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Old 02-16-2005, 06:23 PM   #1
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Question AOD Transmission Guys: Harsh shift into Overdrive - what's the problem?

I have a 91 Gt that had an AOD in it when I bought it. I have since swapped it out for a t-5, however down the road I might swap it back to the AOD. When it shifted at lower speeds from drive into OD while letting off the gas it would clunk and shift extemely hard into gear. It was so bad that I didn't even like to drive the car. If you would stay on the gas it would shift decent, but still clunk, and if you were really on it in drive and it shifted into OD it seemed fine. I figured it was shot, so pulled I converted it to a t-5. I have since been told that it was possibly the TV cable or a 3-4 servo. But if it was the TV cable wouldn't it do the same thing in all the other gears? It shifted perfect into every other gear. Also I should note that when in overdrive and coming to a stop, it didn't want to downshift until I was almost stopped. If I pull the valve body and look at the 3-4 servo piston is there something I should looks for?
After taking the trans out I noticed it had a hardened input shaft and a 9.5" converter, and I was told that it was recently rebuilt, and if it's an easy fix I might down the road throw it back if I break the T-5.
Thanks for any replies
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Old 02-17-2005, 12:44 AM   #2
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To shift into OD, the OD band applies and at the same time the forward clutch
releases. If the clutch releases B-4 the band applies, the trans is in neutral
a short time. Under lower throttle, it might be lagging enuf and under not as
high a tv pressure so when it "catches" it clunks or bangs.

Another possibility, if the band applies B-4 the clutch releases, there's a temporary
hyraulic "park" situation where it tries to stop the input shaft from spinning till
the clutch releases.

I was thinking also that maybe the governor is sticking, but you're right it should
then affect all the shifts just like too high of a tv pressure. That throttle valve is also
known for sticking if it doesn't have the later stiffer spring, I think it's supposed to be blue.

So, the first thing to check probly IS that 3-4 shift valve. AODs are notorious for
sticking valves in the vb. I've had that happen twice after a rebuild even when
the vb was apart and very carefully cleaned and checked for free valve movement.
With the higher tv pressure and/or line pressure of higher rpms & full throttle, it maybe overcomes the valve sticking.

Do you know which valve that is? I've got a service manual showing a breakdown
of all the valves and parts if you need, I've got it scanned and can email.
macxx1@excite.com

If you're thinking of reinstalling that AOD at some point, here's some info that
may interest you as long as it's out.

http://www.lincolnsonline.com/article111.html
http://www.lincolnsonline.com/article80.html

Good luck!
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Old 02-17-2005, 12:46 AM   #3
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PS - maybe it's something as simple as the servo sticking under lower
tv or line pressure.

I was thinking along the lines of something wrong with the wave plate
in the forward clutch, but I've got the wave plate totally out of mine
and there's no apparent effect except a little firmer engagement out
of neutral and from OD back into direct like in kicking it down under
throttle.
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Old 02-17-2005, 10:28 AM   #4
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hardened shaft and 9" converter = street/strip unit

the V/B in all likelyhood has been altered with some form of shift kit and larger O/D servo...these are likely the culprits to the harsh shift.

why not just get a good, used V/B from a local tranny shop and start from scratch...that way YOU know what goes into it.

you should be able to pick one up for 50 bucks or less...
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Old 02-17-2005, 10:55 AM   #5
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The last time I had sticky valves, mostly the throttle valve, I bought a
reman vb for $85.

I think transact online has them available.

A used one, with the AOD's frequent valve problems, might not or
might have problems, too.

As far as shift kit goes, I wholeheartedly recommend
Baumann's kit. In the first place, it doesn't do stuff
to the vb that would do things like that. In the second,
it's adjustable for a number of different levels of
shift quality and shift points.

I've had 2 and wouldn't have anything else.
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Old 02-17-2005, 01:40 PM   #6
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I have the same problem. As a result, I manually shift into OD at all times. And, manually shift out of OD while decelerating. I suspect the 3-4 valve as well. Manual shifting has worked fine for me until I can get time to clean it up.

Nearly identical situation sounds like.
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Old 02-17-2005, 02:55 PM   #7
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It is my belief that the TV cable/setting is one of the main causes of OD clunk. After removing my upper intake and pushing the TV cable around a little bit, I put it all back together and my car started clunking. I moved/wiggled the cable and it didn't do anything, so I toyed with the TV setting until it went away. I ended up having to adjust it lower by about 2 notches or so until it went away.
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Old 02-17-2005, 03:03 PM   #8
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Thanks alot for the replies guys. Now when you say 3-4 shift valve do you mean the 3-4 servo in the case, or the valve in the valve body? I know where the 3-4 servo is, but I don't know where the 3-4 valve is if it's in the valve body, could you breifly explain so I have an idea where to look.
Thanks alot
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Old 02-17-2005, 03:57 PM   #9
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You would need an ATSG manual to find the correct valve or have someone scan the page so you could see what they are talking about.. I had the shift VALVE stick at one time.. The car had been sitting up for a few months and wouldn't shift into OD at all.. took out the valve and worked it in and out to free up the motion, reinstalled it with a gob of vaseline and it works great now.

After installing my Baumann shift kit I got the OD clunk but it was because I modified the valvebody to shift harder and faster so instead of a smooth soft shift into OD it now applies quick and will clunk if I accelerate to speed and then back off just as it shifts into OD.. If I am still accelerating a little when I get to overdrive it will not clunk or atleast not nearly to the extent... basically it is a normal occurance and isn't doing any damage.. it's just annoying.. If your VB is stock your TV pressure might be a little high.. try getting it checked out.. it shouldn't clunk under stock conditions.. everyone I know who has the clunk is because of a modified valvebody.
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Old 02-17-2005, 08:50 PM   #10
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Quote:
When it shifted at lower speeds from drive into OD
Ok, this is a no-no unless you;ve got some wild gearing in teh rear where you need OD to bring the rpm's down at low speeds. OD should never be used at low speeds, especially in an AOD. There is a momentary pause (loss of line pressure) when going from D to OD, and from OD to D. OD should only be used for highway speeds.

Quote:
After taking the trans out I noticed it had a hardened input shaft and a 9.5" converter, and I was told that it was recently rebuilt
Im willing to bet that the VB is either aftermarket, or someone put a shift kit in it and that is whats casuing the "clunking".

Depending on what you mean by "clunking", is may, or may not be a bad thing. For example.. with my Lentech AOD, if im on the highway and I start to slow down, and I hit about 50 mph, ill hit the OD delete switch (to turn OD off) and it feels just like downshifting a 5-speed and dropping the clutch to slow down.) It will make you lunge forward. There is no "clunking", but the downshift from OD to D is hard, fast and if you're not paying attention, it can catch you off guard.

I dont think there is anything wrong with your AOD. I think the parts that are in it, are the cause.
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Old 02-17-2005, 09:40 PM   #11
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Blk89GT- the car has 3.73's, and usually I did only use Drive when driving around town just so it wouldn't have to shift to OD, however if I was going over say 45 mph then I would manually shift to OD, and it would clunk into gear. All of the other gears shifted hard, and would jerk your head if shifting at a low rpm, but overdrive was different if it shifted at the same rpm as all the other gears. I mean it shifted HARD and something in the trans clunked. I am going to take the valve body off and check all the valves, and clean it out, and just look for wear. I'm also going to look at the TV cable, and everything. Thanks for all the replies. If anyone is interested in the trans, since there seems to be nothing wrong with it and also the converter, I'd be willing to sell it, and put the money toward a tremec or a G-force T-5. Otherwise I'll probably put it back in if I break the T-5 that's in the car, since it has good parts and supposedly less than 10K on it.
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Old 02-18-2005, 11:28 AM   #12
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there's noting "wrong" with the tranny right now...don't waste your time taking it apart...

this statement says it all: "All of the other gears shifted hard, and would jerk your head if shifting at a low rpm, but overdrive was different if it shifted at the same rpm as all the other gears. I mean it shifted HARD and something in the trans clunked."

the VB definitely has a shift kit in it and it has raised all pressures to levels that you find "unacceptable"...this is typical of kits that offer various "levels"...and yours is obviously the harder/hardest shifting that was attainable...

the clunking is just a "fact of life" for the 3-4 shift when going to higher pressures(due to the power flow directly off the motor thru the 4th gear)...there is actually a spring inside the tranny around the drum that is called the "anti-clunk" spring that reduces this sound/feel with stock pressures and apply rates...yadda,yadda...yadda

bottom line...tranny is fine...shift kit is too firm for your liking...

get another shift kit or valvebody to solve it...or sell the tranny
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Old 02-18-2005, 01:59 PM   #13
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351Freak- Good enough, everything makes sense. I just figured that it shouldn't be like that. If I do use the trans again, I will probably pick up another VB, that is if I use it. Again if anyone is interested in this trans with the converter and all the other auto parts that I have just PM me or email me at dzartman@nbn.net I would rather sell this trans that seems like it is built more for the "strip" than for the "street" to someone who it would suite better than me, and then I'll just buy a tremec.
Thanks for all the quick replies and advice
Derek
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