Hey y’all, I’m getting frustrated, so I came to the brain trust lol.
Truck is in my sig, and here’s what’s happening. I have been chasing electrical gremlins for a while now. Replaced 2 of the 3 connectors at the PCM, replaced the PCM, thought I was good to go. Then, I ended chasing an intermittent “no bus” after getting stranded on the side of the road a couple times. I finally got it to fail at home so I could troubleshoot. Followed the steps in the FSM and it pointed to a bad CTM. Found a used one and installed it, started the truck and had no issues…. For about 30 mins. Then I get P1762 and P0206. I double check my connections at the PCM and found a few wires weren’t properly seated, fixed that and went to check codes only to get the “no bus” again. This time it cleared itself in about 2 minutes and I had 0 codes. I gave the wiring a once over and took it for a test drive. I made it 12 miles before the check engine light came on, and it pulled P0743, P0351, and P0622. At this point I’m in a pharmacy parking lot. I shut it off, go in and do my shopping, come out and again get met with “no bus”. It cleared in roughly 2-3 minutes and the only code was P1491. Go to my next (and last) stop and truck is running fine. Come out, start the truck and it’s running like crap. Rough idle, sluggish, backfiring, etc. check codes again and have P0753, P0351, and P1491. I make it home and it dies as soon as I pull in the driveway. I try to restart only to get the “no bus” again, and again it cleared in 2-3 minutes and the truck ran perfect the rest of the way down the driveway (long dirt driveway) with only the P1491 code.

 

Radiator fan relay control? That shouldn't show up, as there ISN'T a radiator fan relay....... unless it was repurposed, and not renamed?

 

Yeah, that kinda screwed with me a bit too. I have no clue what that’s about. It showed up with the old pcm too. That was one of the many reasons I was thinking I had a bad pcm. Turns out my truck is just nuts 😂

 

There was another one of these in the not so distant past..... (was that you too though?) May wanna search the second gen section for that code, and see what pops up.

 

No, it wasn't me. granted I have had my fair share of odd codes I've asked about. You're correct there are a ton of threads with that code issue and none of them seem to have ever listed any kind of resolution. I saw some going back 7-8 years. I just updated the software on my scan tool in case there was some kind of miscommunication between the PCM and my scan tool. I'm also going to try 2 other scan tools I have and compare the results. As for the other odd codes that pop up then disappear and the random "no bus" is there a possibility a loose/corroded/broken ground could be wreaking havoc with the PCM? I have a lifetime warranty on the PCM but it's only been in the truck a couple weeks, so I want to make sure I completely eliminate everything on the truck before I harass them about a warranty repair.

Oh, for what it's worth, the FSM does list the P1491 in the diagnostic codes, but that's the only place it appears. I'm starting to wonder if that's preinstalled on the PCMs and only supposed to be active on certain models that use the same PCM, like the 2000 dakota with the dual fans.

 

That wouldn't surprise me..... Did the FSM have a troubleshooting tree for that code, for a full size truck???

 

Not that I could find. I couldn't find anything fan related in the wiring section, and in the cooling section (section 7) the only thing fan related discusses the viscous fan clutch and the fan blades. I did read somewhere that it may possibly be a faulty coolant temp sensor or wiring issue causing the PCM to misread and think it's seeing an electric fan circuit.
This is the only time that code appears in the entire FSM (used the text search feature). "P1491 Rad Fan Control Relay Circuit - An open or shorted condition detected in the radiator fan control relay control circuit. This includes PWM solid state relays." that's in section 25, page 12.
I did find this code that relates to electric fans "P1498 High Speed Rad Fan Ground CTRL Rly Circuit An open or shorted condition detected in the control circuit of the #3 high speed radiator fan control relay." damn, the more I look the more codes I find related to the non-existent electric radiator fan. P1487, P1489, P1490.

This one is odd... "P1479 Transmission Fan Relay Circuit - An open or shorted condition detected in the transmission fan relay circuit." Is there an option out there for a transmission fan? The trans coolers have always been integrated into the radiator, or bypassed and had an aftermarket trans cooler mounted on the front of the condenser, I've never seen a specific transmission fan.

Regardless, I'm definitely wanting to find out why this code keeps popping up in some many of these trucks. I'm going to make some phone calls today while I'm working on the wiring in my truck and trying to find out why I keep having DTCs just appear and disappear at random.

 

You won't find anything in the wiring diagrams for an electric fan, because these trucks never had one..... Nor did they have a trans fan that I am aware of. The heavy duty packages had an aux trans cooler, in addition to the one in the radiator, but, there was never a fan there either. Simply wasn't any room.... Likely features that other vehicles might have had.

Since your truck is an 01, you only have one temp sensor, and that's for the PCM. The PCM tells the cluster gauge what to display.... and if temp was off/not working properly, it would set a code for that, not a relay.....

I have seen this code mentioned before, but, as you say, I have never seen WHY this code was coming up on a second gen, nor how someone fixed it.....

I see where it could possibly be something to do with the EGR system as well, but, your truck doesn't have EGR either......

 

Yeah, I’m completely lost on that. I’m going to call a couple dealerships and possibly an independent shop who seems to be good with troubleshooting and see if they have any ideas. I did start to wonder if it’s possible they made these truck for export and for whatever reason those versions had electric fans.

as for the other stuff, I’m even more lost on that. Well, more overwhelmed than lost. I don’t know where to begin looking. I went out to the truck, stuck the key in the ignition and got the “no bus” again. I let it sit for 10 minutes with no change. So I swapped the CTM back out to my old one and same result. While that no bus is showing I can’t connect with the pcm, so I’m just going to take the PDC apart and check everything in there, then I’m going to chase down all the grounds, and if necessary inspect every damn inch of wire under the hood and behind the dash (already have to pull it for a heater core).

 

If dash lites work, and the fuel gauge comes up, the PCM is getting power, and it's just a problem with the communications bus.