Sounds like the clutch is worn out.

 

Is there anyway to confirm before removing the transmission? Or is it obviously from the symptoms?

 

Guide for troubleshooting and testing
http://www.globaltransmissionsupply...om/pdf/transmissions/zf-transmission-zf-s6-650_6s-750-troubleshooting-guide.pdf

There is also a plastic bushing that gets worn out at the pedal pivot point/push rod to clutch cylinder under the dash. You checked the clutch fluid? Any leaks?

 

The fluid is full and the pedal has plenty of pressure. The clutch point is almost at the floor. I'll try and check that bushing. It makes sense about being a clutch as I'm not sure when it was replaced if at all.

I'm not very familiar with how it should be since I've never had a manual truck just manual cars.

 

250k miles I'd bet the clutch has been changed at least once due to the poor design of the clutch fork on the earlier super duties.

I'd change the fluid to pennzoil synchromesh (I use it in ALL of my manual transmissions) and see if it helps with the grinding after a few hundred miles.

I'd also look at the clutch pedal bushing and do the helm joint mod if it is compromised - that little bit of play can make the clutch not disengage all the way when shifting.

Finally my last resort before pulling the transmission to do the clutch would be to replace the slave/master cylinder as a pre-bled unit which is around $225 or so

 

There is a company that makes a metal version not the plastic factory version. I forgot the company name

 

A quick way to test the clutch is to stick it in 3rd. Hold the brake with your toes and press the throttle with your heal and Rev it just a little. Then release the clutch slowly. If it lugs the motor and almost kills it. Your clutch is good. If it slips. It's bad.

Definitely check the bushing and the hydraulics before pulling the trans.

 

Thanks for the advice I'll do that 3rd gear and brake test before going any further.

 

UPDATE: I performed the clutch test by putting truck in 4th gear while holding brake and parking brake set and it cut off pretty quickly after releasing the clutch pedal. So I'm assuming the clutch is good.

Now there's another issue which developed while doing the clutch test. Brake is soft and all fluid leaked out from above/around the fuel tank area. It wasn't leaking to my knowledge before the test. Could this be the cause of the grinding since it could of had a small leak or a weak system that wasn't sealed good?

 

Sounds like you blew a brake line....unrelated to the clutch system.

 

That brake line is common to rust out.

Sounds like the clutch is OK. I would double check the linkage

 

I was hoping that they were some how connected so it would fix my clutch issue. It's never that easy. I read about the brake lines rusting out and I'm so grateful it happened while parked instead of driving.

 

umm glad to see another zf6 out there.

your problem is the pointed next to check the fluid. I almost bet its got motor oil in it. sounds like the syncro rings are getting shot. I have the same problems but mine is when its cold. going from 3rd to 4th always leads to a **** a la marde gear grind, and 4th to 5 th is a gear chirp that I can feel skip/click into gear. However fully warmed up, no problems to shift without clutch, like butter two finger operation. id pull the fill plug and see what's in there since you do not know. what rpms are you shifting at? low 1200 or higher 2000?
zf6 like to be shifting like a tractor, not a Porsche.

 

Did you try changing to Pennzoil Synchromesh? 240k and my ZF6 shifts great. Also no problems in the cold (below zero) with the slightly thicker fluid compared to ATF.

 

trans fluids all depend on what kind of temperatures you are driving in and what kind of temps your trans runs at.
Me in Northern Canada towing, would be different then in mexico.

Heavy towing, id use gear oil and make sure to have proper warm times, and shift slowly until trans heats up.
heavy towing, but in cold temps -20/-30 Celsius, Synchromesh
Just daily driving and or in California weather, aft is just fine.

main thing for me is, if your not using it like a truck. aft
if used to haul like a mini semi, gear oil

 

Thank you for all the replies. It ended up being the shift fork and the clutch was worn as well so it got replaced.

It got replaced with a oem grade clutch because at the time I'm tight on cash and couldn't afford a nice southbend. The truck does have a ts performance 6 pos tuner and is straight piped. I have changed careers so I won't be hauling too much with it besides a boat and a few small trailers here and there. I'd like to add some power to the truck eventually when funds allow. I know gauges are first, and then I'll need stage 1 injectors since the stocks can't flow enough. Other mods I read about are the intake heater plug and the exhaust restriction mod where there's some butterfly inside near the turbo hot side that restricts flow for cold starting.

Sorry for rant but with these mods will the oem grade clutch hold up if I'm not dogging it all the time and I'm conservative with the power?

Also you mentioned shift speeds. I usually start in 2nd. Is that okay to do or bad for it?

Going from 2-3 it likes low rpms it seems like below 2k, 3-4 I can go above 2k and it seems to like it better, 4-5 same above 2k and into 6th I can be anywhere.

What's the recommended shift rpms for the zf6?

Thanks again!