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Robert R
Robert R, ASE Certified Master
Category: Dodge
Satisfied Customers: 4533
Experience:  Dodge Master Certified, Dodge Sprinter Certified
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When towing up hills in tow/haul mode, coming down the other

Customer Question

When towing up long hills in tow/haul mode, coming down the other side, tranny will sometimes drop out of gear entirely. Last trip, the tow/haul quit engaging at all (no light, no function) during a downhill stretch. Later that evening, after cooling off, the tow/haul seemed to work normally again. I'm thinking it's a heat issue but the tranny fluid looks/smells normal. Any thoughts?
JA: Just to clarify, what are the exact lights you're talking about? What is the model/year of your Dodge?
Customer: Sorry... 2004 Dodge 2500 Cummins diesel, 48re auto. The only light is the tow/haul light in the instrument cluster.
JA: Are you fixing your B2500 yourself? What have you tried so far?
Customer: Have only checked the fluid so far. Depends on the fix... if an old shade tree man can do it, I may give it a shot. If not, then a qualified mech or dealer. One thought... our old 2002 Dodge Grand Caravan developed a shift issue... turned out to be a firmware upgrade at the dealer. How would I find out if this is a similar issue?
JA: Anything else you want the mechanic to know before I connect you?
Customer: 103k on the truck. Tranny fluid last changed at 80k. No leaks. Runs normally otherwise. Did I miss anything?
Submitted: 2 months ago.
Category: Dodge
Expert:  Robert R replied 2 months ago.

Hello, welcome and thanks for asking your question. My name is*****'m a Certified Auto Technician with 30+ years experience. I will do my best to assist you. please keep in mind that I may need to ask questions of you before I can answer. When you reply please feel free to add as much detailed information as possible pertaining to your concern.

I'm not sure what you are referring to. Can you explain exactly what is happening? After the downhill stretch can you get the tow/haul light to come on and resume normal operation? Has the check engine light came on recently? Has it been scanned for codes?

Can you post the last 8 digits of the VIN so that I can do some research on this concern for you?

Keep me posted...

Thank you,


Customer: replied 2 months ago.
VIN: 3D7KA28634G130923. If you have a way to attach a file, I have a pdf of the Dealer Options Report on the truck. If I understand it correctly, it has the maximum tow package available for the year. That's why we bought it.
Just ran a code check on it. Came up with a P0633 Generic. Immobolizer Key Not Programmed - ECM/PCM. No check engine light. Google search suggests that the PCM was replaced and the proper programming not done. We did have to replace the PCM last year. Couldn't afford the dealer markup, so bought one online, sent them the VIN for programming, and have had no problems since. Articles suggest that this could result in a no-start condition, so at least we know the possibility exists and what to do about it. I will clear the code and see if it re-sets.
Back to the issue at hand... I have two distinct symptoms. First, after a long climb in the tow/haul mode, coming down the back side of the hill and still in tow/haul mode, the tranny will sometimes drop out of gear entirely. Once at the bottom of the hill, putting power back on seems to resolve the issue and the system runs normally. We tow several times per year and experience this symptom only occasionally. The truck is rated to tow just over 13,000 lbs but the hitch system factory installed is only rated at 10,000lbs. Our trailer is 9600GVW but I estimate our tow weight btw 7k and 8k.
Second symptom has happened only once, on our last trip. After a*****up highway 101 in northern California, after the last hard climb, coming down the back side of the hill, the entire tow/haul function disappeared... no indicator light on the dash, no downshift out of overdrive... no response at all. I had to use my brakes alone instead of in conjunction with the engine/tranny helping to hold back. We got home with the tranny in normal mode, seemingly shifting normally, but again, could not get a response from the tow/haul mode. Parked the trailer and unhitched. About 3 hours later, took the truck to a friends house. Tow/haul mode had returned and seemed to operate normally and has since (about 4 weeks), though with no towing involved. We'll be taking the trailer out again in the morning (Friday, Aug 11) for a trip down highway 101 again to Santa Rosa and back on Saturday. Will be interested in seeing if the symptoms and code return.
There has been no check engine light since before we replaced the PCM and the IPM about 7-8 months ago. Will be happy to report back if the system lets me. Any other questions, please let me know.
Thanks again... Jim Norton
Customer: replied 2 months ago.
Ahh... just found the "add files".
Thank you.
Expert:  Robert R replied 2 months ago.

Hello, Thanks for the great explanation of the concern... As for the P0633 I HIGHLY recommend to visit the dealer for programming to avoid the possibility of being stranded in the future. With this code present, the PCM could lock out the fuel system and create a start and stall condition and possibly a no start all together.

Do this asap... This likely has nothing to do with your actual concern though.

Although when there at the dealer, I HIGHLY recommend to have them scan the COMPLETE VEHICLE for codes. It is possible that there may be an issue with the Tow/ Haul Button or circuitry. This problem doesn't always show up on a "Generic" scan tool. The dealer scan tool can access all modules and I would think that if there are any codes stored in any other module such as the Cluster or TCM we may come up with a clue that can steer us in the right direction...

The Tow/Haul button basically "turns on" a different shifting schedule for the transmission and if it comes out of this mode it is normally caused by either the Tow/Haul button itself or a wiring issue and lastly a possible issue with communication between modules or a faulty module. A scan with a dealer scan tool is the best bet in successfully testing the button operation input signal.

Let me know if you visit the dealer. Have them give you a vehicle scan report and if possible post it here and I'll look over it and let you know what I think may be the problem...

Keep me posted and let me know if you have any additional questions...

Thank you,