Hard downshifts while in 4-lo

[Mojavejohn]

Hello everyone, I was directed to this forum by Jr_Explorer (I didn't see that user name in your registry). I'm normally on the Expedition Portal forum (same user name). Mike was very highly recommended.

My truck is a '02 Chevy Duramax/Allison, cc/sb 4X4. I have 173,000 miles on it.



When I'm out wheeling and in 4-lo I'll get a really hard downshift into 1st gear when I go down hill. I know that the truck is supposed to down shift like this when in tow/haul mode, but I don't wheel in tow/haul mode. I've had the truck since it was a year old, and it's always done this. It's only into 1st gear that it's really jarring. I've done quite a bit of wheeling with her and this is my only complaint. I just want to know if this is normal and if so is there a way to turn it off? The only work I've had done in that area is transfer case pump rub fixed, a few months ago, but as I said, this has been acting like this for the entire life of my truck. Thanks and any help is greatly appreciated.

 

[TLyons90]

How fast are you going in 4lo?

 

[Mojavejohn]

How fast are you going in 4lo?

I'm just crawling, under 5MPH, going up and down rough and rocky trails.

 

[TLyons90]

Hmm... I know in 4lo it usually stiffins up everything and makes everything real hard feeling but it doesnt seem like the truck should be shifting at all if you are under 5mph...

 

[Mojavejohn]

It happens when I'm in second and it'll downshift to 1st. Just when going down hills. I'm not sure of the exact speeds, but I'd be surprised if I'm going much more than 5 MPH. I really think the tranny is doing what it is programmed to do, I think it's just that the gearing is so low that it really feels harsh. I wonder if there's a way to re-program the tranny to not engage the grade braking while in 4-low.

Hmm... I know in 4lo it usually stiffins up everything and makes everything real hard feeling but it doesnt seem like the truck should be shifting at all if you are under 5mph...

 

[duramaxzak]

You could have ur tcm programmed to go no lower than 2nd gear in 4lo.

 

[TLyons90]

It happens when I'm in second and it'll downshift to 1st. Just when going down hills. I'm not sure of the exact speeds, but I'd be surprised if I'm going much more than 5 MPH. I really think the tranny is doing what it is programmed to do, I think it's just that the gearing is so low that it really feels harsh. I wonder if there's a way to re-program the tranny to not engage the grade braking while in 4-low.

Good possibilty. I would check with your tuner and see how they have your TCM programmed...

 

[DAVe3283]

...
I wonder if there's a way to re-program the tranny to not engage the grade braking while in 4-low.

If you don't want the grade braking, you could always leave it in 4 hi. I almost never wheel in 4 low. I can spin all 4 tires in 4 hi without much effort, and I prefer to foot brake (not engine brake) off road because it is smoother / more controlled (for me).

Or, as duramaxzak said, have the trans programmed to stay in 2+ when in 4x4. But this basically defeats the purpose of 4 low, so might as well be in 4 hi.

OR... I seem to recall from the last time I tuned my trans, that you could program it to stick in only 2nd when the gear lever is on 2. That might be the best of both worlds. You get 4 low, 1st when you want it, and 2nd the rest of the time.

 

[Mojavejohn]

Thanks for all the good input.

I like the idea of being able to lock it into 2nd gear, and not allowing it to downshift into 1st unless I manually select 1st. I don't want to lose 1st gear in 4 lo, I really like the control of crawling along that the lower gearing gives me.

I know absolutely nothing about engine and trans tuners. I've seen the engine programers/tuners advertised, but really haven't ever looked into them to much. Can you buy a tuner just for your trans.? This definitely sounds like the way to go.

I don't want to sound like a moron, but whats a TCM? Transmission control module????

 

[Ne-max]

Yes and Yes. You can tune trans with efi but from I have heard and read its not worth it.

 

[Mojavejohn]

If you don't want the grade braking, you could always leave it in 4 hi. I almost never wheel in 4 low. I can spin all 4 tires in 4 hi without much effort, and I prefer to foot brake (not engine brake) off road because it is smoother / more controlled (for me).

Or, as duramaxzak said, have the trans programmed to stay in 2+ when in 4x4. But this basically defeats the purpose of 4 low, so might as well be in 4 hi.

OR... I seem to recall from the last time I tuned my trans, that you could program it to stick in only 2nd when the gear lever is on 2. That might be the best of both worlds. You get 4 low, 1st when you want it, and 2nd the rest of the time.

Yes and Yes. You can tune trans with efi but from I have heard and read its not worth it.

Why's that?

 

[duramaxzak]

A lot of pullers program the tcm to start in 3rd gear, 4lo.