1/2 ton towing/ freewheeling problem

N900WM · September 10, 2003

Hey Guys,

We have a 2000 1/2 ton 4x4 that gets used to tow on occasion.

When loaded, the truck will accually start freewheeling on a down hill grade when you let off on the throttle. It acts like it goes into neutral! It's pretty un-nerving!

It doesn't seem to matter if the tow/haul is on or off. But I would think that in tow/haul the tranny should hold the gear and help brake?

Anyone else notice this or have any solution? Other than an Ally tranny.

stlmaj · September 10, 2003

My old 00' 1500 extcab Sierra and my new 1500hd crewcab both automatic transmissions both do it and sometimes even do it on a step grade while not towing anything. It's normal all the trucks and cars I've everowned have done this to some degree.

Shaners · September 10, 2003

About all you can do is pull the truck down into "3" to help engine brake your truck when going downhill.

robsawatsky · September 10, 2003

I thought tow/haul would shift down when you touched the brake, but maybe that is only in the D/A.

Anyway, as far as I know - all other GM truck auto tranny's, you've got to shift down manually to get any real engine braking because of the way in which the overrun clutches engage.

99-2door · September 10, 2003

Any automatic will do that, exception: Allison.

1999GMCZ-71 · September 10, 2003

only the allison downshifts when you touch the brake. If you leave your transmission in third it should brake some downhill. OD is designed to freewheel when you let off the gas to get better mileage.

greg.bender · September 11, 2003

Mine did that too on a recent trip. Setting the cruise control kept mine right where I wanted it. Once I turned CC off it'd start creaping back up.

100R1 · September 11, 2003

My '01 Yukon with 5.3, 3.73 axle factory tow options on a recent trip towing a 5,200 lb travel trailer using tow/haul mode in the mountain grades of the smoky mountains would automatically down shift while decending the mountains to hold speed down. I still had to use my brakes, But the tranny automatically down shifted when it sensed that the decent speed was to high for Overdrive/4th gear. I don't remember if it down shifted into second from third , But defently from overdrive/4th to 3rd gear.

I noticed it did this with-out my manually down shifting. I found the Tow/Haul feature to work excellent for me in all aspects of towing.

HillHound · September 11, 2003

Tow/Haul on the 1500 I had didn't do much unless I was pulling the trailer with the engine/tranny under load, not holding it back. Like Shaner's said the only thing to do is manually put it in third and watch the rpms. When I had my Silverado 1500 I even used to put it in second sometimes to hold it back on secondary roads.

My F250 engine brakes very well when you switch off overdrive and make 3rd the highest gear, but on the downside there's no "tow/haul" mode and the only way to manually put it in 3rd is to switch off OD. There's OD, 2 & 1 on the transmission but no spot for third.

BlackSilver · September 11, 2003

My '01 Yukon with 5.3, 3.73 axle factory tow options on a recent trip towing a 5,200 lb travel trailer using tow/haul mode in the mountain grades of the smoky mountains would automatically down shift while decending the mountains to hold speed down.

Unless your car is equipped with an Allison (and I'm sure it's not), then you are mistaken. Your transmission will NOT automagically downshift on a downgrade.

stlmaj · September 11, 2003

To my knowlage the tow/haul mode only changes shift points and nothing else.

N900WM · September 11, 2003

We must have something wrong. And there must be something up with the tow/haul.

If I manually shift it into 3rd, it will freewheel. If I leave it in OD, it will brake a little.

If I switch on the tow/haul, it just freewheels all the time.

I'll tell you what, once you drive the allison with a few ton behind you, there is no comparison. The allison rules.

1999GMCZ-71 · September 11, 2003

Tow/Haul changes the shift points and the shift firmness.

robsawatsky · September 11, 2003

We must have something wrong. And there must be something up with the tow/haul.
If I manually shift it into 3rd, it will freewheel. If I leave it in OD, it will brake a little.

If I switch on the tow/haul, it just freewheels all the time.

Well, that is really weird.

I believe the freewheel, overrun clutch is all controlled hydraulically within the tranny, not by electronics. So, even more weird.