1971 FMX Transmission Hesitates Shifting Into 3rd

[Rumblefish]

I bought a FMX transmission (was told it was recently rebuilt) and installed it on my 1971 Mustang Mach 1 with a 351C. The transmission shifts fine going from 1st to 2nd gear and at low/moderate RPMs, it shifts into 3rd okay. When I come to a stop light and manually shift it into first and stomp on the accelerator the car takes off fast, no problems, it shifts into second instantly with the gas pedal floored. The problem is going into 3rd gear, when I shift it into 3rd, it hesitates and the RPMs increase before it grabs 3rd. It probably takes a full second to go into 3rd. Any suggestions on what the problem is and how to fix? TY

 

[1sostatic]

There's a vacuum assist pipe between carb and transmission -- could the rubbery bits be perished?

 

[rvrtrash]

Check your shifter linkage adjustment. If it's off a little, and you're pulling back on the shifter when you shift manually, it might be holding it from going into 3rd.

Steve

 

[Don C]

How about when you start in drive, and let it shift automatically under full throttle?

 

[Rumblefish]

There's a vacuum assist pipe between carb and transmission -- could the rubbery bits be perished?

I bought a new steel vacuum line and installed it with new rubber hoses at the carb and modulator. I'll recheck those connections. TY

 

[Rumblefish]

How about when you start in drive, and let it shift automatically under full throttle?

If I put it in 'Drive' and cruise it around at normal driving speeds, it starts out and shifts fine. Not much performance there. I can't even spin the rear tires if I keep it in 'Drive'. Only way I can turn tires over is to manually shift in down to 1st, then I can do a little burnout. This is a new motor with 200 miles on it, so I'm still breaking it in. I'm just testing to see what kind of performance I can get. I'm still taking it pretty easy.  I forgot to mention, I haven't installed the kickdown rod yet, I still need to buy that.

 

[Rumblefish]

Check your shifter linkage adjustment. If it's off a little, and you're pulling back on the shifter when you shift manually, it might be holding it from going into 3rd.

Steve

I will look at the linkage and see if it needs adjusting. TY

 

[Hemikiller]

Flare or overspeeding is typical of either low line pressure or something worn. I would definitely make sure all the above mentioned is squared away - modulator line, shift linkage adjustment and proper fill level. Once you are set there and if you still have the issue only at WOT, my thoughts would be first you need the kickdown linkage to be installed and adjusted properly to boost the line pressure for proper shifts. If it still persists, it could be a sticky valve in the valve body or something similar. The FMX does not use a band for direct drive, it applies the front and rear clutches, so there's nothing to adjust for this issue.

 

[Rumblefish]

Update: I adjusted the linkage, rechecked my connections with the vacuum line from the carb (Edelbrock 1406), topped off tranny fluid, then road tested it. No change. When I put it in 'Drive' and stomp on throttle from dead stop it automatically shifts into 2nd and engine RPMs climb until it hits 3rd gear. Not much performance there. I just ran over to Rose City Mustang and bought the kick down rod. I'll install that and see what happens. Could it be the modulator?

 

[Hemikiller]

Doubtful, since it shifts normally at part throttle.

 

[c9zx]

If the modulator has been replaced it may not be exactly the same as OEM and may require a different modulator pin length to function properly. The Ford manual will specify length with a plus or minus tolerance. Also check to see the correct modulator was installed, I think there are two types depending on year. Chuck

 

[kcmash]

I can't believe you are putting that kind of acceleration test on a 200 mile old engine.  I would be waiting until you reached at least the 500 mike mark.

For the FMX.  I remember the same problem with mine way back in the 80's when I bought it.  I was really insecure to manually shift and hammer it because it would "Stick" in second until I let off.  Part of why I always wanted the 4-speed option.

I never did look into or figure mine out.  I just stopped manually shifting.

kcmash

 

[Spike Morelli]

I'm no transmission specialist, but if you ever have the trans gone through, you might consider a reprograming kit, more commonly called a "shift kit". I had my FMX completely re-built some 20 years ago, at which time I had the company "kit" my trans for firmer, more positive shifts. I don't race it, never have, but I do tow a lot in the summer. The Reprogramming Kit was from a company called Trans-Go. It was one of the best purchases I've ever made for the car. I run a stock stall converter and added a trans cooler. It's still going strong.
I know all of this is of no help , as you are still trouble shooting, but for any kind of performance, I reccomend that you "kit" your trans. Just my 2 cents.

 

[Bentworker]

For future folks…
https://7173mustangs.com/threads/fmx-valve-body-info.39371/