Occasional thunking sound in 3rd gear

[SamC777]

Howdy y'all, I have a 94 Ranger, 3.0, standard, 4wd. Transmission was rebuilt a few thousand miles ago (May of last year).

The other night, on my way home from work, I put it in third gear. About 5-7 seconds after accelerating, I heard and felt a loud *THUNK* and it kept on moving. I slowed down and shifted into 3rd again and sure enough, after a few seconds, *THUNK*, albeit with less force. This did not happen in any of the other gears.

Oddly enough, the *THUNK* did not happen again in the days following, despite my efforts to replicate it. I thought it must've just been a fluke, but today it happened again, once again in 3rd gear. Yet again, however, it stopped doing it for the rest of the trip and I couldn't replicate it.

Some things to note:

-The first time this happened, I began to hear a whining sound that rose and fell with the RPM, even with the trans in neutral and the clutch disengaged (I opened the observation port and had my friend press the pedal a few times with the engine running to verify that it was not the throw-out bearing). This sound has been determined to be the idler pulley, though I'm not sure how/if this could be related to the *THUNK*, as it has only happened in 3rd gear.

- The ring gear in my diff is missing a tooth, but this has never caused me any problems in my almost 4 years of ownership.

- The weather here has gotten pretty darn cold, dipping into the single digits. I have full-synthetic 5w30 inside my transmission, so I try to baby it for the first few miles while the trans warms up.

- The transmission was rebuilt last May, and only a few thousand miles have been put on it since may (the whole truck is sitting pretty at about 326k). If there were something broken in there, wouldn't the *THUNK* be happening more regularly and be easier to replicate?

- The u-joints are pretty firm, but there is some slight play in the slip yoke. But I'm not entirely sure if that's the cause of the issue because the sound is happening well after the power has gone from the engine to the ground. If that were the issue, wouldn't I be hearing it right at the beginning?


Any advice would be appreciated, I'm honestly kinda stumped.

 

[don4331]

Isn't the M5OD supposed to have ATF in it?! 5w30 sounds like bad idea.

I can remember off hand if engine oil is compatible with yellow metal (brass) parts in transmission. But wrong oil will eat brass, and ruin synchros, etc.​

​

p.s. Single digits isn't cold - we're -30s + windchill.

 

[alwaysFlOoReD]

Yes, Trans is supposed to have ATF in it.

 

[SamC777]

Isn't the M5OD supposed to have ATF in it?! 5w30 sounds like bad idea.

I can remember off hand if engine oil is compatible with yellow metal (brass) parts in transmission. But wrong oil will eat brass, and ruin synchros, etc.​

​

p.s. Single digits isn't cold - we're -30s + windchill.

Yeah, I read up on that. It's the conventional or conventional/synthetic blend that'll do that, from the sulfur I believe. Full synthetic, from what I've researched, will not, which is what I'm running.

As for putting the ATF in it, that's initially what I was doing. But as I looked further into it, 5w30 seemed to be the better option. Plenty of resources supporting it's use in this application

 

[Shran]

Engine oil is fine, gear lube is not.

Do you have a one piece rear driveshaft or two piece? Carrier bearings on the 2 piece shafts can make noise.

 

[SamC777]

Engine oil is fine, gear lube is not.

Do you have a one piece rear driveshaft or two piece? Carrier bearings on the 2 piece shafts can make noise.

It's a two piece. It's got a little bit of play, but not a ton. If that were causing my issue, wouldn't that clunk as soon as I let off the clutch instead of the 5~seconds after giving her gas?

In other news, the thunking is now much more rhythmic and is present while the truck is in motion, in gear or not. I just now jacked her up, I think I may be dealing with a wheel bearing issue, as the drivers side front wheel has a bunch of play compared to the passenger. I just replaced those a couple years ago.... :/

 

[lowspeedpursuit]

As for putting the ATF in it, that's initially what I was doing. But as I looked further into it, 5w30 seemed to be the better option. Plenty of resources supporting it's use in this application

Curious where you're getting that. I've seen internet arguments to the ends of the Earth where people swear by everything. Someone's putting mayo in there and saying "it works for me". 5w30 is still twice as thick as Mercon cold.

 

[Shran]

Curious where you're getting that. I've seen internet arguments to the ends of the Earth where people swear by everything. Someone's putting mayo in there and saying "it works for me". 5w30 is still twice as thick as Mercon cold.

Probably from my past posts.

Synthetic 5w-30 "fixed" some shifting issues I had on ONE transmission, namely a 1st -> 2nd grinding problem that I would attribute to a worn synchro. That particular oil in that transmission made no change in how it shifts in cold weather, it was just nice and smooth year round.

Trans #2, will grind from 2nd -> 3rd in really hot weather when running ATF. Also suspect a worn synchro. I put non synthetic 10w-40 in that which cured the problem but it's noticeably stiffer and harder to shift in sub zero temps until the oil warms up.

Trans #3, has no issues but I decided to experiment and run synthetic 5w-30. It made absolutely no difference and I went back to ATF.

Trans #4 and #5 - severe grinding on 1 -> 2 shift on one, 2 -> 3 shift on the other. I tried ATF and 5w-30 in these and there was not much difference. Synchros were probably just too far gone.

I won't swear by anything, as there is not truly ONE fix for every problem. IMO if you have a healthy M5OD, Dex III/Merc ATF is what you should use. MTF/Synchromesh fluid or synthetic 5w-30 won't hurt but won't help either. In a high mileage, somewhat worn transmission, those two oils may help by taking up some slop in worn parts. If said parts are too far gone, your oil choice will simply not do anything.

All of that said, really thick oil should not be used in an M5OD regardless of type or ambient temp because it will not get flung up into the oil trough properly and potentially starving the input bearing of oil.

Just my experience and opinion, take it for what it's worth.

 

[SamC777]

Probably from my past posts.

Synthetic 5w-30 "fixed" some shifting issues I had on ONE transmission, namely a 1st -> 2nd grinding problem that I would attribute to a worn synchro. That particular oil in that transmission made no change in how it shifts in cold weather, it was just nice and smooth year round.

Trans #2, will grind from 2nd -> 3rd in really hot weather when running ATF. Also suspect a worn synchro. I put non synthetic 10w-40 in that which cured the problem but it's noticeably stiffer and harder to shift in sub zero temps until the oil warms up.

Trans #3, has no issues but I decided to experiment and run synthetic 5w-30. It made absolutely no difference and I went back to ATF.

Trans #4 and #5 - severe grinding on 1 -> 2 shift on one, 2 -> 3 shift on the other. I tried ATF and 5w-30 in these and there was not much difference. Synchros were probably just too far gone.

I won't swear by anything, as there is not truly ONE fix for every problem. IMO if you have a healthy M5OD, Dex III/Merc ATF is what you should use. MTF/Synchromesh fluid or synthetic 5w-30 won't hurt but won't help either. In a high mileage, somewhat worn transmission, those two oils may help by taking up some slop in worn parts. If said parts are too far gone, your oil choice will simply not do anything.

All of that said, really thick oil should not be used in an M5OD regardless of type or ambient temp because it will not get flung up into the oil trough properly and potentially starving the input bearing of oil.

Just my experience and opinion, take it for what it's worth.

That is good information, perhaps a fluid swap is in order. Mixing the ATF with what little oil will be left in there isn't gonna cause any issues, will it?

 

[Shran]

That is good information, perhaps a fluid swap is in order. Mixing the ATF with what little oil will be left in there isn't gonna cause any issues, will it?

Mix away. These transmissions are not high tech, super sensitive things... it's just a bunch of gears and bearings that spin. The main thing is not to use gear lubes that are too thick which cause oil starvation or hard shifting, or any oils that contain sulphur, phosphorus, or other compounds that are corrosive to brass or bronze. In a nutshell, oil is oil to these things, but which one you should use depends on the ambient temp and how worn the transmission is overall.

Nice thing is, trying out various fluids is cheap, if it works better, great! If not, you're only out maybe $20-30.