Cooling fan keeps running - where is the sensor ?

[Julio]

Hi all,
Saw a few posts on the subject in other sections of the forum but never saw it on the GT350 (and also never saw it resolved).

My cooling fans keep turning, even at low temperature. The engine never gets out of normal running temperatures, so rather than any sort of engine overheating I wonder if I have a sensor-related issue.

As I’m in Europe (GT350 not imported here) I lack specialists of the car around me. Would you have any advice on how to check for this and where can I found the sensor ? And the reference of it ?

Thanks a lot !!

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[JAJ]

So, here's the thing, the only temp sensor on the engine is the cylinder head temp sensor, and the ECU uses it for all engine temperature functions. If your dashboard readout for cylinder head temp is in the normal range, then the sensor is fine and something else is wrong. Most likely candidate is one of the three relays that run the fans. The way the controls work is that one relay (FC1 on the diagrams) is wired to deliver power in a "low" cooling situation. In that situation, the two fans are connected in series and run slowly because each one only has 6 volts across it. When "high" cooling is required, the other two relays (fC2 and FC3) switch together and that undoes the series connection and instead gives them each a full 12 volts.

The ECU has the ability to leave them off, run them slowly or run them fast. They are not programmed to run all the time when the AC is turned off. They may run all the time if AC is on.

If relay FC1 is stuck in the activated position, the both fans will run at low speed all the time even even if they're not activated by the ECU. They will continue to run at low speed if the ECU commands "low", and they'll switch to "high" on command as well.

If FC3 is stuck, one fan will run fast all the time, and if the ECU commands "low", nothing will change - the second fan won't start. At "high" both fans will turn fast.

If FC2 is stuck, no fans will run all the time, but when the ECU commands the "low" setting, one will run fast and the other will stay stopped. Both will turn fast if the ECU commands "high".

It's essentially fail-safe - you might have fans when you don't need them, but there's no single failure that disables them.

So, knowing all that we can troubleshoot:

1. Turn AC off. If the fans stop, you're done. It's working as intended.
2. If they're still running, then check the CHT value in the instrument panel readout. If it's above 200F/90C, then let it cool down.
3. When CHT is below 200F/90C, then check to see if both fans are running or just one.
4. If it's both, pull FC1. If the fans stop, then replace FC1.
5. If it's only one fan running then pull FC3. If the fan stops then replace FC3.

Your Mustang Owner's Manual has a diagram of the fuse box that's located beside the engine. FCI = 89, FC2 = 77, FC3 = 91.

 

[Julio]

Wow Jaj, thanks so much for this detailed reply !!!
Will test all this and report what was the failure.
Thanks again

 

[Julio]

Update
I followed troubleshooting process from JAJ and indeed, both fans were turning since I started the engine, and both stopped after pulling fuse FC1 (89).
I’m away in weekend, so I guess I can switch fuses 76 and 89, 76 being rear window defroster on my diagram and showing similar features (size, reference, etc.)

Right ?

 

[JAJ]

Update
I followed troubleshooting process from JAJ and indeed, both fans were turning since I started the engine, and both stopped after pulling fuse FC1 (89).
I’m away in weekend, so I guess I can switch fuses 76 and 89, 76 being rear window defroster on my diagram and showing similar features (size, reference, etc.)

Right ?

From the wiring diagram, it looks like the relays in positions 76 and 89 are interchangeable, so give it a try.

Now, if that doesn't fix it, then the problem is either in the ECU itself or in the wiring between the ECU and the relay. That's a dealer repair job.

 

[Julio]

From the wiring diagram, it looks like the relays in positions 76 and 89 are interchangeable, so give it a try.

Now, if that doesn't fix it, then the problem is either in the ECU itself or in the wiring between the ECU and the relay. That's a dealer repair job.

Only did a quick check.... but it seems indeed that switching fuses does not solve the issue and fans keeps turning slow even at start....
will buy a few fuses and change them just to be sure the defroster fuse is not dead also (who knows, but yeah, very unlikely). Otherwise will bring it to Ford and hope that this part of the car is similar enough to the 5.0l to be a known environment for them.

Thanks for your help, will keep you posted

 

[JAJ]

Only did a quick check.... but it seems indeed that switching fuses does not solve the issue and fans keeps turning slow even at start....
will buy a few fuses and change them just to be sure the defroster fuse is not dead also (who knows, but yeah, very unlikely). Otherwise will bring it to Ford and hope that this part of the car is similar enough to the 5.0l to be a known environment for them.

Thanks for your help, will keep you posted

Too bad it didn't work. From the wiring diagram on the maintenance manual, all of the V8's are wired the same, so they should be able to figure it out. It could be programming, but there is a new ECU code update out from Ford (that I just installed on my 2016 GT350) and that may change it if it's a programming issue.

Keep us posted on what you learn - thanks!

 

[Anthony 05 GT]

Having the windshield defroster on will keep the fans on with most cars

 

[Dominator961]

This is going to be confusing.
But this is the only way I can post this because my program is a subscription and not shareable.

If you need additional help pm me.

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