We are on vacation pulling the 5th wheel. On the last of the three days driving the transmission starting doing something I have never seen. When running about 60mph and starting up a grade, when I push the pedal and it should shift from OD down, it bucks really bad. It only does it under those conditions and only when towing. It does it when the tow/haul is on and tow haul is off. Also, in the afternoons on the last two da,ys the A/C has started to turn off. The fan still runs but the cooling cycles off. After a while it comes back and is ice cold but then cycles off again. It only seems to do this when the engine fan is running to cool off the engine under load. I have it scheduled at the local dealer for a transmission service and A/C check but I thought I would check here first to get a good idea if anyone has one. As always, thanks in advance

 

Any transmission codes... have them scan for them before they do anything. Changing fluid will help some.

Are you tuned with a programmer?

A/c with no cooling is a clutch thing, maybe its freezing up?

 

What were the coolant and oil temps when this happened? Computer could have shut off the A/C because the temps were too high.

 

Many times engine problems "feel" like transmission issues. I suspect you're overheating. I'd be very easy on it and get it looked at ASAP. Reducing speed and dropping down a gear Manually with the gear selector can often reduce temps on longer hills. Don't shut it down too quickly after a harder pull, let it idle a little. I know it sux to be limpin around on vacation but ignoring this and driving normally could cost a bunch of bucks.

 

A transmission service is never bad, but it isn't going to fix the engine problem you have. There is nothing wrong with the trans, I suggest getting them to look at the engine instead of servicing the transmission.

 

Im thinking you have an injector going south on you. I would have them run an uncompensated power balance test to see if that is in fact what is going on.

 

Thanks for all the suggestions but I think I should have added some more info. It is completely stock no tuner. No puking no head stud problems. I use Torque Pro to monitor everything. My ECT runs about 204 to 210 and not over 218 on the steepest grades. My EOT never goes above 7 or 8 degrees above that because I have a new oil cooler. The transmission hasn't gotten over 170 the whole trip and that was coming down a long grade. The trans has been running about 165. It doesn't have any codes other than the occassional P0404 for EGR that it sometimes gets when towing. My VGT is right on. My injectors have about 15k on them and I know how it feels when one of those is going down because its happened before. Everything else is normal except for just this one point. At about 60, only while towing and starting up a grade, when its time for it to shift down, it bucks. If I push the throttle a little more it shifts down and pulls normal. It doesn't slip at all, and I'm in the mountains so I think I would be able to tell by watching the tach. I'll have them check for codes and go frome there. I do agree that it seems like the A/C is shutting down becasue the ECU is telling it to shut down based on some parameter.

 

OK. No transmission codes, no engine coes, just a code for the brake light not working right which has already corrected itself. The transmission guy at the dealer said if it had any trans problems the CEL should have come on and the tow/haul light should have flashed. So I got the transmission serviced and I will go from there. As far as the A/C goes it seems that the magnets in the clutch are worn and there is too much space between them?? It has been working perfect for the last few days. I really won't know anything until I hook the 5th wheel up and tow again next Tuesday.

 

This sounds really similar to a problem my truck had a couple years ago. It ended up being dirty sensors. I would start with cleaning the MAP tube and sensor. A tube comes off the top of the intake manifold about straight over from the oil fill cap. Follow that tube up to a sensor at the top of the firewall area. Disconnect the sensor from the plug and the tube and clean it with some carb cleaner. Get a coat hanger or some stiff wire and shove it in the tube to clean that out too. It gets all gunked up from deposits from the egr system. Put it all back together and you should be good to go.

A couple other sensors you could look at too if you have time are the IAT2 sensor, about the same part of the manifold but on the other side, and the EBP sensor under the degas tank. They can get gummed up too and cause issues, but my guess is your MAP sensor is dirty.

Not sure about the AC issue. The MAP sensor readings are used to control how your truck shifts and how the turbo works among other things, so can cause all sorts of issues if it's dirty.

 

Clean all these sensors and then do a full EGR, transmission and VGT relearn procedure. This is exactly what I had, except a bit of black smoke before shifting down, took a new EGR and cleaning all sensors and full relearn. Perhaps the cleaning of sensors and relearn, then if still an issue, try unplugging the EGR to rule it out. I agree with these guys, I pointing you in the right direction.

 

Well we made it home last night. On the way back, 1,000 miles, it did the same thing. The A/C was the same but only while towing so I am convinced it is just the extra engine heat making it work too hard and the magnets are going south. As far as the jerking, it is almost perfectly predictable and I am leaning toward the EGR valve and sensors. It runs and tows perfect in all conditions except this: Starting up a grade running about 63, if I push the throttle easy to maintain speed, at the point where it should shift down, it jerks and bucks. Its almost as if it says, "I'm shifting down, oh wait, no I'm not." If I let off the throttle, it will pull perfect and slowly go down to about 50-52mph then downshift smoothly and start to pull again. If I floor it when I want to maintain the uphill speed it will shift down and go, but I don't like to run it hard like that. Either way, it only does it when towing so I am taking it in in 500 more miles when it needs service to try to get it sorted out. I did lean on it pretty hard yesterday on the way to the storage lot while towing and I am getting black smoke. That with the P0404 tells me the EGR valve needs cleaned or replaced.

 

That does sound like an EGR issue to me. Thanks for the update.

I hope you had a nice vacation despite the truck acting up a bit.

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Thanks. The vacation was great despite driving 1600 miles of it towing with the intermittent A/C. It was 91 degrees yesterday so it did get warm when the A/C shut off every few minutes and we had to "window down". I could work around the jerking problem if I just let the truck run down going uphill and downshift itself but it did make for a little slower trip. Now I get to spend the entire afternoon cleaning it out and washing it after 2 weeks of hard towing and hauling 2 dogs and 5 people halfway across the good 'ole USA. I would like to add that when the 6.0 is running right, which it did for the first 600 or so miles of this trip, I am a very happy camper.