C4 transmission shift control

[WPS 73 Bronco]

What controls the shift timing on a C4? Is it the transmission modulator (?) or something else.

My Trany was rebuilt a while back and i noticed that it ”shifted early”, especially on hot summer days. For example, if I’m entering into a parking garage at near idle speed, it will shift from 1st to 2nd very quickly. Then at times it will shift back and forth between 1 & 2, but only under idle speed as I’m creeping along.

When I’m at a stop light and driving away from the light, it seems to shift like it should.

Before I take it back to the Trany shop, I want to be knowledgeable about how it works.

thanks,

 

[WPS 73 Bronco]

This is similar to what I have but not the exact one.

 

[Broncobowsher]

Multiple things control the shifting.
The modulator controls the shifting based on engine torque. The more vacuum, the less torque going in. Less vacuum, higher loads, more torque.
The governor is what gives a speed bias to the shift points.
I'll skip the manual lever and kickdown.

The valve body is a mechanical analog computer. Sees pressures generated by both the modulator and the governor and depending of both pressures it will cause the shift valves to move. This is what causes the shift.

The modulator is the one most people look at. It is the easiest to access, adjust, modify, and the one that fails the most (diaphragms in ATF and sucked on with engine vacuum saturated in hydrocarbons). But anything that affects engine vacuum can cause shift timing/harshness issues.

I did mention that the modulator sees engine vacuum as a factor of torque. This is what deals with shift harshness. At idle, a soft shift is fine. Low line pressure that applies the clutches softly. Since the engine torque is low, it is still a fairly quick engagment but slow enough it is smooth and doesn't bang. At high loads this slow, soft engagment will be a lot of slip and wear. The higher line pressure speeds up the clutches engaging, witch is countered with the engine making more torque trying to make them slip. Balanced out it is a quick shift, but still smooth. If the engine isn't making the torque that the modulator sees (low vacuum and low torque production) you get a transmission that applies quick but doesn't have the torque to slip into a smooth engagment. Bangs into gear. There is more to some of the shifts, accumulators that have fill and drain rates that can also play with shift timing. So hard/soft shifts are not always 100% a factor of modulator. But the modulator does control shifts across the board. The modulator is the only signal the transmission has to how hard you are demanding power (kickdown is a second, basicly a total override of the vacuum system. But it is only an on/off signal).

 

[WPS 73 Bronco]

So what are your thoughts about my problem? It’s not right for sure. It jumps back and forth between gears when it should only be in one or the other. It’s as if the Bronco can’t decide where it should be, when in reality it is just creeping along with no throttle at all but shifting gears.

 

[DirtDonk]

Is the engine original and stock? Or rebuilt and perhaps modified?
Modulator connected to full manifold vacuum on the intake, or maybe one of the ports on the carburetor?
Not sure if any of those would result in what you’re experiencing, but wondered about it.

 

[BroncoJimbo]

So what are your thoughts about my problem? It’s not right for sure. It jumps back and forth between gears when it should only be in one or the other. It’s as if the Bronco can’t decide where it should be, when in reality it is just creeping along with no throttle at all but shifting gears.

FWIW, my C4 has always done that. I just accepted it as one of the inherent foibles of the C4. Mine is running full manifold vacuum, a TransGo shift kit and a RV torque converter.

 

[Slowleak]

Is your kick down lever properly adjusted and functioning?

 

[tpatton75]

Mine does that too, but I thought it was low vacuum from big cam issue. I just shift to second if staying that speed .

 

[Viperwolf1]

The primary valve in the governor can't decide where it wants to be. A Bronco governor will have the lightest primary valve already but stretching the spring might help.

 

[WPS 73 Bronco]

So what I’m hearing is it’s not due to my rebuild. That’s what I was worried about.

Kick-down is not hooked up.

Vacuum is from the manifold. Rebuilt engine.

Where is the governor?

 

[Viperwolf1]

You would need to remove the transfer case and adapter to get to it.