FMX Down for the Count

[cheezsnake]

I believe the FMX transmission in my 71 is on its last leg. Whining, grinding noise that comes and goes in Park or Neutral. Put it in Reverse or Drive and it doesn't move right away. Slips in and out of gear while braked at the stop light. Surges and causes the engine speed to bounce when you get above 40 mph. I'm sure the tranny is original and hasn't been touched. Fluid and filter change didn't help.

I see there are rebuild kits out there for the FMX. I don't know a lot about automatics. I'm assuming there could be parts that get damaged that are not included in a rebuild kit? But, this isn't a job I really want to tackle anyway. So, I'm probably taking it somewhere. I also see there are remanufactured C4 and C6 transmissions available. I haven't found a place that sells FMX. Swapping a transmission is something I probably could take on.

I'm wondering, if I would have to pull and have the FMX rebuilt anyway, would it make sense to just swap to a remanufactured C4 or C6? Assuming a complete rebuild is necessary, what is the relative cost? What are the pros/cons of a C4 or C6 swap? And, what's involved with a swap paired with a 351C? Some possible things that come to mind ...

Torque converter? (which probably needs replaced anyway)

Flexplate?

Bell housing?

Cooling lines?

Shifter & linkage?

Kickdown?

?

Thanks!

 

[Carolina_Mountain_Mustangs]

I do not know how original your car is or if you want to keep original. The transmission has a partial of your VIN# stamped into it so once it is gone then not numbers matching. Might not mean anything to you just info.

If the FMX lasted this long don't see how switching to C-4 or C-6 would help??

 

[Mister 4x4]

I'd say just get the FMX rebuilt, if you can find a shop that'll do it for a reasonable price.

I could've rebuilt mine, but instead went with an AOD from an '89 5.0 GT because... well... it was basically free. The guy gave it to me for the price of I would cover his $32 bill for shop fees when he swapped out his AOD for a T-5 (he wanted a manual instead). Yeah... no brainer.

I also had to swap the flexplate, get new cooling lines, tranny crossmember/mount, AOD output shaft/yoke, adapter U-joint, and modify the center console to fit the AOD shifter. Swapping in a different transmission will take some research and a bit of modification. Fortunately, C4 or C6 swaps are pretty common - there are some places out there offering kits for those swaps.

Here's one: https://transmissioncenter.net/shop/

 

[cheezsnake]

Thanks for the input. Since the transmission has to come out either way, I'll have it looked at to get a repair estimate. Would be nice to keep it original if it can be rebuilt and is not too expensive, compared to a swap.

 

[luxstang]

I had an FMX in mine and I really liked it. Swapping it for an AOD was one of my biggest mistakes in hindsight.

I don't know why the FMX has such a "B grade" reputation and everyone wants to get a C4 or C6 instead. (AOD is a different matter because overdrive)

Like Eric said it may take a lot of mods to swap to a different tranny. In my case I even had to have the drive shaft shortened, so the AOD swap was a one way street. I don't know if a C4 or C6 will require it too but expect some snafu.

I'd say just get your FMX fixed and you're good.

 

[72HCODE]

the fmx is basically a c6 just heavier. the ratios are the same as a C6

if the FMX is original to the car i would rebuild it.

going to a C6 as an example will require a different floor shifter linkage, you can reuse the original FMX neutral safety switch under the floor shifter.

you will need a shorter drive shaft and there will be messing around find the correct U joints. the yoke will have to be changed as well from what i remember.

i think the trans cross member is the same for a fmx and C6, not 100$ sure. then there are things like the torque converter etc..

my car was original an FMX converted over to a C6 badly and i spent some years getting the correct parts to make it right.

 

[will e]

A lot depends on how you use the car and future plans. People swap in c4/c6 because they can be built to take a lot of torque. The aftermarket support is much better too.

As mentioned an overdrive is a nice feature. If you want to swap in a 3.50 or so rear end you can get the faster off the line but still have a nice cruise speed with a AOD.

If you plan on selling the car in a few years, no need to mess with the upgrades. It won't add value and may detract.

I would go with an AOD but I plan on keeping the car forever and I like it to be performance oriented. I originally swapped in a C6 because AOD's were not available and I already had a built C6 ready to go.

 

[Mikes73]

An AOD swap isn't all that bad but as mentioned you will need a shorter driveshaft. I ended up using the Lokar throttle valve cable setup, it was easy to adjust and works great. The AOD plays nice with my 3.73's. in my other Mustang.

 

[turtle5353]

Easiest and most cost effective thing to do would be stick with your FMX and have it rebuilt and reinstall it. But if you go for a swap, go with an OD tranny. Same amount of work goes into putting a c4 or 6 as it does an OD tranny.

 

[Omie01]

I had my FMX rebuilt and "bullet-proofed" My tranny shop had no issues getting parts, I have a 3000 stall converter, and it works VERY well!! It only cost me $800 for a complete rebuild, with shift kit and high quality parts!! I looked into a C4-C6 swap but felt all the mucking around just was not worth it. I think the FMX is a great transmission, and it has no problems with a 3:50 rear end. 2800 rpm at 60mph.

 

[cheezsnake]

So, while I'm researching an AOD conversion, I got some quotes on an FMX rebuild. One shop quoted $1500, and another quoted $1200. Those include pulling and reinstalling the transmission. If I pull and install the trans I can probably save a couple hundred. Prices too high? I'm guessing I could put in a rebuilt AOD for around $1500.

For those of you who have put in an AOD, I've read the following are compatible with the FMX setup ...

Flexplate

Cross member

Cooling lines

Shifter

And the following modifications are needed ...

Lokar cable system to carb

Shorten drive shaft (AOD is longer)

Different yoke

Possible clearance issues with header pipes (AOD is wider)

Does all that sound about right?

 

[will e]

The cost of having the shop pull and install the transmission is not going to be much compared to the rest of the bill. If you go with the FMX have them do it. They may not honor any warranty work if you do the R&R. Honestly, they can do it much faster since anyways.

 

[Mister 4x4]

So, while I'm researching an AOD conversion, I got some quotes on an FMX rebuild. One shop quoted $1500, and another quoted $1200. Those include pulling and reinstalling the transmission. If I pull and install the trans I can probably save a couple hundred. Prices too high? I'm guessing I could put in a rebuilt AOD for around $1500.

You're probably looking at around $500-800 for everything needed to swap in an AOD.

My answers are in bold below:

For those of you who have put in an AOD, I've read the following are compatible with the FMX setup ...
Flexplate nope

Cross member not without modification

Cooling lines nope - cooling ports are in different places

Shifter maybe - you'll need conversion parts since the AOD shifter lever is different than the FMX

And the following modifications are needed ...

Lokar cable system to carb not necessarily - get one from a junkyard car

Shorten drive shaft (AOD is longer) yes, by about an inch or so

Different yoke yes - along with a 'conversion' U-joint if you want to use your driveshaft

Possible clearance issues with header pipes (AOD is wider) yes it is, but it's not a problem

Does all that sound about right?

As I mentioned, mine came from an '89 5.0 Mustang, and the guy gave me everything AOD from the car - the only thing I didn't get was the throttle/TV cable bracket from the engine, because he still needed it for the throttle cable. He gave me:

• AOD
  
• shiter
  
• TV cable
  
• neutral safety switch/reverse light switch cable
  
• torque converter
  
• flexplate
  
• bell housing shield
  
• inspection plate shield
  

I made the AOD shifter work in my car, rather than klooge something together to make a stock '71 shifter work, which I would've had to buy anyway since my stock shifter was toast.

The AOD's TV cable was routed up to the carb, and I used a Mr. Gasket universal cable bracket to replace the factory bracket he kept.

I got a flex plate recommended to work with the AOD and 351 Cleveland - probably could've used the FMX or AOD 'plates, but I just went with something new.

I'm using the AOD's torque converter (don't forget, you'll need one of those, too.).

I had to splice the AOD's neutral safety switch and reverse light wiring to the FMX harness so it would be a clean set-up. The FMX's NSS and reverse light switches are in the shifter, whereas the AOD's are inside the transmission itself.

I hadn't found any '71 FMX-to-AOD conversion cooling lines, so I grabbed some new '71 FMX lines, and just cut them short of the tranny-side ends. Dave left me the cooling line fittings and some length of the AOD's cooling lines on the tranny (he just cut them when he pulled it out), so I flared both the AOD's lines and the new FMX lines I cut the tranny-side flares from (which came up short, BTW), and ran some 6" lengths of fuel injector hose with worm clamps. The cooling lines aren't under any ridiculous pressures or anything, so they should be fine.

I picked up the wrong aftermarket bracket - it simply didn't fit into the car correctly for my liking. I wound up notching the FMX bracket to clear the tranny pan - worked like a champ.

If you click on my garage (below my sig pic), there are pics of the underside with showing the AOD installed. You can clearly see the notched FMX mounting bracket, how close the exhaust is to the AOD, etc. If you check out the link to my Facebook album, you'll see more pictures of when I was installing the AOD into the car, along with some of the things I had to do to get it in there.

There's quite a bit to do to swap in an AOD because of technology and configuration differences. The big plus though is that the 1-2-Drive gears in the AOD are the same ratios as the FMX... but it has an overdrive gear.

 

[Mister 4x4]

I'm not saying "don't do it." Just letting you know what you're up against.

 

[cheezsnake]

Great information, Eric! Thanks for all the details. Very helpful. I'll be sure to check out the pics and link. As far as cost is concerned, I guess I was thinking an apples-to-apples swap, meaning a rebuilt AOD vs a rebuilt FMX, as opposed to comparing a used AOD vs the rebuilt FMX. In any case, sounds like more involved with the AOD than I thought.

The other thing to consider is how the rear end comes into play. My car has a 3.00. I suppose if you're running something like a 3.55 or 3.91, that overdrive gear would make a huge difference. I see that Omie01 is running a 3.50 and turning 2800 on the highway. As soon as I figure out this carb or vacuum problem I'm having at the moment, I need to check that out. But, using Omie01's numbers, mine should be running under 2500 at 60 mph.

 

[Mister 4x4]

I did some quick math using yet another gear ratio/rpm calculator I found and came up with AOD in overdrive at 65 with 3.00 gears should be running somewhere around 1700rpm. My 2012 Ram runs in that neighborhood on the freeway, with 395hp and gets something like 21-22 mpg all day.

http://www.wallaceracing.com/calc-gear-tire-rpm-mph.php

The AOD's final drive ratio is .67:1, which puts the 3.00 rear gears final drive ratio at an actual 2.01:1 reduction. I came to that conclusion using a regular calculator figuring that 3.00 multiplied by .67 equals 2.01. Using the RPM Calculator on the link above with 2.01 gear ratio, 26 diameter wheels, and 65 mph, it came up with 1688.40 rpm.

The reason this is acceptable to me (everybody seems to like their engines spinning over 2000 rpm at highway speeds) is because the '89 5.0 Mustangs came with either 3.25, 3.00 (most common), or 2.75s ('verts). Those cars came with significantly less power than my 400-ish (around 225), so I don't think mine will struggle to maintain the rpm and speed or anything.

Everybody also thinks a higher stall speed TC is needed, but I always ask 'why?' because the AOD ran just fine in the '89 5.0 it came from with that TC and 2.75 rear gears (since it was a 'vert). The need for the higher stall speed TC is lost on me because I'm not into racing (on the track, light-to-light, or otherwise).

I just want a good running car that won't pass everything but a gas station (since West Texas is literally the middle of nowhere and gas stations aren't plentiful between towns), and afford me a burn-out now and then.

I think I'll be OK. If not, I'll stuff in some 3.25s or 3.50s or something. No biggie.

 

[Bill73Ragtop]

If you decide to go with the AOD, invest in the AOD constant pressure valve body. Worth every penny since there will be no more worries about trashing the AOD if pressure is lost. Using the CPVB, the cable hookup will only determine shift points.