Creeping neutral and selector valve problems

Selector Valve scratches

What Chris sees on the bench

• A little creep in neutral on old transmissions that stops as long as you hold it. It doesn't take much to stop it. Chris puts that down to the bushing in the case, and calls it normal for these old units.
• Scratches on the selector valve on a transmission that had no pressure.
• Pressure that stayed the same in reverse, neutral and forward on the test stand, which told Chris the problem was probably the selector valve.
• A plunger rusted in on the bottom. Sometimes if you get water in a transmission, that happens, and it won't give you the pressure you need.

AS15-71C 1017-012 2.91:1 tested

1017-001 Bushing Style Case

Velvet Drive AS1-72C Early style 1017-004 (1017-003)

That bushing design is also why a rebuild alone won't cure the creep. A bushing-style case and planetary won't hold a solid, positive neutral no matter what you rebuild inside it. It takes the later sealing-ring case and planetary to actually stop it.

The factory's own neutral trouble list backs this up: if a transmission drives forward on its own in neutral, that points to broken sealing rings or bushings, warped forward plates, or a blocked control-valve exhaust; if it drives reverse, it's warped reverse plates or clutch failure, or a blocked exhaust; noise only in neutral usually means worn pump gears or low oil.

Fix

If the creep is light, stops when you hold it, and there's no abnormal noise in reverse, neutral or forward, that's normal for an old bushing-style case. If pressure is missing or reads the same in every position, pull the selector valve and check it for scratches, and check the plunger for rust. When you reinstall the valve, line it up with the bolt on the bottom. That gives you neutral, so the ball lines right up and you don't have to fiddle with it.

There's no break-in period on any of these transmissions, so a rebuilt unit that creeps into forward in neutral right out of the gate needs to be diagnosed, not driven through. Most of the time that kind of forward creep traces back to warped or dished clutch plates.

Two related things that can look like a neutral problem

A flattened filter cover O-ring on ZF units. The O-ring should be round. When it goes flat, it doesn't make good contact with the housing, air gets sucked in, the pump neutrals out, and you're dead in the water. Replace the filter and O-ring yearly, or at least every two years.

A melted baffle in an overheated cast-iron Velvet Drive. If you ever have an overheated cast-iron transmission, disassemble it, because you're going to find that baffle melted.

When to call

If pressure reads the same in every gear or you've got no pressure, call (804) 333-3677, Monday through Friday 9 to 3 Eastern, or email sales@halemarine.com.

Related reading: low transmission pressure and water in your transmission fluid.