1990 AX-15 Transmission Fluid Report - Jeep Forum - Forums and Owners Club
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Old 06-25-2020, 07:21 PM
Gary747 Gary747 is offline
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Location: Lancaster, CA
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Model and Year: 1990 Wrangler
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Default 1990 AX-15 Transmission Fluid Report

I now have about two years on my second AX-15 manual transmission, with about 400K miles on the Jeep. I had problems with the synchronizers not working very well, especially when hot. The gear grinding tendency was bothersome when downshifting, and it was difficult to engage higher gears when accelerating.

I decided that, what the heck... it's only a transmission, so I replaced the 75W90 gear oil with Mopar automatic transmission fluid.

After the first 100 miles to "wash" the thick oil out of the system (synchro rinsing, if you will), I replaced it again with fresh auto-trans oil. One year later, just to examine the oil effects, I again replaced that transmission oil with auto-trans fluid.

Here are the results:

After a year, the drained auto-trans fluid was as bright of red as new. No evidence of degradation.

At the first replacement I noted an immediate improvement to synchronizer meshing. My hard and grinding shift issues went away. Two years later, the transmission still works great.

I park on a concrete surface, and I've no trans fluid leaks on the driveway. The AX-15 seals all seem to be compatible with auto-trans fluid. I check the trans oil level at every engine oil change with no evidence of fluid loss. It stays full, right up to the bottom of the filler hole on the right side.

Cold weather driving is the same as warm weather in terms of transmission operation. It always shifts smooth. Interstate or desert dirt roads all shift easy, either up or down shifting. By the way, I live in the high desert in Southern California, with summer temps exceeding 112 F, and winters down to 17F

The thinner oil seems to provide plenty of internal lubrication, although I've had no reason to drop the transmission for any maintenance. No unusual sounds or vibration. When the time comes, I will pay close attention to the internal shafts, gears, etc for any indications of heat or lack of lubrication.

My biggest problem is having to explain all this to a shop before they discover the "wrong" oil and replace it with 75W90. Also, some oil change shops won't service the 1990 Jeep with fluids not approved by the manufacturer... even on a 30 year old Jeep.

I would be interested to hear other owner experiences concerning similar oil substitutions. As for me, in the absence of any new problems, I'm going to use automatic transmission fluid in my manual transmission forever more, happily ever after.

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