Peter,
I guess now is a little late to be posting this, but I had the same problem about 6+ months ago with my Horton in a 1998. 5 24 valve ISB.
The Horton Fan Clutch is a 3rd party item distributed through MOPAR. Since that is true, it also carried the DC MOPAR warranty. Since I got my fan through my local Dodge dealer, they agreed to "help me out" and get me a replacement Horton fan and radiator under warranty. This did take a little persuading since it was an owner installed part and they could have blamed it on faulty installation. It took a little while and they had to put the factory fan back on the truck with the new radiator while waiting for a new Horton fan that was on national backorder.
Horton apparently has at least two different versions of their fan circulating through MOPAR. The MOPAR parts number for the 24v is now 82205124. The older part number was in one of the back issues of TDR, but I don't have it handy. I couldn't get an answer why the part number changed and if there were any changes between the old kit (with the old part number) and the new kit(with the new part number).
The kit I got was the older part number that was obtained from another dealer since the Fan was on national backorder. It only had a single page insert that showed a wiring change for the ISB. There was no other insert about a problem with a bracket or anything else to do with the 24 valve engine.
The installation of the fan went smoothly and everything worked well. I did notice the clearence was pretty tight and I used a block of wood and a mallett to get the wiring bracket and it's P strap as close to the radiator as possible. About two months after installation the fan did the same as yours; it engaged after coming to a stop at an off-ramp and drove the wiring bracket into the radiator.
Luckily (if you can say that after a problem like that) the dealer was only about a mile away from where the truck broke down. I had it towed to the dealer seeing $$$ flash in front of my eyes for all the cost of a new radiator, fan, etc. I left it at the dealer and just told them to fix it.
After talking to the dealer a couple of days later, they said they would cover it all under warranty. This included a replacement Horton fan but was not supposed to include the installation of the replacement. They put in the new radiator and reinstalled the factory fan to get me on the road. A new Horton was ordered but it was on national backorder. I wanted to wait for the latest and greatest Horton fan so I didn't look at inventory at other dealer locations this time.
After a couple of months, the Horton cam in and I called the dealership. They said to bring the truck in and they would take care of it. They weren't supposed to put the new Horton on but they goofed and the mechanic had already put the fan on before they caught it. I think I would have rather done the re-installation myself to compare the new to the old, but the Mrs. had that "you messed it up the first time" attitude and wanted the dealer to do it. The new fan with the updated part number has at least an inch of clearence between the wiring bracket P clip and the fan. That's about 3/4" more than what the old fan had. I'm convinced that there was a design modification done between the new and old fan but can't prove it since the dealer couldn't find all the paperwork for the new fan.
See if the dealer will cover anything under warranty since it's a MOPAR part. If they won't, go talk to Horton about their warranty.
Let me know if I can help you with anything.
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98. 5 2500 4x4, QC, SB, Cummins ISB, Auto Trans, SLT,
3. 54 w/ LSD, Horton Fan Clutch
Falcon, Colorado