Is it ok to leave the Selec-Trac Full Time 4WD System on?

[ShadowsPapa]

So you are saying that the hub disconnects?

That's news to me. I'm happy to delete my post if I'm wrong. Can you explain more?

My understanding was that in 2H, the only thing that disconnected was the transfer case. Which would leave the entire front drivetrain being pushed by the road.

Thanks - Don

Front axle disconnect. With it in 2H, the front right axle shaft is split in half. This means that the front pinion and ring gear and thus the front drive shaft are not rotating.
AMC did a lot of testing back in the 80s - there IS a difference in MPG because you are spinning the ring gear, pinion and drive shaft with the right axle not split.
In 2H that axle is split and you are only turning the side gears and spiders (The carrier gears) while the carrier remains stationary.

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[foo.c]

So you are saying that the hub disconnects?

That's news to me. I'm happy to delete my post if I'm wrong. Can you explain more?

My understanding was that in 2H, the only thing that disconnected was the transfer case. Which would leave the entire front drivetrain being pushed by the road.

Thanks - Don

Not really the hub, it's on the axle, but does the same thing. Saves a bit of fuel.

 

[dcmdon]

I don't know. I thought we were politely and respectfully discussing how the AWD system works. Trying to flush out the correct answer.

 

[dcmdon]

Front axle disconnect. With it in 2H, the front right axle shaft is split in half. This means that the front pinion and ring gear and thus the front drive shaft are not rotating.
AMC did a lot of testing back in the 80s - there IS a difference in MPG because you are spinning the ring gear, pinion and drive shaft with the right axle not split.
In 2H that axle is split and you are only turning the side gears and spiders (The carrier gears) while the carrier remains stationary.

That makes total sense. Thank you. I didn't realize the hub or axel disconnects. And yes, this will help fuel economy since like you said there are parts that aren't turning when in 2H.

Thanks again.

Not really the hub, it's on the axle, but does the same thing. Saves a bit of fuel.

Thank you.

 

[ShadowsPapa]

Well, not exactly...

In 2H, the RH side front axle disconnect is disconnected. This means the LH front axle is turning in the direction of the tire, but the driveshaft is stationary, so the RH inner axle is turning in the opposite direction of the tire and the RH outer axle is turning in the direction of the tire.

Because the driveshaft is not turning in 2H, it is supposed to be slightly more efficient.

I found that in 4HAuto on the highway in dry conditions, there is slightly more noise than in 2H. It almost sounds like slightly louder tire noise that occurs when on the throttle versus off.

Yes, FAD. AMC used a solid right axle on the Eagle in 1980. It was full time. To meet CAFE, they put in FAD and split the right axle to keep from spinning the whole carrier, pinion and front driveshaft and put in "select drive" (with a cool emblem on the glove box door) In the end, it was more expensive for them so they went back to no FAD - but they left the 2w/4w choice and it ended up being shift on the fly without the FAD starting about 85 or 86 in the Eagle.
In 2wd, I am not turning the pinion or ring gear or front driveshaft because of the split right axle. In 4wd, the axle reconnects then the transfer case engages 4wd.
All that spins up there in 2 wheel drive is the INNER half of the right axle, the left axle, and the side gears and spider gears inside the carrier. The carrier remains stationary.

 

[Kazels03]

I have and still drive my 1998 Cherokee that I ordered from the Factory especially so I could have Select-trac. You could leave it on all the time, BUT WHY? As others have mentioned, it allows when you need to use 4WD to not have to worry about the road condition. By my house I can be on Snow packed roads, but it can vary once on the interstate due to gusting winds in spots and or vice versa.
The Select-trac is different than Command-Trac (Which cannot be engaged on Dry Pavement). Back in the Day Select-Trac was not even available on the Wranglers. Only recently while looking at Gladiators did I notice it now being available. I stopped looking at Wranglers because I did not want Command-Trac and even wondered if I could swap-out my transfer case in order to have it.

I had an Isuzu Rodeo that I ran into binding issues because of the variation of road conditions.

Anyway, two folks in service told me that the 4H Auto should never be used on dry hard surfaces. I told them that my understanding is that it’s intended to be used when you’re driving on-road in wet or icy conditions but they were adamant that wasn’t the case. I think they were wrong, can anyone confirm?

 

[brianinca]

Don't forget the occasional burnout!

Just to expand on the less efficient aspect.

You are paying the price in efficiency whether its turned on or not. The fact that you have front driveshaft and an axel with a differential and CVJs is what costs the efficiency.

So if you have a 4 Auto setting, there is literally zero costs to leaving it on all the time since you are already paying for the weight and friction of having all that stuff in the car anyway. Unless you like oversteer, then 2H is where its at. ha.

 

[aldo98229]

I already did my research in the manual and on this forum and it looks like it’s fine to leave it on since 4 Hi when in auto would only be activated due to a loss of traction. It also sounds like there’s a potential decrease in MPG if I leave it on, though I’m not sure if that’s been totally proven.

Anyway, two folks in service told me that the 4H Auto should never be used on dry hard surfaces. I told them that my understanding is that it’s intended to be used when you’re driving on-road in wet or icy conditions but they were adamant that wasn’t the case. I think they were wrong, can anyone confirm?

The folks in service are full of shit. They must have confused 4H Auto with 4H Part Time.

I engage 4A when the rains start in the fall here in the PNW, and leave it engaged until spring. It provides a more surefooted, more planted driving. I could keep it on year round, but there is little benefit in summer.

The ONLY instance I found best not to be in 4A was on the trail. The constant engaging and disengaging of the 4A on the trail overheated the clutch pack and made it smell.

So, now as soon as I hit the trailhead, I engage out of 4A and into 4H Part Time. Haven’t had any issues any more.

 

[jac04]

Yes, FAD. ...

Exactly. Thanks for taking the time for the detailed response. I was away from my computer yesterday afternoon & evening, thus my lack of reply.

 

[legacy_etu]

Great, thanks for confirming. I’m still learning everything about my new Jeep and how vehicles work in general and even though I thought the two guys at the service department are wrong I didn’t want assume that’s the case without checking elsewhere.

Good on you for reading and learning the features of your vehicle. You are already more knowledgeable than the dealer

 

[TreyDrier]

4H Auto (not 4H Part-Time) should be OK to leave it in when there's rain, light snow, when the wheels can 'slip' while turning. I believe that system uses a 'clutch type' system so you don't bind the front wheels. It shouldn't hurt it on dry pavement, but why have it on when it's not needed? It's great for light snow when some streets have snow and other streets are clear. Now Command Trac is different, a
Part-Time system for deep snow/sand etc and should not be used on dry pavement. It doesn't use the clutch system and the wheels won't slip and could bind up (metal things can go BING!). Thanks, JW.

Huh??? That's the whole point in have 4Auto, so that you don't have to worry about changing gears if the weather suddenly turns. 4Auto means it *automatically* shifts to the proper gear setting depending on conditions. If you do it manually like you state there is no point in having it.

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