Failsafe mode in modern cooling fans

Hi, all,

Question about how do cooling fans controlled by the ECU know that there is an open in the signal wire?

Typical four wire connector:
-One constant main feed
-Main ground
-supply V from source after KOEO
-signal from ECU

Ok, so the ECU receives input, then ground side switches the signal to modulate the cooling fan. 

Possible problem is is that the fan runs all the time with KOEO or engine running. 

Apparently, this is a failsafe mode. The ECU detecting it has an open in the signal wire, unable to switch the fan on, has the fan running all the time. 

Right, but how on Earth does the fan know that it's supposed to run all the time?

That's what I can't understand. Cars running, constant feed and ground, and the supply needed for the ECU to ground. 

But it cannot because there's an open in the signal wire. So backprobing the fan connector we see a flat line 12v on the signal wire
 

But that's the thing, there's ALWAYS a flat line 12v on the signal wire just waiting to be grounded by the ECU. That's when the fan gets switched on, when the signal is pulled down. 

But, apparently, that's a failsafe when the ECU sees an open in the signal. 

But... What's controlling the fan then?

I’m not entirely sure I understand your question so forgive me if I’m wrong, and if someone else out there knows better than me, please correct me. This sounds a lot like the way that alternators work in a lot of modern cars, they work on a Lin bus (a single wire communication network generally between two modules, a master and a slave) as the slave module and the pcm is the master in the system, if these alternators don’t receive any message from the pcm about when and how much they’re supposed to charge, they go into a fail safe mode and simply keep the system voltage at 13.5v all the time. I believe the system you’re talking about works the same, where if the cooling fan (slave module) never receives a command from the pcm (master module) then it is programmed to simply run all the time as a fail safe. The fact that it never receives a command, IS the command.

What kind of car are we taking about here?

Thank you both for chiming in.

Mathew, that's an interesting comment, I have heard the more modern cars operate that way. The design I am talking about is in-between a typical thermoswitch and the system you are talking about.

I must admit I have a very disorganised way of writing questions... that's why everything is muddled up. Sorry.

Noah, There isn't a specific car in question, however, we can follow this YT video that I'm basing this question on. It's called VW/Audi Fan Controller - Case Study. I'll post the wiring diagram for this type of PWM controlled fan. Here is a link to the video. 

BTW- on the wiring diagram, at the top, we see the pin 2 wire red/gry. There's supposed to be V there with KOEO/running. I suspect that that is the V for the module itself that gets sent to the ECU for it to be grounded when the fan is supposed to switch on. Am I right? 

AND one of the first comments on that YT video reads that "However it should be known that most of the Audi fan modules have an internal fail safe to run 100% if the pwm signal is missing".

In the video itself it turned out that the fan was running on all the time because it was, just like with Paul's 2003 Mercury Grand Marquis, backfeeding the main 12v and making the fan run all the time even with the key out. The module was shorted.

Now, I understand the operation of the system, but it's just weird because if we back probe the signal wire at the controller KOEO/running, we should expect to see system V flatline because the fan is commanded OFF.

When the ECU wants the fan turned on then it will start pulsing that signal V to ground in order to switch the fan on.

This is understand.

HOWEVER, I don;t understand that supposed "failsafe". The fan apparently turns on full throttle if the fan module doesn't detect a PWM signal (e.g. signal wire open).

But on a fully functioning system, with KOEO/running, we'll see 12v flatline AND NO PWM SIGNAL because the fan isn't being turned on! So, how on Earth does the fan know when to switch on if there is an "open" in the signal wire?

Because in both cases the signal wire looks the same, system V flatline, waiting for a pulsed signal from the ECU.

I hope I phrased that a bit better!

Now, I've heard that if the car is running and the signal wire is shorted to ground then the fan would run continuously, that's obvious, it's as if the ECU was providing a constant ground.

And also, if you have the keys out, and apply 12v to that small pin 2 V wire, apparently that also makes the fan go full throttle... I have simply no idea why.

Ok! I think I've finally managed to solve the issue and have an explanation!

On ScannerDanner's Mercury video, you have a 3 wire connector. Constant 12v, main GND, and a signal wire that reads 12v flatline WHEN THE FAN IS OFF. It's a groundside switched design, so it's waiting for a GND from the ECU to switch on.

Now, the four wire design WOULD WORK the same way IF there wasn't that built in failsafe mode.

The signal wire would read 12v flatline waiting for a GND from the ECU.

Now, that's not the case. With the key removed, you're supposed to see 12 + GND MAIN, and no V on the smaller wires.

With KOEO, you'll now see a PWM signal on the signal wire and 12v on the other smaller wire that's supposed to be a supply from a relay that powers the fan module itself.

I MADE THE MISTAKE OF THINKING THAT THIS WIRE SUPPLIES THE V NEEDED FOR THE ECU TO GROUND IT. It doesn;t. The V on the signal wire comes from the main 12v feed.

Now, why a 5-10% duty cycle on the signal wire with fan commanded off?

BECAUSE OF THE FAILSAFE. WIth the fan commanded OFF, the module WANTS TO SEE that 5-10% duty cycle PWM signal. That means the ECU is working and that there are NO issues with it and the signal wire.

The fan remains off, it's too low of a duty cycle.

Now, let's say there were to be an issue with the ECU/signal wire.

With KOEO/running, you'd see a FLATLINE 12v on the signal wire (coming from the module) and supply V coming from the relay on the other smaller wire.

THIS is what triggers the failsafe mode.

In other words, the fan module thinks "if I'm getting a supply V from the relay, that means the car should be on/running, but I'm not recieving a PWM signal, therefore, there must be something wrong so I'll switch on full throttle not to overheat the car until the issue get's fixed".

Which is why if there is a problem and you get V on the supply wire with the key OFF (eg some sort of backfeeding etc.) you'll get the fan running full blast until the battery dies out.

Because the module is being woken up but, with no ECU activity (key's out), it thinks there's an issue and so, the failsafe.

JUST LIKE YOU SAID MATTHEW, no PWM signal IS the signal. It means something is wrong.

Hence the need to have some sort of PWM signal to keep the fans OFF. Wierd, I know.