I have a 2005 Dodge Ram 2500 2WD with a Cummings 5.9L diesel. Once I drive for about 20 minutes the transmission will not shift down to 1st or 2nd gears. I've tried putting it in park and reverse and even shutting it off for awhile but unless I turn it off and allow it to completely cool down the transmission will not downshift. I changed the filter and fluid but no change. Any ideas?

It isn't specified. There is more than one fuse that covers transmission. You would need an auto electrician or a transmission expert to connect a pro grade scan tool computer and diagnose if the switch or solenoid is faulty. A scan tool at this level is bi-directional and can command solenoids to operate and confirm that they do.

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Fuse Box Diagram Dodge Ram 1500/2500 (2002-2009)

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Fuse box diagram (location and assignment of electrical fuses) for Dodge Ram / Ram Pickup 1500/2500 (2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009).

2006-2009 Dodge Ram 1500/2500/3500 Fuse Box Diagram

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Nov 16, 2018 - 2006-2009 Dodge Ram 1500/2500/3500 Fuse Box Diagram ; 5, 20, except Base: Sunroof ; 6, 40, Diesel ('06): Wastegate Solenoid, Drive Fan Radiator ; 6 ...

2006 5.7 ram 1500 fuse box diagram - DodgeForum.com

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Fuses and relay Dodge Ram 2002-2008

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Governor pressure controls the 1st and 2nd shifts. After that is the 3 and 4 solenoids. you would need to look at the pressure when driving(takes a scanner). pressure is 20 to 30 PSI from 1st to 2nd, depends on acceleration. After that the pressure will jump to 100+ psi. Did you replace the governor solenoid and sensor? They are on the passenger side of the transmission and sit in a block that detaches from the valve body.

What is the governor pressure sensor reading look like.
This sensor tells the transmission controler what kind of load the engine is under while driving. This tell the transmission what gear to be in. That's why the transmission starts in 2 nd gear
..With most transmissions now if you don't have the live data in front of you to see if all the sensors are in range and test the values that are out of speciation.

Heres a belt diagram for a 2005 dodge ram 2500 series, with the5.9 diesel engine hope this helps.

sounds like trans has gone bad, last service of filter and fluid? clogged screen or low/burnt fluid would effest but if clunking sounds like trans is shot

You have not down a scan of the TCM (transmission control module) for fault codes, you need that done first before throwing away money on parts that don't fix the problem.

your mechanic is right coz there might be a problem with your 2nd or 3rd gear.

Have someone scan the transmission for bad shift sensors.

I have a 1999 Ram B 1500 Van, auto transmission, 5.9 L engine.
Same problem, trans hesitates in 2-3 upshift, sometimes 1-2
to 2-3 upshift. Posts trouble code P1756 - Transmission Govenor pressure too low at below 15 to 20 Psi. Transmission pump shot or valve body leak stops trans from reaching proper pressure to shift, which is minimum 15 Psi.
Either trans is leaking in valve body, valve is stuck, shift solenoid on valve body is sticking, or pump is shot.