On my 2006 Dodge Caravan SE 2.4, the cooling fans won't come on.

It doesn't seem to be overheating although it may be running a bit hot, but I don't idle for long periods of time either. What I first noticed was that the AC was getting warm at stoplights, even though it works fine otherwise.

-Check engine isn't on
-changed relay module expecting a quick fix
-changed ECT sensor again expecting a quick fix
-found that unplugging the ECT coolant sensor causes some sort of safe mode and both cooling fans instantly kick in on high thereby proving the computer CAN turn them on

-checked relays and fuses (seems more or less pointless since it does work in failsafe mode)

-concluded that the PCM must be the culprit and swapped it out for a refurb which didn't change a thing

Frankly, I'm stumped.

However, after a LOT of searching, I did find another forum posting with a user describing exactly the same symptoms here:
http://www.300cforums.com/forums/gen...wont-come.html
It is a 2005, but the ECM looks identical in pics I found. Unfortunately, no solution was ever posted.

What should I be looking for at this point?

Luke

 

I would look for a mechanic who knows how to use a voltmeter and factory schematic wiring diagrams to properly trace the problem to determine root cause. You need to confirm function and connectivity of particular circuits, interlocks and sensors to the computer. Trying to troubleshoot an electrical problem by throwing parts at the car is a costly potshot that does not always work. If you keep driving it long with this defect present you could possibly blow a head gasket or ruin the engine. Do NOT use the A/C without fans or the compressor could overload from excessive head pressure and fail. This fan circuit defect might cost you $200-$500 labor for a trained tech to fix at your Dodge dealer. However the delays associated with fiddling and failing to quickly repair it could cost you $2K+. You tried the simple stuff with no luck, so it's time to bite the bullet. If you disconnect the battery and reconnect it then the problem goes away, then the engine computer is defective. I would personally not do that because it will not permanently fix the trouble and that will make it impossible for a trained tech to find and fix it.

 

The reply you got was good advice, I went through a similar case with my 2006 Caravan, replaced the sensor, & finally the complete fan Assembly got off lucky.

Now with my Ram 1500 different story, engine kept quiting on me and spent over $1000.00 trying to isolate the problem, with no luck. A friend of mine knew a trouble shooter on wireing and in under 2 hours he had the problem solved. Turns out it was a corroded wire in the under-hood harness, along with a Fuel Sensor.

I was told the Computer was the issue and like you replaced it with a refurb, plus many other Sensors & 3 Tow jobs still no difference.

So for what its worth get a good wire Tech, had I done this it would have saved me plenty.

 

Would you like the wiring diagrams? Send me a private message and include your email address. Click "User CP" at upper left then "private messages" in left column.

 

I really appreciate the offer for the wiring diagrams. I'm going off the one in the Haynes manual which seems to be accurate as far as I can tell, so I think I'm good there unless you have something better.

I do have a question that might shed some light. Given the exact nature of the problem, there really just aren't that many ways this could be happening, and I have pretty much eliminated all of the usual and not so usual suspects unless....

Does the engine cooling fan come on in response to AC pressure instead of with the AC compressor? That seems like a really unusual setup, but if that is the case, it would point to a bad AC pressure switch/wiring which might explain everything, and that is the one thing I haven't checked yet.

 

Runs based on a/c pressure. The fan turns on when coolant temp hits 220 deg. F, a/c high side pressure reaches 250 psi or transmission fluid temp reaches 204 deg. F. The fan turns off when coolant temp drops to 214 deg. F, a/c high side pressure drops to 248 psi or transmission fluid temp drops to 192 deg. F.

 

Tizzy1, You're the greatest!

I'll check what I can of the AC pressure wiring/sensor. I don't do air conditioners, so I'll have to take it in to change the switch if that is necessary, but that could explain the whole problem.

Except that it doesn't explain why I haven't been able to catch the cooling fan coming on for engine temps, but I don't have a way to actually check the engine temps, and it doesn't seem to be overheating, so maybe I'm just not catching it running or it hardly ever gets that hot.

 

I checked the wiring diagram and the coolant sensor and A/C transducer share a ground that goes to the computer. If it is corroded or otherwise compromised, it might be acting as a resistor and changing the values enough to make it malfunction without throwing an error code.

Hopefully tomorrow I will get some time to work on it and get it sorted out.

 

The most likely cause is the coolant fan resistor. The other way to have it check is with a scanner. Try to actuate the fan and if it goes on, the resistor is good.

 

If the a/c high pressure switch is bad, there is a shredder valve under it so it can just be unscrewed and the new one installed.