3rd member swop

7173Mustangs.com

Help Support 7173Mustangs.com:

Superbond

Well-known member
Joined
Sep 6, 2017
Messages
96
Reaction score
27
Location
Thunder Bay
My Car
72 mustang
So
I just swopped out my 2.75 3rd member with a 3.50. I took her for a ride and my speedometer was off by 20 mph. I had a 17 tooth speedometer gear. So I ordered and swoppped it out with a 21 tooth gear. I’m still out 5-6 mph
I have on order a 23 tooth gear. I looked all over but it looks like no one makes a 22 tooth gear. Oh well!
But my real problem is my shifting pattern seems to be off. It’s an FMX transmission and I am looking and may have a aod for next winter project.
But I’d like to get my FMX shifting normal or as close to normal as I can. What should I do?
Should I adjust the lockdown lever at the carburetor?
 
Last edited:
Joined
Jul 27, 2021
Messages
820
Reaction score
598
Location
East Texas
My Car
1971 Mustang Mach 1 M code 351 4 speed
It is probably shifting really early, correct? To adjust that you need to work with the governor in the transmission. I have never worked on a governor of an FMX, so I have no idea where it is or how to get to it. For your T400, T350, 700R4, there are governor kits. The governor has weights with springs, and these weights move in a circular motion. The weights basically work on centrifugal force, as the governor spins faster the weights open up and that is what makes the transmission shift. by adjusting the weight of the weights and or the tension of the springs you can have the transmission shift sooner or later.
 
Joined
Jan 2, 2019
Messages
273
Reaction score
174
Location
Wisconsin
My Car
1972 Mustang convertible, 351C 2V with FMX (#'s matching)
Currently equipped with 351C 4V 4BM, .060 over, roller cam, Sanderson block huggers, -AC
So
I just swopped out my 2.75 3rd member with a 3.50. I took her for a ride and my speedometer was off by 20 mph. I had a 17 tooth speedometer gear. So I ordered and swoppped it out with a 21 tooth gear. I’m still out 5-6 mph
I have on order a 23 tooth gear. I looked all over but it looks like no one makes a 22 tooth gear. Oh well!
But my real problem is my shifting pattern seems to be off. It’s an FMX transmission and I am looking and may have a aod for next winter project.
But I’d like to get my FMX shifting normal or as close to normal as I can. What should I do?
Should I adjust the lockdown lever at the carburetor?
I did same 2.75 to 3.50 and also swapped fmx to c4. Largest gear I could find is 23 and it has me under recording by 5mph which is fine by me.
 
Joined
Jan 1, 2012
Messages
8,215
Reaction score
961
Location
Springfield, OR
My Car
1971 Mustang Sportroof M code
Definitely check and adjust the kickdown linkage. Make sure the vacuum modulator holds vacuum. While the FMX did not come with an adjustable vacuum modulator most replacements were adjustable. Turning the screw in (clockwise) will increase control pressures and make shifts firmer, out will be softer.
Here is the shift speed chart for an FMX with 351C-2V. Although it doesn't have a 3.50 gear listed you can make educated assumptions on what yours should be by looking at the difference between 3.25, 3.00 and 2.75 ratios.
FMX Shift Speeds.JPG
You can look up the tire diameters and calculate your tire diameter with the spreadsheet in this thread. The original tire sizes are in the "Original Sizes", others are in the "Tire Data" tabs
 

Superbond

Well-known member
Joined
Sep 6, 2017
Messages
96
Reaction score
27
Location
Thunder Bay
My Car
72 mustang
it is shifting early.
I do have the original vacuum modular.
I get it if I have to run it like this until I update my transmission to an aod I’ll deal with it until next winter. That usually when I do upgrades. That is if I find an aod before winter.
I will look at the kick down but I don’t know what I should do. Maybe disconnect it all together. I don’t know
 
Joined
Jul 27, 2021
Messages
820
Reaction score
598
Location
East Texas
My Car
1971 Mustang Mach 1 M code 351 4 speed
My understanding is that to adjust the kickdown linkage the FORD Shop manual indicates to "hold the throttle shaft lever in wide open position and adjust the downshift screw to 0.05-0.07 clearance". I don't believe that this will resolve you issue, but adjusting the kickdown can't hurt. The one thing I would do is see if you have an adjustable modulator, look inside it and see if you see a screw in there, if it has a screw inside it, then start screwing in clockwise and it will keep the car in gear longer. If you do not have an adjustable modulator get one and adjust it to keep the car in gear longer. My experience is mostly with GM and Chrysler transmission. On GM transmission the modulator will move the shift points, but not by a lot. I do not know how much of an effect it will have on an FMX, but it will have an effect. When you have an issue like you are having on a GM car, you usually cannot fix it by the modulator, you need to remove the governor and make changes to it.
 

Hemikiller

Well-known member
Staff member
7173 Mustang Supporter Member
Moderator
Joined
Jul 12, 2010
Messages
3,759
Reaction score
1,028
Location
Killingworth, CT
My Car
71 Mach 1
65 coupe
The factory gear for a 3.50 geared FMX car is the 21 tooth driven gear, those 23 tooth gears don't last.

You can try an adjustable modulator, or play with increasing the modulator pin length. Make one from a piece of 1/8" diameter rod or wire at 2 15/16" length and see where it takes you. Beyond that, you'll have to get into the valve body with a shift kit.




ModulatorPinChart.jpg
 
Joined
Jan 2, 2019
Messages
273
Reaction score
174
Location
Wisconsin
My Car
1972 Mustang convertible, 351C 2V with FMX (#'s matching)
Currently equipped with 351C 4V 4BM, .060 over, roller cam, Sanderson block huggers, -AC
I can't speak of how long a 23 tooth gear will last in the C4 w/3:50 trac-lok I put in last Sept and have 1500ish mi on her so far. I went through the whole range of 17-21 teeth in the kit I ordered from NPD and the aftermarket 23 is as close as I can get to accurate. I've said this earlier.. I'm ok being under the "actual" at 4-5mph. The 23 is still working and takes 3 minutes to change out (if/when if fails), wipe up the few oil drops, and hammer down.
 
Joined
Apr 24, 2020
Messages
352
Reaction score
220
Location
Pittsford, NY
My Car
My all time favorite vehicle is our 1969 Shelby GT500
Definitely check and adjust the kickdown linkage. Make sure the vacuum modulator holds vacuum. While the FMX did not come with an adjustable vacuum modulator most replacements were adjustable. Turning the screw in (clockwise) will increase control pressures and make shifts firmer, out will be softer.
Here is the shift speed chart for an FMX with 351C-2V. Although it doesn't have a 3.50 gear listed you can make educated assumptions on what yours should be by looking at the difference between 3.25, 3.00 and 2.75 ratios.
View attachment 63001
You can look up the tire diameters and calculate your tire diameter with the spreadsheet in this thread. The original tire sizes are in the "Original Sizes", others are in the "Tire Data" tabs

Definitely check and adjust the kickdown linkage. Make sure the vacuum modulator holds vacuum. While the FMX did not come with an adjustable vacuum modulator most replacements were adjustable. Turning the screw in (clockwise) will increase control pressures and make shifts firmer, out will be softer.
Here is the shift speed chart for an FMX with 351C-2V. Although it doesn't have a 3.50 gear listed you can make educated assumptions on what yours should be by looking at the difference between 3.25, 3.00 and 2.75 ratios.
View attachment 63001
You can look up the tire diameters and calculate your tire diameter with the spreadsheet in this thread. The original tire sizes are in the "Original Sizes", others are in the "Tire Data" tabs
This (get and adjustable Modulator and adjust it) is what I was going to suggest, before dinking with the governor. The aftermarket (adjustable) Vacuum Modulator may be able to provide the degree of adjustment needed to get you back on track.

As for the AOD swap, we did that in out 73 Mach 1 last year. Our Mach 1 has 3.5:1 TractionLok gears in it, which are great for decent around town spirited driving performance. And at 60 MPH the engine RPM was not all that bad, about 2,900 RPM or so. But, with the AOD in 4th/OverDrive at 60 MPH the engine RPM is now just about 1,900 RPM - which makes for a far nicer cruising experience. You are going to LOVE it.

But, the AOD needs to be set up correctly with the Throttle Valve (TV) pressure at the carburetor. This is likely something new to you, unless you have worked with AODs in the past. Otherwise it may/will sound spooky, and perhaps it should because if you do not set the TV pressure high enough you can smoke your clutches. I can assure you, this is not a difficult thing to do, and in our case instead keeping the initial setting of the TV pressure to 35 PSI I reset it to 38 PSI. And I am about to re-reset it again, this time to 41 PSI. Why? Well, I am glad you asked...

First of all, setting the TV pressure higher than factory spec at 35 PSI is not going to hurt anything. I am doing it to move the light throttle shift speeds up a little, as the upshifts are occurring a little sooner than I like. Also, with the higher TV pressure setting the upshifts will be a little firmer - which I also want. I doubt I should need to go higher than 41 PSI, but if I do it is not going to hurt anything.

I have attached a few files showing the parts and vendor lists I used, and another file with a bit of background on our swap experience. In the document with a sketch of our experience I have a bunch of YouTube videos I tool of our swap project. I really think you will find it useful. Included in the files and YouTube videos is a section on where we got the pigtail for the AOD Neutral Safety/BackupLamp Switch (NSS), as it is nothing at all like the NS/BU Switch on the C4, FMX, and C6 transmissions. In one of the videos I show where I spliced the AOD NS/BU Switch into the old C4 NSS harness (not into the automotive harness, I left that along). It worked perfectly.

Another aside re: the NSS... If you have a 1973 Mustang the original NSS has 6 wires. Two are for the Neutral Switch, two are for the Backup Lamp Switch, the the last 2 are for the likely no longer in use Seatbelt Interlock circuit that was no doubt disabled (legally) as the Interlock circuits proved to be a debacle and drivers across the country revolted - and the federal government allowed them to be disabled. I never see 1973 cars with them working, and the AD pigtail will not have wires to connect to the InterLock system anyway. Nothing to worry about. Just follow the direction on the one YouTube video I have the link for in one of the attached document files and you will be fine. I did solder my pigtail into the NSS harness, and used shrink wrap tubing to protect it from moisture. I do not drive the Mach 1 unless the road are dry, but every once n a while there is standing water just waiting to splash the car. I knew if I used simple crimp connectors the day would come where the NS/BU Switch leads may end of having a problem.

If you run into any questions or concerns re: your AOD project please feel free to email me (or post in this forum as I have been scouring the new entries for the past few months and plan to continue to do so for as long as I can). My email is [email protected].
 

Attachments

  • 1973Mach1_AOD Transplant Parts List_20210728.pdf
    1.3 MB · Views: 2
  • 1973Mach1_AOD_Transplant_Chronology_20210619.pdf
    149.4 KB · Views: 2

Superbond

Well-known member
Joined
Sep 6, 2017
Messages
96
Reaction score
27
Location
Thunder Bay
My Car
72 mustang
thanks mrgmhale!
It’s great information but there was one thing without me watching your videos just yet.
What year transmission did you use and out of what car?
I read if you modify the shifter linkage you don’t need to purchase a wire harness. If I’m not mistaken it was a 88-92 years and I chose those years because their supposed to have the beefier internals. But what do I know! Hahaha. Just stuff I read on the net!
 

Superbond

Well-known member
Joined
Sep 6, 2017
Messages
96
Reaction score
27
Location
Thunder Bay
My Car
72 mustang
I looked up those adjustable modules. There were different color bands around them. What is the difference and which one should I get
 
Top