... the temperature gauge isn't linear, it's basically a 2 stage idiot light. cold, ok, meltdown impending. I think anywhere between 75 and 105 or 110C it just shows dead centre (90C on a canadian cluster).

Then once it gets over 105 or 110C or whatever it takes off towards redline.

[B6 Dude]

The 4-pin sensor at the back of the head would be the engine outlet temp. The 2-pin sensor in the lower radiator hose would be the radiator outlet temp.
I was wondering about the meaning of "specified" as there's these 2 values on measuring block 131...
- Engine Outlet Temp (actual)
- Engine Outlet Temp (specified)

Yes, the taking off towards redline worries me, because I had this issue some years back and did that. I later plotted OBD-II coolant temp vs. the needle and found exactly that -- that the need is designed to stay perfectly centered through a wide range of temps. By the time it starts to move higher than center, the engine is already overheating.

[5ktq]

Yeah. Stupid design choice. Wish you could fix the firmware on the cluster so that it reads true. I think all the vag stuff from that era is the same. My GTI was, for sure.

[rocket1420]

Found the Vag-com, but was chasing down the cylinder misfire, which turned out to be a bad coil connector.

I never used the measuring blocks before, and it's pretty neat. With actual engine outlet temp up to 100-102 deg-C, Radiator outlet temp at 39 deg-C, Thermostat duty cycle varying from the low teens to low 30's %, Fan 1 activation duty-cycle was fixed at 11 %. Idling for a while, radiator outlet temp went up to as high as 80 deg-C, but fan 1 activation stayed at 11%. BTW, at 11%, the left fan was off. I put on the A/C and fan 1 activation DC went to the 30's %, the fan was visibly running, and radiator outlet temps dropped back to the 30's deg-C. Engine outlet temp did stay at no more than 103 deg-C though.

I originally raised this concern as the temp gauge went just past centerline, which is not normal on this car. I did not push it to that point this time until I first know where the threshold is for me to be concerned. Anyone know what that temp is? And is it based on radiator outlet temp or engine outlet temp?

BTW, what is "Engine Outlet Temp (specified)"?

Thanks.

This sounds pretty reasonable. Operating temperature tops out at either 105 or 110. The thermostat seems to be keeping it below that. Was this all at idle? I assume so since you were visually monitoring the fans. Take it for a slow spin around the block and see if you can make the engine outlet temperature go past 110.

You could also try tricking the ECU into thinking the engine temperature is like 200 C. Or whatever it's capable of measuring. I assume if you unplug the rear sensor, the MB shows 0 C. This sensor is probably a 5v sensor. So you short the wire carrying the constant 5v to the signal sensing wire, and the temperature read by the ECU should shoot up, causing the fan to turn on. It will probably also turn on when you unplug the sensor, since it goes into a sort of "panic" mode when it can't read one of the sensors.

[old guy]

Repeating some of what has already been said: The temperature in the display reaches the 12:00 position at 175°f and stays there until the temperature passes 230°f. Definitely not a linear instrument.

The Engine Outlet Actual is the temperature of the coolant leaving the head at the rear coolant flange. The Engine Outlet Specified is the temperature that the ECU is trying to maintain under the specific circumstances. If the Specified is lower than the Actual the ECU will increase the thermostat heater duty cycle until the actual drops to meet the specified.

The fans are activated by the input from the sensor in the lower coolant return hose combined with the signal from the ambient temperature sensor. It really doesn't matter what the Engine Outlet temperature is as long as the radiator is producing coolant at a low enough temperature.

Unplugging any of the sensors will put the ECU in "protection mode". This includes a full duty cycle to the thermostat heater along with the radiator fans on high.

[rocket1420]

Unplugging any of the sensors will put the ECU in "protection mode". This includes a full duty cycle to the thermostat heater along with the radiator fans on high.

Essentially, I believe he's worrying about absolutely nothing. You don't get great coolant data at idle. Isn't there a way to manually move the thermostat in VCDS?

[B6 Dude]

So I actually don't have a problem. I did notice the coolant level drop since I filled it some weeks ago, so I'm guessing I had air bubbles in the system, which worked it's way out. Drove around for a couple hours with the A/C off and temps are good, and coolant level is staying fixed now. Woo-hoo!

[B6 Dude]

Yes, this was at idle. I was worried because I did see it go past 12 o'clock once, and I've had past experience with the horrors of overheating years ago. Just being overly cautious.

Repeating some of what has already been said: The temperature in the display reaches the 12:00 position at 175°f and stays there until the temperature passes 230°f. Definitely not a linear instrument.

The Engine Outlet Actual is the temperature of the coolant leaving the head at the rear coolant flange. The Engine Outlet Specified is the temperature that the ECU is trying to maintain under the specific circumstances. If the Specified is lower than the Actual the ECU will increase the thermostat heater duty cycle until the actual drops to meet the specified.

The fans are activated by the input from the sensor in the lower coolant return hose combined with the signal from the ambient temperature sensor. It really doesn't matter what the Engine Outlet temperature is as long as the radiator is producing coolant at a low enough temperature.

Unplugging any of the sensors will put the ECU in "protection mode". This includes a full duty cycle to the thermostat heater along with the radiator fans on high.

Excellent. Thanks.