C6 transmission whining when hot and losing gears

[mat in tn]

the testing gauges for transmissions are usually able to read high pressure like 300 plus.

[viven44]

I've driven another ~50 miles without any incidents (knock on wood).

I did another 1/2 turn in (total 1 turn in so far) on the vacuum modulator, the 1-2 shifts are where I want it to be. I do have a little lag in 2 to 1 downshift. It won't happen unless I am more or less stopped. Kickdown lever will solve this but I'd probably want this to be addressed without using that.

And I did do a couple of good pulls in reverse.

[Gary Lewis]

Good progress. But the kickdown lever only is part of the equation at WOT. Otherwise it makes no difference.

[viven44]

Right... when you are wanting to accelerate fast from a near dead stop, unintentionally one can get close to WOT. It was nice having one on my 84' Bronco with a 302 which was a dog. It actually kicked down slightly before WOT to I had some unintentional and annoying downshifts. I also setup a kickdown lever on a 84 F150. It is kind of a tricky thing to make it work just right with the edelbrock carb so I want to avoid if at all possible.

[Gary Lewis]

That's why I've not bothered with a kickdown lever.  You can always pull it down via the shift lever and it doesn't make a difference until close to WOT, so why bother?

[viven44]

Agreed

[viven44]

Well transmission started acting up again… slipping. Dropped pan again. Found debris .. but not as much as last time. Chunks looked like soft material (band?). No metal chunks.

I replaced with fresh fluid (about 5.5 qts that came from the pan... I had just drove it a bit... usually after sitting there should be more fluid). No snake oil as there should be remnant from the heavy dose last time in the torque converter. No more slipping for now.

I guess repeat this a few more times until 1) it’s fixed , or 2) it stops moving altogether

[viven44]

I think we have hit normalcy for a while now.. but I cannot be sure ever. And the last time I dropped the pan, I realized this unit may have been rebuilt once already... so I don't think we are dealing with ancient internals.

My only issue at the moment is that the transmission is unable to downshift to 1 automatically. It was working ... sort of.. I'm having to manually drop it down.

I have the following working well
- nice engine braking in manual 1st gear, manual 2nd gear
- Automatically shifts to 2nd and 3rd
- I think it downshifts from 3rd to 2nd well

Is the only fix to remove the transfer case and maybe clean the governor ? I love the simplicity of the 2WD trucks

[mat in tn]

the governor normally regulates pressure relevant to up shifts but if there is a screen anywhere its possible to be clogged.

[viven44]

I think I'm going to drop the pan again shortly. Replace it with a 2WD pan with a drain plug and corresponding filter, more fresh fluid. The 4WD pan I have with its pick-up tube is picking up gritty fluid from the bottom due to the fluid turbulence. I don't need a 4WD pan to go to the grocery store and work ;)

About the downshift, I'm hopeful that with time it will work itself out. I've just been manually dropping to 1 and it has been working well. I feel like I'm driving a one-leg operated manual truck

I agree with the approach so far of cleansing the transmission. I think most people I've seen post online just commit transmissions to 'rebuild' when they hear slip without actually troubleshooting it. The troubleshooting process can cost some but at least I have the satisfaction of trying my best before letting someone tear into my truck and hoping they put it back together just right.

[viven44]

I found a good tip on how to adjust vacuum modulator in this book. The book says never take vacuum modulator adjustment for granted based on how it shifts.

I will definitely be checking line pressure at different vacuum levels
https://a.co/d/1VheMeK

[viven44]

I tested the fluid pressure, here is a video on it. This looks to be in spec with the current vacuum modulator...

https://youtu.be/XzCPPjZGiHA?si=PlZyKncwFdbDYH4D

[viven44]

Installed the transmission pan with drain plug and Motorcraft FT52 filter (without that pick-up tube).
https://shop.broncograveyard.com/C-6-Chrome-Steel-Transmission-Pan/productinfo/21104/

The pan isn't as shallow as the stock pan and actually needed the same amount of new fluid back as the old "deep" pan.

I must say everytime I dropped the pan, it looked cleaner each time. You guys were right I should have focused on getting new fluid in. I did add a small quantity of snake oil this time, hoping this is the final fluid change for a while.

The shifts are definitely firmer now, and I am getting better downshifts... but if I want anything better than this, I probably need a shift kit which is what I'm pretty sure Big Blue 2WD has.

If I get any sort of reasonable life out of this unit after seeing what all I got out of the pan, I gotta say the C6 transmissions are tough sunnagons!

[Gary Lewis]

The C6 is really one tough son of a gun.  Stands up to the 302 and 460 and everything in between.  Not the most efficient, but strong.  

Glad it is working for you.  

[viven44]

Thanks to all for the help and advice so far!!

[viven44]

Hi all, after about 1000 miles of driving I was happy, but I got really happy when all 4 BBLs of carb were unlocked. Sure enough, I think I have burned up the transmission in short order

After about 25 miles of driving today, stop and go, the 1st gear is locked up. It won't move if I manually select 1st gear or if it is in D, but shifts 2-3 and 3-2 just fine. The fluid temp is fine, it was hovering around 195 or so. The supercooling radiator is doing its job... but an auxiliary cooler may have prolonged the life possibly.

I have honestly been waiting for this day so I can get the transmission rebuilt. The amount of mud I got out of the pan I have been shocked to see how tough the C6 has been.... will add an extra cooler as well (i'm sure the shop will recommend this). If I can get it to shift like Big Blue 2WD's I would be very happy.

Well, needless to say I will drive Big Blue 2WD to the truck show. Probably for the best it happened before I set out long distance.

[Gary Lewis]

Yikes!  Glad it quite now instead of part way up here in just over a week.

You might consider a torque converter with less slip.  They are available and with the 460 I doubt you need much slippage.  The reduced loss will help on MPG and reduce the temp of the fluid.

[viven44]

Just to make sure I understand, you mean a converter with low stall speed, right ?

Sorry I'm bad on the terminology... I don't like automatic transmissions as I am a control freak and I feel I'm praying all the time driving one of those (obviously today ) and I wish they made new trucks with manuals.

[Gary Lewis]

Yes.  Monster has the C6 Torque Converter 1300-1650 Heavy Duty Low Stall torque converter.  I'm not recommending Monster as some have had problems with them IIRC, but the idea is the same.

[viven44]

OK I will ask for that style!

I wish I knew exactly what was in Big Blue 2WD. Even with the 3.08 gearing it is a beast. The torque converter is blue. That is all I know. I has been rebuilt, It most likely has a shift kit.

I'm planning to drive up Big Blue 2WD to the transmission shop and tell them "that's what I want".

The shop that will be doing the work is "Plano Automatic Transmission". They have been in business since 1966, the year C6 was introduced. The owner Jeff builds all the transmissions personally.... I think it will be in good hands.