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Old 01-11-2018, 11:42 PM
Hoss5504 Hoss5504 is offline
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Default 6 speed questions

I just bought a 99 7.3 with 6 speed Manuel 248k miles, I was driving around 55 or so went to down shift from 6th to 5th and it made a grinding noise.....now if I am below 50 to 45 it well go in super smooth, all other gears are smooth so I am wondering if this is normal or should I be concerned
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Old 01-11-2018, 11:54 PM
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blacksmoke.r.us blacksmoke.r.us is offline
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A lot of synchro issues come from the condition of the fluid. I would check the fluid level and the condition of the fluid.
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Old 01-12-2018, 12:06 AM
Hoss5504 Hoss5504 is offline
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I am going to change it tomorrow but I just thought it was weird it shifts great going through the gears and if I slow below 45 or so it down shifts to 5th great also
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Old 01-13-2018, 02:30 AM
Hoss5504 Hoss5504 is offline
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What fluid do you guys recommend and any additives?
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Old 01-13-2018, 03:47 AM
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In my 5 speed (ZF5 you have the ZF6) I run Redline MTL a little over full which helped my old worn synchros and overall noise.
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Old 01-13-2018, 09:29 AM
Hoss5504 Hoss5504 is offline
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Just redline mtl when I google that there are a bunch of options
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Old 01-13-2018, 09:50 AM
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I've heard alot of good things about Pensoil SynchroMesh in the ZF transmissions. I've heard from several people that it works wonders. I've never ran it personally, my ZF6 has just mercon ATF.
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Old 01-15-2018, 06:24 AM
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I can't tell you for sure without actually seeing the condition of the clutch, clutch hydraulics, or the guts in the transmission.

Obviously you can't get a reliable diagnosis from a Doctor over the Internet for a complicated medical problem like brain cancer and the same rule applies with this.

However- speculation is free, but not necessarily applicable. Your mileage may vary.

That said, there are a few things to consider before getting after the transmission.

Is your clutch hydraulic system getting weak from an air bleed?

Is your clutch disk getting thin?

Are the fingers on the pressure plate wearing down?

Is the clutch fork bent or cracked? (99's and 2000's had this defect)

Pilot bearing could be worn out, damaged.

Nylon bearings in your clutch pedal tree axle could be worn, or the clutch Master cylinder pivot stub on the clutch pedal is also worn, nylon bearing is shot, usually both. You can't believe how much this will affect your clutch working correctly.

That said, new oil isn't likely to fix worn out synchros or bearing preload slop from high mileage, especially IF that trans has never been rebuilt. Different/or new oil is no replacement for new bearings, synchros, and shift forks where needed.

Having oil changed often in the ZF transmission series is recommended by the ZF factory, which means it's not a 'fire and forget' situation.

However, some logical conclusions can be made given the mileage. Most stock clutches are replaced at about 100,000 miles, some a little more. Depends on the driver, use, etc.

More often than not, the importance of the clutch pilot bearing is overlooked. People spend gobs of money on the clutch and then install an absolute predictable failure point in their job, the pilot needle bearing.

Most don't even question it.

But some do.

New whiz bang 'snake oil' Kevlar pilot bearings suck (that's Dupont nylon to Skippy the 'Hall Monitor'), and the OEM roller bearings are even worse. Don't get me started on that crappy Okie input shaft sleeve fix.

The #1 problem with many modern transmissions is from the pilot bearing running out of lube- then becoming too sloppy and worn out. That problem is across the board, I don't care what manufacturer we're talking about. You'll see the same grief with all other makes and brands.

I also rebuild my own toploaders and I rarely see problems associated with input shafts because they use Oilite bronze solid bearings. Same thing with old school T19 truck transmissions, and the like. Input shafts typically get replaced because of a worn syncho mesh surface, NEVER the input shaft, unless some jerk didn't replace the solid Oilite pilot bearing with a new one when they changed the clutch. But that wears out the syncho ring, not the input shaft surface.

With the ZF series transmissions, when the pilot bearing is worn, that also wears out a lot of stuff down line on the input shaft, and then mainshaft components.

Doesn't matter if it's a Ford or GM Diesel or a Corvette, same situation with ZF's.

Needle bearings are very hard. Generally speaking, they are much harder than the surface of input shafts. Heat, rust, then more heat- creates surface pits, which in turn, makes them a cutting device.

It's a vicious cycle. Your input shaft, some synchros, and bearing races definitely suffer as a result.

At 248,000, your 5th gear synchro sounds like it's getting very worn. You shouldn't have downshift problems if it is working within spec; that's what a sychro is supposed to do. It acts like a clutch on the other gear to get the two to turn at the same speed for proper meshing.

If either is not spinning at the same speed, you get grinding.

The caveat to that is this: speed shifting. Some transmissions can handle it, some can't. But that's more of a driver problem, not necessarily the transmission. You can't use a mule to win the Preakness. Wrong tool for the job.

Now if you are having clutch/pressure plate issues, this downshift problem may be the 'shot over the bow', so to speak. As time goes on, you may notice other problems starting to crop up that seem to surround the clutch. Don't be fooled.

I lost a snycho spring and retainer plate and that !)($&!)($& would not downshift shift from 4 to 3 at all. And over a few more weeks, I started having problems with even putting the vehicle in gear. Time for a rebuild/overhaul of the entire system. I found lots of problems in the entire drive-train, fixed them all, but it was a long haul.

So have patience.

I also run Mercon 5; no issues with it.

Like I said, I have rebuilt my transmission, modified my pedal clutch tower with Oilite bearings and a heim joint, replaced the entire hydraulic system, new Val-Air HD clutch pack and I use a solid bronze pilot bearing (Dorman 690-039) in my 99. Plainly put, it was all 'due'.

Like I said, your mileage may vary.

Good luck.
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Last edited by Dave_Nevada; 01-15-2018 at 06:33 AM.
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