C-4 third gear issues

[junkyardjeff]

I had three C-4s in my Sunliner that all ended up with third gear issues,wants to slide into third at light throttle so talked to a trans shop and was told it sounds like a vacumn issue.  Hooked up a vacumn gauge and went for a drive,with higher vacumn it wants to slide into third and when I mash the pedal and vacumn gets low it goes into third nice and firm.   I think I have all vacumn lines hooked up right but wondering if the engine could be doing something to cause this.

[chimp koose]

Manifold vacuum to the modulator ?

[junkyardjeff]

Yes

[kb426]

What was the reading on the gauge?

[junkyardjeff]

It shifted into third the best when the vacumn was below 10.

[Crosley.In.AZ]

The problem is likely low pressure causing the 2-3 soft shift. Or the clutch pack clearance is too wide.

Make sure the modulator pin length is 1.620 minimum.  If the pin is that length , add .050 to the length.   welding rod of proper diameter or a 8 penny nail can be used to cut to length for new pin.  This pin is between the inner modulator valve and vacuum  modulator.  

The green stripe push in modulator is better to use.  If the vacuum modulator is a thread in , the white stripe unit is better to use.  Do Not use the modulator design that has two hose nipples on it.

The added length will keep the inner modulator valve in further against the pressure and will increase the main line pressure which should help the 2-3 shift firmness.

[kb426]

^^^^^
The expert has spoken.

[junkyardjeff]

I got it working better but still not quite right,the trans is a used one I picked up and the first shift when cold into third seems to be delayed so I baked off the modulator trying to get it to shift sooner which caused it to slide into third at light throttle much worse then the old trans.  I screwed the modulator in 3 to 4  turns but it still has a very slight slide into third at light throttle,the recently installed trans has the modulator from the old trans that I thought I really hurt when the fluid just started to smell burnt but was till red and had third gear issues.   Since I now think it could have been the modulator that was the problem with the old trans I ordered a new one and will make a slightly longer rod when I install it,my local NAPA sold me the wrong tailshaft seal so its low and could be part of the problem but will be full when I try it again.

[Crosley.In.AZ]

A used transmission to me is just a core... Full of unknown issues.

[junkyardjeff]

I will most likely have the one I removed gone through again.

[junkyardjeff]

Working much better now.