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Transmission doesn't shift


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'73 Mustang
351 Cleveland 2V
FMX transmission

Hi All;

    My old '73 Tang has an FMX tranny and when I first start it up it shifts to all three gears.  After warmup it shifts to one and two (and going to #2 takes higher RPM than it should)
      Could it be a vacuum line problem?  The Vac-lines need sorting-out and the Vac Schematic is useless.  It only has a few of the lines represented.

     I  rang-up one transmission place and they said the FMX tranny is very hard to find parts for rebuild.  Is this true?

Any ideas as to what could be causing the tranny not to shift other than it needs rebuilding?  $$$$$$$$

TNX

Roy

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Posted (edited)

Not only the vacuum lines, but also the vacuum modulator and the kickdown linkage.

Hopefully you'll be able to find a vacuum diagram here that matches yours

Vacuum Hose Diagrams - Engine, Transmission, Drive Line, Etc - 7173Mustangs.com

For FMX parts

FMX: Oregon Performance Transmission

Edited by Don C
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“If you can't explain it simply, you don't understand it well enough.”

--Albert Einstein

 

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Roy

The parts supply for FMX transmission was bad for long but it is getting better every year. If you want to fix it by yourself, get in contract with Kenneth Collins. He runs a business called badshoe and sells also tutorials how to fix your FMX. www.badshoeproductions.com.

I'm not a technician and I got it managed to fix mine by myself.

Frank

1972 Mach 1, 351C-2V, wimbledon white, blue all vinyl luxury

born in Dearborn, grown in the district of San José, spent a lifetime in California, moved to Germany in 2010

Mustang_Mach1_wallpaper_300_150.jpg proud member of clublogozusammen.JPG.59fb4a10d15cfff9ec756235059135b8.JPG

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Excellent !!!     Thanks Don.

The Vac lines are so mucked-up that the heater blows hot air no matter what the setting.

Where exactly is the Vacuum Modulator located?  I looked up the image but didn't recognize it.

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That's the modulator:

fFord_fmx_transmission_for_small_block__1_593873afe464f.jpg.2ea2a2b3703c08cde46dc6afdc5bfce6.jpg

Cheers Frank 

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1972 Mach 1, 351C-2V, wimbledon white, blue all vinyl luxury

born in Dearborn, grown in the district of San José, spent a lifetime in California, moved to Germany in 2010

Mustang_Mach1_wallpaper_300_150.jpg proud member of clublogozusammen.JPG.59fb4a10d15cfff9ec756235059135b8.JPG

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Complete engine vacuum diagrams are at the bottom of this page:

https://www.mustangbarn.com/technical-information

The 73 has a dual area modulator, that raises the shift points when the EGR system is active, which is why there are two lines shown in Vicus' photo above. Full manifold vacuum should be applied to the center or rearwardmost vacuum port on the modulator. 

IMO, if it shifts fine cold, but has issues hot, that points to failing internal seals and a rebuild is in your future. 

 

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from what you wrote about the FMX:

as it warms up you have more issues shifting. this is because the Transmission fluid is reaching temperature and becoming thinner in viscosity. at the same time you have the transmission components heating up and expanding. as this happens the  fluid pressure is dropping through leaking o-rings.

even without vacuum on the modular the speed governor overrides for gear shifting. the trans would be shifting without the vacuum modulator however because the internal seals are gone you can't build enough pressure to hold the clutches and so it never shifts.

you can test this buy sticking a transmission fluid modifier in like Lucas Transmission fix, mechanic in a can this will boost the pressure by trying to fill in the o-ring seals and increase the viscosity of the fluid. 

however its going to need a rebuild to fix. the clutches might be fine but you have to crack it open and look around to know. you can get seal rebuild kits for the FMX but you won't know how complete it is until you start the rebuild.

i found the complete kits for the C6 were not complete and i had to buy another kit to address the missing seals in the first kit from another manufacturer.

a good complete FMX kit is going to run like 300$ in materials.

just the seals is going to run around 100$ 

 

keep in mind thanks to Covid supply chains are a mess and many kits not not be available at the moment.

 

as for the modulator itself you can test it easily. put a hand vaccum pump the modulator and just pull some vacuum and see if it holds. if it holds no vacuum then you know it is blown and should be replaced however i believe your issue is dried out seals internally not allowing proper pressure to hold the different fluid circuits inside.

 

if you do it yourself i also recommend www.badshoeproductions.com.

i used  Kenneth Collins video to fix my 3rd member and my C-6 

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Does the transmission shift as expected if you put it in the positions for first gear and then drive and bump it up to second gear around 15-20mph?

The vacuum modulator should get you shifting gears if its hooked directly to manifold vacuum.  At least for diagnostic purposes, you can skip all the extra valves and junctions.

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Sorry for hijacking this thread...I'm wondering what would cause the opposite, with a C4 transmission shifting from 1st to 2nd gear when cold and stays there for about 20 minutes (while idling and or driving) and finally shifts into 3rd. Thanks in advance!

71-73 Mustangs never die, they just go faster!

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Yes, G.  It starts out in first and then will not shift but I can manually go to second.  It will not, however go to third manually.

I really suspect what Tech Advisor (above) says it probably the problem.  This Tranny is old and is going to need a rebuild.  I would like to get a bit more use out of it before that but it looks doubtful. 
   The engine is pristine.  Re-manufactured by some really good professionals.  When the tranny is done, then the whole power train should like new.

TNX a bunch.

Roy

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