Don't stone me. 4wd questions

wiggles

Original poster
Member
Dec 4, 2011
34
I've never driven a vehicle before my trailblazer with a 4wd switch on the dash. I have some questions and I bet I'm not the only one....atleast I hope not.:redface:


Do I need to stop the vehicle and put it in park before I switch to 4wd.

A4WD means automatic 4wd correct? as in it switches from low 4wd to hi 4wd on it's own?

I have it set on 2 hi for normal driving.

What is the top speed for 4wd without messing anything up on a trailblazer?

I hope everyone doesn't think I'm dumb as a rock for asking.
 

Ghoster

Lifetime VIP Donor
Member
Nov 18, 2011
1,444
While I am sure someone will pop in and give you good direction, it sounds like you need to spend a little quality time with your owners manual. Most of the questions you asked are covered in there. In case you don't have one, try this link.

[PDF]gmtnation.com/staff/2002_chevrolet_trailblazer_owners.pdf[/PDF]
 

wiggles

Original poster
Member
Dec 4, 2011
34
That answered my questions about when I can change over to 4wd ( I found the answers surprising) but I'm still wondering about speeds.
 

Wooluf1952

Member
Nov 20, 2011
2,663
Milwaukee, Wisconsin
wiggles said:
I've never driven a vehicle before my trailblazer with a 4wd switch on the dash. I have some questions and I bet I'm not the only one....atleast I hope not.:redface:


Do I need to stop the vehicle and put it in park before I switch to 4wd.

A4WD means automatic 4wd correct? as in it switches from low 4wd to hi 4wd on it's own?

I have it set on 2 hi for normal driving.

What is the top speed for 4wd without messing anything up on a trailblazer?

I hope everyone doesn't think I'm dumb as a rock for asking.


A4WD is 2WD until the rear tires start to slip. Then it turns into 4HI.

Don't use A4WD as a normal driving position. Only use it if you are driving in slippery conditions.

Even then, if its slippery enough that you need 4X4, use 4HI.

Also, remember: 4X4 helps you go. It does not help you stop.

If you need 4X4, go slow.
 

wiggles

Original poster
Member
Dec 4, 2011
34
I've driven 4wd before I just didn't quite understand the a4wd and apparently was completely off.
 

navigator

Member
Dec 3, 2011
504
I don't remember what the manual says about the top speed for switching to 4Hi but I would do it when going slowly as our splinned disconnect is fickle. On 4Lo, you want to be in Neutral, ideally moving about 2-4 mph.
For either moving slowly helps things gently slide into place.
 

bigredtank

Member
Nov 21, 2011
31
yeah mines 4wd but i never use it. the only time i did was pulling a boat out of water and im pretty sure i didnt do it right. from what i read on the "past" site. id stop, put in in N, switch to whatever setting. reverse for a second to engage and then drive. but i dont know. i drive a slammed 4x4 on SS wheels. sorry Roadie!:lipsrsealed:
 

mika

Member
Dec 6, 2011
82
you can switch to a4wd or 4hi at any time at any speed, that doesn't to flip to 4 hi at 70mph, if you are driving that fast there is no need for 4hi, 55 to 60mph it no problem, as for the 4awd position, me personaly from what I have read in the OS, if the conditions are bad enough for 4awd use 4hi, the constant poping in and out of 4hi whit the thump it dose will damage the bearing. Roadie has made a few posts about this, I don't if he brought them over to GMT yet?

As for 4lo you can switch to it while driving less than 5mph. swithing back to 2hi it is best to be rolling less than 5mph or even better ead stop and in neutrel.
 

TB_n00b

Member
Dec 19, 2011
121
Queens, NY
Ideally you should be driving slowly (5mph or less), with the front wheels straight when shifting into and out of A4WD or 4HI. This guarantees happy gear engagement. You can do this at any speed, but honestly why would you need to engage 4HI when driving at speed? All 4WD does for you is help you start and keep moving in bad conditions. It's not helping you steer, not helping you stop.

^^Edit: I know the manual doesn't say any of that. It's just good practice, particularly if you are using 4WD a lot.

For 4LO your wheels should be straight, the transmission MUST be in Neutral and the truck MUST be rolling between 2 and 4 MPH or the transfer case will not engage. Same goes for taking the truck out of 4LO. As the manual says this is for EXTREME conditions only.

This reminds me, I need to exercise my TC.
 

HARDTRAILZ

Moderator
Nov 18, 2011
49,665
TB_n00b said:
the truck MUST be rolling between 2 and 4 MPH or the transfer case will not engage. Same goes for taking the truck out of 4LO. As the manual says this is for EXTREME conditions only.

As one that uses 4LO alot, for full days at a time...I am not typically rolling when I engage or disengage it. You are mistaken. It will engage and disengage at a stop, but the truck must be in neutral.

Extreme conditions...mmmm may need some defining. The lower gearing of 4LO can be helpful in weather, towing, offroad, steep slopes. These are extreme conditions, but just be aware that 4LO can be quite helpful in many situations. I virtually never use A4WD or 4HI. If I need 4x4, I use 4LO.
 

TB_n00b

Member
Dec 19, 2011
121
Queens, NY
HARDTRAILZ said:
As one that uses 4LO alot, for full days at a time...I am not typically rolling when I engage or disengage it. You are mistaken. It will engage and disengage at a stop, but the truck must be in neutral.

Extreme conditions...mmmm may need some defining. The lower gearing of 4LO can be helpful in weather, towing, offroad, steep slopes. These are extreme conditions, but just be aware that 4LO can be quite helpful in many situations. I virtually never use A4WD or 4HI. If I need 4x4, I use 4LO.

My mistake. I admittedly have not used 4LO on this truck. I haven't felt the need or maybe don't grasp the concept. Maybe it would have helped me last year when I was trying to get out of my driveway through a 3' tall snow bank during one of the blizzards to get to a fire call? I was in 4HI but with no traction I felt it was a mute point.

Do you believe there is benefit to engaging 4LO while slow rolling over being stopped?
 

HARDTRAILZ

Moderator
Nov 18, 2011
49,665
TB_n00b said:
My mistake. I admittedly have not used 4LO on this truck. I haven't felt the need or maybe don't grasp the concept. Maybe it would have helped me last year when I was trying to get out of my driveway through a 3' tall snow bank during one of the blizzards to get to a fire call? I was in 4HI but with no traction I felt it was a mute point.

Do you believe there is benefit to engaging 4LO while slow rolling over being stopped?

They say it helps things bind easier, but I have not noticed any issue, so I have not really bothered trying to be rolling. I just put it in nuetral, flip the switch and listen for all the good sounds of things working, then go.
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
The internal structure of the passenger side splined disconnect sometimes delays full engagement if the two major gears aren't perfectly lined up. The collar is spring loaded and will slip over the two of them only when they're lined up. Often, when I'm stopped, even moving a tiny bit straight ahead won't engage them, and I need a slight turn to get RELATIVE movement between the two gears to let the collar slide on.

Here's the two gears with the collar in the DISENGAGED position, showing a slight misalignment.

2hi.jpg


Here's the collar in position to lock the two gears together, with the shift fork pushed upwards. In the vehicle, up would be INWARDS, towards the driver's side. The front axle actuator pushes on the right side of the shift fork to accomplish the task

4hi.jpg


For more reading: Offroadtb.com Front Axle 4WD Disconnect

4LO gives you a 2.7-to-1 gear ratio advantage, but as you discovered, no improvement in traction.

As to changing modes on the fly - you can go from 2HI->A4WD at any speed, but it's usually not a reasonable thing to do. James Downing was the first to post an analysis that convinced me - but it's subtle. In 2HI, the transfer case isn't spinning the front driveshaft (from the transfer case to the front differential). So the driver's wheel is turning the CV shaft which is turning the end gear. In this drawing, which isn't specifically our unit, let's say the driver's side is at the right.

View attachment 17358

If the pinion isn't moving, neither is the ring gear, and neither is the differential "case" or otherwise known as the carrier. So the spinning end gear will spin the spider gears it touches, and that ends up driving the opposite side (left in the picture) intermediate shaft BACKWARDS. So if you're rolling at 55 MPH, I did the math once, and IIRC the tires are spinning about 600 RPM. So when you engage the front axle actuator, the collar is going to be spinning at 600 RPM on one of the disconnect's gears, and you're asking it to slide over a gear connected to the intermediate shaft spinning 600 RPM the other direction, and it has to stop the spinning mass of the intermedate shaft and its end gear, spider gears, and start to spin the differential carrier, ring gear, pinion gear, and front driveshaft up to speed, at the same time the transfer case is engaging its clutches to help this process. You're putting some unknown amount of stress on the splined disconnect for no purpose. So I recommend not engaging the system while moving fast. :wink:
 

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mika

Member
Dec 6, 2011
82
:
the roadie said:
The internal structure of the passenger side splined disconnect sometimes delays full engagement if the two major gears aren't perfectly lined up. The collar is spring loaded and will slip over the two of them only when they're lined up. Often, when I'm stopped, even moving a tiny bit straight ahead won't engage them, and I need a slight turn to get RELATIVE movement between the two gears to let the collar slide on.

Here's the two gears with the collar in the DISENGAGED position, showing a slight misalignment.

2hi.jpg


Here's the collar in position to lock the two gears together, with the shift fork pushed upwards. In the vehicle, up would be INWARDS, towards the driver's side. The front axle actuator pushes on the right side of the shift fork to accomplish the task

4hi.jpg


For more reading: Offroadtb.com Front Axle 4WD Disconnect

4LO gives you a 2.7-to-1 gear ratio advantage, but as you discovered, no improvement in traction.

As to changing modes on the fly - you can go from 2HI->A4WD at any speed, but it's usually not a reasonable thing to do. James Downing was the first to post an analysis that convinced me - but it's subtle. In 2HI, the transfer case isn't spinning the front driveshaft (from the transfer case to the front differential). So the driver's wheel is turning the CV shaft which is turning the end gear. In this drawing, which isn't specifically our unit, let's say the driver's side is at the right.

View attachment 1869

If the pinion isn't moving, neither is the ring gear, and neither is the differential "case" or otherwise known as the carrier. So the spinning end gear will spin the spider gears it touches, and that ends up driving the opposite side (left in the picture) intermediate shaft BACKWARDS. So if you're rolling at 55 MPH, I did the math once, and IIRC the tires are spinning about 600 RPM. So when you engage the front axle actuator, the collar is going to be spinning at 600 RPM on one of the disconnect's gears, and you're asking it to slide over a gear connected to the intermediate shaft spinning 600 RPM the other direction, and it has to stop the spinning mass of the intermedate shaft and its end gear, spider gears, and start to spin the differential carrier, ring gear, pinion gear, and front driveshaft up to speed, at the same time the transfer case is engaging its clutches to help this process. You're putting some unknown amount of stress on the splined disconnect for no purpose. So I recommend not engaging the system while moving fast. :wink:

:thankyou: That makes perfect sence and is totally logical. I'll remember that.
 

RayVoy

Member
Nov 20, 2011
939
the roadie said:
As to changing modes on the fly - you can go from 2HI->A4WD at any speed, but it's usually not a reasonable thing to do. James Downing was the first to post an analysis that convinced me - but it's subtle. In 2HI, the transfer case isn't spinning the front driveshaft (from the transfer case to the front differential). So the driver's wheel is turning the CV shaft which is turning the end gear. In this drawing, which isn't specifically our unit, let's say the driver's side is at the right.

View attachment 1869

If the pinion isn't moving, neither is the ring gear, and neither is the differential "case" or otherwise known as the carrier. So the spinning end gear will spin the spider gears it touches, and that ends up driving the opposite side (left in the picture) intermediate shaft BACKWARDS. So if you're rolling at 55 MPH, I did the math once, and IIRC the tires are spinning about 600 RPM. So when you engage the front axle actuator, the collar is going to be spinning at 600 RPM on one of the disconnect's gears, and you're asking it to slide over a gear connected to the intermediate shaft spinning 600 RPM the other direction, and it has to stop the spinning mass of the intermedate shaft and its end gear, spider gears, and start to spin the differential carrier, ring gear, pinion gear, and front driveshaft up to speed, at the same time the transfer case is engaging its clutches to help this process. You're putting some unknown amount of stress on the splined disconnect for no purpose. So I recommend not engaging the system while moving fast. :wink:
I disagree just a little.
As I understand the procedure, the transfer case shifts into 4hi without engaging the clutch. A signal is sent to the axle disconnected, commanding it to engage. Once engaged, a 2nd signal is sent back to the controller informing of the engagement, the clutch is then locked into 4hi.

While this is happening, the right front wheel, and the outer half of the front right axle, are traveling at road speed, the inner half of the front right axle is not turning. When the disconnect engages, the inner half of the right front axle comes up to road speed. With both front axles turning at road speed the ring and the pinion gears are being turned at road speed; this, in turn, spins the front driveshaft at road speed.

About this time, the transfer case internal clutch engages.

This sequence should protect the drive line from a lot of stress.
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
RayVoy said:
I disagree just a little.
As I understand the procedure, the transfer case shifts into 4hi without engaging the clutch.
The only major item that moves inside the transfer case for this transition is the clutch. There is no 4HI without the clutch being engaged. Offroadtb.com Transfer Case Operation: NVG 226 (NP8)
A signal is sent to the axle disconnected, commanding it to engage. Once engaged, a 2nd signal is sent back to the controller informing of the engagement, the clutch is then locked into 4hi.
I can't tell from the shop manual that there is any sequencing going on between the front axle actuator and the transfer case encoder motor, but I concede there could be. I'd have to pull out my oscilloscope and look at the timing of the commands.
While this is happening, the right front wheel, and the outer half of the front right axle, are traveling at road speed, the inner half of the front right axle is not turning.
Not sure what you mean by the inner half. The CV shaft is one piece. The splined disconnect is on the side of the oil pan. The intermediate shaft goes through the oil pan hole from the splined disconnect to the differential. The intermediate shaft's rotation is the mystery. I think it's rotating backwards. Slight residual friction in the disconnect may have it tend to rotate forward. Seal friction will tend to keep it at rest. Not sure which of these competing forces wins out.
When the disconnect engages, the inner half of the right front axle comes up to road speed.
Agreed.And it happens in an instant.
With both front axles turning at road speed the ring and the pinion gears are being turned at road speed; this, in turn, spins the front driveshaft at road speed.
Agreed.And the engagement of the splined disconnect, that gets the intermediate shaft spinning, AND the heavier differential carrier, ring gear, pinion, and driveshaft, is doing a lot of heavy work.
About this time, the transfer case internal clutch engages.
Hmmmm, all depends on timing, but the rotation of the intermediate shaft is still a mystery.
This sequence should protect the drive line from a lot of stress.
The goofy designers forgot about the engagement stress of the A4WD mode when it detects wheel slippage, and the pre-2005 transfer cases did break a few internal bearing supports. A4WD mode is still something I avoid.
 

RayVoy

Member
Nov 20, 2011
939
the roadie said:
Not sure what you mean by the inner half.
Just trying to simplify the picture, so the reader could picture a part of the axle connected to the wheel; and a part of the axle connected to the diff. I was talking about the intermediate shaft. And, I agree timing is all important if we are to understand the rotation sequence.



the roadie said:
The goofy designers forgot about the engagement stress of the A4WD mode when it detects wheel slippage, and the pre-2005 transfer cases did break a few internal bearing supports. A4WD mode is still something I avoid.
I am with you on this, after I felt the first engagement thud I stopped using A4WD.
 

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