M5OD Grinding While In Gear
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3rd Gear
Joined: Jul 2025
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M5OD Grinding While In Gear
I recently bought this truck. It has the inline 6 with a manual transmission (M5OD) with 4wd. I drove down from the mountains about 3.5 hours and after we got off the mountain, the transmission started to grind while in gear. It does not pop out of gear at all. I replaced the fluid, which was low, and I also replaced the shifter bushings. And that didn't fix the issue. Next step was to do a clutch swap. I replaced the clutch, flywheel, slave cylinder, rear main seal, and pilot baring. Yet I am still having the issue. I'm thinking it might be the forks or synchros.
Anyone have this issue before? The grinding is pretty bad. And it usually happens when I let go of the gas while in gear. If I am pressing the gas it won't do it. Mainly does it in second and third gear. But honestly it has done it in every gear.
Anyone have this issue before? The grinding is pretty bad. And it usually happens when I let go of the gas while in gear. If I am pressing the gas it won't do it. Mainly does it in second and third gear. But honestly it has done it in every gear.
Cargo Master
Joined: Mar 2007
Posts: 2,825
Likes: 1,030
Not certain what year truck you got, but if the manual transmission was low, it's probably has been low a really long time. People tend to check things when there are problems.
I would assume your transmission is worn out. Remove it one more time and go through it. It could be totaled if the fluid was really low for a long time.
I would assume your transmission is worn out. Remove it one more time and go through it. It could be totaled if the fluid was really low for a long time.
Lead Driver
Joined: Sep 2003
Posts: 7,015
Likes: 208
From: SoCal
Also make sure to use only Mercon ATF, not gear lube, in the M5OD, and most modern manual transmissions.
Gear lube is too think for the tiny oil pump on the input shaft that supplies oil to the pocket bearing between that and the main shaft. That's usually where things start to go bad. If that bearing wears out, it usually damages both shafts, and will require replacement, which requires a full rebuild.
Check to see if it grinds in 4th gear. This couples the input shaft directly to the output shaft, so there should be no speed difference between the two. If it's quiet in 4th, but grinds in all the other gears, that would strongly suggest the pocket bearing.
If it still grinds in 4th, then there are other problems.
Gear lube is too think for the tiny oil pump on the input shaft that supplies oil to the pocket bearing between that and the main shaft. That's usually where things start to go bad. If that bearing wears out, it usually damages both shafts, and will require replacement, which requires a full rebuild.
Check to see if it grinds in 4th gear. This couples the input shaft directly to the output shaft, so there should be no speed difference between the two. If it's quiet in 4th, but grinds in all the other gears, that would strongly suggest the pocket bearing.
If it still grinds in 4th, then there are other problems.
Thread Starter
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3rd Gear
Joined: Jul 2025
Posts: 3
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Also make sure to use only Mercon ATF, not gear lube, in the M5OD, and most modern manual transmissions.
Gear lube is too think for the tiny oil pump on the input shaft that supplies oil to the pocket bearing between that and the main shaft. That's usually where things start to go bad. If that bearing wears out, it usually damages both shafts, and will require replacement, which requires a full rebuild.
Check to see if it grinds in 4th gear. This couples the input shaft directly to the output shaft, so there should be no speed difference between the two. If it's quiet in 4th, but grinds in all the other gears, that would strongly suggest the pocket bearing.
If it still grinds in 4th, then there are other problems.
Gear lube is too think for the tiny oil pump on the input shaft that supplies oil to the pocket bearing between that and the main shaft. That's usually where things start to go bad. If that bearing wears out, it usually damages both shafts, and will require replacement, which requires a full rebuild.
Check to see if it grinds in 4th gear. This couples the input shaft directly to the output shaft, so there should be no speed difference between the two. If it's quiet in 4th, but grinds in all the other gears, that would strongly suggest the pocket bearing.
If it still grinds in 4th, then there are other problems.
I put in synchromesh oil hoping it would help. And yes it grinds in fourth gear. It even grinds in reverse once the temperatures are up. I'm thinking it might be time to swap in a ZF5. But for now I am looking for a quick solution just so I'm able to drive it a little bit. Do you think swapping over to atf might help? I was also thinking of putting in a quart of the Lucas Oil Stabilizer.
Thread Starter
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3rd Gear
Joined: Jul 2025
Posts: 3
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Not certain what year truck you got, but if the manual transmission was low, it's probably has been low a really long time. People tend to check things when there are problems.
I would assume your transmission is worn out. Remove it one more time and go through it. It could be totaled if the fluid was really low for a long time.
I would assume your transmission is worn out. Remove it one more time and go through it. It could be totaled if the fluid was really low for a long time.
The truck is a 1990 F150. I honestly think the transmission is done. Just trying to buy some time so I can swap in a ZF5.
Lead Driver
Joined: Sep 2003
Posts: 7,015
Likes: 208
From: SoCal
Synchromesh oil is gear lube, and would be too thick for this type of transmission.
Steady grinding noise could be from a failing bearing or a failed bearing that is allowing gears to not align.
It does sound like the transmission is toast.
Since you're going to replace it, you should pull the top cover off and see if there are metal pieces in there.
Steady grinding noise could be from a failing bearing or a failed bearing that is allowing gears to not align.
It does sound like the transmission is toast.
Since you're going to replace it, you should pull the top cover off and see if there are metal pieces in there.
Cargo Master
Joined: Mar 2007
Posts: 2,825
Likes: 1,030
Or the syncros won't do their job and hold the gear in place. No amount of additive will fix a worn out bearing, syncro, or gear. I would not invest any money in attempted to this this without at least taking the transmission out.
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