Help troubleshooting radiator fan not coming on while driving

[TheFleshRocket]

The radiator fan on my '03 Cobra doesn't come on during normal driving, and I'm trying to figure out why.

History:
I bought the Cobra in 2020 from a seller in Texas. I flew down and when he picked me up at the airport in it, it overheated. We discovered that the radiator fan physically would not spin; one of the blades had cracked and wedged itself against the housing. He bought a new assembly, we installed it, the car checked out fine after, and I drove it 800 miles home.

A year ago or so, I started noticing the radiator fan occasionally not working. As in the coolant temp would creep up over 200 under normal driving conditions (fan is set to come on around 185 or 190), but it would usually recover and overheating wasn't an issue.

The issue got worse, to the point that the radiator fan no longer kicks on at all during normal driving. If stuck in low-speed traffic, the coolant would get to 220F or more, so I stopped driving the car except for trips where I knew there would be minimal traffic and when it was cool outside. I figured it was the fan resistor so I installed this: "Radiator Resistor 1R3Z-8L603-AA Fit for Ford Mustang 4.6L V8 2001-2004" from Amazon--for some reason I can't post a directly link.

No change. So as a test, I flashed the ECM because I know that when the flash is being applied, the radiator fan kicks up to full speed. The fan did come on, but it sounded like it was only at low speed.

So then I jumpered the fan directly to the battery, and both the low speed and high speed circuits worked properly, so the fan itself is presumably fine.

More Googling suggested that the CCRM could be bad. In addition to the radiator fan, the CCRM controls the AC (it worked fine when I started driving home from Texas when I first bought the car, but by the time I got home, it had gotten where the air coming out was barely cool; I haven't diagnosed the issue, since the car is a convertible and I pretty much never need the AC), and it also controls the fuel pumps (the car drives great 99% of the time, but occasionally when doing a pull, it'll completely cut out for a split second--I figured it was a fuel pump issue and this was on my to-investigate list).

I ordered a CCRM from Late Model Resto https://lmr.com/item/LRS-37517/mustang-ccrm-relay-pcm-fuel-pump-fan-01-04-4-6 but figured I would post about the issue here to see what I should check next, if the CCRM doesn't fix the radiator fan. Thanks in advance for any input!

 

[01yellercobra]

How does your plug look? I had a Dorman replacement for a long time and for whatever reason the plug was starting to melt. I never had overheating issues and the only reason I found it is because I swapped fans.

 

[MG0h3]

I’d def do a visual of the plug.

I’ve heard of these failing over time under normal operation

When your fan blade broke and seized the motor, it put a huge load on the circuit as it tried to pull more amps to run the fan.

It should’ve popped the fuse; make sure you check that.

It also would’ve caused a lot of extra heat to the entire circuit. There is additional heat created wherever there is a connection, like the plug.


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[03' White Snake]

Turn the AC or defroster on. This should turn the fan on. If it doesn’t, good chance the CCRM is bad. Like they said, check fuses and the connector.

 

[hotcobra03]

I had connection issues also

Mine was fan side of plug .
The barrels were to far back,just needed to push them forward after plugging connector in .

 

[TheFleshRocket]

I installed the new CCRM and there was no change. When flashing the ECM, the fan only spins up to low speed. (I also tried the AC and it still doesn’t work, either.)

So I swapped back to the original CCRM. I then decided to check the wiring at the fan plug. I did another ECM flash and checked all three leads on the harness side. Only one of them lit up, which I’m assuming is the low side. If that’s correct, and the high speed lead isn’t getting power, what should I check next?

 

[01yellercobra]

Do that same check with the A/C on. That should power the high speed side. The ECM flash only turns on the low speed.

 

[TheFleshRocket]

Do that same check with the A/C on. That should power the high speed side. The ECM flash only turns on the low speed.

Are you positive about the flash only activating the low-speed fan? I am pretty confident that the ECM flash did turn on the high-speed fan--I remember it sounding like a plane was taking off whenever the flash was loading (and seeing the voltmeter drop into the very low 12s or high 11s due to the extra current draw) and it's definitely quieter now.

The AC doesn't work so I'm hesitant to turn it on, in case running it causes the compressor to get beat up.

 

[TheFleshRocket]

I found this thread: https://stangnet.com/mustang-forums/threads/03-radiator-fan-wont-turn-on.911298/

I also looked up the fan wiring diagram in the service manual.

You really should be "bothering with the wiring" at this point. CCRMs and Temp sensors are pretty simple devices; they don't fail that easily. I personally replaced a CCRM on mine when dealing with an A/C clutch that wouldn't come on, and it turned out to be the clutch connector (a $14 part). I've also had issues with my fan connector that caused the high speed fan to fail to start.

Check the fan plug first; in my case one of the wires had started to back out of the connector (the rubber seal on the back was out just enough to break the circuit). But, it's also a high amperage circuit, so damaged/burned/etc fan plugs aren't that uncommon.

If the connector is good, get a multimeter and start testing the wiring. Even a simple tone test would be helpful. Some information that might help:

Wires at the Fan plug:
Black - Ground
Red/Orange - Low Speed +12V
Orange/Light Blue - High Speed +12V

Wires at the CCRM:
Red/Orange (2 wires, should be pins 1 and 2) - Power out to Low Speed Fan
Orange/Light Blue (also 2 wires, should be pins 6 and 7) - Power out to High Speed Fan
Yellow/Black (again, 2 wires, pins 3 and 4) - Power in from Junction Box (50A fuse under hood) for fans

Check for continuity from the CCRM to the Fan (on the Red/Orange and Orange/Light Blue wires at each end).
Check for power at the CCRM (on the Yellow/Black wires).
With the engine hot and running, check for power at the fan plug (low or high should be on, but not both).

This definitely can be a little time consuming, and annoying, but it's difficult to repair this sort of issue without proper diagnosis, so the time is worth it.

Interestingly, it states that checking for power at the fan plug, either low speed or high speed should be on, but not both. So if I get power at the low-speed side, then I shouldn't be getting power at the high-speed side. What this means is that if I am getting power to the low-speed fan side when doing an ECM flash, I shouldn't be getting power to the high-speed fan side (corroborating what 01yellercobra said), and meaning that the ECM flash isn't a valid test of the high-speed fan activation.

I used a 12V test light and checked the two yellow / black wires going into the CCRM which are supposed to be hot at all times, and they both are.

I used an ohmmeter and checked the two orange / blue wires going into the CCRM which are supposed to have continuity to the high-speed side of the harness plug, and they do.

I checked the two red / orange wires going into the CCRM which are supposed to have continuity to the low-speed side of the harness plug, and they do.

So, the CCRM has power going to both of its internal fan relays (2 x yellow / black wires), the wiring from the CCRM to the low-speed fan harness is good (2 x red / orange wires), and the wiring from the CCRM to the high-speed fan harness is also good (2 x blue / orange wires).

Looking at the wiring diagram, the only other wiring that should affect the high-speed fan activation is the light green / violet wire between the PCM (the high-speed fan controller) and the CCRM.

At this point, I think I may not have been properly testing the CCRM via the ECM flash, so it's possible that the old CCRM's high-speed fan relay is indeed bad. I'm going to reinstall the new CCRM and take the car for a test drive tomorrow.

 

[01yellercobra]

I don't remember details, but the low speed signal is reversed. Meaning it applies a signal to the low speed relay to turn it off. Ford does this so that if something happens to the circuit the fan will default to on. The reason the fan comes on during the flash is the signal to turn off the fan is not there.

If you have a hand held I wonder if you can set the fan temps to something really low, like 50 degrees, so they come on? You can set them both to the same temp and check power at the same time.

 

[TheFleshRocket]

Thanks for the explanation about the low-speed fan signal.

I have an SCT Livewire but the car has a custom tune that I can't edit. But.. I guess I could adjust a canned tune to have really low fan temps, load that in the car, and just turn on the key to see if the fan kicks on.

I drove the Cobra this morning and the fan did not turn on (at least not high speed) during regular driving. (Fortunately I avoided traffic enough that coolant temp never got up to 200F.) I also turned on the AC and that had no effect on the fan. I could hear the compressor cycling, so apparently it's not dead.

Any idea where else I should look, assuming that the fan doesn't turn on after I load the tune with very low fan temps?

 

[TheFleshRocket]

I did some testing by setting the fan speeds in a "base" tune I had on the device. (Canned tune support is disabled, but I had one "base" tune that does let me change parameters.)

Basically, whenever the coolant temp is below the high-speed fan temp, the fan runs in low-speed mode. As soon as the high-speed fan temp threshold is passed, the fan shuts off.

It appears that my regular tune has the low-speed fan turn on somewhere in the 170s, with the high-speed fan set to turn on at 190. The fan runs until the temp hits 190 and then shuts off.

Looking at the wiring diagram, as far as I can tell, the only thing left that could be causing the problem is the light green / violet wire that goes between the high-speed relay on the CCRM and the high-speed fan controller on the ECM itself. I'm guessing I should check for continuity between the ECM and the CCRM on that wire.

Or I should disconnect the fan harness, let the engine get up to 190F, and see what I see on the high-speed fan wire. The reason I haven't done that yet is the engine has huge aftermarket reservoirs for the engine and intercooler coolant and in order to get to the fan harness, I have to push them out of the way (not easy when they're full of hot fluid, and they usually get pressed up against the engine.. can't do that when the belts are spinning).

 

[01yellercobra]

At this point I'd jump the fan from the battery. Make sure the high side works. If that's good then I'd let the engine idle up to temp with the fan disconnected and use a DMM to check the power at the plug from the CCRM. Make sure it's sending power when it's supposed to. If it's not then check the continuity. If continuity is good then you have to make sure the inputs to the CCRM are good. Which unless something went south with the ECU I don't see being an issue.

 

[TheFleshRocket]

At this point I'd jump the fan from the battery. Make sure the high side works. If that's good then I'd let the engine idle up to temp with the fan disconnected and use a DMM to check the power at the plug from the CCRM. Make sure it's sending power when it's supposed to. If it's not then check the continuity. If continuity is good then you have to make sure the inputs to the CCRM are good. Which unless something went south with the ECU I don't see being an issue.

I've already confirmed that the high-speed setting of the fan works if jumpered directly to the battery. I've also already confirmed continuity between the harness at the CCRM and the harness at the fan, for both the low-speed and high-speed fan modes.

I'll next check to see if the high-speed lead goes hot once coolant temp gets up to 190F. I have a feeling that it won't. Thanks for the guidance!

 

[01yellercobra]

In the words of a certain dad rock band....."One step closer...."

 

[fordmaster302]

You can also test the fan output using the forscan app it can test different outputs on ford vehicles.

 

[TheFleshRocket]

You can also test the fan output using the forscan app it can test different outputs on ford vehicles.

Never heard of it.. but sounds handy. I'll look into it.

 

[TheFleshRocket]

I disconnected the fan harness and drove the car for about 10 minutes to get it warmed up. With the coolant over 190F (the temp at which high-speed operation should begin), only the high-speed lead was hot according to my 12V test light. (In my picture above, it's the lead that's at the 1:00 position. The low-speed fan lead is in the 10:00 position.)

What this seems to indicate is that everything is working fine up to the fan harness connector. Since the fan comes on if I jumper power to its high-speed circuit directly from the battery, the problem must reside at the connector itself--apparently the lead from the harness is not making a connection to the lead into the fan.

Both connectors look good so I'm not sure why they're not making good contact--I'm going to let the car cool off before inspecting any further. But I'm just about positive I have the cause of the problem figured out.

 

[TheFleshRocket]

I used the pointy tip of my 12V test light to push in around the edges of the connector on the fan side, hoping that it would give better contact with the harness-side plug. After plugging the fan back in, I fired up the car and turned on the AC. The fan started spinning! Assuming that the AC triggers the high-speed fan setting, the problem appears to be fixed. I then shut off the AC and let the car idle up to 184F, at which point the low-speed fan setting kicked on. Fingers crossed, pending some driving to confirm, I think the problem may be fixed. How irritatingly simple and stupid of a fix!

Thanks to everyone for their input!

 

[venom1997]

Awesome just a connectivity issue nice!!!! Electrical issues are a pain to track but a huge relief once remedied!


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