C4 2-3 shift flare

[turbotim2]

77, 351w, rebuilt C4 has about 4k miles.

I have an issue developing with my bronco C4. Sometimes I get a significant 2-3 shift flare when it's cold but not always. The other day I started it cold and drove 25 miles to work and it worked perfectly. Got in at the end of the day to return home and shift flare for several miles until it warmed up. This has happened on a number of occasions this season, you neverknow whatit will do. Fluid is full and clean, reset band adjustments, VM was new at rebuild. Any further suggestions?

 

[FordBronc]

Check the vacuum modulator and the vacuum line that goes to it. Check the line for kinks or other problems (dry rot) , make sure it hasn't come off.
If the line checks out, take the line off where it connects to the modulator. If trans fluid leaks out, its bad. If no fluid, it still could be bad.

They're simple to replace, disconnect the vacuum line, remove one bolt that holds the clamp in place and the modulator comes out. It has an o-ring seal. Modulators are identified by the color of the stripe. Ford 73-79 Ford trucks...pretty much the "go to" one is the black stripe. If they don't show that one in the catalog, go with the green stripe. Avoid the pink stripe. That one is a dual diaphragm version that has two vacuum line going to it.

If by chance you have one of these, replace it with one of the other two and use the vacuum line that goes to direct manifold vacuum, cap the other line. When you pull the modulator out, be careful. There is a pin that goes in the modulator. Sometimes it comes out with the modulator, sometimes it stays in the tranny. If it comes out, don't lose it.

 

[Slowleak]

Could be a valve body problem. A lot of rebuilders put shift kits in these things and it can really mess them up. Every 2-3 flare problem I have had was resolved by cleaning the valve body and putting it back to stock. That might mean replacing springs, check balls, and the separator plate. Do you know if it has a shift kit?

 

[turbotim2]

Yes, I rebuilt the transmission and it has in service for 5 years with a mild shift kit. It has a green stripe VM and the lines are good. My understanding of the VM is that they effect down shifts and not up shifts but I could be wrong, it down shiftsquick and strong. This is a new issue that just arose.

 

[Quick & Dirty]

A 2-3 flare can be from one or a combination of: a loose intermediate band adjustment, loose direct drum clutch pack clearances, a sluggish or leaking servo, low line pressure, a sluggish valve in the valve body.

 

[79INA69]

Yes, I rebuilt the transmission and it has in service for 5 years with a mild shift kit. It has a green stripe VM and the lines are good. My understanding of the VM is that they effect down shifts and not up shifts but I could be wrong, it down shiftsquick and strong. This is a new issue that just arose.

What servo piston did you install or have? 2-3 shift as I understand uses line pressure to release the piston and apply the direct clutch. If your servo piston is not releasing because it is sticking from not enough line pressure or spring pressure it will hang 2nd. If your direct piston is sticking and not applying it could be delaying third. There are really so many variables in a condition like this. If you have a pressure guage it can be helpful if you know what to look for.

 

[Quick & Dirty]

Fluid flow has to move the intermediate servo to the release position and apply the direct clutch at the same time. If the servo uses too much fluid from a loose band adjustment or a leaking seal, there isn't enough flow to the direct clutch so it goes neutral for a moment. Excess clearance in the clutch pack also uses more fluid to apply direct gear.

 

[turbotim2]

I have the stock H servo in it. It's quite an inconsistent condition. I did check band adjustments but have not checked the pressure recently.

 

[Broncobowsher]

Did you dial down the vacuum modulator for softer/earlier shifts at any point?
I can see how a harsh shift kit on top of a stock Bronco valve body would be objectionable and fixed with adjusting the vacuum modulator. Now you have too much torque for the modulator calibration and you get a flare. Adjust the modulator for more line pressure, flare goes away but the shifts are now later and harsher. Right on the edge of it being a problem so fluid temp is noticable.

Just my thoughts anyway...