Strange intermittent free-wheeling downhill !?! - Off-Road Forums & Discussion Groups
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post #1 of (permalink) Old 12-17-2000, 06:51 PM Thread Starter
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Strange intermittent free-wheeling downhill !?!

There is a strange and scary problem I'm having with my '91 YJ, I-6, AX-15, NP231, 2.5" lift, otherwise stock. 4wd works well uphill, 4H or 4L, climbs like a billy-goat. When going downhill, I put it in 4L 1st gear and let it idle down the hill it just lets go for a couple of seconds kinda free-wheeling then catches up again without me doing anything. The whole time it stays in 1st gear, doesn't pop out with the wheels turning not skidding. It feels like if I just press in the clutch for a second then let it out again. Needless to say this scared the s* out of me and it seems to be pretty consistent. First I thought that it was the clutch cause it seems to grab a little too quick, - it's hard to do a smooth engagement - but I couldn't get it to slip on level ground or uphill. Another thing is, that this seems to happen when the idle drops to really low as the downhill eases out and the engine almost stalls for a second until it catches back up. Could it be that because of the too low idle the vacuum drops in the front disconnect disengaging it, making it run in 2wd temporarily (half the braking power on only the rear axle) ? I'm going to check on this theory with some more testing with somebody walking along and watching for this when the weather is better. Any other clues ? This is the last thing I need to fix to make it run like a champ! Please help. Thanks.

'91 YJ, I-6, 3" lift, 31s
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post #2 of (permalink) Old 12-17-2000, 07:35 PM
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Re: Strange intermittent free-wheeling downhill !?!

The front end disconnecting wouldn't do that, you would still have the same gear reduction at the rear. YOU ARE SURE the trans isn't jumping out of gear? The only other thing I can think of is the chain in the t-case jumping a tooth or two, how many miles, have you looked at the t-case chain an gears?

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post #3 of (permalink) Old 12-17-2000, 07:59 PM Thread Starter
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Re: Strange intermittent free-wheeling downhill !?!

125K miles. Didn't have the tcase open ... yet. I'm sure it's stays in gear, since through the whole thing I don't touch the gear shifter or clutch or brakes and when it catches back up again it drives and engine brakes like normal. I'm basically using engine braking through the whole time. Now, when it let's loose it doesn't make any strange noise or anything. It really feels like if I just pushed the clutch in for a sec, it's kinda a smooth feeling. Wouldn't a slipping tcase chain would make some sort of noise or vibration? How much trouble would it be to look at the chain? Why doesn't it slip up hill then? I don't have any problems up hill. I'm really lost with this one...

'91 YJ, I-6, 3" lift, 31s
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post #4 of (permalink) Old 12-18-2000, 10:50 AM Thread Starter
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Re: Strange intermittent free-wheeling downhill !?!

C'mon guys, surely someone has some ideas about this!

'91 YJ, I-6, 3" lift, 31s
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post #5 of (permalink) Old 12-18-2000, 11:02 AM
 
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Re: Strange intermittent free-wheeling downhill !?!

Does it happen just at the start of the hill? If so it probably just the back pressure in the engine building up. If its not the clutch slipping(which it doesnt sound like) and its not slipping in and out of gear, in the T case or the tranny(which you would hear, clunking as it disengaged and engaged) that it seems like it is a back pressure issue with the motor.

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89 YJ
If at 1st you dont succeed, your replacement will try and try again
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post #6 of (permalink) Old 12-18-2000, 11:25 AM Thread Starter
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Re: Strange intermittent free-wheeling downhill !?!

Well, the longest hill I've been on so far was about 30-40 feet. I'd say it happens at about 15-20 feet down the hill and the engine seems to idle very low when this happens. Now, let's assume that I lose vac while in 4L. Would the collar just disengage on it's own then? Doesn't it need vac imput for movement for both directions? Per the Haynes manual the shift motor should hold vac for 30 sec. I'll check for that. If it's the engine back pressure what can be done about it?

'91 YJ, I-6, 3" lift, 31s
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post #7 of (permalink) Old 12-18-2000, 11:56 AM Thread Starter
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Re: Strange intermittent free-wheeling downhill !?!

Well, here's an explanation I got off the news groups. It makes perfect sense to me since I don't have lockers and have fairly limited flex. I bet that when I have somebody watch me during this "lurch" they'll see the in-air tire spin backwards for a sec until it's on solid ground again.
Gotta work on that flex and get me some lockers. What locker goes into a stock D35c without backlash adjustment and special tools?
>>>>>>>
This sounds like the Lurch. Here is a good quote from the following article
that describes what is happening.

http://www.4x4now.com/bb0797.htm
"The lurch happens when the two tires that are giving resistance on the
descent slide or catch air. By now we have the understanding that 4WD is not
4WD unless you have lockers. So, there are usually only two tires holding
the rig back. When they hit the loose stuff or find air, the other two tires
which are static and just rolling along will not help to keep the rig from
moving forward very quickly. This "lurch" forward a few feet is a bit
unnerving unless it is anticipated.

On loose trails, wet, rocky, sandy, soft slick rock, etc, i.e., almost any
trail, the lurch is a common thing. The thing to do is not panic and jam on
the clutch or brakes like at a stop light. When going through such sections
expect it and drag the brakes lightly or engage the lockers."

'91 YJ, I-6, 3" lift, 31s
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post #8 of (permalink) Old 12-18-2000, 12:01 PM
 
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Re: Strange intermittent free-wheeling downhill !?!

Your rear wheels would still be turning the same speed though. So unless the rear tires were actualy sliding due to the lack of traction at the front, you shouldnt notice much difference.

Jeff
89 YJ
If at 1st you dont succeed, your replacement will try and try again
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post #9 of (permalink) Old 12-18-2000, 12:09 PM
 
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Re: Strange intermittent free-wheeling downhill !?!

Ahhhhhh, that makes sense, I wasnt thinking open diffs,( its been many moons since Ive ran open difs in the rear). Yes if one of your tires breaks free with open diffs it will dump the engagemnet to the traction side,(not good when 4 wheelin).
You could go to something like the Tra-Tek(Detroit Manufacturer) "Gearless Locker" in the rear and still retain your stock carrier and gear set up. They are supposed to be a very good locker as far as lunch box lockers go.

Jeff
89 YJ
If at 1st you dont succeed, your replacement will try and try again
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