It's been awhile since I pulled anything (like 20 years to be exact) and the
last thing I pulled was a 1000lb boat.

Finally hooked up the Boy Scout trailer to the truck (the reason I
gave the wife as to why the Tahoe ). Once I get up to
35-40, starts to buck. If I let off the gas, the bucking diminishes.
What the heck is going on?

Trailer (pics below) is a double axle trailer and it's 15' (to the tip of
the hitch). I'd guess it's about 3500-4500lbs with all the gear in it.

The dealer hooked up a Tekonsha P-3 Trailer Proportional Brake Controller
for me and I think I've set it correctly.

I was pulling on level even ground (smooth roads) and can't understand
the bucking. The trans got to around 160 (never higher) and I stayed in
drive the whole way.

I know it's not going to pull like my little boat diod way back when, but I didn't expect this at all!

Any ideas??

Edit - It is very windy today and I was bucking on open roads. Perhaps I'll try it
in M4. It almost seems like it's slipping in and out of overdrive.

 

Well, I've pulled my Jet boat with it (see here ), and I've not had any issue at all. Can barely tell it is back there. Now, the difference is you have a trailer brake. I would bet money it is set wrong, maybe just a little, but it sounds like it starts engaging too early and you get an oscillation going. Make it slightly less sensitive and see how works.
There is no "overdrive" on this, the trans is much more complex than that. If you think it is shifting between two gears, watch the tach when it is bucking, shifting would be accompanied by a jump/drop in RPMs.

 

Naperville? I saw Cheap Trick play Rib Fest there in the 1990s...

 

The tach does not flucuate and I thought there was no overdrive on it
either.

How would you set the trailer brakes? I set it to not lock up on
straight, flat area, but just. Do I have it set to high? I can turn it
down and see what happens. When I was bucking, I watched the brake
controller and it was outputting 0.0 so I figured it wasn't that.

BTW, nice boat!!

 

Thanks! I hope someone likes it enough to buy it...

If it was outputting zero, then that probably is not it. If it continues, make an appointment to drive it with the dealer, they would probably be interested in this. I very much doubt this is normal behavior. I've pulled mine in Santa Ana Winds and not had a problem.

 

Oh yeah - Ribfest gets all kinds of acts!! This year it's Charlie Daniels Band and Sammy Hagar. I like going, but it's getting very crowded.
Maybe it'll get better when they move it to the other end of Naperville in a few years (Nike Park off of Diehl and Washington).

You can get details here if you are interested ... http://www.ribfest.net/

 

I'm not sure what you mean by bucking but I don't see a sway bar attachment in the picture you posted. If your problem has to do with sway, that might be the answer. I had real sway problems with a trailer about that size and my old Silverado and a sway bar fixed it.

 

The bucking is not a side to side motion, but a bad forward/backward
motion, like a bad surging/falloff in rapid succession.

 

At 35-40 mph, the trans will select either EVT Mode 2, Fixed Gear 2 or 3 (depending on load) (side question, what rpms does the tach indicate when this is happening?). Too fast for Mode 1, not fast enough for Fixed gear 4. If it's slipping out of a fixed gear due to a clutch slipping the Engine rpms are going to move around. If you see a stable tach, then I would agree it's probably the trailer brake setup.

 

I was runnning aroun 2000 RPMs, so I assume I would not be in EVT mode (I was accelerating). The tach was stable and I was on smooth flat roads so now I'll check the brake controller. I will be hauling it back to its storage area this afternoon so I'll check that out and report back. I will also see if M4 makes any difference (I doubt it, but will check).