Help me figure out what's wrong with my C6

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jjr329

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Sorry if this seems kind of long...trying to include anything pertinent.

1973 Mach 1

351C

C6 - stock rebuild est around 2000, B&M 2400 converter installed in 2004 as well as a B&M shift improver kit. I estimate no more than 3000 miles on it in that time (most of that time the car was in pieces).

As long as I can remember I've had to manually shift the car. Specifically coming to a stop and then accelerating again the car would not be in the right gear and would stall, especially when cold. So I manually go from D to 2 to 1 when coming to a stop.

I should note that the shifter is "off" in the car. Meaning the selector doesn't actually line up perfectly (ok it's way off) with PRND21. The car only starts in N and not in P. It has a newer NSS circa 2009.

Lately the trans has been acting funky. I had a misfire and some other engine maint stuff going on so I wasn't really concerned until I fixed all the engine stuff. I did plugs, wires, rotor, cap and an oil change. Electronic ignition so no points or condenser to worry about. So I take it for a test drive and it seems to act more unusual than it had been. Specifically if I'm in 1st I'm not necessarily in 1st, I'm actually in 2nd. While accelerating from a stop being in 2nd but 1st selected after about 100ft the trans decides that hey you really wanted me in 1st and it dropped down to 1st and then let me shift into 2nd manually. And then not back down to 1st at the next light. It kept seeming like 1st was just a suggestion and if it felt like it, it would make it available, otherwise 2nd or 3rd only.

So I get home and think gee I haven't check the trans fluid in a long time. Mind you it does not leak (or at least didn't prior to today). So I did the standard practice for checking fluid level and there was nothing on the stick. It ended up taking 2.25 quarts to bring it back up to normal level (added slowly and after re-warming it up). I do have a cooler and a deeper pan if that matters any. Adding the fluid didn't change the behavior at all.

So today I figure I might as well get it over with and drop the pan to see how bad the damage is. First thing I notice is trans fluid on the garage floor, that's unusual as it's never leaked before. So I drain the fluid (pan has a plug) and it comes out looking perfect. Nice color and no contaminants and no burnt smell. Pan is as clean as a whistle. That's good news but not what I was expecting to come out given how it's been driving.

Drivers side of pan and linkage a little wet:



IMG_20130805_201510_821 by JJR329, on Flickr

Magnetic drain plug:



IMG_20130805_201918_144 by JJR329, on Flickr

And the filter, other than a couple "grains of sand" looks perfect:



IMG_20130805_211429_976 by JJR329, on Flickr

If you look in the linkage pic there appears to be something not even hooked up. Is that the kick down linkage?

I followed what looks like vacuum hose (looks pretty old) off the trans to a hard line that continues into a "T" that goes to the choke (I think) and the front metering block on the carb. I pulled the line and it was dry with no evidence of fluid being sucked in.

So I'm not sure where to go from here. What would cause the leak to suddenly appear now that it was filled? I checked it hot, went through the gears, and with it idling so I don't think it's overfilled. What do I do with that linkage? How do I fix the shifter alignment and can the start only in N problem be fixed?

Thanks!

 

caspianwendell

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You have to check fluid while engine is running and warmed up to get an accurate reading. I am no trans expert but could be modulator valve not functioning properly, but my best guess is valve body contaminated, or piston in valve body malfunction

 
K

Kit Sullivan

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Your description is kind of vague, but I have a few guesses:

1) sounds like your kickdown rod has not been hooked up.

2) if the shifter gear indicator does not line up with the gear you are in, the shift rod is not adjusted correctly. If the detent is off enough while you are driving, you will get severe wear.

3) your vacuum servo may be bad, possibly due to the backfire. I have seen backfires destroy them, or sometimes just blow the hose off.

6) pan gasket leaks are easy to fix...other leaks are more problematic. Determine where the leak is coming from.

7) check anti freeze in radiator for ATF leaking from trans cooler. Pink-ish foam is a sure sign.

A C-6 is a very durable trans. If it was rebuilt(correctly) recently then it may just need a few simple adjustments by a good trans guy.

Good luck!

 

jjr329

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I followed the standard operating procedure for checking the fluid, warmed up from driving, level surface, idling, go through the gears, etc.

1. My carb doesn't have the ford kickdown on it, so there's nothing I can do about that for the moment. That would only affect it dropping down from 3rd to 2nd on WOT, correct?

2. Correct, its way off. I don't even look at it, I go counting. Severe wear on what? The shift rods or something else?

3. Is that the small module on the back where I followed the hose to the carb? I was looking at replacements suspecting maybe its bad but there seem to be 4 kinds - green, blue, black, and red (adjustable) stripe. How do I know what I have? When I looked at it, it's color was "rust" lol.

6. The pan did not leak prior to filling it to normal. Is there something else higher up on the drivers side that could leak? I'm going to be doing a new filter and gasket anyway since I dropped the pan.

7. Cooler is not stock and is external to the aluminum radiator I have, so that's not it.

Thanks for the responses so far.

 

CZ-75

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Check for leaks in the line connections... it's not uncommon for the fittings to get loose.

 

Don C

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Higher up on the driver's side would be the seal for the shifter shaft, where it goes through the case. Pretty common on C6s for them to leak.

 

jjr329

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I'll check the lines on the trans and shifter shaft seals after work.

I'll try and describe it better as well. 90% of the time 1st gear is not available to me. Doesn't matter if its in D or 1. There is no grinding or unusual noises. If I come to a complete stop in D, the car takes off in 2nd. If I stop and put it in 1, the car takes off in 2nd. If I'm in 2nd manually and a select 1 (while slowing down) it doesn't shift into 1st. Occasionally and randomly 1st will work. If it's in D and 1st does decide to work, I would say it works at an improper time, meaning it should have been in 1st already and upshifting to 2nd, but instead it was in 2nd and then downshifted to 1st and is now winding out 1st which would make me shift to 2nd manually for fear of something breaking if I leave it.

 

Widowmaker00

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I would try changing the shift module, the thing you traced that vacuum hose to. Just to eliminate that as a possibility. I'm not sure which one you would need, but I'm sure someone here can tell you. They are inexpensive.

 

72HCODE

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1. My carb doesn't have the ford kickdown on it, so there's nothing I can do about that for the moment. That would only affect it dropping down from 3rd to 2nd on WOT, correct?

yes the kickdown throws the trans down into 2nd from 3rd when you stomp the pedal.

its for passing another car at highway speed. allows you to accelerate faster.

2. Correct, its way off. I don't even look at it, I go counting. Severe wear on what? The shift rods or something else?

maybe your shifter is just out of position because you have to constantly shift with it and the linkage got loose and is now mis-adjusted. easy fix put the trans in a known gear like park or neutral then loosen the one bolt under the car on the shifter linkage and move the shifter into the correct position on the floor board then re-tighten the bolt then check the shifter will shift correctly though all positions, verify your reverse switch is working with the reverse lights and then check neutral safety is working by starting the car in park or neutral.

you could have some kind of wear as well there are plastic bushings on the floor shifter base that could wear out but if you pop the shift linkage bolt and wiggle the shift rod around it will tell you how worn that is.

3. Is that the small module on the back where I followed the hose to the carb? I was looking at replacements suspecting maybe its bad but there seem to be 4 kinds - green, blue, black, and red (adjustable) stripe. How do I know what I have? When I looked at it, it's color was "rust" lol.

The shift rod that goes into the modulator you keep and reuse with a new modulator.

the modulators are adjustable with a small flat screw driver.

however you should test it first with a hand vacuum pump, use the pump to generate a 10-20 HG vacuum and make sure the modulator is not leaking or trans fluid is not coming out of it.

now the modulator has a big adjustment allowance. when working properly you take the car for a spin. get a nice straight road and accelerate briskly, you don't have a kickdown so you want to feel the modulator. if she shifts through all 3 gears instantly then you need to counter clockwise turn the screw in the modulator until you get the shift points you want.

if she takes forever to shift through the gears and your almost redlining the engine then tighten the screw clockwise until the shift points speed up.

that is assuming you don't have more problems which is possible.

Shift kits can cause problems. was it assembled correctly in the valve body, did a spring break, that can happen.

now you can pull down the valve body and take a look at it. maybe a fluid circuit is clogged up or somebody forgot a check valve when they put it together.

if you pull down the filter your half way to pulling the valve body out and checking it or rebuilding it. new gaskets and then you could inspect the springs or make sure things are installed correctly.

again what you told us is vague. also if you have a blown modulator it could show itself as a vacuum leak

the modulator is a vacuum sensor override to the transmission basically there is a mechanical aspect to shifting gears then depending on vacuum load the shift points are altered, finally the kickdown is the master override to everything. all highly adjustable.

6. The pan did not leak prior to filling it to normal. Is there something else higher up on the drivers side that could leak? I'm going to be doing a new filter and gasket anyway since I dropped the pan.

yes, the band adjuster and you have the linkage you could also have a leak from the back of the torque converter.

the only seal you can change is the band adjuster the other seals require the entire transmission to come out of the car because you have to remove the valve body and then you have to pull part of the clutch pack out to get to the fork adjuster and remove it to change the seals.

the vacuum servo is also on the drivers side and could leak.

on the passenger side you have trans cooler lines, dipstick as possible leak points.

the leak you described seems pretty bad, i don't think there is a vent hole on the drivers side if you over filled the trans. so you will need to really inspect where you think the fluid is coming from. you might have a cracked case. Actually there are some C6 that do have a vent hole on the drivers side there is a loose metal cap over it, its designed as a last resort if the trans is over filled. but you said it was low.

----

since the trans is open and drained pull the modulator then pull out the little rod insert it back in the modualtor then hook up a vacuum hose and suck on it, the little rod should move in and when you release move out. if there is no motion check the adjustment screw and see keep adjusting it until the rod moves if it still doesn't move replace the modulator and try again.

the modulator is tied really close to what vacuum your engine is making. some people retune the engine then do not adjust the modulator and it causes all kinds of shifting issues. if the modulator is turned off then the shifting takes forever and you might find yourself manually shifting the trans.

Again this is just an idea you might have mechanical damage.

 

jjr329

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The leak is coming from the selector shaft. The NSS was only hand tight, also the selector shaft outer seal looked like it wasn't seated right. So I pulled and inspected it, still nice and soft with no cuts or nicks and just put it back together for now. It sounds like the inner can be replaced with the trans in the car. To me it looks like both the inner and outer seals are leaking, so if I can find a kit I'll just replace both.

I didn't get a chance to play with the modulator. I messed around with the linkage and something still wasn't right. If I got the needle to line up with the center of any of PRND21 I then didn't have all the gears because the detents weren't lined up right. So I put it back the way it was for now. Can the shifter be adjusted inside the car as well? I noticed a nut under the shifter bezel. The shifter has side to side play, is that the bushings bad?

 
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72HCODE

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is the C6 original to the car? its like maybe somebody used the FMX shift rod or a C4 or something.

the shape of the rods are all a little different.

 

jjr329

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I was trying to adjust it myself. I'm going to have a buddy come over and try again. If I still have trouble I think I'll pull it out to take a photo so we can ID it. The cars had at least 5 owners and who knows how many different people that have worked on it over the years, there could be wrong parts on it.

I've been looking for new seals for the selector shaft. Chasing down part #s has been a pain. Finally found what should work on ebay, inner and outer for $8.

 
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72HCODE

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i had trouble getting C6 seals also i cannot remember the name of the company i used it was a long time ago now. many seals don't come with the rebuild kits.

I remember when i first got my car, it originally had a FMX trans somebody swapped to a C6. one thing i remember was the FMX shift rod was always hitting my floor board and it made the shifter stick out of park my reverse lights were all miss adjusted also, see they left the original FMX version of the floor shift with the neutral safety inside the car and not on the C6 underneath like usual.

the neutral safety was still factory adjusted and worked correctly but the FMX rod to the C6 was all wacky.

eventually a freind found me a C6 shift rod from another mustang. once i swapped it in the shifter worked right but i just had to do some adjustment with the bolt and the shifter had a sweet spot so i figured out put my trans in reverse and then setup the shifter inside the car in reverse so my reverse lights worked, then i taped the shifter in place and with the reverse lights still working went under and made a final adjustment. worked perfect after that. the shift rods for the C6/fmx look almost the same they are slightly different. the C4 is very different it is much shorter.

the sucky thing with my setup is the steering lock no longer works with the transmission lock because they just took a late model C6 and put it in with the completely incorrect shift lever for a mustang. just the main transmission lever is correct the rest is different and i would of needed to drop the trans to change the lever to a correct one for a mustang/cougar but i just said forget it.

 

jjr329

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I believe the car came with a C6. It is a Q code, although the motor was blown at the dragstrip by one of the previous owners and is now a 2V with Aussie Powerheads.

Shift rod is about 17 3/4" long:



IMG_20130807_212217_900 by JJR329, on Flickr

Bottom of shifter (nasty, and there were popcorn kernels in there as well):



IMG_20130807_212153_672 by JJR329, on Flickr

Connector bar:



IMG_20130807_212047_552 by JJR329, on Flickr

The bushing was paper thin and what was left just literally turned to dust:



IMG_20130807_212146_785 by JJR329, on Flickr

This bushing needs to be replaced too. It doesn't sit flush and looks like its starting to come apart:



IMG_20130807_212108_914 by JJR329, on Flickr

 

jjr329

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Ended up being the shift kit was not installed properly and the valve body gasket was bad (blown out between the two rectangular ports). Waiting for my buddy to come over to do some final adjustments but it moves again and I have all my gears back! Thanks for your help.

 

Bill73Ragtop

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"If she shifts through all 3 gears instantly then you need to counter clockwise turn the screw in the modulator until you get the shift points you want.

if she takes forever to shift through the gears and your almost redlining the engine then tighten the screw clockwise until the shift points speed up."

Shouldn't this be - clockwise turns the screw inwards (extending the shift points to higher rpm) and counterclockwise (out) speeds up the shift points ?

 
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I didn't read all of the response. Sorry.

I swapped out an fmx for a c6 in my 73 as well. Mine was from a torino. The shifter/transmission lever didn't match up. I ended up lengthing the shifter rod.

When my shifter bushings wore out I had the same problem as you. I could NOT get it to go into first from a stand still. Sometimes, in '1' at about 10 mph it would say 'oh, you wanted to be in first and would do so'. I think the pressure would build enough to allow it to slip into first.

I don't have a kickdown. Without the kick down my transmission shifts normally when driving normally. In drive it just acts like you would expect. But it will NOT kick down a gear at speed unless I move the shifter into 2 or, at slower speeds, into 1. (again, with the bad bushings it would sometimes not slip into 1 either)

When I did an engine pull I accidently disconnected the vacuum servo. If I remember correctly the transmission would not shift out of first but it has been a while.

My suggestion is you get the shifter aligned correctly. We ended up welding a little bit of extra onto the floor shifter to make it work. Also, as I did read, swap out the bushings... WOW what a difference that can make.

Once you have that set, then see how it runs.

 

MeZapU

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When my car had its original C6 I remember it being finicky trying to find the proper gear no matter how many times I adjusted the linkage. Turned out that the shifter itself had some play of about 1/4-3/8" of movement before the linkage would start to move. If you take apart the top of the shifter there is a nut that I had to tighten. The slop was gone and the detents then lined up perfectly with the PRND21 once a final linkage adjustment was made.

 
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