01 V6 Swap - Fan not coming on

Apr 5, 2019
4
0
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49
Magnolia TX
#1
I recently bought a V6 body (01) that has a rebuilt and slightly modified 4.6L in it. Not sure of all the modifications, still waiting on the ppwk for the engine, but it is running fine. The problem is that the fan does not come on. I disconnected the coolant temp sensor to jump it and the fan started running....without jumping it! WTH? I'm scratching my head on this one. If I understand how this thing works, the wiring would need to be jumped in order to send the signal to the fan....or am I missing something?

Any input is appreciated at this point.
 
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wmburns

SN Certified Technician
Aug 14, 2009
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373
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Houston Texas
#2
To correctly trouble shoot this one needs to know what the PCM "thinks" the engine coolant temperature is. What is the easiest way to do this? With an ODB2 scanner. Here's some information on an affordable Windows based unit.

ForScan ODB2 scanner w ELM327 USB
https://www.stangnet.com/mustang-forums/resources/forscan-odb2-scanner-w-elm327-usb.57/

With a powerful ODB2 unit with two way communications it's possible to "command" the PCM to turn on the low speed and/or high speed fan at will. Think how this will simplify the trouble shooting.

If looking for an educated guess? Some model years of V6 Mustangs have a single speed cooling fan. Some have a two speed cooling fan. All GT's have a two speed cooling fan. I suspected that there's an incompatible mix of V6 and GT parts. It's quite possible that the PCM is trying to turn on the fan but because the wrong fan has been used there's no wire to accept the signal.

It's also possible that the wrong body wiring harness has been used and the wire for the the low speed fan is missing. IDK but that's why some trouble shooting will be necessary.

OBTW, if you want to know WHY disconnecting the ECT causes the fan to run it's because the PCM knows that the ECT circuit is open and it engages "fail safe" mode. During "fail safe" operation the PCM commands the high speed fan to run since the PCM does not know what the engine temperature is. So the fact that the high speed fan comes on "proves" that the PCM is able to control the HS fan. What we don't know is IF the PCM is able to control the low speed fan (which is what I think is wrong).
 
Apr 5, 2019
4
0
1
49
Magnolia TX
#3
Thanks for the info WM! You explained the problem perfectly. I did fail to mention that the car has an SCT tuner on it. Again (my often flawed thinking) would tell me that the PCM should be getting the same readings that the tuner gets, correct? The tuner will get up over 190 with still no fan action.
 

wmburns

SN Certified Technician
Aug 14, 2009
5,069
373
164
Houston Texas
#4
Soooooooooo.

Sounds like the ECT is sending "reasonable" values. But one still needs to know the PCM fan mode (off, low speed, high speed) to conclusively determine that the PCM is doing exactly what it's supposed to be doing.

But I suspect that the PCM is calling for low speed fan BUT because of V6/GT parts interchange the low speed fan isn't actually working.
  • Single speed V6 fan (2 wire) installed in a dual speed (3 wire) application
  • V6 body wiring harness is missing the low speed power wire.
  • V6 CCRM that doesn't have the support for two speed operation.
Of course the problem could be a bad low speed relay in the CCRM. Tests and visual inspections will be needed to trouble shoot.
 
Last edited:
Apr 5, 2019
4
0
1
49
Magnolia TX
#5
Turns out the SCT tuner has a diagnostic tool and it was set to wait until the temp was over 200 before it cut the fan on. Of course I didn't learn this until after I ordered and received the scanner. No worries, though, surely it will come in handy somewhere down the road.
 

wmburns

SN Certified Technician
Aug 14, 2009
5,069
373
164
Houston Texas
#6
Note, 200 degrees F is not too hot. The stock 198 degree T-stat is barely fully open. Expect problems IF the low speed fan cut on point is set that low. The PCM is trying to run the fan for the shortest time possible. The aim here is to control the motor's temperature using just the T-stat and then and only then use the fan.

While I don't know off the top of my head what the low speed fan turn on point is, if asking for an educated guess I would say 210 F. 220 for the high speed fan.

Have you cross checked the PCM temperature reading with an external infra red temperature scanner?

Are you POSITIVE that the motor is actually overheating? What are the other "signs" of over heating? Are you positive there isn't any air inside the motor?

I have helped more than one person who thought they had an over heating problem that turned out to be a system voltage/grounding issue that CAUSED the dash temperature gauge to falsely read high.

Some of the people I have helped have changed EVERY part in the cooling system before reaching out for help.
 
Last edited:
Apr 5, 2019
4
0
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49
Magnolia TX
#7
Wow! Good info. Thanks so much for your help. I guess I'm just too old school and I am used to 180 being the "normal" running temp. The car has not been showing signs of running hot, I just noticed that we were around 190 with still no fan action. That just did not seem right to me as I have driven other performance cars that were always running 180-190....or course I have had a couple that liked to run around 200-210, but it always seemed to be more sluggish when running at that temp compared to 180ish.
The stang has started leaking coolant from somewhere (yet to be identified) in the front of the engine....I'm assuming the water pump, but for now have just been adding water every couple of days. I am hoping to find the time to find the leak sometime this week, but my 18 year likes to drive the car more than work on it. Thinking I'm going to have to just take his keys until the leak is found and corrected.
Thanks again for your help!