C4 hard shift from first

[johnbeck]

I had my 1973 C4 rebuilt while it and my 302 were out. I got both back in and now the C4 trans shifts too hard from first to second. The trans rebuilder replaced the original two line vacuum modulator valve with a single vacuum line modulator valve. Could that be a problem? I was told the vacuum modulator valve establishes when the first to second shift occurs(based on vacuum) but doesn't control the harshness of the up shift. If I can adjust the shift so it occurs at a higher rpm(2500 ?), would that likely make the up shift easier? Second problem: When the rebuilt, recently installed trans sits over night, starting the engine and putting it in gear often has a momentary lag of a half second as if the fluid has to fill something internal. Any thoughts? Thanks, John B

 

[blubuckaroo]

The modulator and governor work together to make the shift.
The modulator's job is to delay the shift until the engine vacuum cones up to a preset amount.
The modulator is adjustable. The adjustment screw is inside the vacuum port.
The longer the modulator delays the shift (higher RPM) the harsher the shift will be.

 

[Rightpace]

Loss of vacuum at modulator will make the shift slam into gear. Test vacuum present on hose above idle speed with a vacuum guage. If vacuum is present, then adjustment can be turned with a small flat blade screwdriver in half turn increments until desired shift is obtained.

 

[Rightpace]

Turn screw counter clockwise to soften the shift. Only half turn to full turn at a time.

 

[Broncobowsher]

Double check you have the modulator connected to manifold vacuum, not ported.

Engine rebuilt? does it have a big cam in it now? How is vacuum at idle?

The purpose of the modulator is to sense torque supplied to from the engine. The more vacuum, the less torque. Less vacuum, more torque.
The more torque the transmission is expecting, the harder the clutchs will be applied to handle it. The less that is expected, the lighter the clutch apply (smoother, less parasitic loss, runs cooler, etc.)
Adjusting the screw on the modulator changes the vacuum to torque offset. That is what the transmission is thinking the engine is making for torque compared to what it really is.

Shift points are controlled by the governor, but not exclusively. The signal is modified by the modulator. If there is a lot of vacuum, little load/torque, the shift point happens sooner. Less vacuum, the shift point isn't as advanced. And with the kickdown engaged, the modulator adjustment is ignored and max RPM shift points via the governor signal.

 

[spap]

I don’t know as much as the others but did you talk to the builder could they have put a shift kit in the c4 when they were rebuilding it ?

 

[jckkys]

Broncobowsher is correct, but for the sake of understanding the word load could be substituted for the word torque. So the more load the less intake vacuum and the later/harder the shift. Another thing to consider is the habitual installation of shift kits by many transmission shops. The OE C4s in EBs didn't need a shift kit. The shifts were plenty firm as built, and the unwise,and in fact, unthinking shift kit installation will cause your symptoms.

 

[johnbeck]

Where are the modified shift kits located in the transmission - in the valve body? How is the Bronco's C4 differentiated in designation from C4s of the same time period found in other Ford models? I appreciate your help. You've made reference to vacuum pressures and unfortunately I haven't picked up a vacuum gauge yet, hope to get one today. Much thanks, John B

 

[jckkys]

The Bronco C4s are late pan fill heavy duty types used in trucks and large cars. They were effectively the same as the others in this category, but with a short output shaft. The shift kit involved modifications to the valve body in the transmission. They removed some parts and of those some were replaced with parts included in the kit. Some were removed and not replaced. The separator plate got a hole drilled in it too. It's difficult to impossible to return the valve body to the original configuration. When I found out what was done to mine, the only way was a replacement valve body. A real PITA. With the stock valve body, it now shifts the way it should.