When testing the relay make sure the engine is running. Chrysler is pretty good about adding many fail safes in their software and won't let the cooling fan turn on via the A/C with the engine off.

 

Good to know, I have tested it both ways but the fan stays off. Any insights on why I'd see 5.6 on 86 instead of a solid ground? Bad ECU, or does this line just naturally float until it gets clamped?

If I had hair I'd be tearing it out

PS: Maybe more important; would just jumpering 86 to ground fix the problem? I can't seem to find a fat 12V supply line in the PDC that's switched by the ignition otherwise I think I might be tempted to uses a Siamese splice and feed power directly to 87, bypassing the relay. That'd run the fan whenever the key was on so I wouldn't be able to sit in the car listening to the radio while my ex-wife went shopping if I did that though, but I'd still be able to get the darned thing back on the road. It's been two years or more I've been trying to fix this and I'm on my third ECU, second fan and I don't even know how many times I've swapped out the relay.

What are the odds a mouse chewed through the control line from the ECU to the PDC you think? Maybe a bad ground on the ECU? I figure that would cause other problems too though so not likely...

 

Hi,VERY new here but have same issue going on with my nephews 2000 dakota and wonder if you fixed yours?

 

Sorry for the delayed reply, I didn't get a message in my eMail for this.

I fixed the symptom but not the problem. I ended up running a solid ground wire directly from the negative battery terminal to the control input on the fan relay (pin 86). Now, when the ignition is in the on position, the fan starts and stays on until you turn off the key. It remains off in the ACC position so you can sit in the car with the radio on and not run the fan.

Another thread on this subject suggests checking the quality of the connections on the underside of the PDU, one person reported a loose spade connector poorly crimped to the relay socket. Re-crimping the wire fixed the problem for him. Personally, I just gave up and jumpered it.

I thought I'd left a complete write-up on this thread, but it must have been on a different forum. Now I can't find it. But the short story is the one above.

 

OK, here's the full report:

For the record, I had (actually stiil have) the very same problem; I can't get my fan to come on with the A/C "on" method and the car overheats at idle. After testing the relay, both fuses (hi and low V sides) and replacing both the PCM and the fan, I gave up and ran a jumper from relay pin 86 to the battery negative terminal. This turns the fan on (using the relay so the ground wire can be light, I used 16G copper) whenever the key is in the "on" or "start" position. It means sitting in the car listening to the radio is going to drain the battery fast since the fan's always on. UPDATE: This is wrong. Using pin 86, the fan is only engaged in the "start/run" ignition position, not in the ACC (accessory) position. So I can still sit in the car with the stereo on and not drain the battery.

I have a call in to the outfit that gave me the replacement PCM (the one that didn't fix my problem) to see if they have anything useful to say on the subject. According to my manual the control signal comes out of pin 17 on the white (center) connector block on the PCM, but I can't find anything that shows me where it comes in to the PDC. Unless I can find both ends it's pretty hard to ring out the problem with the usual tools, so I guess I'm stuck with a hacked up http://www.sears.com/shc/s/p_10153_12605_SPM7063867803 for the duration. It appears the PCM is somehow putting a constant 5+V on the control pin at the .

This is what I see at the relay with the key on, ignition off (koeo), A/C in the "Max A/C" position:

Pin Relay Circuit Voltage
---- -------------- --------
1 85 12V
2 86 5V
3 30 12V
4 87a 0V
5 87 0V

If I understand the theory of operation correctly (which is a very large assumption) I should see a solid ground on relay circuit 86; this should fire the relay and connect a 30amp fused 12V signal to relay circuit 87, which powers the fan. To prove this theory I directly connected pin 86 to the negative poll of the battery, sure enough the fan fires right up when the key is turned to "on". Position of the A/C selector is unimportant with this kludge; if the key is on, the fan is on.

The odd part is the wiring diagrams show pin 86 tied to fuse #9 , a 10amp fuse in the dashboard fuse box (left of steering column). From what I see, it should have a 10amp fused 12V signal on it all the time. It doesn't of course and I can't figure how it could ever work right if it did. Even more confusing is the same wire supposedly connects to pin 86 on the Fuel Pump relay, but that pin shows 0V koeo. If the wiring diagram is correct, both of these pins should have the same potential on them. They don't, so something is not right in the state of Denmark.

This whole thing is way too peculiar and I've read lots of reports from people having this same problem. The only thing I can think of is there's something wrong with the way the PCMs on these vehicles are programed, maybe something wrong with the way they were wired originally, and there's a whole batch of them running around the country with messed up controllers. For now, I can't find any way to fix it.

PS: I had nothing to do with the shameless "Sears Dual Fan Controller" link in the text, it was inserted during the post. I can't even remember what word is supposed to go there.