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My C6 shifts through the gears really early (under normal acceleration) putting me in high gear at very low RPMs and low speeds. It also stays in a high gear rather than down shifting at slow speeds, downshifting requires a harder than normal acceleration.

 

So any recommendations on what I can do to delay shifting until higher RPMs and get it to downshift earlier?

Jim

 

M code 71 Mach 1, 351 4V Cleveland, Ram Air (not factory), C6 Trans, 3.5 rear

 

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Jim,

Did you check the transmission vacuum lines to the modulator for leaks at the modulator itself and in the engine compartment where it connects to the intake vacuum tree? This modulator controls the shift points based on engine vacuum. You could have a bad modulator or it's out of adjustment. If you pull the hose off at the modulator and look inside the modulator there is a flat adjustment screw. In keeps in gear longer, out gets it out of gear sooner. Every turn should make about 2 mph difference. Have you had this issue for a long time or has it just recently started?

-john

-john

(jbojo)

351C 4V cc heads, 10.5 : 1 CR, 290 Herbert cam, Flat top forged pistons, forged connecting rods, Atomic efi,

C6 with Gear Vendor overdrive, 3.89 Tru Trac, Hooker Super Comp with 2 1/2" Pypes Exhaust.        

 

Some Mod pictures can be seen at: [button=http://www.7173mustangs.com/forum-garage?filterxt_uid=2026]Bojo's Garage[/button]

 

 

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+1 on jbojo's post regarding the modulator. The length of the "pin" that the modulator engages in the transmission is critical in determining part throttle shift point. A longer pin causes a higher shift point. Average length of the pin is about 1.680. Min. is about 1.625, Max about 1.750. These values assume a 1966-1973 C6 with a screw in modulator. WOT shift point is controlled by the governor. Less weight causes a higher shift point. This mod is a bit tricky and requires a trail and error approach. Rebuilders are notorious for using what ever governor is handy rather than the correct one for the specific car/engine combination. To my knowledge there is not "kit" available to recalibrate the Ford governors like there are for GM governors. Let us know what progress you make. Chuck

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something to remember,, not all modulators are adjustable.

 

originally Ford did not have adjustable modulators, they had modulators with paint stripes on them. Green, White, Purple, Orange, black,, etc...

each one of these were set for a specific Vacuum to activate, there was also 2 versions sea level and high altitude(like for cars in Denver).

there was also screw in pre late 72 and push in late 72 and on...

 

that said you can pop off the vacuum hose to the modulator and with a flash light peak inside, if you see a flat head screw inside the port then you have an adjustable modulator. if nothing inside the port then you have a non-adjustable modulator.

 

you can change a modulator, but be prepared for a flood of transmission fluid. do not lose the pin for the modulator, when you swap them fast the pin might fall out or get stuck in the old modulator. so get a big tub under the modulator if you swap it and have trans fluid ready to refill the transmission as you will loose fluid fast just like a drain plug, make sure you fill it with correct fluid after if the transmission was rebuilt then don't use type F.

 

at any rate lets say your modulator is adjustable.. use a small jewelers flat head screw driver, and it will be hard to get in there around the cross member.

 

turning the screw In keeps in gear longer, turning the screw out gets out of gear sooner.

 

there is a range and you can customize it to how your motor is making Vacuum.

 

 

it is kind of a pain to keep adjusting the modulator since you have to get back under the car to make a change and that usually means jacking the car up.

i remember i did it about 4 times and finally just left it as good enough even when i felt the shift was still a little short. of Course my transmission just blew up so all that work was for nothing.

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Jim,

Did you check the transmission vacuum lines to the modulator for leaks at the modulator itself and in the engine compartment where it connects to the intake vacuum tree? This modulator controls the shift points based on engine vacuum. You could have a bad modulator or it's out of adjustment. If you pull the hose off at the modulator and look inside the modulator there is a flat adjustment screw. In keeps in gear longer, out gets it out of gear sooner. Every turn should make about 2 mph difference. Have you had this issue for a long time or has it just recently started?

-john

 

John, I haven't checked anything, I'll have in on a lift this weekend to try and determine where my rear end leak is coming from so thought I'd try and tackle this while on the lift. It's been an issue since I've owned the car, not a sudden one, it shifts great just shifts to early. Hoping backing the adjustment screw out a few turns resolves it. Thanks for the information.


+1 on jbojo's post regarding the modulator. The length of the "pin" that the modulator engages in the transmission is critical in determining part throttle shift point. A longer pin causes a higher shift point. Average length of the pin is about 1.680. Min. is about 1.625, Max about 1.750. These values assume a 1966-1973 C6 with a screw in modulator. WOT shift point is controlled by the governor. Less weight causes a higher shift point. This mod is a bit tricky and requires a trail and error approach. Rebuilders are notorious for using what ever governor is handy rather than the correct one for the specific car/engine combination. To my knowledge there is not "kit" available to recalibrate the Ford governors like there are for GM governors. Let us know what progress you make. Chuck

 

Thanks for the additional information Chuck, I'll let you guys know what I find out this weekend.


something to remember,, not all modulators are adjustable.

 

originally Ford did not have adjustable modulators, they had modulators with paint stripes on them. Green, White, Purple, Orange, black,, etc...

each one of these were set for a specific Vacuum to activate, there was also 2 versions sea level and high altitude(like for cars in Denver).

there was also screw in pre late 72 and push in late 72 and on...

 

that said you can pop off the vacuum hose to the modulator and with a flash light peak inside, if you see a flat head screw inside the port then you have an adjustable modulator. if nothing inside the port then you have a non-adjustable modulator.

 

you can change a modulator, but be prepared for a flood of transmission fluid. do not lose the pin for the modulator, when you swap them fast the pin might fall out or get stuck in the old modulator. so get a big tub under the modulator if you swap it and have trans fluid ready to refill the transmission as you will loose fluid fast just like a drain plug, make sure you fill it with correct fluid after if the transmission was rebuilt then don't use type F.

 

at any rate lets say your modulator is adjustable.. use a small jewelers flat head screw driver, and it will be hard to get in there around the cross member.

 

turning the screw In keeps in gear longer, turning the screw out gets out of gear sooner.

 

there is a range and you can customize it to how your motor is making Vacuum.

 

 

it is kind of a pain to keep adjusting the modulator since you have to get back under the car to make a change and that usually means jacking the car up.

i remember i did it about 4 times and finally just left it as good enough even when i felt the shift was still a little short. of Course my transmission just blew up so all that work was for nothing.

 

Thanks for the information. You mention not using type F fluid if the transmission was rebuilt, I'm not sure how I would know that since I'm the third owner. So what's the safest fluid to use if one doesn't know if it's been rebuilt or not?

Jim

 

M code 71 Mach 1, 351 4V Cleveland, Ram Air (not factory), C6 Trans, 3.5 rear

 

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.

suck on the vacuum line going to the modulator . . if you can suck air, the line or the modulator is bad . . the modulator is for fine tuning . . adjusting it will make very little change.

 

if the modulator is good then you ave a problem because it obviously should downshift on its own.

 

installing a shift kit will change the shift points too.

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I lean the other way. Always use type F fluid unless you know that it was built with '76 and later dex/merc clutches and bands.

 

Black stripe was supposed to be the most aggressive modulator. I got a couple from rock auto. The fram modulators I got were adjustable.

1973 Mach 1 Q code 351 4V, 9A paint, standard interior, 3.50 rear, C6 trans.

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Good points noted above. One thing you may run into is that (at least in my experience on my car), the exhaust runs close below the modulator location. So after making a quick drive around the block after the initial adjustment you decide -- 'maybe another turn or two in' and immediately jack up the car to make the adjustment...

 

That exhaust pipe at that location is HOT! Can't tell you how many times I scorched my forearm reaching up and over to adjust the screw....

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Ok, shifting issue resolved, shifts at higher RPM's now and downshifts sooner. I replaced the vacuum tree on the engine,( it appeared a little corroded inside) replaced the vacuum hose which was showing it's age and two turns on the adjusting screw of the modulator. Not sure which one of them it was but the fix was easy and it's fine now.

 

Still have to deal with the leaking pinion seal (?) on the rear end and while under there discovered an axel leak onto my wheel and brakes, same one I had repaired last year. I'm just going to let a shop (different one) take care of those. Good thing is no leaks from the C6 or engine and it's shifting at the right points now. It always seems as soon one thing is fixed, new issues pop up.

 

Thanks for all rapid and great advise provided by everyone, that's what makes this my favorite hang out. Every time I have an issue, there's someone out there that has a solution.

Jim

 

M code 71 Mach 1, 351 4V Cleveland, Ram Air (not factory), C6 Trans, 3.5 rear

 

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It never ends. Not sure whether to replace the drive shaft or try and have this one repaired.

 

Trouble.jpg.3af0471787f55387bb919ccf612463f8.jpg

Jim

 

M code 71 Mach 1, 351 4V Cleveland, Ram Air (not factory), C6 Trans, 3.5 rear

 

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If there is a local shop that can make and balance a driveshaft, they can replace the end easily and at a lot less expense than a whole new driveshaft. What does the other end look like? Chuck

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If there is a local shop that can make and balance a driveshaft, they can replace the end easily and at a lot less expense than a whole new driveshaft. What does the other end look like? Chuck

 

Chuck, the other end is also in bad shape, guess that explains the slight drivetrain vibrations I would randomly get but could never pin down. Yes, there's a local shop that advertises they repair drive shafts and balances them. I did some checking and CJpony has new balanced ones with front and rear u-joints for $254.00, guess I'll get an estimate from the local shop to see which way to go.

 

http://www.cjponyparts.com/driveshaft-52-steel-v8-1971-1973/p/DS2/

Jim

 

M code 71 Mach 1, 351 4V Cleveland, Ram Air (not factory), C6 Trans, 3.5 rear

 

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If the local shop has a good reputation and they are anywhere close in price to the CJ price I'd go with the local shop because they are more easily held accountable if there is a problem/vibration. Just my opinion.

 

Chuck

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If the local shop has a good reputation and they are anywhere close in price to the CJ price I'd go with the local shop because they are more easily held accountable if there is a problem/vibration. Just my opinion.

 

Chuck

 

yup!

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Retube and balance for mine was $330 from Capitol Driveshaft, Jim.

1973 Mach 1 Q code 351 4V, 9A paint, standard interior, 3.50 rear, C6 trans.

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Retube and balance for mine was $330 from Capitol Driveshaft, Jim.

 

Thanks Pat, that's who I'll be getting an estimate from on Tuesday.

Jim

 

M code 71 Mach 1, 351 4V Cleveland, Ram Air (not factory), C6 Trans, 3.5 rear

 

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They're pricey... but the cheaper alternative in Yelm no longer does good work.

1973 Mach 1 Q code 351 4V, 9A paint, standard interior, 3.50 rear, C6 trans.

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OK Capitol Driveshaft is taking care of the driveshaft for $292.00, should be done first of next week but today I found out I need to replace both Axels. Both axel bearings were spun which scarred the axels. Plus who ever changed the last bearings cut into the axel on one of the axels. Bottom line need new axels and while I'm at replacing the 3.00 (highway gears) with 3.5

 

Here's the damages caused by who ever removed the last bearings.

 

Axel1.thumb.jpg.2008b99abbf73aad8fe80be4ab06123a.jpg

Soooo here's the specs on the axels I need to replace. 2.8" O.D. bearings, Approximately 32" right side axel and 28" left side, 28 spline. I noticed there are some on Ebay but my concern is they be as scarred as mine. I've reached out to Don to see if he has any.

 

On a side note the last time I had a axel seal replaced the shop used a 31 spline seal according to the part number and they told me it I was wrong when I thought they were 28 spline, but I was right. Not sure what the differences in the seals are if any.

Jim

 

M code 71 Mach 1, 351 4V Cleveland, Ram Air (not factory), C6 Trans, 3.5 rear

 

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.

If you are referring to those two lines on the machined part of the shaft, don't worry about them . . if the bearing fits more than finger tight on the axle, it will be fine.

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Jim, If that is the story that needs to be told to get to the 3.5:1 gear I understand:P. As Barnett said those two marks are "mullet marks" left by the unskilled use of a cold chisel trying to remove the bearing ring. If you do need to remove the bearing ring to change the bearing, use a drill bit smaller in diameter than the width of the ring and drill a divot in the ring. Do not go so deep as to touch the axle. Now using a cold chisel, placed across the width of the ring, strike the chisel with the hammer with enthusiasm. The ring will break and the bearing can be pressed off.

Make the gear change a "While I'm here I may as well.....".

 

Good Luck,

Chuck

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28 and 31 spline small bearing axles use the same seal.. they are the same size at the bearing area.

 

Those grooves likely are what allowed them to leak, since they are in the area the seal rides on.

 

Alloy's from Summit racing aren't too bad price wise.

 

http://www.summitracing.com/parts/msr-a28-71must/overview/year/1973/make/ford/model/mustang

1973 Mach 1 Q code 351 4V, 9A paint, standard interior, 3.50 rear, C6 trans.

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Jim, If that is the story that needs to be told to get to the 3.5:1 gear I understand:P. ... Make the gear change a "While I'm here I may as well.....".

 

Good Luck,

Chuck

 

Chuck, you have me pegged! Why put it back together unless it's what I really want. Never let a need for a repair go to waste.

 

toredown.jpg.6b529b2a7058738cc764d811e7d8aa59.jpg


28 and 31 spline small bearing axles use the same seal.. they are the same size at the bearing area.

 

Those grooves likely are what allowed them to leak, since they are in the area the seal rides on.

 

Alloy's from Summit racing aren't too bad price wise.

 

http://www.summitracing.com/parts/msr-a28-71must/overview/year/1973/make/ford/model/mustang

 

 

Pat, thanks, that looks like a way for me to go so I can get it back on the road.


.

ok, didnt think about the seal riding there . . you can fill the groove with mig weld then just get the area machined on a lathe.

 

I thought about repairing them but both axels have scarring and groves around the seal and bearing areas, I'm just weary of going through the cost and effort trying to repair them and then having them leak again. They did their service over 43years, and obviously have been repaired (beat on) more than once, time to retire them.

Jim

 

M code 71 Mach 1, 351 4V Cleveland, Ram Air (not factory), C6 Trans, 3.5 rear

 

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