Slow Shifting

[chris piazza]

I got 892's w/Allison's. My port trans shifts immediately however Sb side takes 2 sec, both forward and reverse. Last year both were a little slow and I changed fluids and filters and port works fine but the SB side is still slow. This weekend I noticed that pressure in the SB trans was running about 20-30pounds less than port. In previous threads I have read something about a "Internal" screen that needs to be cleaned? Could I get some guideance as to how to do this if this is causing my problem? Also I did check the oil, running while hot and it is right at the full mark and just as clean as when I changed it last year.

Chris
Superior Nights 53C

 

[SKYCHENEY]

There is a three bolt flange where the big oil line hooks to the top(side) of the the trans. Pull the three bolts and the screen is under there.

 

[chris piazza]

Thanks Sky, Do I need any gaskets before I do this? and do I just clean the screen with some fuel and re-install?

Chris
Superior Nights 53C

 

[Genesis]

Check the screen and make sure when you replace the flange you use a new gasket and there are NO AIR LEAKS on the suction side.

If you still have a problem there is an adjustment for pump pressure on the hydrico pump. It is EXTREMELY sensitive (1/16th-1/8th turn will result in a 20-30psi change in output pressure!) and is under the long nut on the lower left side of the pump, looking from the rear. Remove the long nut (counter-holding the locknut) and under it is a screw; hold that, loosen the locknut, adjust, tighten locknut.

This should be done with the engine running and a MECHANICAL gauge on the gear pressure you can see so you know where you are.

DO NOT allow the pressure to go over 135-150psi, even momentarily; you can blow seals in the gear. More is NOT better beyond specs (125ish)

If the pressure is ok in neutral but refuses to hold in gear either the pump is bad, flow is improper (e.g. screen is plugged, air leaks on the suction side, etc) or there are internal problems in the gear that are leaking down fluid pressure (e.g. seals on the clutch pistons.) If the hydrico pump is "hissing" the usual cause is air leaks on the suction side. If the pump is suspected of being bad you can swap it with the one on the other engine for confirmation.

Check for air leaks and check/swap the pump before ripping the gear apart, as the latter is a real pain in the butt while swapping the pump and air leaks are very easy by comparison. The flange on the strainer is a very common source of air leaks.

 

[chris piazza]

Thanks Karl, I will start with the screen first. Is it OK to make my own gasket or do I have to track down an OEM part.

 

[SKYCHENEY]

I keep a couple of OEM gaskets on the boat, but it's just a paper gasket and you could easily make your own.

 

[yachtsmanbill]

That sounds just like my situation after the rebuild last winter... after warmed up, about 50 psi at idle, at 1000 rpm and above, around 90 psi. It was like this on the first start up and is still the same after 15 hours on it.
With 50 psi it will go into F/R after a few seconds, at 1000 rpm a tad faster. I separated the screen flange and fingered a film of gasket eliminator on the faces to no avail.
After I finish the fuel polishing, I think I'll put a gauge on the top and adjust the pump. The overhaul was almost a 12 day job!! As posted previously, I had the pump apart to remove the broken tach cable stub, and had to make a new gasket that was .001 - .002" thicker; I hope I'm not loosing pressure from that! ws

 

[yachtsmanbill]

This kinda shows the cap over the adjstment... ws

 

[Genesis]

Chris - its just a paper gasket; you can cut one from gasket material.

Make sure the surfaces are clean and true before you put it back together. If in doubt a VERY SMALL amount of RTV or other gasket sealant can be used with the gasket.

The important thing is that you can't have any air leaks on the suction side. If you do you normally can hear the pump "hiss" from the cavitation; this will affect shift times and if you get enough air entrained in the suction (it doesn't take much!) you can damage the pump or get pressure that won't build and hold properly.

This is the most common problem with that set of symptoms; the hydrico pump is pretty simple internally; its just a positive displacement pump with a spring-loaded pop-off valve for pressure regulation (which is how you adjust the pressure.)

BTW if you snap a tach drive cable it is USUALLY the worm gear train in the offset housing that is at fault. They get sloppy with age and then bind, causing the drive cable to break. The bad news is that there's really no good way to fix those and they're f^#)(#@ expensive - I think I paid $250 for one? I do remember swearing over the price.

 

[yachtsmanbill]

Yer right Karl... that and all the pieces are proprietary GM numbers. I have one bad gear in mine so I can either drive the cable or the tach generator, but not both!
DD in NOLA has the units, and 2 years ago the were about $250. The stem on the gear is about 1/4" for perspective. Both the driving and driven gears are chipped and will bind the unit tighter than a chinese finger! ws

 

[chris piazza]

OK i got the gasket material on the boat so I'll make one of my own. It looks pretty straight forward and I'll do it this weekend. However just one more question, when I take this apart how much oil should I be prepared to clean up? I assuming its not under pressure and what I get will just be out of the hose? Do I have to purge the air out of the system? I guess that's two questions.

Chris
Superior Nights 53C

 

[Genesis]

Usually little or none - the oil will usually drain right back into the gear.

The pump will reprime itself.

 

[questover]

Karl,

When adjusting the screw -would clockwise increase pressure? Thanks Russ

 

[Genesis]

Yes.

VERY SMALL changes..... little moves make big differences.