To flush, drain and fill, or hope and pray?

Looking for the hive mind opinion.

Just bought a 1997 2-door Tahoe. 350 and the 4l60e with 269,000 miles....

Actually runs pretty good, no smoke, good power. (Shocks and brakes were gone). After a decent cleanup, oil change, new plugs, new shocks and brakes, she's going to be a decent hauler. But.......

I'm not sure about the transmission. Shifts ok, really firm on 1-2 when cold. Pretty decent when warm, but kinda slow when shifting from reverse to drive. Plan on a 400 mile trip tomorrow morning and shake out the cobwebs. Fluid is dark. Not too smelly. No idea if it's the original transmission or even the fluid.....

What would you do? Change the fluid? Flush and fill? Grab a bag of popcorn and watch her blow? If this shakeout trip turns out ok, I'm going to tow my 6x12 cargo trailer with it. (About 3300 lbs in a giant square box).

I'm ok if it goes as I'm kinda planning on replacing it with a beefier unit in time, but want to keep that as far down the road as possible.

What does the hive mind think?

-Rob ( response will be delayed from me as we drive to Dallas and practice karts tomorrow)

change the fluid and filter with some of the latest Dexron fluid (i think they are up to 6 now)... don't flush it.

There is more than one method of flushing an automatic transmission.

What I've been doing to transmissions for years is draining the fluid, changing the filter, refilling the fluid, and before starting the engine, disconnecting the cooler hoses and setting them in a drain pan. Then I start the engine and let it pump out all the remaining old fluid. As soon as you see new ATF come out of the cooler hoses stop the engine, reconnect the hoses and set trans fluid to correct level.

For a trans as old as the OP's, a bottle of Lucas transmission stop slip snake oil couldn't hurt.

HappyAndy wrote: There is more than one method of flushing an automatic transmission. What I've been doing to transmissions for years is draining the fluid, changing the filter, refilling the fluid, and before starting the engine, disconnecting the cooler hoses and setting them in a drain pan. Then I start the engine and let it pump out all the remaining old fluid. As soon as you see new ATF come out of the cooler hoses stop the engine, reconnect the hoses and set trans fluid to correct level. For a trans as old as the OP's, a bottle of Lucas transmission stop slip snake oil couldn't hurt.

I've done that too - but I take the return and stick a hose+funnel in it and keep pouring new juice in until I see clean stuff coming out the cooler feed line.

It is amazing how much fluid a modern truck trans takes. IIRC my Tundra took something like 17 quarts.

I expect the Allison in my chevy to be a monster capacity unit. It's due the minute the weather allows me to work outside in short sleeves.

I agree with the above (a manual drain and fill is fine too, if a lot more tedious) but I would avoid the Lucas trans-fix stuff until the fluid change makes no difference. It can work as a last resort but I wouldn't put it anywhere it didn't have to be.

As long as you do not under any circumstances do the "POWER FLUSH" (high pressure machine flush) you should be fine. THOSE will blow out every seal in your trans and stir up all kinds of crap in a hurry...

I wouldn't flush it. I would drain, change the filter, and refill.. and then after a couple of weeks, do it again. this should get 90% of all the gunk out of the trans without hurting seals, bearings, or dislodging anything of the gunk that is holding stuff together

Really firm shifts often mean that the computer is jacking things around to compensate for slipping shifts elsewhere... And it's usually a firm 1-2 because of a slipping 3-4.

Flush the fluid, you've got little to lose. It'll either buy you time, or it will make the trans shop happy that they're working with a clean trans instead of one filled with burnt goo. (And DON'T tow in overdrive!)

While working for a GM dealer I "rebuilt" a lot of transmissions with a simple filter and fluid change. The filter is the important part. It gets plugged and reduces the flow and pressure.

I towed with my KJ in overdrive all the time. It simply down shifted when needed.

SlickDizzy wrote: As long as you do not under any circumstances do the "POWER FLUSH" (high pressure machine flush) you should be fine. THOSE will blow out every seal in your trans and stir up all kinds of crap in a hurry...

I've done probably 200 trans flushes on cars ranging from 30k miles to 300k miles. NEVER had an issue. If its a decent flush machine it regulates the pressure to whatever the trans pump is putting out to the cooler. (Ive seen between 5 to 60 psi). I say flush it. If it dies, its better dead in your driveway than out on the road where it has to be towed.

iceracer wrote: I towed with my KJ in overdrive all the time. It simply down shifted when needed.

4L60s don't run fluid through the cooler when in 4th gear. Most of the heat in the transmission comes from the torque converter. If you're in OD and the torque converter unlocks for whatever reason, you cook the transmission. It's not normally a problem when not towing because the loads are so much less, so you're less likely to unlock AND you're putting less load on the torque converter when unlocked.

The 700R4 which the trans is based on would not allow 4th gear in WOT at any speed, unless you had a Corvette. So it made sense at the time.

Thanks for the explanation, knurled! I've always wondered why people didn't recommend towing in overdrive...

Is that a problem on the ford equivalent?

underpowered wrote:

SlickDizzy wrote: As long as you do not under any circumstances do the "POWER FLUSH" (high pressure machine flush) you should be fine. THOSE will blow out every seal in your trans and stir up all kinds of crap in a hurry...

I've done probably 200 trans flushes on cars ranging from 30k miles to 300k miles. NEVER had an issue. If its a decent flush machine it regulates the pressure to whatever the trans pump is putting out to the cooler. (Ive seen between 5 to 60 psi). I say flush it. If it dies, its better dead in your driveway than out on the road where it has to be towed.

But were these 200 cars that you continued to drive for years afterwards or customer cars that might have blown up a few months later without your knowledge?

Knurled wrote:

iceracer wrote: I towed with my KJ in overdrive all the time. It simply down shifted when needed.

4L60s don't run fluid through the cooler when in 4th gear. Most of the heat in the transmission comes from the torque converter. If you're in OD and the torque converter unlocks for whatever reason, you cook the transmission. It's not normally a problem when not towing because the loads are so much less, so you're less likely to unlock AND you're putting less load on the torque converter when unlocked. The 700R4 which the trans is based on would not allow 4th gear in WOT at any speed, unless you had a Corvette. So it made sense at the time.

This is great info Knurled. It answers a question I have always had about why my trans temps were higher in OD than in 3rd while towing. Since replacing my trans, I have towed exclusively in 3rd and have had no issues at all. Thanks for clearing it up!

dculberson wrote: But were these 200 cars that you continued to drive for years afterwards or customer cars that might have blown up a few months later without your knowledge?

They were not MY cars, but customers cars. Seeing that I've only worked at 3 shops in the last 17 years, I usually see the same car for a while. I've seen flushes help some transmissions and NOT help some others, but I've never seen it make one worse. I'm sure it can happen though.

I've never seen it happen.

The theory is that the valve body's spools and the bores they ride in are worn out and it is only varnish that is keeping everything sealed, and the new ATF washes away the varnish.

To which I say: So what. Modern transmissions don't have valvebodies like that, and even if it did, if the trans is that worn where it is only working because of some sludge, the pin was pulled a long time ago and changing the fluid isn't going to make it any worse because the damage is done.

Knurled wrote: ...if the trans is that worn where it is only working because of some sludge, the pin was pulled a long time ago and changing the fluid isn't going to make it any worse because the damage is done.

My thoughts exactly!

I ran tranny shops for 7 years. Allow me to give my opinion. Take it or leave it. None of my customers wanted to hear this, but its truth.

Flushes are a bit pointless and can actually do damage. The clutch material gets brittle and crusty over time. As parts of it wear off, it saturates the fluid (and filter). If you change all 15-ish quarts, its like changing dish water in the sink with fresh new hot soapy water... great for dishes, bad for ailing transmissions. What can happen is that the fresh fluid (with its huge capacity for dissolving things compared to the old) can quite literally dissolve the clutch material. I always recommend pan/filter and 5-6 quarts.

Put it this way... if it can be fixed with a flush, its not fixed.

Having said that, I don't think your issue is clutch wear. If you had excessive slipping, shift flares, or really slow cold shifts, I'd say clutches. 4L60Es have issues with the valve body. Several things to check:

1) you say intermittent 1-2 hard shift. That is almost certainly valve body. The piston bore gets galled and the valve sticks. Then when it finally shifts, it slams

2) There are a few holes in the separator plate that have check balls. The separator plate gets worn from the balls and sometimes the balls get stuck in the plate, or in rare cases go the whole way through. That could explain the slow R-D engagement.

3) The fact that things are different hot and cold also suggests another common 4L60E issue: the fluid temp sensor. You'll get really funny looks if you ask your tranny part supplier for a 4L60E fluid temp sensor because there isn't one. Its part of the internal harness. A semi-reliable way to check is with a good scanner that can read tranny temp. When it fails it usually reads -241 degrees or something like that.

Basically, if it were mine, here is what I would do.

Buy a reman valve body, internal harness, separator plate (be careful, many variations), and while you're at it, both the TCC and PWM solenoids. You won't need it all, but this is all cheap and pretty easy to access at home. There is nothing suggesting that you need TCC and PWM solenoids, but they're a common failure point and not to expensive.

If that doesn't fix it, then you probably have servo seals and a few thousand other little fiber seals and o-rings that have just given up. Have it rebuilt, but rest assured the shop won't be calling you saying "well we also need blah blah for another $800"

I guess what I'm saying is this... if you were having it rebuilt, its not much more in parts to update all of those other pieces since they'll fail soon anyway. I'm saying to try those first before involving professional help.

I will also say this: DO NOT sit back with popcorn and wait. It will never be cheaper to fix than right now. That is SO true with transmissions. Right now it might deserve a rebuild with a couple hard parts. Letting it go means you until it blows means heat and metal shavings.. .which means you'll need an input drum and shaft, a forward sprag, and possibly a new shell.

i was gonna add my $.02, but ^^^ that pretty much sums it up.

-J0N

Man I don't understand slushboxes at all...curtis73's post sounds like the "turbo encabulator" video to me

is there anything that can be done about the transmission not using the cooler in overdrive? I noticed this as well, and seems like a sadistic design feature. what was the advantage to doing that?

belteshazzar wrote: is there anything that can be done about the transmission not using the cooler in overdrive? I noticed this as well, and seems like a sadistic design feature. what was the advantage to doing that?

4L60E does, as do most of the ones I know. They are reduced flow, but with the TC locked it is still usually adequate. Non-lockup pressure is around 30-40 psi, lockup flow is more like 20-25