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Discussion Starter · #1 ·
Hello all I'm having some trouble with my magnum. For some reason my cooling fans for radiator are not kicking on when up to temp but do turn on with ac on. I've put Brand new fan assembly 620-039XD( comes with control module on it) checked all fuses and all relays for fan assembly. Brand new coolant temperature sensor, I've ohm tested the wiring harness 4 pin connector from bottom of fuse box straight to fan connector and all checked out. I have no check engine light on dash or codes. Checked alternator and battery, battery sits at 12.8v car off and 13.5-14.1v on with and without ac on. Brand new jetcool 180° thermostat, no coolant leaks anywhere or shorts anywhere. Have tested the relays with a tester at orileys and all 3 tested good. Only thing I can think of now is fcm maybe? Again im getting no check light or codes, headlights work, turn signals work, horn works, wipers work, ac works (halfassed) after clutch kicks on and then cooling fans it bogs down motor, maybe another symptom of fcm?
Last question if I get a used fcm aslong as that top p0xxxxxxxxak number matches will it be just plug and play or need vinned to my car? Thank you all.
 

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Before adding more parts to your car, have you run a jumper to your fan? You mentioned that the fan comes on with ac, but there are 2 fans and only one is in the ac circuit. Have you run a jumper from the battery to the other fan to make sure it works? In case one of the switches breaks ground make sure that you use 2 jumpers, 1 to the positive for the fan the other to the ground side. From there trace power back from the fan with the engine at temperature. Testing for power should locate the exact place where you are losing power. Even though you replaced switches and sensors, it would not be the first time that one was bad out of the box.
 

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Discussion Starter · #3 ·
Before adding more parts to your car, have you run a jumper to your fan? You mentioned that the fan comes on with ac, but there are 2 fans and only one is in the ac circuit. Have you run a jumper from the battery to the other fan to make sure it works? In case one of the switches breaks ground make sure that you use 2 jumpers, 1 to the positive for the fan the other to the ground side. From there trace power back from the fan with the engine at temperature. Testing for power should locate the exact place where you are losing power. Even though you replaced switches and sensors, it would not be the first time that one was bad out of the box.
That's the really odd part, is does have the dual fan setup but when ac is on both fans kick on not just one fan. fully engaged and do not respond with the temp. Might throw out there I just had to replace ac compressor 2 weeks ago.
 

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It sounds like someone who didn't know what they were doing did some rewiring on the fans. If both fans come on with ac, it may be that neither one is hooked up to the engine temp sensor. If you are getting overheating in heavy traffic with the ac off, the fan wiring may be your problem.

Be careful of the overheating. One problem that has been mentioned often about the 3rd gen hemi engines is the tendency to drop valve seats when they overheat. If both fans come on with the ac, you may want to consider running the ac all the time until you have time to check the wiring. If the outside temp is too cold to run the ac, just turn on the heater with the ac.
 

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Discussion Starter · #5 ·
It sounds like someone who didn't know what they were doing did some rewiring on the fans. If both fans come on with ac, it may be that neither one is hooked up to the engine temp sensor. If you are getting overheating in heavy traffic with the ac off, the fan wiring may be your problem.

Be careful of the overheating. One problem that has been mentioned often about the 3rd gen hemi engines is the tendency to drop valve seats when they overheat. If both fans come on with the ac, you may want to consider running the ac all the time until you have time to check the wiring. If the outside temp is too cold to run the ac, just turn on the heater with the ac.
As far as I know and have seen everything was Stock when I bought the car like oe harness wraps that are all crumbling and such on a 16 year old car would. I've never had this issue until recently down towards Dallas my ac compressor seized up and temp started sky rocketing, turned ac off and everything cooled down for me to get home. New ac compressor and all that crap I noticed getting warm on i20 around 210-220 and didn't hear my fans come on at all. Turn ac on Temps dropped enough 190s to get home. (Jetcool 180 Stat in it). Since then I've replaced the 2 prong harness going to the fans them selves and just replaced the 620-039xd fans and same issues along with new temp sensor. I'm not sure how I can test out the power distribution system in the engine bay or fcm. I know the fcm controls the rad fans ac fans lights ect ect. When fans kick on with ac on it seems like when fans engage the whole car bogs down to almost stalling out. Could the fcm be sending the wrong signal completely and causing that or most likely unlikely? Yeah I've read and learned about these valve seat crap after I bought the thing ergo I'm so anal about the Temps getting to even 210. When I bought it I replaced entire coolant system and ac system to try and prevent any failure like so.
 

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Discussion Starter · #7 ·
I use software to actuate each motor to test- just recently did that with AppcarDiag FCA. You can control each fan and set each speed for testing. Then just click the box and the fan should kick on (or not...)
I finally figured it out and ended up being software?? I have a diablo tuner for it never hooked up and decided to see what the parameters were set at and the settings were 240°F low 255°f medium and 268°f high. I never seen my car get that hot before and changed all to low 195 med 203 high 206 and they turned on and cooling perfectly fine now. I'll probably drop a little more being in texas but the main overheating is resolved.
 

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I don't think I would drop your settings any lower than where you have them now. I had a 1987 Firebird Trans Am, and had my fan set to come on a 195 and cut off at 180. It worked fine even in the Mojave Desert even if idling in traffic or climbing Cajon Pass in mid-summer. I could watch the fan cycle on & off by the temp gage. The temp gage on my Magnum R/T doesn't cycle as quickly as the gage on my Firebird did. Also, my Firebird had numbers on the temp gage, but my Magnum only has lines.
 
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