Technical Trans issues that are probably not the trans.

I have a 351-w/C-4 in my Sunliner,its on the fourth trans with two being rebuilt and none shifted correctly in third gear. The two common parts are the engine and the steel vacumn line going to the modulator,the steel line is going to be replaced this week so if that does npt fix the problem where else should I look. I really do not think its the trans since all four had similar issues.

 

Do you have the “ kick down” lever hooked up?






Bones

 

C-4 Trans is probably the easiest trans to rebuild.
Explain just what they are doing or not doing.
Bill

 

Mine did that with a bad modulator.
Check the vacuum to make sure it’s pulling the correct amount.
The kick down could be not adjusted correctly or stuck.

 

You might also want to describe what the symptoms are....

and, what rear gear ratio is in the car?

 

I had an interesting one recently. Engine was smoking when running, engine running rough, trans wasn't shifting right. All on a solid car that had been in storage too long.

Ended up being a bad brake booster, screwing up the vaccum and actually pulling trans fluid up into everything else.

 

The vacuum reading at the modulator end of the pipe should be the same as your full manifold vacuum reading. The pipe itself doesn't tend to plug up, but anything's possible. It also can develop a vacuum leak due to rusting or if the pipe has been rubbed thru due to improper routing or mounting. Also make sure the rubber hoses on the ends of the pipe aren't cracked, split or collapsed.

Sometimes the OEMs would place a restriction or crimp the pipe at one end to "dampen" the vacuum signal to the modulator to fine tune the shift timing and feel. The restiction occasionally could get plugged with a bit of dirt. It's a simple matter to remove or bypass this vacuum restriction to ensure that the moduator sees the full manifold vacuum signal.

Beyond that, adjusting or replacing a defective modulator should improve the shift timing and feel. A properly working governor is also part of the equation.

 

Are you sure it wasn't pulling brake fluid from a leaking master cylinder thru the booster?

 

What do you mean by, "none shifted correctly in third gear"?

 

Third gear never felt firm enough and kind of wanted to slip in gear in all the trans at times including the latest rebuilt trans but it will go in firmly when manually shifted.

 

All four trans had what I would say is a wimpy shift into third which is controlled by the modulator so most likely vacumn related. Its driving me nuts and before I complain to the trans builder would like to see if there is another problem.

 

If there's too much vacuum, then it would shift soft and early. If there's not enough vacuum, it will shift late and hard.

Most modern modulators have an adjustment screw.

What rear end gears does it have? That will affect shift feel. How much stall speed does it have? That will affect shift feel.

 

Shift points are not controlled just by the vacuum modulator, but also by the kickdown linkage. Is the kickdown linkage hooked up and adjusted properly?

I hate to say it, but with all the trouble you've had trying to get this problem solved (four transmissions?, and two of them "rebuilt"?) it's time to take the vehicle to someone who knows what they're doing.

 

Bright Red fluid in vacuum lines to the carb from booster, surprised me. Pulled them off, blew thru them onto white towel - red not rusty. Trans is powerglide

Swapped from single to dual rrservoir master, had that original booster rebuilt versus an aftermarket generic version, the master cylinder itself didn't get additional diagnosis.

 

Squirrel said it best. If it stacks shifts early and soft too much vacuum or weak modulator setting. Kick down lever on a C4 is only for exactly that. It will also delay up shifts on a wide open throttle.

 

Well that sure sounds like A/T fluid then. By some chance was the line for the vacuum modulator plumbed into the vacuum hose for the brake booster?

 

3.00 gears and a stock converter.

 

The kickdown linkage on a C4 is only for kickdowns. It only has any control over shift points at wide open throttle (when the kickdown valve is depressed). In all other driving, shift points and quality are controlled by the vacuum modulator. The transmission balances modulator pressure against governor pressure to determine shift points.

 

Here is the history on the four trans,#1 came from a junkyard and had a wimpy third gear shift so I just figured it was worn. #2 was rebuilt and much better but third gear shift was not the best and got that trans hot and lost third completely until I changed the fluid and third gear shift was still wimpy. #3 was used since I had planned on putting. U block back in and did not want to waste money and this trams was a total disaster and ended up having governor issues. #4 is trans #1 rebuilt and third is not as firm as I would like but the shift points delay when the trans warms up. It has a Lokar kick down cable system but is disconnected right now as I try to figure out what is going on.

 

you need a modulator.
fluid from the trans can only enter trough a bad modulator.

 

Have you pulled any of the transmissions down and looked at the clutches? That would give you a clue as to why it might be slipping in high gear. Also, before removal, it would also be informative to check transmission pressures during operation. That will tell you if the modulator is adjusted and/or working correctly.

I had a C6 in a tow vehicle, F250, that started slipping only in high gear when pulling my trailer. If I took it easy, it wouldn't slip. I pulled it and expected to find worn or burned clutches in the high and reverse clutch. What I found was the forward clutch was starting to fail. It is always on in 1st and 2nd gear, but only slipped in high gear. The C6 clutch arrangement is the same as a C4 so the same clutches/bands are applied in each gear. I would want to autopsy the transmission and find the source of the problem.

 

I had an off topic pickup that started running rough, smoking, fouling one spark plug.
Turned out, the vacuum line to the modulator was terminated on a single runner on the intake. That happened to be the cylinder that was fouling the plug. I didn’t realize what was happening until the trans was so low on fluid that it started to shift/move funny.
The modulator diaphragm had failed, letting the engine suck trans sauce into that single cylinder. Changing the modulator and topping the fluid fixed all the issues.

 

Will double check that. Booster was also - definitely bad.

Thank You

 

I had a Dodge Chavan (Way Off Topic) that was shifting really terrible and sounded like a bucket of rocks. Got on line locally and adds said they could fix my trans in a couple of days for around $700.A friend of mine came over and checked it out and GUESS WHAT!!!. It wasn't the trans but a bad diode in the alternator. Seem that the ECM and the electronic shift trans doesn't like AC voltage. Saved myself a lot of money for a couple of beers. Wife drove it until the wheels fell off LOL.

 

It just seems strange that even though there were other issues all had soft third gear shifts so there is something I am missing.

 

14 inches of vacumn so something wrong and exhaust smells rich.

 

If it has a lokar shifter as well I would make sure it is aligned correctly. That lokar stuff isn’t the most accurate. It could be between detents in the valve body but still sounds like a modulator issue to me. Get an adjustable modulator, pocket screwdriver in the vacuum port and screw it in a turn or so at a time until it’s acceptable.

 

In order to get the vacumn up to close to where it should be I had to crank up the timing and now its pinging and barely starting,took it for a drive and it seems to be shifting better so it looks like the cam had been installed retarded. It looks like I had a vacumn issue with the engine the whole time,it was supposed to be a RV grind but since it starts to come on above 2500 I doubt its what I ordered ans all I know it has a 302 firing order so time for a cam thread.

 

More vacuum equals softer/earlier shifts

 

Is this a engine that would of had a EGR valve?