AX-15 gets extremely hot at freeway speeds

Jeep450r · May 13, 2013

4.0/AX-15/4.88's/33's

With the above setup, at 65 MPH, my speedometer reads 90 MPH (even though the tach stops at 85 mph). At 75 MPH the needle points straight down.

With that said, my transmission is spinning like crazy at highway speeds and gets EXTREMELY hot. Hot to the point that the floorboards heat up enough to make my right leg feel like the hairs are being burnt off.

I have put stick-on dynamat style insulation under my carpet on the trans tunnel, and even on the underside of the trans tunnel between the transmission, and my entire center console still gets hot enough to put my iPhone into overheating mode when its resting by my e-brake.

Do you guys have any ideas as to what I can do to drop the temps in my transmission? Should I switch to a thicker gear oil? That's really all I can think of. Changing gears or increasing tire size is not an option.

Any thoughts or input would be greatly appreciated.

77_Jimmy · May 13, 2013

I have a stock 91 and my transfer case was getting hot. changed the fluid out to synthetic and problem went away. Make sure your using a gl-5 oil and not a gl-4 . gl-4 will burn up your sycros.

IslanderOffRoad · May 13, 2013

Are you sure it's not your cat?

xpedition xj-eh! · May 13, 2013

77_Jimmy said:
I have a stock 91 and my transfer case was getting hot. changed the fluid out to synthetic and problem went away. Make sure your using a gl-5 oil and not a gl-4 . gl-4 will burn up your sycros.

I think you are confusing transfer case with transmission? The transmission has brass syncros and requires a yellow metals safe fluid. GL-3 is yellow metals safe while GL-4 and GL-5 are not. GL-3 is hard to find these days and many people run 10w-30 motor oil instead. The transfer case takes ATF.

77_Jimmy · May 13, 2013

right ,the transfer case takes atf. both my shop manual and owners manual specs gl-5 for the transmission.

ChiXJeff · May 13, 2013

77_Jimmy said:
right ,the transfer case takes atf. both my shop manual and owners manual specs gl-5 for the transmission.

It's been discovered that the Jeep docs are just plain wrong for the manual transmissions. You can't use GL4 or GL5, the sulfur based additives will corrode the brass synchronizers. GL3 is fine. I've been running Redline MT90 in mine for years.

blondejoncherokee · May 13, 2013

my tranny tunnel has always gotten crazy hot. I always figured it was the long drives on 4.88s and 35s/36s and the auto getting hot and the exhaust adding to the mix.
last fall I put the grand cherokee heat shields for the exhaust under the floorboards. ripped out carpet, cleaned up the floor, used high heat paint, then a layer of thermotechs sticky heat mat, then a layer of automotive 1/2" thich heat insulation, then new carpet with mass backing. that helped a ton. I did two passes on the tranny tunnel with the heat mat stuff, and lined the under side of the center console as well. the parts of the center console without insulation get hot to the touch still. I run a decent sized tranny cooler out of line. I know its not a manual, but the same ideas might help. also change fluid in tranny and t case with some fresh stuff and see if that helps.

joe_peters · May 13, 2013

77_Jimmy said:
right ,the transfer case takes atf. both my shop manual and owners manual specs gl-5 for the transmission.

AND THEY ARE BOTH WRONG.

Use Pennzoil Synchromesh, Redline MT 90, GL-3, or a full-synthetic 10w30 motor oil.

Or you can go to the dealer and pay $12 a quart for their synthetic for the NV series transmissions.

Johnnie Walker · May 14, 2013

I would definitely get a temp gun and check temps underneath at different areas, especially exhaust. A fluid change may be in order as well.

Jeep450r · May 15, 2013

Ill plan on changing the fluid and while I'm at it ill throw something a little heavier in there. And ill go from there I guess.

joe_peters · May 15, 2013

Jeep450r said:
Ill plan on changing the fluid and while I'm at it ill throw something a little heavier in there. And ill go from there I guess.

I ran Mobil 1 syn 10w30 for over two years--way better than GL-3 in the cold, but Pennzoil Synchromesh was on sale at NAPA last service so I went with that, and it is better than GL-3 or Mobil 1.

ChiXJeff · May 15, 2013

I've had Redline MT90 in mine for years, works pretty well in the cold, not like shifting in peanut butter.

Ecomike · May 15, 2013

Jeep450r said:
Ill plan on changing the fluid and while I'm at it ill throw something a little heavier in there. And ill go from there I guess.

Wrong direction, you do not want heavier, go with thinner. Mine would not even shift after 3 days, freshly rebuild, and I had GL-4 in by mistake. I switched to MT-90 and never a problem, that was about 6 years ago!!! It is very thin, but runs cool and shifts like silk since I switched. Well worth the money!!!

Matthew Currie · May 15, 2013

I've had two AX-15's that happily went for over a quarter million miles, and used various things in them. The one thing they hated was the specified GL-5, which was noisy, destructive and hard shifting for the brief time I tried it.

I used full synthetic Valvoline GL-5 in one, which it was fine with for over a hundred thousand miles. On another I ran non-synthetic "synchro" gear oil for a hundred thousand, up to 250 or so. Later, when that transmission got a noisy reverse from a clutch failure that required slam shifting in traffic, I put the other box in, this time with 10-30 motor oil. It was fine with that too. About the only thing I haven't used is Dexron, but many manual boxes are fine with that too. Just stay away from the recommended GL5 gear oil.

Ecomike · May 16, 2013

Some of the GL-4 is also extremely bad on the brass and causes terrible shifting problems too. It only one tank of fuel to fill mine up with brass flake before I figured out the mistake.

holysirsalad · May 17, 2013

~2500 RPM isn't *that* fast. With that much wind it shouldn't be a real problem. I would suspect the cat making a lot of your heat there.

Also a major +1 for Redline MT-90. Found it on the shelf at my local NAPA one day. Legend has it that Royal Purple 75w90 achieves the GL-5 rating without the harmful sulphur EP additives, but I can't remember if I read that in one of their product datasheets or elsewhere. Any synchro-specific fluid is what you want in there, synthetic is definitely good, especially if you feel that much heat.

I ran GM's Synchromesh fluid in my AX-15 for a while... seems far too thin for how much wear mine has.

Jeep450r · Jun 24, 2013

Ecomike said:
Wrong direction, you do not want heavier, go with thinner. Mine would not even shift after 3 days, freshly rebuild, and I had GL-4 in by mistake. I switched to MT-90 and never a problem, that was about 6 years ago!!! It is very thin, but runs cool and shifts like silk since I switched. Well worth the money!!!

Why would I want thinner oil instead of thicker?

Matthew Currie · Jun 25, 2013

Jeep450r said:
Why would I want thinner oil instead of thicker?

Thinner oil circulates better. As long as lubrication is adequate, you want oil that moves quickly and doesn't get in the way. The cooling effect of oil requires that it can flow quickly out of hot spots and carry away the heat.

Try to picture how a synchronizer works. A ring, usually a tapered brass ring, engages a matching surface to bring gears up to speed before the final engagement of the gears themselves, which occurs when the "hard" gear slides forward on the synchronizer. The synchro ring is basically a quick-acting little clutch. If the oil is thick, especially when it's cold, the oil film may be so thick and gooey that the rings cannot engage quickly all the way, and it will be hard to complete the shift, or it will continue to slip and you'll get a crunch.

Bryan C. · Jun 25, 2013

Ecomike said:
Wrong direction, you do not want heavier, go with thinner. Mine would not even shift after 3 days, freshly rebuild, and I had GL-4 in by mistake. I switched to MT-90 and never a problem, that was about 6 years ago!!! It is very thin, but runs cool and shifts like silk since I switched. Well worth the money!!!

Not sure if you noticed, but Redline MT-90 is a GL-4 spec trans fluid.
http://www.redlineoil.com/product.aspx?pid=46&pcid=7

I run a Redline MTL in my NV 3550, I also run the same stuff in my AtlasII 5:1, both for over 50,000 miles with no problems. GL-4 spec with no sulphur additives.

http://www.redlineoil.com/product.aspx?pid=45&pcid=7

Ecomike · Jun 28, 2013

IIRC MT-90 meets the GL-4 spec with out any brass eating sulphur, and it is synthetic.

AX-15 gets extremely hot at freeway speeds

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