I have a 98 6.5 TD that I tow a 26 foot travel trailer with. The trailer weighs just under 6000lbs loaded up. I am able to tow quite nicely in OD keeping the rpm low and in the torque range but I am having trouble losing overdrive occasionally. It will cutout and drop to 3rd and the engine rpm go up to around 2600. When this happened the engine light went on . I can pull over and let the truck sit while turned off for about 5 minutes, start it up again and have overdrive back again - until the next time. This happens off and on again while I am towing. I had the code read and it is a P1870 (transmission component slippage). I am thinking it could be heat but I have a tranny cooler - should I add a second inline cooler? What could the code mean? Anybody else have this problem with the 4L80E?

I hate to tell you this, but your tranny is hurt. I had the same problem on my 94 just before it paked it in. What is happining is the computer is sensing that there is excessive difference between the input shaft rpm and output shaft rpm. Rebuild it now before it gets really expensive and seeing as you are towing, make sure to use upgraded components like the frictions and a shift kit. :grd:

Well you were right - it needed a rebuild. I guess I can't complain too much the truck had 237,000 km on its original unserviced tranny. The tranny company added some upgraded components when rebuilding (on their advice). Thanks for the response!