Doug Hampton

Hello Gentlemen,

I was hoping to get some input about an issue with a transmission symptom I am having that was apparent before and after I just got done overhauling my 4l60.

I had my transmission looked at and was told the band was worn. This made sense as trans had a horrible 1-2 shift. The other symptom I was hoping to cure was what I perceive as a flare up in 4th gear with the converter locked up. If you are driving 60 mph or so and poke the gas pedal fairly hard but not hard enough to make it down shift it feels like it flares up for just a second, increases line pressure and then grabs on. If you accelerate gradually but firmly it will not exhibit the symptom.

The car did this before and now after the overhaul. The trans works exactly like I hoped it would except for the flare.

Any thoughts?

Thanks!

Doug

mantycarguy

Could be a leak at the apply piston in the servo. This is the outer piston just under the cover. A cut oring here would cause the symptoms you are describing. Also a leaking 3/4 accumulator piston can cause this to happen. Check the servo oring first as this is the easiest

mrvedit

I would also check the servo pin length; too short a pin could cause a flare.
You can test before removing the servo - put a crow bar on the cover, depress it and measure/estimate how far it moves in - 1/16" to 1/8" is the suggested amount. Much more and your pin is too short.

If this clearance looks OK, then follow mantycarguy's suggestions.
BTW - The 3/4 accumulator is under the separator plate and therefore involved removing the valve body.

72blownvert

Most likely the 3-4 accumulator hole is worn out. Especially since it had this problem before and after overhaul. But it doesn't hurt to check the servo as well.

Doug Hampton

Well before I overhauled it I replaced the entire servo with an aftermarket "Corvette" servo to if it would improve my 1-2 shift problem and the flare. It did seem to help the 1-2 but only when it shifted with light throttle, it had no effect on the flare. After that I took it to a friend who drove it with a tech 2. He said the band was worn out/taking too much time to apply and it need to come out.

The overhaul kit I bought came with a 3rd servo that is in the car now, 3 servos with the same result. The servo bore was in great shape...

My tech that overhauled the trans measured the band apply pin and it is within spec.

The 3-4 accumulator that came out of the car was worn as was the pin. It was replaced with a sonnax pinless accumulator piston which has 2 seals on it and the piston bore was cherry...

After talking with my tech that went through the trans about your suggestions he is thinking it may be an input issue like a skewed Mass air flow sensor, calculating the load incorrectly. When he drove the car after first putting it back together it was pinging pretty badly on hard acceleration. After a couple of test drives that seemed to subside. I had experience the same thing when I first bought the but changing the fuel filter made a massive improvement as did cleaning the mass air flow sensor...Good to the point I thought it was fixed.

I truly appreciate your insight guys!

Doug

Doug

mrvedit

With almost any transmission problem it is best to hook a pressure gauge up, visible while driving, and observe the pressure when the problem occurs.
The pressure should go up as throttle increases, reaching 175+ psi under high throttle. If you have correct pressure, that indicates that the engine sensors are working and the PCM is command the right pressure.

Sounds like the 3-4 accumulator should be fine.
When you install a Corvette or other servo, the outer 4th apply piston is generally not included and you just reuse the old one. Or are you sure you tried different 4th pistons too?

Doug Hampton

You are right, I don not believe the outer piston was replaced. I am pretty sure the seal was in the kit and replaced. The servo cover is a new machined billet piece. It is easy enough to take a peak at.

Otherwise it sounds like it is time for a road test with a pressure gauge. I'll report what we find.

Thanks again for you input!

Doug

mrvedit

If there is enough clearance to remove the entire servo, just hold the 4th piston in your fingers and move it through its range in the empty servo bore. Try to feel whether it is always sealing. Then firmly twist it in your hands looking for cracks. Best, put in a new piston and seal.
Otherwise, I hope the road test with pressure gauge will be informative.

jimmyblue

iTrader: (11)

You need to distinguish between flare (bands / clutches
in the case) and TCC slip. You can scan for this. The TCC
(especially, stock) is set up to have a small continuous slip
and you can get on the wrong side of that with weather,
or just a happy foot and the TCC will slip before it gets to
the point where is unlocks by command.

Tip-in misfires will also unlock the TCC momentarily and
with a higher stall converter, this might feel like flare.

mrvedit

Jimmy: I thought the stock trans only slips the TCC during engagement and not once fully engaged. And replacing the PWM TCC valve in the valve body with an On/Off valve gets rid of that slippage; which is highly recommend with all aftermarket converters.
Please elaborate.

Also, I think the OP's slippage is more than this.

jimmyblue

iTrader: (11)

TCC slip learning seeks a low, non-zero slip (because
you can't close a control loop around a one-sided
goal; without negative slip possibility, you can't
settle to zero).

TCC duty is jacked by the learning to get "just enough"
TCC apply pressure. According to yesterdays' news, so
tomorrow with slightly different air you could see more
part-throttle locked slip than you'd like.

If you set TCC min duty to 98% or so, this kills the
effect of "slip learning".

mrvedit

Jimmy: Thank you for the explanation.

Doug Hampton

Sounds like I will be looking for a tech 2 on Monday as well.

I have never actually been able to differentiate if it is converter or 4th gear slip.

The servo cover can be removed but I do not believe the entire servo can be removed without dropping the back of the transmission down which my tech seems pretty practiced at.

I will be looking at the tcc slip and line pressure when I goose it!

Best regards,

Doug