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TCC lockup problem?

br1anstorm

NAXJA Forum User
Location
United Kingdom
I asked a while ago in this post http://www.naxja.org/forum/showthread.php?t=975842&highlight=auto+lockup
about what I described as "driveline shunt" in my 93 4.0 auto XJ. When in 'D' and up to 4th, above about 40-45 mph, lifting off the gas while cruising along produces a definite clunk. Felt as if the tranny was kicking out of lockup. But replies suggested the TPS, or maybe a worn/failing U-joint in the driveshaft.

Well it's definitely not a U-joint - there's no play or movement when I try to push, pull or turn the driveshaft by hand under the vehicle.

Now I've noticed another symptom. When cruising in 4th above about 40 mph, if I touch the brake pedal even the tiniest bit, the revs kick up by about 150, then drop back again. It's clear that something (electrical switch?) linked to the brake pedal is making it do this. NB I don't have cruise control. But it feels like touching the brake pedal is causing this brief rise in the rpm.

So I went searching, and in a post here http://www.naxja.org/forum/showthread.php?t=972809&highlight=auto+lockup

I found this quote:

"If you have an auto, this switch [on the brake pedal] also tells the tranny computer to disengage the torque converter lockup."

Now I wonder, are these two symptoms related? And if so, does this mean the TCC lockup solenoid is faulty, or the switch on the brake pedal? Or both? Which should I look at first?

And if the symptoms point to the solenoid, can anyone tell me where to find a simple step-by-step guide to checking and if necessary replacing it?

Any advice would be welcome!

br1anstorm
 
That rise in RPM when touching the brake is exactly what is supposed to happen. The TC will unlock even before the brake lights come on in some situations. Other than the rise in RPM, you won't ever feel the converter unlocking, especially not when you let off the gas.

So, no, not related.

EDIT: when you checked those U-joints, did you do it with the Jeep off the ground? If not, there is still loaded torque on the joints, and you wouldn't be able to turn even badly worn ones by hand.
 
Last edited:
Thanks, urbexx.

After I'd posted I kept searching, and in fact I found another forum post which said exactly the same as your reply - that tapping the brake pedal with your left foot while cruising with gas pedal steady makes the lockup disengage, and the revs rise briefly. Nice to know it's normal. I'm just surprised that in 15 years of driving this Jeep I don't remember noticing it before!

Which takes me back to the still-mysterious driveline shunt when I lift off the gas at speeds above 45 mph. In a '93 with 125,000 miles on the clock, I'm suspecting something, somewhere is worn or loose...

Thanks anyway, for the reassurance on the brake-switch operation.

br1anstorm
 
EDIT: when you checked those U-joints, did you do it with the Jeep off the ground? If not, there is still loaded torque on the joints, and you wouldn't be able to turn even badly worn ones by hand.

Aha - urbexx, I was typing my reply while you were adding that Edit!

And you're right on the money. Of course I tried to wiggle the driveshaft/joints with the Jeep sitting foursquare on the ground. Dumb or what? Thanks for pointing that out (even though I now feel a fool). Will now get out there with the jacks and lift the Jeep off the ground and see whether the U-joints - or the diff??? - have slack. But I do have one slight problem: I don't know how much movement, rotational or lateral, is normal.... Still, I need to try it and see.

Cheers

br1anstorm
 
Haha...nah, don't feel bad. Never occurs to most people that the Jeep needs to be off the ground to check the joints. Jack the Jeep up, and grab the driveshaft in one hand, and the yoke connected to the axle or t-case in the other hand and try to twist. You have no movement rotational or lateral in the joints. If you feel any movement at all, replace the joints.

To check the diff, rotate just the driveshaft back and forth. Make sure the t-case is in neutral for this. You may want to engage the parking brake, so you know you're only feeling the slack in the diff. Some slack in the diff is normal (keep in mind the slack between the ring and pinion, between the spiders, and between the side gears and axle shafts). I'd say no more than 5 degrees of rotational twist if you have an open diff or factory limited slip.

Speaking of which, you don't have a locker in that axle, do you?
 
Thanks urbexx - great advice, which provides all the guidance I need to check things out. I'm away early tomorrow for a day or two at working meetings, so this checkup may have to wait until next weekend...

And no, I don't have any lockers - my Jeep is stock, pretty much as it left the factory. It hasn't been stressed that heavily off-road. But I remember reading somewhere that XJ driveshafts/halfshafts/diffs did tend to wear out or become troublesome after a while.
 
Ok, just wanted to make sure. Lockers, especially some of the lunchbox units (Powertrax, etc) can make clunks like that, as well have a lot more slop in the diff that's normal. Just wanted to make sure I wasn't going to send you chasing a non problem :)

Let us know how it goes.
 
No real need to jack the vehicle up. Just put the parking brake on and the tranny or xfer case in neutral so there no load on the drive shaft. YOu might also have a look at the tranny mount and engine mounts.
 
No real need to jack the vehicle up. Just put the parking brake on and the tranny or xfer case in neutral so there no load on the drive shaft. YOu might also have a look at the tranny mount and engine mounts.


True enough. But then getting it off the ground gives me a chance to easily spin the driveshaft around to easily check the condition of all the caps snap rings, along with do a quick lube job while I'm under there. Not to mention, we bigger guys have issues trying to slide under a stock Jeep :D
 
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