Improving synchro shifting problems by using different gear lube weights or additives?


Bryan

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Feb 12, 2021
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84050
I have synchro problems in the 95 YJ that I just purchased. It grinds mostly when shifting into 3rd and 4th. Has anyone had experience improving synchro shifting problems by using different gear lube weights or additives? I believe there is hope, because when first starting out and the transmission/transfer case are cold, I have very little grinding issues. So, the colder thicker lube makes a difference. Thanks in advance for the help Bryan
 

jeepjoe43

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Dec 4, 2020
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Black Canyon City, AZ 85324
Fluid isn't going to do it.
Sounds like the clutch isn't disengaging completely and you're just on the bleeding edge of that.
The clutch system probably just needs to be properly bled.
 
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Bryan

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Feb 12, 2021
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Fluid isn't going to do it.
Sounds like the clutch isn't disengaging completely and you're just on the bleeding edge of that.
The clutch system probably just needs to be properly bled.
Oh, OK appreciate the information. I just put a new clutch in it when I installed the remanufactured engine,. I didn't replace the clutch slave cylinder, but it sounds like I should have bled the old clutch anyway. I'll give that a shot.
 

jeepjoe43

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Dec 4, 2020
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It's always a pain to bleed the clutch and it's common to do it several times. One little bubble getting back into the slave can mess things up.
You are right, it is a pain and takes multiple attempts.
Not a fan of hydraulic clutches, but it's got what it's got.
I recently read somewhere that the way to bleed the clutch system is to force fluid into the bleeder (requires a hand pump).
Then the trapped air is pushed into the reservoir.
I have a very nice pump kit I bought for doing brakes, but had't tried this method, yet.
While mine is disengaging, it still doesn't feel right.
 
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machoheadgames

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May 19, 2020
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Dallas, TX
I wouldn't bother bleeding a clutch linkage on a 94 or 95, it never goes properly in my experience. I just buy new assemblies. That said, usually you'll have issues in all the gears if that is the problem. If it's in 3rd or 4th only then you probably have issues specific to the transmission.

The best fluid I have found is Redline MT90, but it's only fluid, it isn't a miracle worker and won't fix a transmission suffering from broken or worn out parts.
 

Gary747

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Nov 7, 2020
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Lancaster, CA
I have an AX15 and had similar synchronizer issues when hot. Two things... 1.) Chrysler-Jeep came out with a service bulletin that recommends replacing the transmission 75W90 gearbox oil with 30 weight engine oil. Interestingly, my rebuilt TRANSFER gearbox has a warranty based upon using 30 Weight Engine Oil in it instead of the factory red automatic transmission fluid. Using the lighter weight oil in the AX15 helps the synchronizer issues, and using 30 weight in the transfer gearbox reduces oil loss due to vaporization. I use 30 weight in my transfer gearbox, which leads me to 2.) My synchro problems disappeared when I drained the 75W90 oil from the AX15 and replaced it with Red Mopar Automatic Transmission Fluid (ATF). After the initial fluid swap, I ran for 100 miles or so to "wash" the internals with fresh red fluid, drained and refilled it again with ATF. I've operated the Jeep as my daily driver with this fluid for over two years now in the Mojave Desert area with NO problems... no fluid leaks and the smoothest shifting since I bought the Jeep in 1999. I've checked the transmission oil level at every engine oil change and have not had to add any ATF. After the first year, I drained the ATF just to "have a look" for contamination, overheating, etc, and the fluid looked, felt and smelled like new. Refilled the AX15 with fresh ATF and never looked back. Maybe others can comment, but I know I'm not the first to run an AX15 on ATF. My 1990 Jeep now has 401,000 miles on it.