67 C4 slams into Reverse from P or N - Ford Muscle Forums : Ford Muscle Cars Tech Forum
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post #1 of 16 (permalink) Old 12-20-2018, 07:14 PM Thread Starter
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67 C4 slams into Reverse from P or N

The C4 has a transgo shift kit installed.
After years of dealing with the condition i was advised it was worn check ball-balls in the valve body or a broken low-reverse servo spring.
I plan to pull the valve body to replace the check ball-balls with plastic ones.
The trans is a 6 cylinder C4. Yes the engine has a cam which requires a 1000 rpm curb idle. The trans has a looser converter.
Does the valve body have 1 ball or 2??
While the body is down how can i tell if the spring is broken??
Since i have the valve body out would you pull the cover of the low-reverse servo piston & replace the piston & another spring to be sure. Thanks Bill

2003 lightning daily driver.
1967 mustang 200 six cylinder, with many mods.

Last edited by wsa111; 12-20-2018 at 07:16 PM.
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post #2 of 16 (permalink) Old 12-20-2018, 07:46 PM
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Re: 67 C4 slams into Reverse from P or N

The high idle is likely the major cause of the slam into reverse,
tho there can be other things too.

Try this...

With engine at idle, pull the shifter down into manual low. All the way down to "1".

Allow idle to settle, then, quickly put the shifter to reverse.

Still harsh into reverse? or better?

Report please.
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post #3 of 16 (permalink) Old 12-21-2018, 01:22 AM
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Re: 67 C4 slams into Reverse from P or N

Never seen a broken reverse servo spring. As for checkballs if the valvebody is infact a 67 with the PRND21 patter it will most likely have 2 checkballs. But that may depend on exactly which kit has been used
I'm not buying that as a reason though , very much doubt the checkballs will change anything.
Where is the reverse band set at ?
How much clearance in the high/reverse clutch?

As mentioned at 1000 rpm idle will be a big factor

Any slack in the driveline / Uni joints ? Diff?
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post #4 of 16 (permalink) Old 12-21-2018, 08:13 AM Thread Starter
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Re: 67 C4 slams into Reverse from P or N

If i put the shift lever into L, 2nd or D & then put into R there is no problem.
I should mention the idle speed in Drive is 800-850 rpms.

2003 lightning daily driver.
1967 mustang 200 six cylinder, with many mods.
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post #5 of 16 (permalink) Old 12-21-2018, 08:17 AM Thread Starter
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Re: 67 C4 slams into Reverse from P or N

Quote:
Originally Posted by gregaust View Post
Never seen a broken reverse servo spring. As for checkballs if the valvebody is infact a 67 with the PRND21 patter it will most likely have 2 checkballs. But that may depend on exactly which kit has been used
I'm not buying that as a reason though , very much doubt the checkballs will change anything.
Where is the reverse band set at ?
How much clearance in the high/reverse clutch?

As mentioned at 1000 rpm idle will be a big factor

Any slack in the driveline / Uni joints ? Diff?
Greg, no slack in driveline. I have adjusted the bands, by the book & even tighter.
I have no idea on the clearance in the high/reverse clutch.

2003 lightning daily driver.
1967 mustang 200 six cylinder, with many mods.
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post #6 of 16 (permalink) Old 12-21-2018, 09:38 AM
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Re: 67 C4 slams into Reverse from P or N

I need Greg to "have my back" on this because I am going from memory. Reverse has the highest reaction to line pressure of any of the "gears" in the transmission. Part of this "may" be caused by what has to take place to change directions. High Idle speed creates higher line pressure and that increases harshness even more. The "stall" converter should soften engagement "some" but obviously not enough in your case. You "might" want to replace the pressure regulator spring ( back to the original) since it was probably spec'd for a V8 which could benefit from higher line pressure ( durability wise). Greg could shoot all my ideas down and that is OK. He is more well versed than I am . I've "maybe " done 10% of the amount of transmissions he has. I wish he wasn't so darn far away!
Randy

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post #7 of 16 (permalink) Old 12-21-2018, 12:51 PM Thread Starter
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Re: 67 C4 slams into Reverse from P or N

Are there different low reverse servo springs depending on the engine??

2003 lightning daily driver.
1967 mustang 200 six cylinder, with many mods.
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post #8 of 16 (permalink) Old 12-21-2018, 06:15 PM
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Re: 67 C4 slams into Reverse from P or N

Ok, not sure if this will help... but here goes...

What happens when you select reverse in a C4?

2 things have to happen, the high/reverse clutch has to apply, and the low/reverse band must apply.

Here's the deal...

Those 2 things happen "almost" at the exact same time.

BUT... If the high/reverse clutch applies first, that gets the mass of the gear train spinning.

A fraction of a second later... the low/reverse band grabs and stops the spinning gear train. WHAM!

The trick is to get the L/R band to apply FIRST. this holds the gear train.

Then bring on the high/reverse clutch, hopefully in a gentle manner.

If clearance is tight in the high/reverse clutch, and the low/reverse band is adjusted on the loose side,
you will get a harsh reverse, even on a stock trans.

When I build a C4, or any trans with a L/R band, I adjust the band fairly snug.
Sometimes as tight as 1/2 turn.
I believe the factory adjustment is 4 turns. Waaaaay too much.

I also fuss with the high/reverse clutch clearance.
Generally going for as loose as I feel comfortable with.
Usually around 0.070" to 0.080" clearance.

"Normal" rule of thumb on clutch pack clearance is 0.010" per clutch.

Soooo, with a loose Hi/Rev clutch clearance, and tight L/R band clearance,

What happens is the band applies first (very important) and then the clutch comes on, giving a nice feel into reverse.

With a modified trans (shift kit) all bets are off.
It depends on the guy that installed the kit,
and whether or not he did any "tweaks" or "tricks"... or even if he installed it correctly.

Plus the afore mentioned clutch and band clearances. How did he set it up?

A good test to try...

You can try tightening the L/R band adjustment really snug so it locks the band "on" at all times and see how reverse engages.

DO NOT DRIVE THE CAR!!!! ONLY DO THIS AS A TEST!!!!

Basically, you are locking the band "ON" and then the only thing applying in reverse, is the Hi/Rev clutch.

If it is still harsh, you need to get out a pressure gauge,
and see what mainline pressure is in reverse, BEFORE going into the valve body.

Knowing the pressure ahead of time lets you make an informed decision about what to modify in the valve body,
instead of just guessing about what to do.

HTH.
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post #9 of 16 (permalink) Old 12-21-2018, 06:47 PM Thread Starter
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Re: 67 C4 slams into Reverse from P or N

Thanks will check the front band adjustment. I will back off 1-2 turns & see what happens.

2003 lightning daily driver.
1967 mustang 200 six cylinder, with many mods.
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post #10 of 16 (permalink) Old 12-21-2018, 07:23 PM
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Re: 67 C4 slams into Reverse from P or N

Quote:
Originally Posted by wsa111 View Post
Thanks will check the front band adjustment. I will back off 1-2 turns & see what happens.
The front band adjustment has absolutely zero to do with reverse.

Tighten the rear band.
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post #11 of 16 (permalink) Old 12-21-2018, 11:33 PM
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Re: 67 C4 slams into Reverse from P or N

Quote:
Originally Posted by GT350HR View Post
I need Greg to "have my back" on this because I am going from memory. Reverse has the highest reaction to line pressure of any of the "gears" in the transmission. Part of this "may" be caused by what has to take place to change directions. High Idle speed creates higher line pressure and that increases harshness even more. The "stall" converter should soften engagement "some" but obviously not enough in your case. You "might" want to replace the pressure regulator spring ( back to the original) since it was probably spec'd for a V8 which could benefit from higher line pressure ( durability wise). Greg could shoot all my ideas down and that is OK. He is more well versed than I am . I've "maybe " done 10% of the amount of transmissions he has. I wish he wasn't so darn far away!
Randy
I reckon you're right on it ... The term shift kit is very loose . It could be of a couple different types .
The 1000 rpm idle sure wouldn't be helping. As you suggested a stock pressure spring may help.

Also the term "loose" converter is very broad. Depends what you have . You really won't see much cushioning of the shifts till the stall speed is 3000+

I set rear bands at 2 to 2-1/4 turns out . The high /reverse clutch depends how many clutches but usually no more than .045-.050 if 5 or 6, if only 4 a tad tighter.

I'm never far away , I actually have visited Anaheim many times . Not sure when next trip is but usually go to a few cruise nights , donut derelicts etc
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post #12 of 16 (permalink) Old 12-22-2018, 09:30 AM
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Re: 67 C4 slams into Reverse from P or N

The other thing you could do is "always" select a forward gear (Drive) before selecting reverse.
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post #13 of 16 (permalink) Old 12-22-2018, 04:30 PM Thread Starter
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Re: 67 C4 slams into Reverse from P or N

That is what i do to prevent the condition. Thanks for your help & comments.

2003 lightning daily driver.
1967 mustang 200 six cylinder, with many mods.
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post #14 of 16 (permalink) Old 12-24-2018, 05:00 PM Thread Starter
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Re: 67 C4 slams into Reverse from P or N

If i remove the valve body can i air test the low-reverse band servo to check the spring or???
I don't plan to R & R the trans so unless i find something so be it.

2003 lightning daily driver.
1967 mustang 200 six cylinder, with many mods.
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post #15 of 16 (permalink) Old 12-24-2018, 05:16 PM
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Re: 67 C4 slams into Reverse from P or N

Quote:
Originally Posted by wsa111 View Post
If i remove the valve body can i air test the low-reverse band servo to check the spring or???
I don't plan to R & R the trans so unless i find something so be it.
Yes, with the valve body removed you can air check the L/R servo,
as well as the forward and direct clutch packs, and the front servo.

It is very un-likely that the spring will be broken in the L/R servo.
And even if it was, it's further un-likely it would cause the slam into reverse.

Seriously, just do the test I posted above.
It's so simple to do.
Tighten the rear band... check how the trans engages.
That will help narrow down the possibilities.

If you continue to ignore advice from someone that has 45+ years rebuilding and trouble shooting transmission problems...

Well... then I can't help you anymore...
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