AX15 trouble/questions

[Schmitty]

Having some intermittent issues with the trans on the TJ: on hot days or after driving it for a while it sometimes won't go into 1st or 2nd gear. I'll have to let the Jeep roll a little or bump into 3rd then over to 1st before it'll go into gear. I'm thinking either a syncro is toast or a dog ring needs replacement.

One thing to note is that I recently replaced the fluid (which was very dark and dirty) with new MTF.

I've read that rebuilding an AX-15 is better left for a shop with the proper tools and remanufactured ones cost about $1200 after core deposit. What are y'all's thoughts?

[Jim]

MTF - manual trans fluid?

As you said you just changed fluid and you're now having issues. I think I'd change fluid one more time (edit: with a different fluid) and see.

Here's what I have listed for my AX15 5-speed manual trans fluid change info:
ATF Fluid (any type), 15-25 ft-lbs drain/fill plugs, 1-3/16" socket; Drain & Fill plugs to 20 Ft-Lbs

[FINOCJ]

One thing to note is that I recently replaced the fluid (which was very dark and dirty) with new MTF.

Did you use GL-5? Novak has this to say:
The factory manuals call for 75W90 GL-3 gear oil for the AX15 transmission. This fluid is essentially obsolete; superseded by GL4, then GL5. However, many GL5 fluids have friction modifiers that are not compatible with synchronizers, so make sure to check for compatibility.

The old transmission and TCs found in my old jeeps have this issue as well...I found StaLube gear oil from NAPA that is good for the yellow metal synchronizers and blocking rings seems to work really well....

[newracer]

I ran Redline MT-90 in mine, smoothed out the little crunching in 2nd I was having.

[Java]

https://www.novak-adapt.com/knowledge/transmissions/manual/ax15

They say 50w, I personally use 10w-30 Mobil 1 in everything from the engine to the axles.

[JandDGreens]

Did you use GL-5? Novak has this to say:
The factory manuals call for 75W90 GL-3 gear oil for the AX15 transmission. This fluid is essentially obsolete; superseded by GL4, then GL5. However, many GL5 fluids have friction modifiers that are not compatible with synchronizers, so make sure to check for compatibility.

The old transmission and TCs found in my old jeeps have this issue as well...I found StaLube gear oil from NAPA that is good for the yellow metal synchronizers and blocking rings seems to work really well.... What he said. I made the mistake of changing my oil. Did not know that I needed to use a specific oil. Once I bought the correct oil (Redline MT-90 not cheap!) and changed it out it was all good. I was lucky that there wasn't too much damage to the synchro's. It's been good for 4 years now.

[Steve-O]

MT-90 in my AX-5. Good stuff!

[Schmitty]

The factory manuals call for 75W90 GL-3
That was my reference... hmm foiled for being a nerd. drats!

I'll try what Jim suggested before investing in Redline. The best oil is clean oil.

Thanks for all the feedback, I'll change the fluid and relay my findings.

[Jim]

I'll change the fluid and relay my findings.

:thumb:

[Trevor?]

I have no idea what was in my AX-15 when I bought it, but it did have a hard time with 1-2 shifts until I drove it a few miles and it warmed up. I tried 10w-30 after Paul mentioned it and it does shift much better now. It does seem odd, but it appears a lot of folks out there use it.

It took a few weeks before I started to notice an improvement. I suppose there's a chance it may revert to it's old ways once it starts to get cold again, but so far so good.

Edit: It may help to elaborate: I would get a little clash shifting from 1 to 2.... it felt as if the syncro wasn't generating enough friction to bring the intermediate shaft to speed. Your issue sounds a bit different maybe.

[Jim]

This might be a good spot to show the WAY EASY method I use to fill my AX-15...

(I do have a 3" body lift but those with standard body lift should be OK with this)

www.JimWilliamson.net/automobile/2017-09-02--tech--ax15-fill-through-body-drain-hole/ (https://www.jimwilliamson.net/automobile/2017-09-02--tech--ax15-fill-through-body-drain-hole/index.html)

[speedkills]

That’s a pretty great trick.

[Schmitty]

This might be a good spot to show the WAY EASY method I use to fill my AX-15...
Clever...

[Schmitty]

Alright I ran a fresh fill of some ATF for a week, still same problem. Drained and refilled with 10w-30 Castrol conventional oil; It shifts into 1st easier and all other shifts are smoother.

[Java]

Someone please correct me if I am wrong, I really don't want to say the wrong thing here, but I don't think conventional oil is OK in an AX-15 due to yellow metal. I think you need to switch to synthetic or your syncros will be damaged.

[FINOCJ]

Alright I ran a fresh fill of some ATF for a week,

Do you mean you ran Automatic Transmission fluid in it? IIRC the AX-15 is a 5 speed manual....I wouldn't run ATF in a manual trans.

[FINOCJ]

but I don't think conventional oil is OK in an AX-15 due to yellow metal. I think you need to switch to synthetic or your syncros will be damaged.
You can definitely run conventional oil in an an old manual with yellow metal - it just can't be the common modern GL-5 rating. There are a number of GL-3 and GL-4 conventional option out there - but rarely at local parts stores (although the Sta-Lube can often be found at NAPA). Another option is to run 10-30 synthetic motor oil like Mobil 1 - but be cautious of synthetic 90W gear oil as it may not do well with yellow metal either (especially if it says hypoid).

[JandDGreens]

What he said. I made the mistake of changing my oil. Did not know that I needed to use a specific oil. Once I bought the correct oil (Redline MT-90 not cheap!) and changed it out it was all good. I was lucky that there wasn't too much damage to the synchro's. It's been good for 4 years now.

Cost me aprox $70 but it fixed my issue.

[Trevor?]

I am under the impression that the thing to look out for with yellow metal in transmissions is using a lubricant containing sulfur compounds. Its the sulfur that is not compatible with yellow metals and gL4 and 5 oils may contain sulfur. Aside from that it seems that AX15's can be picky about the exact lubricity of the oil used. I thought this was an interesting discussion on the topic: https://forums.4wdmechanix.com/topic/47-ax15-transmission-gear-oil/

I think if you are getting good shifts with the 10w30 and it doesn't seem like its getting excessively hot then you are probably fine.

[Trevor?]

Been doing some more reading. Maybe this is all common knowledge for some, but I found it interesting and I hope others will too.

A good summary on different lubrication scenarios.
https://www.machinerylubrication.com/Read/30741/lubrication-regimes

Boundary lubrication would describe the high pressure sliding contact in a hypoid differential gearset. EP (Extreme Pressure) gear oils are designed for these applications. I believe engine oils are designed more for applications involving full film lubrication. A manual transmission is probably in-between these two applications. It should involve mostly rolling contact with helical and spur gear-sets and greater and greater pressures with increasing torque.

"EP gear oils contain additives that prevent metal surfaces from cold welding under the extreme pressure conditions found in situations where boundary lubrication prevails.
At the high local temperatures associated with metal-to-metal contact, an EP additive combines chemically with the metal to form a surface film that is ductile enough to prevent the welding of opposing asperities and prevent scuffing or scoring that is destructive to sliding surfaces under high loads (Figure 1). Chemically reactive compounds of sulfur, phosphorus and sometimes chlorine, are used to form these inorganic films."
https://www.machinerylubrication.com/Read/496/active-sulfur-ep-gear-oil

Reading further it discusses chemical compatibility issues with reactive sulfur compounds and copper.

API publication with more details on GL raiting designations:
https://www.api.org/-/media/Files/Certification/Engine-Oil-Diesel/Publications/1560-Eighth-Edition-April-2013.pdf

It seems like GL raitings dictate performance critera but not chemical composition, however in order to meet the performance criteria most oil manufactures appear to be using reactive sulfur additives.

It looks like Redline MT-90 and other products that claim to be safe for syncros and yellow metals are EP oils using a combination of additives with this compatibility issue in mind. There is a Pennzoil product out there too. Also one from Valvoline that appears to be priced a little better. I'm sure there are more.

https://www.pennzoil.com/en_us/products/other-oils-fluids-fuel/manual-transmission-fluids-axle-oils/synchromesh-manual-transmission-fluid.html
https://www.valvoline.com/our-products/grease-gear-oil/synchromesh-mtf

I'm getting the feeling that the right choice for anyone is going to be dependent on application. If your jeep isn't particularly heavy and you're not driving aggressively then you're generally generating less torque in the transmission and your gear contact pressures should be lower. The 10W30 path is probably fine.... it does appear to be recommended by Mopar with their substitution of 4761839ad for 4897622aa. If you have a lot of extra weight from modifications, and if you are generating more torque from your engine and drive aggressively than you may stand to benefit from one of these EP oils. There's no clear line in the sand between these two positions.

I'm starting to think about switching away from 10W30 if I can do so without spending MT-90 prices. That Valvoline fluid is going in the mid $7/qt range and claims to meet GL-4. The question will be if it would still make for good shifts. I may try that the next time I change that fluid.

Also I dont know why I keep wanting to type sulfur with a ph. Cant shake that for some reason.