P1491 code 2001 dodge caravan 3.3 L

[Jglosenger]

I have 2001 dodge caravan with 3.3 l engine. The check engine light is on and when scanned I got code for egr valve. Replaced that woth new egr erased codes and rescanned now i have P1491 code, rad fan control relay curcuit. I replaced the rad fan control relay that is mounted on the frame between radiator and grill. The fans do kick on when up to temp and also kick on when a.c. is turned on. Fans seem to be working as they are supposed to but still have the P1491 code.Any ideas what else I can check that may be causing this code to show up?

[Jglosenger]

So after a few hrs of reading forums and tinkering around with it today i have changed out the fuse with new 40 amp new rad fan control relay and visually inspected converters and wires I could see without tearing into the harness. From what I can see everything looks good. No corrosion, cracks breaks or bare wires that i can see. Not really sure how to use volt meter so I'm not gonna mess with that till a friend can help. Is there something I'm missing or overlooking? Does new relay have to fastened down for it to work properly get a proper ground? As engine gets close to normal temp fan kicks on when a.c. is on fans work. I am stumped. Any help much appreciated.

[gromittoo]

I have had randomly appearing P1491 codes for years on my 01 Voyager minivan since it was 4 years old. I have not gotten to the bottom of why it happens, but every time I take it to someone, they want to replace the solid state fan relay.

The part alone costs like $190. It is mounted on the steel bumper, under the bumper cover, which is a b***h to get off. The relay is attached to the bumper via a big rivet, which must be drilled out. The labor is cost is staggering.

First, lets get things straight. The "solid state fan relay" is a semiconductor (a really big transistor). It only has two failure modes: Shorted or Open. If the "solid state fan relay" is "shorted", then the fan will be on at the highest speed all the time. The van will sound like it wants to be an airplane, and want to become become airborne. If it is "open", the fan will never come on under any circumstances. The van will overheat on every trip.

If you can hear your fan going on and off with the A/C, or you can hear the fan running at a low speed, then that very expensive relay is not the problem. If your mechanic suggests replacing the relay, knowing that the fan can run at different speeds (and no overheating), then it is time to take your van to someone else.

JGlosenger may have an intermittent wiring issue, but Mr. Glosenger seems to have looked for that. I looked for one on my voyager, and never found any issue.

The fan in the this generation minivan is a variable speed fan, controlled by a pulse width modulated signal from the PCM. This means the relay turns on and off many times per second, and if it is on 60% on the time, and off 40% of the time, the fan runes at 60%. The algorithm the PCM uses to set the speed takes into account:
- The Engine temperature
- The pressure measured by the A/C pressure transducer
- I read somewhere that Chrysler decided that the fan should turn on when going at a low speed or idle, with the wipers on and A/C or defrost switch on but compressor cycled off. The idea was that rain kicked up onto the hot condenser, on a very humid day, could cause fog to come out of the grill. Owners might panic when they saw fog coming out from the hood, and believe they were boiling over. Turning on the fan is a cosmetic way of eliminating this. I don't know if this is really true.

I don't know what the algorithm the PCM uses to decide the fan circuit is faulty. I have been looking for years for a complete description of the "Fan Relay Circuit test" performed by the PCM. However, I have made some observations:

Observation 1: For the P1491 that I have experienced, it usually is set when the van begins to move after a cold start, AND THE A/C IS RUNNING (remember that Defrost also turns on the AC). The air temperature is usually cool, but always above freezing.

Observation 2: If I get the code, and I am careful not to use defrost or AC for a dozen or so trips, the code will go away. This worked for me, because my inspection is in January in Philly.

Observation 3:
I had a period when I couldn't go without AC (Summer, on vacation), and I was totally annoyed at this code kept coming back. When I got home, I checked the refrigerant pressure in the A/C system. The pressure was a little low, but still high enough to get cold. I topped off the refrigerant to the proper pressure, and the code stopped coming back!

Ever since I have made Observation 3, I check the A/C pressure when I get the code, and add a little refrigerant. It seems to work, but I can't explain why it works. I suspect a bug in the PCM firmware that sets this code when the refrigerant pressure is a little low (with the wipers going?).