C6 transmission whining when hot and losing gears

[viven44]

Hi all, looking for advise on how to approach this

You may remember but I swapped a 460 from a 86 F250 into a 78 Bronco recently and I've driven it may be 100 miles so far... I used the torque converter from the 86 pickup as it looked brand new vs. the torque converter that was on the Bronco... maybe this was a bad idea.. fluid and filter changed at the same time. Up until now I haven't had any issues.. Both 2WD and 4WD..

Unfortunately I have a problem now (both in 4WD and 2WD)... transmission was the only thing I hadn't touched on this Bronco.. except the fluid and filter change.. didn't mess with seals or pumps... but I really did expect problems as the previous owner went and did some aggressive mudding which I understand caused the radiator to clog up and cause the previous engine (a 400) to overheat (most likely the transmission as well). It was a coin toss.

The transmission whines when hot and losing all gears. It might limp along just enough to make it to my garage if close. Else, I have to let it sit for about 10 minutes and then I'm back to normal for a few miles.

I have verified I have good fluid pressure through the cooler when cold. Basically have a lot of fluid leaking out of both cooler input and output when the lines are loosened individually.

When hot, this behavior changes completely... there is very little fluid coming out of the cooler output... but when I loosen the cooler line input, I have good fluid flow out of the input and OUTPUT as well.... Why this behavior only when hot ?

video below shows the whine.. whining when in park or neutral but goes away in gear.. but I don't know if this is consistent all the time

https://youtu.be/VjilFjUqmAw

[mat in tn]

I would suggest running a line in lieu of the in-radiator cooler and see if the whine goes away. if it does then you may need a radiator. this does not exclude the trans but a clogged cooler can kill a trans.

[viven44]

Thanks Matt. Before I put everything back together, I did flush everything out with compressed air.

There is no clog when things are cold. It flows well. But maybe not well enough ? I happen to have an aux trans cooler. Maybe try that ?

[mat in tn]

absolutely. a simple short bypass will prove enough of a noise check and that is mostly what I was suggesting as a means of finding a direction.

[viven44]

I was thinking along those lines as well.

Just to be sure, will I test with the short bypass the entire time or just when it starts acting up ?

[mat in tn]

well. you explain it as an overheating transmission. so having more cooling is the most important thing. a simple bypass is only to prove the viability of the rest and see if the cooler might be the issue.

[viven44]

Another possibility I just thought of. When this Bronco was sitting for a few years, just about every vent hose on the drivetrain was clogged with mud dauber 'stuff'. Does the C6 have a vent ? If it does, and it is clogged, can the fluid build up pressure internally which causes cavitation and/or even flow issues from the oil cooler return ?

The transmission had all kinds of leaks before... like where the shifter lever installs. The O-ring was flat out missing. All these were breathing passages. When I got the vehicle, I noticed the old transmission dipstick was stuck open. It wouldn't go down all the way. I swapped out this tube from the 86 so the dipstick was a tight fit. Maybe it ran all that time without issues due to this and I have made the transmission "too tight". It has zero leaks anywhere.

The pump I assume needs suction ultimately (like a fuel pump)... maybe a wild hypothesis.

I have also been beating myself up on whether I set the torque converter correctly ? I thought I did. 3 clunks, measured the inset. etc, made sure it matched what I measured on both the trucks. Since its a issue 'when hot' maybe not a problem here.

[FordFETruck]

Yes, the C6 has a vent. It's about midway on the case if I remember right on the passenger side. It's a metal hole with a crimped over metal cap like you'd see on some differentials.

[viven44]

Thanks I think I found it. It is impossible to reach anymore ! Need the exhaust pipe out of the way.

[viven44]

Negative on both of them.

Tried venting with dipstick out and also tried a bypass hose at cooler. Pump whining starts when things get warm.

[mat in tn]

the biggest concern for me is not necessarily the whine as I have heard many autos that ran for all the time that I knew them and never knew them to fail. the issue is that your is failing. once warm it is failing to move until cool again. that might be in the torque converter. if it is slipping the clutches or band then it will not reset. if it's a servo failing then it will slip the operated band and only reset a little as long as band friction material exists. is there any chance that you are feeling the stall speed raise as you get it warmer? a symptom of a failed tc. also, this will not burn the fluid yet could contaminate it with metal. much of the heat generated in a c6 comes from the tc due to the way it "works" the fluid.

[viven44]

Called a local transmission shop that was recommended highly. They will do a C6 rebuild for $1635 parts and labor.. which seems like a great deal. But they are of course booked up for classic cars and trucks (due to inside parking needs) and can schedule me after June 1st.

[viven44]

Sorry I didn’t see this reply…


Can a failed TC act up only when warm ? Also would it cause pump to not pump fluid to the cooler ?

Reading some more... this could be a stator bushing wear problem. Anything in the install that could contribute to this ? Angles, etc ? Its a 460 swap and the transmission had to scoot back maybe 1/4 inch to help the motor mounts fit right… or is it just plain wore out from 1978 to now ? If you recall a while back I made a post about how I found rust on the transmission oil filter mating surface and Matt said that didn’t look like a good sign

Probably ambitious for me to think this old C6 can sit behind the 460 and do just fine

[viven44]

Well.. I did the unthinkable. I have been relying more on snake oils these days!

I had a hunch that the "whine when hot" had something to do with the fluid viscosity being lower so I removed some fluid and added lucas transmission fix in hopes to increase the viscosity. As I poured it down the whine went away and after turning off the truck and back on the gears restored immediately.

I went on a drive. The whine returned after a bit... but compared to before it still moves while whining with noticeable loss in power but gears have not been lost totally. It does take a bit more RPM to move the vehicle when this happens. After turning off the truck, and firing it back up the power returns immediately. Just posting these data points in hopes of better understanding whats going on inside. I just don't know what is inside that "black box".

[Gary Lewis]

I know that the C6 is susceptible to problems after sitting for some time.  I don't know if the seals shrink or what, but I've seen it where something like Sea Foam "cured" the transmission after a bit of driving.

Ken from NZ reported years ago that driving as fast as you dare go in Reverse tends to force fluid in places it wouldn't otherwise go, and I've heard people say they tried it and it helped.

So if it was my tranny I'd try all the tricks I could before pulling it and having it rebuilt.

[mat in tn]

changing the makeup of the oil this way may have just "conditioned" a seal or two by having a thicker consistency. I'm not sure if Lucas is a thinner or thickener or possibly a seal sweller. I have not tried it. but it may just be saying its time. as to the tc  then yes, maybe. thicker fluid will respond more positively inside a converter yet if this is happening then it's well worn. they do wear out and need to be rebuilt. most that are available are remanned.

[viven44]

Thanks Gary!

When I dropped the pan a while back, I found some atrocities in it. It was a LOT of sludge. It was really a coin toss. But I was too excited and kept moving! Get everything assembled and baselined and work down the problem list was my approach.

My other 460/C6 example in Big Blue 2WD is a completely different story. I know it is a rebuilt unit. Fluid is clear as can be and has a blue torque converter (those are my clues!) Anyway.... it shifts like a machine gun and there is so much more power in it I can tell. Just doesn't feel like a Stock C6.

The Bronco has the better 460 in it based on compression values and also a higher 3.5 gearing however all the power comes in when the vehicle is already in motion. From a dead stop, Big Blue 2WD is much stronger and I know all that loss in the bronco is in the transmission.

I am just hopeful that after a proper rebuild it would do justice to the 460 in front of it. I had doubts throwing an aged C6 behind it from day 1.

[viven44]

I agree !! Its probably time

After 1 bottle I am still not out of the woods. I don't have a no drive situation anymore but still a whine + loss of power when hot. I think one more bottle would hopefully add more to the transmission until its decommisioned.

I have a car show at my workplace this Friday and I was really hoping to take the Bronco  But Big Blue 2WD probably got jealous

[mat in tn]

hmmm. will big blue feel better towing the bronco home?

[viven44]

That would make Big Blue's day but at this point Big Blue 2wd should be at the car show! If I did go to the transmission shop I will need to tow the Bronco using Big Blue, or use my AAA. UHAUL's towing guide says I can't tow the Bronco with a 1984 F350 but thats kind of BS.