just got a new to me ZJ (42re) and trying to work out some trans issues, it has the 1st gear whine I've dismissed as normal, but I noticed a groan while not moving in reverse and sometimes in drive, especially when its cold (its regularly below freezing here). Originally I thought it might be low on fluid because after checking the dipstick (when warm), it was at the bottom of the crosshatch area. I have a small amount of bubbles on the dipstick, like a single digit amount of bubbles, and sometimes there are none at all. added a quart and there seems to be no change, but now I'm wondering if its overfilled? fluid comes to the max line when warm now but its not hot, can hold the end of the dipstick comfortably.

 

If this is the first jeep you've owned with the 42re in it, be aware that the fluid level must be checked in NEUTRAL, not park. It makes a difference of about a quart because the pump doesn't charge the converter with pressure while in park. Also ATF+4 must be used to stop clutch chatter and slippage.

Bubbles can be caused by a suction air leakage in the filter seal where it goes into the valve body, especially when cold. If the fluid is low, this seal is exposed to air and not covered with fluid. Also remember that the oil cooler in the radiator actually WARMS the fluid so the aux cooler, if you have one, shouldn't be the only one in the circuit. I've made it a point of habit to immediately shift to neutral after a cold start so the fluid can start to circulate and warm the 0-rings and seals before being asked to move the jeep. In extreme cold, there is a thermistor in the valve body that will not allow the trans to shift to 4th gear if the fluid is below 32*F or above 240*F. Most of the heat in the transmission is created by the slippage of the fluid in the converter so it really won't get that hot sitting in neutral and not working. The rad cooler can't warm the trans fluid until the thermostat opens and coolant starts to flow either.

 

I usually put it in N when I start it in the morning but bubbles still show up on the dipstick even after driving on the highway for an hour, they do go away shortly after though or if the jeep has been shut off and sits for a minute or two it seems. Also the fluid never warms up to the point where its gonna burn you if you try to hold it. I also noticed a leak at the aux cooler but its very slow. Ill have to see how it behaves in warmer weather. I am aware I need to check the fluid in N and it looks too full but its hard to tell. also it does have ATF 4+ in it, and from what I can tell 30k miles since it was changed last and still reddish pink.

 

Sorry. Not knowing your history, I had to start with the basics. The not getting hot part worries me. After driving a while, the fluid temp should be at or close to engine coolant temp, unless the rad cooler has been bypassed. If it's overfilled the fluid can be whipped up by the rotating parts enough to create foam, but it doesn't seem like just a few bubbles is caused by that, so I suggest a suction air leak or a vigorous flow back to the pan from the cooler. The torque converter drain valve controls it's pressure and sets the flow rate thru the cooler. It's possible a weak spring in that valve is dumping too much fluid thru the cooler, although you'd suspect soft or delayed shifting in that case, or slippage and excessive heat in the converter. I can't think of anything else.

 

I'm hoping the not getting hot issue is due to it being really cold outside. I noticed the trans lines are leaking a little, would that possibly cause bubbles in the fluid? shifting feels fine and no slipping. the trans was rebuilt about 30k miles ago. fluid is still red, a little dark but not brown and I assume its the same fluid from when it was rebuilt. I also noticed that after a minute of sitting in park the bubbles are gone. if its overfilled I assume it may be by a quart or so. I noticed that the trans made a terrible whine in park when cold starting and added almost a quart and the noise went away. now its just a really quiet moan/whine while in gear when sitting at the light, goes away the moment you hit the gas. it had bubbles before I added the fluid but it was noticeably less then than it is now.

 

The cooling lines are just what is known in the hydraulic industry as a "overpressure return to tank" and since it's coming off the unit that causes the most heat is a good point to add a cooler. There is no suction in that circuit so a leak will only loose fluid, not suck air. An extremely vigorous flow may form bubbles as it splashes back into the pan, so I'm assuming that's where the bubbles are coming from. The main pressure pump may have a slight shaft seal leak that's sucking air and causing the whine and bubbles, but I don't think it's anything to worry about. My XJ has been doing that for 2 years and is functioning perfectly. (AW4) That whine is normal for a GM trans anyway. My Chevys have always done that. (THM 350)

As far as not getting hot, The fluid should be getting as hot as the coolant in the radiator since the thermostat controls engine coolant temp. If the transmission cooling element in the radiator has been bypassed, it won't be being heated by the radiator. My WJ (42re) today wouldn't shift into 4th gear for about 5 miles because the air temp was 7*F today. The thermistor in the trans won't let it shift into 4th if the fluid is below 32*F. I'm running an aux cooler are well as the rad unit.

 

I'm starting to think that most of the noises I'm getting are something resonating, except for the 1st gear whine which I'm told is normal more or less. still got bubbles but not after sitting idling in N, only after driving, but no shifting problems, the leaks are worse now but I am planning on fixing them soon. maybe in the time it takes me to pull the dipstick and touch it the fluid cools down. haven't heard any pump noise really except for occationally on a cold start in park. might be overfilled a little or something or a filter seal issue.