ZF Hurth 630V Shifting Issue

Crash4723

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Apr 6, 2016
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So I've got a 1990 Sea Ray Sundancer 310DA with twin 454's married to the ZF Hurth 630V drives. The starboard motor and tranny sat for 10 years in the boat not used at all. I had both motors rebuilt and they have about 12 hours on them now. The Port tranny has no issues. Starboard shifts fine when cold but after about 45 minutes of cruising I can start to feel a shudder in her. I back the motors down, shut them off, pull the dip stick on the Starboard tranny and it is right at the full mark. Look things over and start them back up. Starboard tranny won't engage. No forward. No reverse. Limp back to the dock on the Port motor and let her sit for a couple hours then she is good to go.

Today I went for a cruise and the same thing happened about 45 minutes in. I backed both motors down to about 1200 to 1500 rpm and I could still feel Starboard tranny kinda shuddering. Slipping maybe. Then all of a sudden I heard something that sounded like the prop eating the rudder. I shut her down and idled in on the Port motor. The Starboard tranny was still turning (looking at the coupler) as I crawled in on the Port motor, so I know I didn't throw a prop. Again let her all sit for a couple hours and then she ran like there was no problem at all.

Mechanic thinks it is bad quality ATF. Anyone had a similar issue? Both tranny were drained and refilled with standard run of the mill Dexron/Mercon III ATF when the motors were put back in 12 hours ago.

Any thoughts on my issue are welcome and appreciated.
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
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Could be bad atf, or dirt in it plugging an orifice
 

ericga

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Mar 4, 2012
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Check to see if the engine is idling and running smoothly. If not, that alone is enough to cause shuddering. Particularly true with a big block.
 

Crash4723

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Apr 6, 2016
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Can anything be used to flush the transmission out? I run Gunk Engine Flush in my truck if I run her over 5k miles between oil changes, is there anything like that designed for the marine transmission?
 

Crash4723

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Engines purrrrr like kittens.. Even for the 454's I know they don't like to idle. And this happens at idle, just above idle, the 1500 range, etc..
 

Crash4723

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Ok. Forget the "flush" question. I just read forum after forum that say no no. I get it. So I think what I'm going to do it suck the fluid out and fill it back up with some better quality ATF. I'm thinking about Valvoline ATF. Anyone else have any recommendations?
 

ericga

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Mar 4, 2012
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Transmission fluid is aready a strong cleaner. That's what I would use to clean my transmission :)

The list of approved ATF fluid is quite long and includes pretty much all major brands. You'll be fine with the Valvoline.

Check the filter area for clogging or damaged oring etc...
 

Crash4723

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Apr 6, 2016
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So today I fired up the motors and starboard tranny won't engage at all. The fluid level was just above full cold. I pulled the filter and it was clean, reinstalled and tried it again. It engaged for about 30 seconds at 800 rpm then it gave up again.
 

Crash4723

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Apr 6, 2016
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Anyone have experience in testing the pressure on the Hurth 630's? I just need to know where to connect the gauge?
 

Crash4723

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Apr 6, 2016
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***UPDATE**** Clogged breather causing a vacuum situation in the tranny. Cleaned the breather and viola!!! All better now..
 

ericga

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Mar 4, 2012
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Thanks for the update. What was causing the clogging and how did you clean it?
 
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