C4 flairs/slips on 2-3 upshift

[pcf_mark]

Everytime I think I am making progress something new happens.

'76 C4 from a Bronco installed in my '68 I just rebuilt with new converter about 70 miles ago. I did the valve body modifications in the tech section on the conservative side (smaller holes). I have just started driving it regularly. Today I noticed slip or flair - picking up rpm then grabbing - when shifting from 2-3. Perfect behavior otherwise.

I tried the search but I either get nothing or every thread with "C4" in it!

I was thinking I may not have adjusted the band properly. I thought I would ask for some advice before I started down that path.

Thanks
Mark

 

[Viperwolf1]

Check the adjustment on the front band, 1 1/2 turns out. Does it have the H servo?

 

[pcf_mark]

I was really hoping you would comment Viper. Thanks.

If I remember correctly I torqued the bolt to some value then backed it off some number of turns per the manual. Then I locked it with the nut. When you say 1 1/2 turns out is that snug then back it off or use the torque wrench method?

I do not know if it has the H servo - I just re-used the stock unit. It passed the air check.

 

[Viperwolf1]

No need to use a torque wrench. Turn the stud in until the band is pulled tight, then turn out 1 1/2 turns. The locknut needs to be pretty tight, 30 ft-lbs or so.

"I just re-used the stock unit. It passed the air check." doesn't inspire confidence in the rebuild. When you rebuilt the trans did you replace all the clutch and servo seals? The servo will have a big letter cast on it.

 

[pcf_mark]

Yes I replaced all the servo seals, plates, bands,piston seals, most of the bushings, torque converter etc. I'll look at the servo housing when I adjust the band tonight but I did re-use the stock servo. When you said H servo I assumed it was some type of aftermarket high performance version.

The nut may have loosened because it was fine for several test drives but now I have rattled the crap out of it maybe it loosened up.

Just to be double sure - "front band" is the one closest to the torque converter?

Thanks

 

[Viperwolf1]

Just to be double sure - "front band" is the one closest to the torque converter?

Yes it is.

 

[pcf_mark]

I checked the band and it was correct within 1/4 turn. I have not test driven it yet - goofing with the shifter now.

The cover on the servo is cast aluminum with a ring around it and four small nubs evenly spaced. How can I tell if it is an H servo?

 

[Viperwolf1]

I checked the band and it was correct within 1/4 turn. I have not test driven it yet - goofing with the shifter now.

The cover on the servo is cast aluminum with a ring around it and four small nubs evenly spaced. How can I tell if it is an H servo?

It'll have a big H on it.

The H servo is important because it has a larger apply and release piston than most servos. If you have something weird on there like a Z, I can see that causing some problems with the shift timing.

 

[pcf_mark]

I just got back to this problem, thanks for the photo. I can not confirm I have an H servo because my headers are too close to get my head in there. I need a mirror.

Additional information - if I am light on the throttle it shifts from 1-2, 2-3 pretty quickly at low speed. I am in 3rd very quickly. The flare is almost undetectable at light throttle. If I use more throttle it holds first longer then it shifts crisply but 2-3 is more if a slip them grab. More throttle causes more slip. It only does it for a second but you know it.

I do not have the kick down linkage hooked up yet if that makes any difference.

Several times today I shift it manually it does not seem to flare just shifts. I did this with more load (hill) and throttle to see if was slipping and did not seem to. It does not snap like 1-2 upshift but it shifts clean.

Could I have a valve body / valve body modification problem? I followed the tech sheet on the valve body mods but I drilled the holes mid way between stock size and the holes in the tech sheet.

Bums me out because so many other things are going right with the rig!

 

[Viperwolf1]

It could be a valve body issue. I would get the Transgo 40-2 kit ($52 at bulkpart.com) and install it exactly like the instructions say. Use the yellow pressure regulator spring. If that doesn't cure it you have direct clutch issues.

 

[Rustytruck]

Since this is a swap in transmission are you using a green stripe vacuum modulator? Are you using a downshift rod attached to the Carb? The down shift rod must have a spring attached to it to pull it towards the Carb. Even if you dont use a down shift rod you must put at least a spring on the down shift lever pulling it forward towards the front of the Bronco. If the lever is left to its own to flop around the shifting gets Goofy. If you are not using the complete 76 motor with all the smog stuff dont use a dual port vacuum modulator use an adjustable single port unit hooked to manifold vacuum. If you must use a dual port unit hook up manifold vacuum to the larger rear port and leave the other port open do not cap this port. The adjustment screw inside the adjustable modulators can be adjusted to shorten or delay your basic shift point.

 

[cs_88]

I would recommend the 40-2 Transgo kit. When I began experiencing a flare with my C4 I removed and serviced the valve body, installed the Transgo kit, adjusted the bands and installed a green stripe modulator.
The C4 has been shifting great ever since.
Shifts are nice and smooth at light throttle and good and firm when on the gas, and it seems to be holding up to the 351W so far.

 

[bronco_daddy]

never tore into a C4 or any automatic..how much work is involved and what else would you do while underneath? Looking downstream here...

 

[pcf_mark]

I'll order the trans-go kit today. Seems like a good idea since I do not know who else may have been in the valve body before me and the transmission did not function when I got it.

I am not running the kick down link and the arm is off the transmission to prevent it from flopping around. I can put the lever on and tie it in that position for a quick test. I am rebuilding a kick down carb so I can get this installed and have the transmission conversion done the right way.

I am running the green striped modulator hooked directly to manifold vacuum.

Thanks I'll keep you posted.

 

[Viperwolf1]

The lack of a kickdown rod won't cause your problem but it's good to have for driveability and long term durability. Lugging the trans in a higher gear when it should automaticly downshift to a lower gear isn't good for it.

 

[Rustytruck]

Without the kickdown rod if you dont tie up the lever it will float around and cause erratic shifting. All the regular shifting still functions normally with the exception of a down shift for passing which you can do a manual down shift for passing. Dont believe me if you wish but the inertia weight in the down shift lever must be controlled.

 

[Viperwolf1]

Without the kickdown rod if you dont tie up the lever it will float around and cause erratic shifting. All the regular shifting still functions normally with the exception of a down shift for passing which you can do a manual down shift for passing. Dont believe me if you wish but the inertia weight in the down shift lever must be controlled.

I'm not disagreeing with you. We're talking about two different things, I think. The kickdown lever is small and bolted to the transmission. The valvebody downshift valve that it acts upon is spring loaded. There is no way the lever/valve can move without the kickdown rod attached.

The kickdown rod probably has enough mass to overcome the downshift valve spring if it's not attached on the carb end and is bumping around. However, the OP said he doesn't have the rod or lever.

 

[joser]

My C4 has done the same thing. I tried adjusting the band. I replaced the vacume modulator.

My trans has worked great if I manually shift and holds in all gears. I just can't really use auto, I typically shift manually or let off the gas to let it shift easier from 2 to 3.

I think I might try the Transgo 40-2 kit.

I am very interested to see if you find something. I have struggled with this for 3-4 years.

 

[pcf_mark]

I drained and removed the valve body today. The fluid stinks something aweful and is no longer a pinkish color. Is there anything else I should check (run air check or similar?) that may be easy to get at now? There is no dirt in the pan or valve body. This is type FA fluid I used but I am replacing with plain old Type F now. It was not slipping much so I was surprised it stunk so bad.

I installed the Trans Go kit and drilled the holes per the instructions. I was considering draining the converter too since 4/5 quarts are out now. But if there is something else wrong and it needs to come out there goes another $20 in AFT.

 

[Viperwolf1]

Do air checks while the VB is out. Make sure all the valves move freely in the bores (no sticking). If the fluid looks burnt it's possible the direct clutch is slipping.