Another 3-2 downshift clunk dilemma

72Chevelle

Member
Joined
Sep 10, 2017
HI all. I see other threads about this, but I have a problem that is vexing me and didn't want to wind up being that guy taking over another person's thread.

I've picked up a (new to me) '81 Z28 with a (supposedly) well built 2004r in it. I'm a little familiar with these transmissions (I have one in my Chevelle that I got sorted out) but only on the surface - not an expert by any means.

Trans shifts great, at good speeds, firm shifts, fluid looks and smells great, pressures look good (below.) 3.73 rear gears. Speedometer is accurate (so mph references are accurate.) It has the 3-2 downshift clunk right about 18-15 mph as I'm slowing down to a stop. TV cable appears adjusted properly. Moving it doesn't really change anything, it still clunks. Clunk behavior is the same in either D or 3 when slowing down.

It has the Sonnax super servo in it, so I figured "great, they left out the cushion spring." So, I pulled the servo out, and it does have the cushion spring and bell in it. So, I checked the pin length. I have another Sonnax super servo in a box from my other 2004r excursions, so I used their directions, disassembled, put the spacer in, put it together, and the clearance was at 0.050 (used a dial gauge to measure and used a tool to push it in/out.) Sonnax instructions said to aim for .110 or so which seems quite loose to me, so I grabbed an extra pin, filed it down and got to .080 and figured I would try there (this way I still had the original pin and setup untouched.) Re-assembled with the cushion spring in place as it was before.

Same exact behavior. Everything works great, just that crazy 3-2 clunk. :(

It has a very heavy cushion spring in it. I do have some other cushion springs (most are heavy, but I have one lighter), along with the shim that Sonnax gives you to increase the cushion spring rate (which I did not put in.)

Any ideas where to look next to figure out what could be causing a 3-2 downshift clunk? I don't know much about the internals of this transmission, and the PO is clueless as he got it from a friend who had it built by someone for another build, a trade happened, etc. etc. etc. So, tracking down the innards isn't easy.

Here are the pressures:

Gear / TV not connected / TV connected at idle / TV at full pull
P 100 / 105 / 260
R 160 / 170 / 310
N 100 / 105 / 260
D 100 / 105 / 260
3 100 / 105 / 260
2 250 / 250 / 250
1 250 / 250 / 250

Thanks!
 

72Chevelle

Member
Joined
Sep 10, 2017
An update: I've been playing with it, and haven't found any solution to the downshift clunk yet. Played with the TV cable some more, no real change.

But, I have discovered that it has a very tight converter in it. It is supposed to be a 2800 stall (and does appear to flash up to that range) but it is very tight.

It idles at 940-960 RPM in Park/Neutral. When you put it in gear, it drops to 770-780 or so and wants to pull the car pretty hard right away.

I have tried lowering the Park/Neutral idle, but it doesn't run as well and when it goes into gear it drops into the 600's and wants to stall.

Could a converter that is too tight cause a 3-2 downshift clunk?
 

72Chevelle

Member
Joined
Sep 10, 2017
Another follow up after more seat time with it.

I have discovered that when the trans is cold, it doesn't clunk. Once it warms up is when the clunking on downshifts starts.

Also, once warm, I have discovered that it clunks audibly whenever putting it into gear (either drive or reverse) while stopped. Not as bad as the downshift clunk, but it's definitely ringing the bell when putting it into gear warm. And I have confirmed it is definitely coming from the transmission itself, not the rear end or the u-joints or anything.

But, I'm still at a loss as to where to look for something in the transmission itself that will cause a bang/clunk when putting into gear, a bang/clunk on downshift even with the cushion spring installed, and apparently only once the transmission is warmed up (and by warmed up, I mean fluid temps above 120.) Maybe a plugged passage somewhere, or something just not assembled right internally?
 

turbocamino1

turboholic
Joined
Jul 7, 2007
Mine was making a "Ting or Ping" type sound dropping into gear..was a u joint..pull the d/shaft to eliminate anything behind the trans..rears can make similar noises?
 

72Chevelle

Member
Joined
Sep 10, 2017
Mine was making a "Ting or Ping" type sound dropping into gear..was a u joint..pull the d/shaft to eliminate anything behind the trans..rears can make similar noises?

Hi. Thanks for the reply. Yes, I've already confirmed, the noise is coming from the transmission itself, and only once the transmission is warmed up. And it still has the nasty downshift clunking only on what appears to be the 3-2 downshift.
 

87geeinn

Buick and AMG pilot
Joined
Jan 17, 2006
Mine has done this as long as I can remember...10+ years or so on a Brian Hofer transmission still going strong and gets beaten on regularly. Send it.
 

72Chevelle

Member
Joined
Sep 10, 2017
Mine has done this as long as I can remember...10+ years or so on a Brian Hofer transmission still going strong and gets beaten on regularly. Send it.

While I have no doubt that the transmission would probably last and survive, the downshift clunking makes the driving experience absolutely suck and makes me not want to drive the car.

I cringe every time I slow down to a stop, waiting to hear and feel the inevitable "ka-bang" from the downshift clunk. It mystifies my why it is still there, even with the cushion spring in place. I was hoping there would be something I could look at/check myself, but so far, nothing short of pulling it out, sending it off and having an expert go through it and sort it out. Which I may just do since it has good hard parts in it (supposedly.)

But, I was really hoping to find something I could try on my own first before going that route.
 

72Chevelle

Member
Joined
Sep 10, 2017
Your min. TV pressure seem a bit high, I don't know if that has much affect.

I just saw HyperV6’s update. He seemed to be having a similar problem as I am, and made a few changes and seemed to resolve his issue.

Since I don’t know much about the innards of this trans, and since I’m quite unhappy with it, I’m willing to try some similar experiments.

To the trans experts out there - what determines the min pressure? Is there something I could change from just dropping the pan to lower the min pressures somewhat just to see if that is my problem? I am open to changing just one thing at a time to see what makes the difference and report back.

Thanks in advance for any ideas or tips to try.
 

INEEDAGN

Seen Your Member
Joined
Feb 28, 2003
You can lower minimum pressure by dropping the pan and removing the pressure regulator valvetrain with an internal snap ring pliers (from the bottom of the pump) and swapping to a weaker PR spring and reassembling. Might not help the 3-2 clunk but would definitely reduce the banging into gear. How much horsepower is in front of this trans again?
 

72Chevelle

Member
Joined
Sep 10, 2017
You can lower minimum pressure by dropping the pan and removing the pressure regulator valvetrain with an internal snap ring pliers (from the bottom of the pump) and swapping to a weaker PR spring and reassembling. Might not help the 3-2 clunk but would definitely reduce the banging into gear. How much horsepower is in front of this trans again?

It's a big block that I was told dyno'd at about 550 hp and 600 ft lbs of torque (at the flywheel.)

Is there a place where I can purchase an assortment of PR springs? Since I'm not sure which one is in there (my first guess would be Sonnax parts since it has a Sonnax servo, but that is just a guess) it would be nice to have a bunch on hand, so I can try one and see what it does to the pressures, and then adjust from there to keep decent pressures but also see what it does to the behavior.

I know these transmissions are very pressure sensitive, so maybe it won't take much of a drop at all. And if the PR spring alone doesn't help, I could also try what HyperV16 did and swap to a smaller boost and/or reverse boost valve too (I just found an exploded diagram and see all have to come out together to change the PR spring anyway.) Of course, I don't know what boost valves are in it, so when I do the first swap of the PR spring, I will measure them and go from there.
 

INEEDAGN

Seen Your Member
Joined
Feb 28, 2003
Weaker spring/bigger valve is better than a stronger spring/ smaller main boost valve. Spring basically sets min pressure. Boost valve affects rise. Let it have a lower base pressure with a more aggressive rise off idle
 

72Chevelle

Member
Joined
Sep 10, 2017
Weaker spring/bigger valve is better than a stronger spring/ smaller main boost valve. Spring basically sets min pressure. Boost valve affects rise. Let it have a lower base pressure with a more aggressive rise off idle

Thank you, that is what I was thinking as well.

I found a website (www.thespringstore.com) where you can specify the parameters of the spring. Based on what I've seen posted, I was going to order up a few different springs of the appropriate length, circumference, wire diameter and wire material to give a few different lb/in ratings in the same range as the Sonnax one (both lighter and heavier since I have no idea what is in there) and then use them to adjust the minimum pressures. Right now, I'm at 100 for min pressures, I'm thinking shoot for 80-90 and see how the trans reacts to that?

Also, for the boost valves, which is better, the ones with O-rings, or the ones without O-rings?
 

INEEDAGN

Seen Your Member
Joined
Feb 28, 2003
Try the tci purple spring 374300. O ring or non o ring is builder preference. I don’t use them but many swear by them. It’ll work just fine either way
 

HyperV6

Member
Joined
May 5, 2004
72Chevelle, you and I seem to have the same problem with the 3 to 2 downshift clunk. I've changed the pump pressure regulator spring and remove the Sonnax sping. I also reinstalled the stock BRF TV boost valve and reverse boost valve. The clunking is greatly reduced but not totally gone when I have the T.V. cable set to the optimum position. In 2022 after I get some racing on my trans I will probably but in the other / #2 short pressure regulator spring in to try it. I am thinking spring #2 will give the lowest pressure at idle. I need to invest in a transmission pressure gauge to know for sure. I got the two other pressure regulator springs from the transmission builder. I did take measurements and used the spring calculator on www.thespringstore.com website for my three springs. Below is the measurements and lbs/inch of my three springs. I did not have access to a spring compressor which would measured the actual force.

Spring: wire diam: outter diam: free length: # of active coils: LBS/INCH
Sonnax .050 .520 1.915 10 8.648 (Hard bang on 3 to 2 downshift)
#2 short .048 .525 1.940 10 7.027
#3 long .048 .525 2.004 10 7.027 (In car now - light bang on 3 to 2 downshift, T.V. optimum)

Spring #2 and #3 calculated to as the same LBS/INCH at 7.027. After I installed the #3 spring I think it actually has a bit more force than spring #2 due to the length. *** Not sure if the calculations below are correct multiplying the actual spring length by the LBS/INCH but I think in makes sense....***

Sonnax length 1.915 X LBS/INCH 8.648 = 16.560
#2 short length 1.940 X LBS/INCH 7.027 = 13.632 (.45 LBS less than the spring in the car)
#3 long length 2.004 X LBS/INCH 7.027 = 14.082

I would suggest you call Dave Husek at 516-285-1103 and see if you can buy a pressure regulator spring from him. He was not the builder of my transmission - My builder sent me the two springs. Many people on this site say Dave is very knowledgeable and helpful. I think you will be much closer to getting it right instead of buying a hand full of various springs. Please post your results and maybe we can get both of our transmissions for clunking.

Thanks, Craig
 

72Chevelle

Member
Joined
Sep 10, 2017
72Chevelle, you and I seem to have the same problem with the 3 to 2 downshift clunk. I've changed the pump pressure regulator spring and remove the Sonnax sping. I also reinstalled the stock BRF TV boost valve and reverse boost valve. The clunking is greatly reduced but not totally gone when I have the T.V. cable set to the optimum position. In 2022 after I get some racing on my trans I will probably but in the other / #2 short pressure regulator spring in to try it. I am thinking spring #2 will give the lowest pressure at idle. I need to invest in a transmission pressure gauge to know for sure. I got the two other pressure regulator springs from the transmission builder. I did take measurements and used the spring calculator on www.thespringstore.com website for my three springs. Below is the measurements and lbs/inch of my three springs. I did not have access to a spring compressor which would measured the actual force.

Spring: wire diam: outter diam: free length: # of active coils: LBS/INCH
Sonnax .050 .520 1.915 10 8.648 (Hard bang on 3 to 2 downshift)
#2 short .048 .525 1.940 10 7.027
#3 long .048 .525 2.004 10 7.027 (In car now - light bang on 3 to 2 downshift, T.V. optimum)

Spring #2 and #3 calculated to as the same LBS/INCH at 7.027. After I installed the #3 spring I think it actually has a bit more force than spring #2 due to the length. *** Not sure if the calculations below are correct multiplying the actual spring length by the LBS/INCH but I think in makes sense....***

Sonnax length 1.915 X LBS/INCH 8.648 = 16.560
#2 short length 1.940 X LBS/INCH 7.027 = 13.632 (.45 LBS less than the spring in the car)
#3 long length 2.004 X LBS/INCH 7.027 = 14.082

I would suggest you call Dave Husek at 516-285-1103 and see if you can buy a pressure regulator spring from him. He was not the builder of my transmission - My builder sent me the two springs. Many people on this site say Dave is very knowledgeable and helpful. I think you will be much closer to getting it right instead of buying a hand full of various springs. Please post your results and maybe we can get both of our transmissions for clunking.

Thanks, Craig

Thanks - this info is very useful. I know my current pressures, but I don’t know what spring is currently in there.

It is cold here now, so I won’t be doing any work on the car until next spring.

My plan was to pick up a bunch of springs of different pressures so that when I do take it apart, I can measure what is in it. I would then be able to select a spring of lower pressure and give it a shot. Right now, my min pressure are right about 100 psi. I’m gonna shoot for 80-85 first I think and see how that behaves. Every change I make I’ll measure the pressures and keep track of the change.

But, since I don’t know what is in it for a spring, it is hard for me to buy just one spring and have a proper one on hand to try without being lucky. So I don’t mind buying a handful of springs to give it a shot. It’s all part of the learning process and they aren’t that expensive. Plus, if I try one and it’s too light or still too heavy, I will have more on hand to experiment with.

I‘ve talked with Dave in the past a few years ago about another 2004r I have in another car and he’s been very helpful. Should I want to get another one built (I have a few cores sitting in my garage, including a last known good working CQ that is still all factory) I’ll probably be ringing him up for a build.

As soon as I have some results I will definitely post them here, but I expect it won’t be for a few months unless we get a couple nice warm days and there is no salt put down on the roads yet.
 

TexasT

Texas, Where are you from
Joined
Sep 10, 2002
I'm looking fwd to the results . Nothing quite like the trans fluid bath to get there either. Keep up the good work.
 
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