Towing issue with GMC 2500HD - RV-Dreams Community Forum

Motoadvrider · RV-Dreams Community Member

Towing issue with GMC 2500HD

Bill and Linda · RV-Dreams Family Member

Bob, I am assuming you are running the truck in Tow-Haul with the trailer? Forgive the question, but that would be my first thought when you mention "shifting."

Second, may I ask, are you sure it has a 3.73 and not like a 3.56 rear end? As it is a 2500 I doubt it has a 3.56 but nonetheless, its a question. (I have some experience with this situation.)

Finally, what does Chevy say in the spec sheet about the this truck? I agree it would seem to be fine with a 5000lb tow. But I just always like to read the manual - salesmen, not so much.

Bill

RodnReal · RV-Dreams Family Member

Bob
It sounds to me like something is wrong. Have you had the trailer checke out to see if brakes are dragging or anything? I would be leary of adding an after market chip until I was positive that there was nothing wrong with both the truck & the trailer. If you add the chip it may void your warranty.

--Rod

Terry and Jo · RV-Dreams Family Member

I would check with a mechanic that you trust about the performance chips. I've been on some forums (more specifically, Ford) and there are comments both pro and con with the chips.

While I like to get the opinion of those on forums, they aren't experts for the most part. That is why I suggest the TRUSTED mechanic.

Also, for the shifting back and forth, is your pickup equipped with a switch that lets you disengage overdrive (sometimes labeled as O/D)? I had a Ford F250 that had the switch at the end of the shift lever on the column. They recommended that when pulling a heavy trailer to disengage (turn off) the overdrive. That prevents the transmission from trying to go into and out of overdrive.

Good luck in finding a solution.

Terry

Motoadvrider · RV-Dreams Community Member

Spec sheet on the truck says it is a 3.73 rear. I tried both with and without Tow Haul on. It did shift more without. Book says you don't need to use Tow Haul if the combined weight of trailer and truck is 3/4 or less of GVW.(16000) My total is at 11,100. But I use TH anyway.

The book says the towing capacity for this truck is 9800lbs.

Motoadvrider · RV-Dreams Community Member

It did not feel like the brakes were dragging but I will definitely check that out. Being the first time I towed it with that truck I may not have noticed it.

Motoadvrider · RV-Dreams Community Member

Actually the service manager at an RV dealer suggested the chip. I used it on my o7 half ton and that is probably why it did so well. The catch is you have to remember to restore the stock settings when you take it in for service. The chip manufacturer claims it will not do anything to harm the vehicle but will not guarantee no issues with your warranty.

rjenkins · RV-Dreams Family Member

Terry you took the words out of my mouth. I own an F150 ('97) and it too has the pushbutton for O/D at the end of the shift lever. When I first got the truck, whenever I would tow my boat I would forget to push the button. After a few times, it became a part of my routine to disengage and it made a difference. Boat fully loaded was at about 6,000 pounds.

Motoadvrider · RV-Dreams Community Member

That OD button is the same as the Tow Haul on a GM.

thebearII · RV-Dreams Family Member

You didn't show if your 1500 was 4 wheel drive. 4X4s get less mileage than a 2 wheel drive.

The chip will help, but like you said until you are absolutely sure there's not a transmission problem no need to go to the chip.

I'm sure you've checked to make sure the hubs have disengaged. I've seen many 4X4s travelling down the freeway with no reason to have the hubs engaged and I can see the front drive shaft spinning. With auto hubs you need to make sure they have disengaged. I switched mine out to manual hubs because I kept forgetting to backup to disengage.

Gas mileage will be affected if your front drive train is engaged, even if you're not in four wheel drive.

Maybe you need to tow at 65MPH like you did with the 1500....LOL.

My old 3500 liked 61MPH that seemed to be where I would get the best overall MPG. The whole truck seemed to smooth out at 61 MPH.

-- Edited by thebearII on Friday 15th of January 2010 02:52:12 PM

Jim01 · RV-Dreams Family Member

Motoadvrider wrote:

Spec sheet on the truck says it is a 3.73 rear. I tried both with and without Tow Haul on. It did shift more without. Book says you don't need to use Tow Haul if the combined weight of trailer and truck is 3/4 or less of GVW.(16000) My total is at 11,100. But I use TH anyway.
The book says the towing capacity for this truck is 9800lbs.

Have you checked what the plate on the door post says about total weight? Don't go by the book. they use the same book for all the trucks. A GMC truck with a 3.73 normally doesn't have much capacity. I know, I had one.

Jim

Motoadvrider · RV-Dreams Community Member

I'll check the door post info. The book lists it by body (crew cab), 2500, engine and axle ratio. Even if it were 1000lbs less I would expect better from this truck.

Bill and Linda · RV-Dreams Family Member

The purpose of “Tow Haul” on all GMC products is to change the shift points of the transmission and to keep it from “shifting too soon or too often.” It does more than just “lock out overdrive.” In fact, it will not “lock out” overdrive as such. It just requires a higher vehicle speed, and less torque load on the transmission, before it will shift into 4^th gear. (4^th gear is OD in the case of the non-Allison transmissions. I am sure you don’t have the Allison. GMC only put that wonderful transmission on the 8.1 liter engines as I recall – and the Diesel Duramax.)

My I suggest you try the vehicle with Tow Haul engaged. It will not solve gas mileage issues as such, but it will improve the transmission’s operation with a heavy load. IMHO, the people who designed the transmission and engine combination know best which gear is best as determined by the vehicle ECM. (Engine Control Module)

I have 7 GMC / Chevy vehicles in a fleet upon which I base this experience and suggestion. So you might want to give Tow Haul a try remembering that a gas engine will need higher RPM’s with a heavy load and one lower gear. Just is what it is.

Please also note that any kind of headwind will increase the effective “load” on the tow vehicle and will significantly impact mileage and transmission shifting.

My 2 cents worth of experience towing

Bill