Things to consider..

What’s your rear axle ratio?

Do you have a vacuum leak on the line from intake to vacuum Canister..?


And yes like mentioned above, is the fluid level ok..?

If it revs in 3rd, as if its in 2nd ..... its slipping. Unless you're just not used to the feel of 3:73 gears.
Wether its from worn bands or incorrect oil level is hard to tell from here.
Not tryin ta be a Richard Cranium, but do you know the procedure for checking and adding oil??

With a 3.73 and normal sized tires (around 25" dia.) it would be turning around 2000 rpm at 40 mph in third gear.  With the stockish gears (2.8:1) it would be turning around 1500 rpm.  So if you aren't used to the lower rear gear it will sound like it's in second.  Get a cheap tachometer and a couple of alligator clips.  Hook the red wire to the battery +, the green wire to the distributor side of the coil, and the black wire to ground.  Be careful to keep the wires out of the fan and go for a drive.  That'll tell you exactly what's happening.  If you push it a bit in third and the rpm flairs, it's slipping.  
Butt (TS&T), check all the other stuff that has been mentioned first.  The early shifting sounds like a vacuum leak to the transmission modulator, as Gaba mentioned.  There are two sections of neoprene hose involved, one at the carburetor and one at the right rear of the trans.  Or, the modulator itself could be bad.

BB

almcgee wrote:

3:73 gears are way too low for a C4 transmission.

Didn't the C4 transmission have 3:00 gears as original equipment? 
 

3.73 is tall but not crazy I dont think .

I was remembering a transmission shop telling me about my transmission, that the 1st to 2nd shift is giverned by the vaccum canister, and the 2nd to 3rd on the c4 is over the bands.

There are band adjusters on the outside of your transmission, I DO NOT RECOMMEND CHANGING THEM if you dont know what you are doing, BUT, I would check them to see if they are screwed all the way out, or all the way in, or if they are loose (by hand) , like mine were before I went in for a rebuild.

here are the band adjusters in the video. This transmission is upside down, so the band for 2-3 on yours is on the passenger side back, and 1-2 shift is drivers side front of transmission

https://www.youtube.com/watch?v=ygxQRtkSMyo

I do not say this to overwhelm or confuse you. I wouldnt even worry about this till after you have
1) Checked fluid level
2) checked vaccum leaks
3) and confirmed with an RPM gauge that you are actually slipping.

 

Last edited by Gaba (5/01/2018 10:52 AM)

I let the car get warm/hot left it running checked it..it was pretty low barely a drop on the stick...also yes im not use to driving anything more than a stock mustang 

50vert wrote:

If it revs in 3rd, as if its in 2nd ..... its slipping. Unless you're just not used to the feel of 3:73 gears.
Wether its from worn bands or incorrect oil level is hard to tell from here.
Not tryin ta be a Richard Cranium, but do you know the procedure for checking and adding oil??

all in all I might head to a trans shop and have them check it out...I did want a AOD but now im just taking on to many projects at the moment for this car lol

Or:  https://www.googleadservices.com/pagead/aclk?sa=L&ai=DChcSEwjPtN2tmOXaAhWEMWkKHW-wBIsYABASGgJpcQ&ohost=www.google.com&cid=CAESEeD2QX1FE-RVPWWvCCtbX6h9&sig=AOD64_0YZrhd_YpR08aSG_w2N45g7dHyvA&ctype=5&q=&ved=0ahUKEwisldetmOXaAhWb2YMKHfO0BZEQ9aACCEI&adurl=
for a reasonably priced tach that will do the job.

BB

some of those would look nice in my car!...im going to look on offer up for one see if I can get a good one.. my car at 40mph sounds like its screaming way more than 2k

Gaba wrote:

Vicfonz wrote:

im running 225/45/17s all the way around...im going to hook up a cheap rpm and try to post a vid when I can..gotta find a cheap rpm gauge first...

TKOPerformance wrote:

Typically when automatics fail the highest gear clutch packs go first, so if its slipping it would start slipping in 3rd.  If engine RPM does not seem to match road speed I would say its slipping.  Post a MPH vs. RPM and let me know your tire size and I can tell you if the RPM is too high for the road speed. 

 

Tire height of 24.97
Rpm at 40mph with your gear ratio of 3.73, like already said above should be at 2000 rpm
And 3000 at 60 mph

Cheap tach:
https://m.autozone.com/gauges-and-gauge-accessories/tachometer?aqs=4294962368~4294961710&productKey=100989&categoryKey=102773

I have that first one permanently strapped on my steering column with a Hose clamp, offset to the left covering the oil light (I use an oil pressure gauge . )

yes indeed it screams starting around 40mph..its gets to 3rd gear pretty quick..probably before we hit 25mph

TKOPerformance wrote:

Yeah, 3.73s are pretty tall for a C4.  I've got 3.50s and they were too tall for a C4.  She would scream on the highway trying to run 80MPH with everyone else. 

Tach installed...im running 3k rpms going about 45-50... im not sure of my mph because my speedo needs to be calibrated

DC wrote:

I'm confused, I thought tall gears were a low numerical number like 2.90 and short gears were high like 4.30?

TKOPerformance wrote:

Yeah, 3.73s are pretty tall for a C4.  I've got 3.50s and they were too tall for a C4.  She would scream on the highway trying to run 80MPH with everyone else. 

 

I have the same problem.  I have found that the 'tall or short' definition depends on who you are talking to.  It is better just to give the actual ratio.

​and 3k rpm @ 50 mph is too high an rpm.  you have to have 3.73's or above.  How do you drive on the freeway at >60 mph?

​I grew up with rear ratios of 3.27, 3.54. 3.78, 4.11. & 4.44.  Boy have times changed.