Detroit 8v-71 clutch advice

[scsmith42]

I need some help in troubleshooting a clutch / transmission problem.

The clutch in the RR Museum's 25 Ton P&H MT-250 crane is acting up.  It is a 1970's vintage crane with a Detroit 8v-71 engine, and a 5 speed manual transmission.

This all started one day when the crane was in first gear going down a 400' long hill. The shift linkage was sticking (not uncommon with this machine) so the operator just pushed the clutch in to let it free roll down the hill so as to not over speed the engine or have to ride the brakes (with the transmission still in gear).  

About a third of the way down the hill there was a "bang", and the clutch reengaged itself.  Subsequently the crane refused to shift into anything other than first or reverse, and that was only with the engine shut off.

Subsequent troubleshooting revealed that the clutch slave cylinder had fresh fluid visible - sprayed on the chassis at the rod end, and the consensus was that the rubber cup inside the slave cylinder blew out while the clutch was being held in, and the sudden pressure loss caused the clutch to re-engage even while depressed.  The "bang" was from the clutch linkage rapidly reengaging.

The master and slave cylinders were rebuilt, but the shifting is not acting right.  If you start the crane in neutral, the transmission grinds significantly when trying to put in gear.  It absolutely refuses to go into gear.

If you start the crane in gear with the clutch fully depressed, it will start w/o the crane trying to run away.  You can let the clutch out and the crane will move just fine; but you still can't shift into another gear w/o grinding.

The shifter will now go through the gears w/o the engine running.  Immediately after the slave cylinder blew, it would only shift between first and reverse, but it freed itself up after working it a bit and with the engine stopped you can shift into all gears.

The travel at the throw out bearing appears to be around 3/8", give or take, but the travel at the pressure plate only appears to be around .030 - .050, give or take.  I'm thinking that the clutch disk may have partially lost the lining when the clutch suddenly engaged at high rpm, and that the loose lining may be preventing the clutch from fully releasing.

Has anybody run into a similar problem and can offer advice?  I'd sure like to know more about what's going on before we take the step of pulling that 1000 lb plus transmission.  It would be disappointing to replace the clutch only to discover that it did not solve the problem.

Thanks in advance for the advice.

Scott

[mike_belben]

Coasting down the hill at speed with the clutch in but shifter still in first gear caused the input shaft to spin so quick it slung the spring hub apart and now the broken bits are hung up in the flywheel pocket, dragging the flywheel and clutch discs so you cant fully uncouple the driveline.  Its not a strong enough coupling to move the truck but it is enough to grind all the time.  Pretty common failure. 



Pull the trans.  And tell the driver to never ever let a truck run away down a hill again.  

[mike_belben]

[Dave Shepard]

Sounds about right, and also best case scenario. If it's not the clutch, then it's in the transmission, assuming slave and master are doing what they are supposed to do.

[scsmith42]

Mike and Dave, thanks much for the feedback.

Dave, the transmission works as long as you start it in the gear that you want to stay in, so hopefully its the clutch.

Mike - great explanation.  Many thanks.  Are all of the clutches for the Detroit 8v-71's the same?  It would be great if I could pick one up in advance.  Unfortunately, there are not a lot of listings for  P&H Crane clutches....

[Dave Shepard]

Transmission will determine diameter and number of splines in the clutch disk (s).

[mike_belben]

The clutch is mated more to the trans than the engine afaik.  What trans is in it?  Is the bellhousing completely circular?  


Bigrigs ran 8v71 and the standard in bigtrucks has always been SAE2 bellhousing and i think 14" clutch so good chance it is that.  But you could have a 1.75" or a 2" input shaft or something different altogether being in a crane.  Plus you dont know if its single disc or dual. Id find the broken part first. 

[mike_belben]

Dave beat me to it

[snowstorm]

if its a 2 plate pull clutch. and almost all are it will have a clutch brake to stop the impute shaft from turning so it will go into gear

[hedgerow]

You have to tear it apart to see what you have for a clutch. Another thing to check when you have it apart is the input shaft. Some times they get grooved by the clutch disc and won't let the disk slide and cause grinding. It probably going to be a double disc with a clutch brake. Might want to pull the top on the trans while out and check the shifting forks for wear or damage.