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Discussion Starter · #1 ·
Built a strong C4 about 2 years ago with lots of help from the folks on this forum. It's still running strong, but since the beginning, it has always slammed pretty hard into reverse. This has never been a problem...yet...but I'm getting tired of it.

The U-joints and rear gears are tight, no sloppiness anywhere. The valve body was modded from one posters homemade version of the Transgo kit. All tolerances in the tranny were to spec when built. All forward gears are fine.

Any ideas what might be causing this? Thanks.
 

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It's probably a fault in the valve body.
 

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Discussion Starter · #4 ·
Idle is pretty low, maybe around 500-600. What would I look for in the valve body? Can't remember any concerns during the mod.
 

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The problem is most likely in the valve body. I can't put my finger on it, but I had a TCI manual valve body that did this. I now have a JPT valve body and it does NOT do it. The TCI valve body also banged second while down shifting from third. The JPT doesn't.
 

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At the outside end of the pressure regulator is a reverse boost valve inside the brass housing that comes out first when the press reg is removed.
If you look closely you can see the feed to it that comes from the manual vlave in reverse.
I used to in my home cobbled
valve bodies eliminate that circuit so there was no reverse boost.
Reverse boost may be bringing line pressure up too high to suit your setup.
 

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Where did you get the JBT valve body??
 

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Discussion Starter · #11 ·
JBT valve body? This was a stock '78 valve body modified thru advice from folks on this forum, primarily TimR. It is similar to the results of using a Transgo kit. The tranny works great with strong and firm shifts, just hits too hard in reverse.
 

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With the trans-go shift kit they supply a different booster valve.
Since the harsh reverse started with the installation of the trans-go kit would going back to the stock valve fix the harsh reverse engagement problem, but do you need the boost for the reverse circuit for proper reverse clutch or band engagement???? Bill
 

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On 2006-11-03 06:24, polkat wrote:
It's a 1978. Mario428, how did you eliminate the circuit?
I have been trying to remember the VB I modified is long gone. I am looking at a schematic and beleive what I thought was cutting off the reverse boost was wrong and I cannot help you, my apologies.
Check out the valving you changed and see if anything is stuck would be my next move.
 

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I did save the pics somewhere you posted Mario if that's any help.
I often wondered if drilling the seperator plate to increase the oil feed for 3rd would cause the hard reverse. i haven't had any real issues of too hard though

<font size=-1>[ This Message was edited by: gregaust on 11/7/06 4:11am ]</font>
 

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The bang into reverse is caused by too little cushion in the shift or too much clearance. You could make sure that your accumulator for the low reverse servo was in the stock configuration, try adjusting the reverse band snug and then back it out 2 1/2 turns instead of 3 turns, the direct clutches may have worn enough that they are too sloppy and allowing reverse to "bang" into gear.
I need to look at my flow chart to see what the configuration of the low / reverse accumulator is supposed to be. I almost alwas reverse the intermediate accumulator but I leave the low / reverse as is unless the tranny is for drag race only - you don't use reverse much and since you start in first it doesn't do a lot of banging because it is cushioned by the forward clutch.
When you shift low to reverse it changes from the forward clutch to the direct clutch and hits the reverse servo too. might be excess play in that direct clutch...
Paul
 

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Discussion Starter · #17 ·
Thanks Paul, I added extra clutches when I rebuilt the trans and measured clearances precisely. So far the trans has seen mostly street duty (I've been building another engine slowly over the last year, and didn't want to push the trans too hard until the engine is ready), so I don't think it's clutch clearance wear. Fluid changes are very clean. Forward shifts are nice, firm, and fast. I'll try the band adjustment you suggest.

<font size=-1>[ This Message was edited by: polkat on 11/12/06 4:04am ]</font>
 

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I always wondered why Ford designed the reverse boost into the valve body myself. Last time I had the pressure gauge on the Mustang, I remember 150 psi in all ranges, 275 in reverse, IIRC. I also remember a few years ago at a $5000 to win bracket race (cool weather in the 50's) that I blew the seal out of the low/rev servo when I went to back the car off of the trailer. I wanted to burn that C4! But, I fixed it that day at the track with a T350 Governor cover o-ring & went a round or two. Laying under the car at the track is not my idea of fun.
 

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Mavman,
did you blow the cover seal? The rear servo uses a molded lipseal on the piston.
If you get pressure spikes that blow the cover seal out of the rear servo the trick is to use a thin film of gasket maker (by Loc-tite) on the seal and cover surfaces when you assemble it. The gasket maker will actually make the rubber seal bond to both the cover and the case elliminating any leakage.
a thin film is between .010" and .020" thick. If that stuff gets into the valve body you will have no end to your problems. It can clog the screen on a C4 too - I cannot stress too much that only a thin film is needed.
Paul
 

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Yes it was the cover seal...sorry should have specified. Never thought about the Loctite trick. 'Course then again, we haven't had the problem since either. Thanks for the tip!
 
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