Harsh 2-1 Downshift '90 4T60.

[High Mileage]

Hey gang,

I added a 45,000mi '90 Ciera to my small fleet of A-bodies. The biggest issue is an earth shaking 2-1 downshift when stopping. All else is well with the tranny,as well as when it 1-2 upshifts. The mounts are good.

Note: When I bought this car,I was told it had a bad tranny. It was 6qts low on fluid.
I changed the filter and fluid. No sign of leaks,and the cars belly is dry after almost 500mi of city/hiway driving.
I'll bet it had a run-in with an incompetent Jiffy Lube.

Any hints on what to check for this harsh 2-1 downshift? Possible accumulator spring? It has been about 7yrs since I have messed with one of these tranny's.

Thanks,

Steve from Albany,NY/Ocala,FL 4 A-bodies.

[SilentWing]

If it was running that low on fluid chances are more likely than not the tranny is shot, especially if the previous owner ran it at all like that. Check the fluid and see if there's metal in it, only real way to tell that I know of outside of dropping the pan.

[turbokinetic]

The problem is either misadjusted TV cable AND/ or misadjsuted modulator pressure.

When coasting down, the engine vacuum is very high and this lowers transmission charge pressure through the modulator. If the modulator pressure is higher than it needs to be, there will be a hard clunk as id downshifts.

You can try to back the modulator down a little bit and see what happens, but if you go too far it will cause soft shifts.

If the TV cable is pulled out too far, it will cause the downshift to happen at too high a road speed, also accentuating the clunk.

Try these adjustments before condemning the tranny. I've seen them run with so little trans oil that it would not move at all; and not damage the transmission.

David

[High Mileage]

I do have an adjustable modulator laying around. Which way to turn the screw to turn the pressure down?

I'm taking the car to work tonite. No probs except for that harsh 2-1 downshift.

Thanks.

[High Mileage]

I do have an adjustable modulator laying around. Which way to turn the screw to turn the pressure down?

I'm taking the car to work tonite. No probs except for that harsh 2-1 downshift.

Thanks.

[turbokinetic]

Oh one more thing - the modulator its self can leak trans oil internally and let it suck into the engine. A slow leak burns drop by drop and you may not notice any exhaust smoke. This could be the way the oil disappeared.

[turbokinetic]

Strange I'm not getting any forum notifications.

There is an adjustment screw in the vacuum fitting. Turning the screw CCW (backing the screw out) tekes pressure off the spring and lowers the charge pressure.

[High Mileage]

You know,I pulled the vac line off of that junction on top of the plenum... Sure enough,the inside of the vacuum line is wet.

I saw that the inside of the end of the brake booster connection at that junction was wet the other day.

It sure would be nice if that orig modulator is the problem.

Later this afternoon I'll be putting that small, adjustable one in. It was in the other '90 Ciera for awhile. I installed it in that one when I did the tranny rebuild on '03. It now has a non adjustable modulator.

Thanks.

Steve from Albany,NY/Ocala,FL 4 A-bodies.

[High Mileage]

You know,I pulled the vac line off of that junction on top of the plenum... Sure enough,the inside of the vacuum line is wet.

I saw that the inside of the end of the brake booster connection at that junction was wet the other day.

It sure would be nice if that orig modulator is the problem.

Later this afternoon I'll be putting that small, adjustable one in. It was in the other '90 Ciera for awhile. I installed it in that one when I did the tranny rebuild on '03. It now has a non adjustable modulator.

Thanks.

Steve from Albany,NY/Ocala,FL 4 A-bodies.

[High Mileage]

Sorry about the double post,group. After clicking on Submit Reply,the site told me to wait and try again. ?

[turbokinetic]

Sorry about the double post,group. After clicking on Submit Reply,the site told me to wait and try again. ?

No problem. Something is all screwed up. I haven't got any forum notices in a week. People probably think I am ignoring them. :-(

[High Mileage]

[QUOTE=High Mileage;30571]Hey gang,

I added a 45,000mi '90 Ciera to my small fleet of A-bodies. The biggest issue is an earth shaking 2-1 downshift when stopping. All else is well with the tranny,as well as when it 1-2 upshifts. The mounts are good.

Note: When I bought this car,I was told it had a bad tranny. It was 6qts low on fluid.
I changed the filter and fluid. No sign of leaks,and the cars belly is dry after almost 500mi of city/hiway driving.
I'll bet it had a run-in with an incompetent Jiffy Lube.[QUOTE=High Mileage;30571]

--



Well,problem solved. Actually,2 problems solved.

I took the vacuum modulator off my other '90 Ciera,and installed it in the '90 mentioned above. Now the 2-1 downshift when coming to a stop is smooth,not earthshaking like before.

When I removed the suspected/bad modulator I turned it upside down. About and ounce of tranny fluid dumped from the vacuum port. (it's not supposed to have tranny fluid on that side of the diaphram)

I heard of this happening a long time ago. But,I was leaning towards a broken accumulator spring as the cause of the harsh downshift. That is what the cause was for the same problem with my '87 Euro with the 125C.

Thanks to that bad modulator,the Ciera sucked enough fluid from the tranny to make the previous owner post it on Craigslist as a car with a bad tranny. It sat there for a year with no responses.

It's mine now. Sorry previous owner. It's a 45,xxxmi survivor that's in good hands now.

Thanks David and others for the help.


Steve from Albany,NY/Ocala,FL 4 A-bodies.

[turbokinetic]

Great news glad you've got a survivor!

[Nick]

Anyone got a picture of this screw to adjust? I'm having the same issue with 42k on the car. every shift is okay except for 2-1 downshift around 18mph

[Duke George V]

Why would you downshift 2-1 at 18 mph?! That's more in the area of 3-2.

[Nick]

Why would you downshift 2-1 at 18 mph?! That's more in the area of 3-2.

whatever shift it is, it's hard :help:
slowing down is starting to feel like a task... release brakes 19mph and cruise until the thump, reapply brakes :sad:

[High Mileage]

If you have the smaller aftermarket modulator,there is a very small screw inside the vacuum port. You'll obviously have to pull the vac line off to get to it. The screw has a tiny hole in the center for vacuum. A small,straight blade screwdriver is all that is needed. But it has to be small enough to get about 3/16in inside the port.

If you have the larger factory modulator,I don't believe they are adjustable.


Steve from Albany,NY/Ocala,FL. 4 A-bodies.

[High Mileage]

whatever shift it is, it's hard :help:
slowing down is starting to feel like a task... release brakes 19mph and cruise until the thump, reapply brakes :sad:

What I did until I fixed mine was throw it in neutral before it banged into 1st when coming to a stop.

[Nick]

Thanks, I'll have to check out an adjustable modulator. No idea what it is but I'll have to find out lol. I checked the fluids and everything checks out fine there.

[High Mileage]

The vac modulator is that can looking thing that is on the side of the trans,pointing towards the front of the car. If you have a 3 spd,you don't have one.

It's held in with a hold down clamp like one used for a car distributor. I believe the bolt is 13mm.

Make sure the o-ring is there on the stem.

You might have to get a small piece of vac line to make the connection since the aftermarket one is a different size.

[turbokinetic]

Modulator is located on front of transmission, pointed towards radiator.
http://68.209.87.173/Temp/adjmod2.jpg

Inside the vacuum fitting, there is a small adjusment screw. Take off the vacuum line to gain access to the screw. You'll need a small "jeweler's screwdriver" to make the adjustment.
http://68.209.87.173/Temp/adjmod1.png

Hope this helps ya!
David

[Nick]

Thanks, I have the 3spd tranny so I guess I can rule this out :(