FMX Transmission disengages at higher rev while cold

Hi all,
My car has been a trouble free driver for the last few summers.
However beginning this year, tranny (FMX) hardly engages and/or disengages from D at stops (red light for example) when the car revs high on choke (1200-1300 disengaged, 900-1000 engaged).
Engages well in R, 2 and 1, so I have to start in 2 then shift in D to move forward, when cold (it stays in D once moving).
Gets back to normal after warming up a few minutes and revs down to normal 650 (engaged), once choke kicks off.
Thanks for advice that will be greatly apreciated.

I would check the vacuum modulator line to make sure it is connected properly. Also check fluid level.

Thanks Royce.
Trans fluid is OK (hits full while idling hot in Park).
Connections look thight in engine bay at firewall tree and merging wilh the metal line running down alongside the tranny.
Remains to be checked underneath down to the modulator, will do tomorrow when room is available in garage for lift up.
If OK, would the next step be replacement of the modulator?
Aanything else I should check before beginning replacing parts and/or taking it to the mechanic?
Thanks again for your help.

If you have a MityVac, you can just apply vacuum to the modulator hose at the tee to verify hose and valve don’t leak. If the modulator valve itself is leaking, it will often suck transmission fluid out of the transmission and you will find it inside the hose.

My original 70 FMX got so bad at cold temps, that I would set the parking brake, drop it in drive, and then go inside for 5 minutes before it would finally go into gear.

Saturday I found the last short bit of vacuum modulator plumbing - the short stub of hose that connects the rigid lines from the firewall to the modulator itself - had been jostled off while moving the exhaust. Do Royce’s visual check first, and please keep us updated.


Update: connections at the modulator were loose both on metal line and modulator ends. Rubber elbow hose was cooked as I.D. was now m ore than 1/4". No fluid in the hose though. So I tried to fix it with clamps, which improved a little but did not work in the end.
I then just installed brand new vacuum hose, it fits much tigther and seems to improve, but tranny still disengaged once while playing betwen P, R and D back and forth. Will see for sure tomorrow when car is truly cold.
Thank you so much Royce for hand given, this is very keen of you.

Back to sqare 1: fluid level is fine, vacuum line connections at transmission modulator are fine, but tranny still disengages in D while idling when not moving (such as waiting on red traffic light) when car is cold on choke. Back to normal when warm.
Any thing else I could check or do prior to pay a visit to the mechanic?

Next step is get a pressure gauge. NAPA has nice kits with hose and various fittings. Hook up on port on the drivers side. Kind of a pain in the ass to do. Now you can verify your line pressures and compare to factory numbers. Should get you going in the right direction to figure out what is going on… Good luck .

Purchased a gauge as suggested.
Manifold Vacuum measured as follow (plugged in free port of firewall tree):

  1. Engine started on choke + quick gaz pedal depression: 900 RPM with 17 in.Hg. Not running very smoothly - needle was vibrating.
  2. Rev. increased slowly by itself; 19 at 1000; 21 at 1250. Was running smoother, needle stopped vibrating.
  3. Engaged Tranny in R, vacuum dropped to 15 for 1 second then back up at 16, ± 850 RPM, not so smooth.
  4. Returned to P, 1250 RPM with 21 in. Hg, then engine was revved at 1500/2000/2500, vac climbed from 21 to 22 at 2000 up to 2500.
  5. Engine was warmed at this point, idle down to 900, 21 in. Hg. Run smooth as usual, but not perfect, needle stable.
  6. Vac, peaked at 25 when I gave it few sudden strong shots up to 3000.
  7. Idle at 900/21 - engaged tranny in R then D, 2 & 1: 17 to 18 in. Hg. at ± 650 RPM. Smooth as usual, but not perfect, needle stable.
  8. Returned to P; 900 RPM at 22 in.Hg. Stable as usual.

Any comments on these results?

When warmed, tranny in D shifts at 10 MPH then 25 to 30 MPH depending on the level of acceleration, are these numbers correct?
It kind of behaved like that since I got the car in 2012, put 23,000 miles on it since then.

It seems to me that fluid may not be moving in the tranny as easily as it should, when it is cold.
Fluid too thick? Clogged filter? Modulator not working as it should?

Comments are more than welcome.

What axle ratio is in this car?

Ok. You got good vacuum. Your engine is obviously in great shape. I recommended a pressure gauge to read your line pressures of your fmx at different points of operation- park, neutral, reverse, drive and first and second. Napa kit 3343 is what I got to figure out my C4. Do you have the factory service manual for your car ? There will be a chapter devoted to automatic transmission and it’ll cover your fmx. Including shift points, line pressures, and vacuum levels. More than likely the seals on the clutch drums are old and brittle. They are unable to hold the line pressures and the transmission slips/ disengages. Look up Ken Collins of Bad Shoe Productions. He has rebuild videos for the fmx. Another possibility is there is sticking valves in the valve body. Again, the factory service manual is worth it’s weight in gold. It’ll have all the info for you. I highly recommend Ken’s videos for a rebuild if it comes to that. Good luck.


Could be the filter is partly clogged and needs to be changed. Likely lots of debris from the clutch and band lining circulating with the fluid has clogged it. Occasionally it fixes the problem you are experiencing if there is sufficient friction material remaining. The fact that it works OK when warm is a good sign.

I would try dropping the pan, cleaning it, replace the filter and fluid.

Thanks for all this info.
I used to have Ford shop manual that I purchased on line but it does not open anymore and the link to the publisher brings me to a 404 error stating the site is gone. I should have purchased a CD.
However, getting to the internals and rebuilding the tranny is at this point beyong my capacity as I am not equipped to lift the car properly to take it out.
I probably will change the filter that has not been since I do not know if it has ever been, and search for dirt and/or rubber particules in the fluid… but this will be PITA since I had to fix a leak a few years back using gasket maker (pan is not 100% flat anymore and new pans are not available).
Additional suggestions are welcome.

Thanks, that is exactly what I am going to do. Saw your message immediatly after posting the previous answer.

I would recommend changing the fluid and filter as well.
Also check the manual linkage to make sure that it is properly adjusted. This is critical to ensure proper shift points and to prevent slippage.

Personally I would check in “D” with parking brake on. You might need a real person in the driver’s seat with their foot on the brake to do this test.
Check the level. If it’s normal then you are probably dealing with a choked up filter or a fried forward clutch. The only test after this is to drop the pan. It really needs to be done by someone who knows what to look for. The pan will contain lots of tidbits that are clues. With the pan off you can do more tests but those usually require an experienced person but have a look. There are some air tests that can be done once you drop the valve body…
But after that? Are you willing to have the trans rebuilt? Very few rebuilders know how to do this trans anymore…

Did check fluid level in D with wife standing on brakes.
Was may be 1/4" lower than full mark, ended adding 150 ml which is nothing to me, It is now may be 1/4" above full.
Will see next time I start tbe car cold but have strong feeling I will need to drop the pan.
Parked the car for now, may end up being done only this next winter. Will update then.

My original Cougar FMX back in 70’s wouldn’t go into gear until fluid warmed up. That was back when trans shops still worked on them for cheap. It was dried up shrunk internal rubber seals that expanded just enough to seal up when warm.

I used to have Ford shop manual that I purchased on line but it does not open anymore and the link to the publisher brings me to a 404 error stating the site is gone. I should have purchased a CD.

Email him. If you can show you purchased it, he will send a new link. I downloaded mine as a book to my IPAD, then printed it. Great to have a hard copy. :slightly_smiling_face: