What causes an auto trans to shudder at shifting RPM but not shift?

[lemming303]

Its a C4. I know, its not a mopar but yall are really helpful and I figure at least one of yall works at a shop that works on all transmissions. Besides, I would think that most auto transmissions are built somewhat alike. Its on my mom's 76 Elite. The car has only 80,000 original miles on it.

I changed the filter and fluid and put Valvoline fluid and one quart of Lucas Stop slip in. The filter is a Pioneer from O'reillys. We're not sure how old the filter or fluid was but considering it only has 80,000 miles I doubt that it has been changed until now. The filter was fairly clogged, or at least it appeared to be. There was not hardly any metal in the pan and no gunk or sludge. The little metal that I did notice was actually more of a metallic powder in the fluid.

When the car gets up to shifting RPM it shudders and won't rev any higher, but also won't shift. The first shift is at around 20 mph and unless you let off the gas it will not shift. It just started doing this recently. If you let off the gas it will shift, but it does it slow. The first time it happened it did it at 55 trying to go into overdrive. Now after changing the fluid/filter it does it at every gear.

I really don't know much about transmissions. Well, I don't know a whole lot about working on cars either but I"m learning. Thanks for the help yall. I know it's not a mopar but my mom loves this car. Her mom bought it brand new and when she passed away she gave it to my mom.

[71chally416]

Maybe the vacume modulator if it has one or a leak in the hose that goes to it. Also might have the wrong fluid. Probably uses type F.

[hooD]

Sounds like it may be your torque converter causing the shuddering when approaching OD speed.

I have always heard that if a trans hasn't had its fluid changed in a long time, it is best to do it in small amounts over a period of time.  When you do the whole fluid change at once it cleans the inside of the trans and it can actually do more harm than good.

I am not a trans expert, but when it was explained to me it made a lot of sense.  Hope someone else here has better news for you.

[Bullitt-]

Sounds somewhat like the issues I experienced on the '98 Dakota I adopted...turned out that there was note enough fluid. I was checking fluid level in Park which is not the same as Neutral which runs the pump  ... Also we had a 79 LTD II that was having shifting issues and a new modulator improved things somewhat. 

[lemming303]

When I was putting the new fluid I would put in 1 quart and shift through all the gears a couple times. Then I would check the level and if it was low I'd put another quart in and shift gears a few times again. I did this until it was full. Then we tried to drive it and it was still doing it. I checked the levels again and they were the same.

I guess I will have to check the vacuum actuator, although I have no idea what that is. I will find out though. Thanks for the help yall.

[lemming303]

Oh also I put Type FA in it Chally.

[71chally416]

Maybe this will help ya indentify where it is and what it does Bro. It's on the Passenger side right behind the pan.  I'd check the vacume line carefully first.

http://www.mustangandfords.com/techarticles/mufp_0605_small_block_ford_performance_transmission/photo_29.html

[tactransman]

It sounds like a bad vacuum modulator,or the vacuum hose has come off at the bottom or the top. If you pull the hose off the modulator down below and fluid runs out of the hose,the modulator is bad. If you take the modulator out of the case,make sure you don't lose the pin inside of it.
 
If the modulator is bad or the hose has come off,it should still shift,but it will be like you have your foot to the floor. (really late)

[tactransman]

Another thing too,when you took the filter off,did it have a spring and check valve under the round tab on the edge of the filter? It will fall into the drain pan and some times you won't see it fall out. It HAS to be in there if it takes one.

http://www.makcotransmissionparts.com/A26741-1K.html

[lemming303]

Is that the filter on the newer C4 that you are talking about tactrans? If it is that is not the right one. I didn't see anything come out and I checked the old fluid pretty well looking for shavings and stuff like that.

I will definitely check the vacuum modulator. I looked up the actual part on Summit so I think I should be able to find it. I'll let yall know what I find when I get back to working on it. It's gonna be a few days though.

[71chally416]

If you take the modulator out of the case,make sure you don't lose the pin inside of it.

Is that the pin that if you forget to put it in it will blow the modulator off the back of the case?? I remember a guy is shop class in HS did that with a Ford tranny back in '73, but I don't know what type tranny it was. The shop teacher was really 

[tactransman]

Is that the pin that if you forget to put it in it will blow the modulator off the back of the case?? I remember a guy is shop class in HS did that with a Ford tranny back in '73, but I don't know what type tranny it was. The shop teacher was really 

No,it will cause very low pressures without the pin.

[71chally416]

Then maybe he installed it wrong or upside down of something. Just putting that out there cause I definately remember it flying through the shop when he installed it wrong. Musta been an older model like a late 60's or early 70's cause this was 1973. I've never worked on one other than installing a boneyard C-4 in a '67 or '68 Cougar for a guy I used to play with decades ago in his garage. His old tranny was slipping like crazy and the boneyard tranny worked great.

The shop teacher fixed the broken tranny case with some PC7 epoxy and the car went down the road. Not the way I would have repaired it, but what else would you expect from a Ford guy?   

[lemming303]

Tactrans, is that the filter for the C4 that you are talking about? That's not the same filter that came off her car. It was a square filter, and more filter material was visible.

[tactransman]

Was it like this one? If so,it is a C-5.