Allison MT643 Auxiliary Oil Cooler. Yes/No?

[Simplicity]

Forewarning, LONG (unintentionally) to reduce the amounts of questions upfront.

I'm in the midst of addressing all my drivetrain issues/concerns on my 2000 T444E / MT643 combo, RE 33' IC flat nose.

I've addressed the oil and air already. I just added an EGT gauge but haven't tested it because of my tranny.

My electric modulator has become the focus of another thread on here, https://www.skoolie.net/forums/f37/m...oad-34679.html. I tested the modulator in and out of the tranny per Christopher's (CadillacKid) instructions and no clunk. I'm awaiting a replacement modulator after an amazing offer from Christopher.

Even though I have checked the tranny fluid, my failing sense of smell and knowledge of what burnt fluid smells like failed to tell me my tranny fluid was a milk chocolate color. I'm not sure if this is actually burnt or simply old??

So, now I'm "planning" (replies from this thread may change that) to not only drain and change the spin on, but drop the deep pan and clean it out and change the internal filter.

Per other threads, it appears the MT643 does not have a torque converter drain, so I'm planning on going with Dexron III for about 5k miles to clean out as much of the old fluid and change it out to Transynd.

Besides the transmission not downshifting automatically (see my posts in above listed thread for details), it upshifts and downshifts in light load situations and manually shifts if needed with no backtalk from the tranny. So, I think that my tranny is still solid, but needs attention and as much help as it can to get "going" (key word) down the road.

That brings me to this question, should I add an auxiliary tranny cooler??

I read other threads about the importance of fluid operational temps and I'm like 98% sure my bus has an integrated tranny cooler with the radiator since it's a 2000 and my engine and tranny temps run pretty close to each other.

My tranny usually runs about 190, yet during my last modulator test run, it got up to 200-205 in 50 degree weather, which I understand it still well within normal. But, again, milk chocolate tranny fluid.

The bus has lived its whole life in Seattle with no high heat (schools out when it's "hot" (85-90) up here for a few days a year) and no passes to climb.

Since the MT643 is a wide range four speed, I'm imagining some higher temps when in hotter climates and long up-hill pulls.

If I were to put an auxiliary cooler in, I would not use a fan. I was thinking, if it's available, to use an internal thermostat and bypass set up to assure I don't run too cool (yes, there's such a thing as too cool).

If you made it through to this point, I dub you Knights of the Long Post. As always, your time, knowledge and feedback are greatly appreciated.

[TJones]

If your tranny fluid is "milky" in appearance you may want to pull an oil sample and see if coolant is present. If the transmission has an oil to water cooler it could be leaking into the transmission fluid. Also check your coolant for signs of oil.

Ted

[Jolly Roger bus 223]

yes check if you have an oil to water cooler.
dumb design to allow the two to mix if a failure.
word of caution and this is for older vehicles not specifically your 2000 but i have learned the hard way when you drop the tranny pan you do not clean out the particles/sludge in the pan. those are pieces of the bands in the tranny and should be floating in the fluid just like fluid in stores that have zinc addittive? if dont take it away you dont have to add it

[s2mikon]

If your fluid is already contaminated do not add a cooler now. It will become contaminated too. Fix the transmission first.

[Simplicity]

I get that before I think about adding a tranny aux cooler I need to make sure I don't have bigger issues. I'm already on that track.

My original question about an aux cooler is based on if your suggestions all check out. I want to take advantage of having the system drained so I can install a cooler if I choose to. Any input about the original cooler question is appreciated.

I'll be sending my transmission fluid and coolant for testing.

As of now, each fluid has a consistent look to it, meaning I don't see any streaking of coolant / tranny fluid floating in either.

I'm a bit perplexed by the leaving the sludge in the pan. Seems to me if there are band particulates floating around, both filters would grab them.

I "think" what you're saying is the band particulates have a similar affect as tranny additives do, so why take the sludge out only to have to add an additive.

I work for a school district as a bus driver, so I'll talk to my mechanics and show them a sample of my tranny fluid and see what they say.

Thanks!

[s2mikon]

If it looks like a chocolate milk shake order some fries. Fried clutch plates.

[s2mikon]

Thank you for your service as a bus driver!

I always wanted to drive a school bus myself. But they insisted I had to take the children. So I declined.

[Mountain Gnome]

Originally Posted by Jolly Roger bus 223

yes check if you have an oil to water cooler.
dumb design to allow the two to mix if a failure.
word of caution and this is for older vehicles not specifically your 2000 but i have learned the hard way when you drop the tranny pan you do not clean out the particles/sludge in the pan. those are pieces of the bands in the tranny and should be floating in the fluid just like fluid in stores that have zinc addittive? if dont take it away you dont have to add it

This is news to me. I have for 20 years cleaned the bottom (inside - and out also, around bolt holes especially, so the bolts all torque evenly, but also to identify any leaks) of the pan, as directed by a master mechanic among master mechanics years ago. We even serviced "non-servicable" Mercedes (POS) trannys (the fluid is rated for 120K miles - then the tranny blows up just after the warranty ends because the fluid is shot, and you need a new one from the dealer!). I have seen copper dust and steel dust in the pan bottom of various different trannies. The steel dust binds to the magnet. It is imperative to clean that magnet, because it can only hold so much dust. That dust will eat your tranny from the inside out. The copper dust I assumed was from bushings wear and tear.
And zinc and or copper will float in oil? And no tranny I ever saw called for a zinc additive, but I'm not a tranny guy (just service them).


I cleaned my Pathfinder's tranny that way most of its life, and at 320K+ miles, it shifted like new - never needed anything.


Same with my Villager's tranny, but less miles.


I always used Valvoline full synthetic.


What I have hear of, is that servicing an older tranny that was not properly maintained can cause problems, because the new fluid (a high-detergent oil) will loosen gunk that builds up where you can not clean it, and then that gunk clogs valves.


I put a tranny cooler on my bus, a nice big one with a fan, and I don't think it helps in any way shape or form. A waste of money and time, IMO, on my bus at least. Maybe a cutout van with a smaller radiator would benefit.

[s2mikon]

Yes the oil to coolant heat exchanger. It makes me cringe, but it does work. At one time Allison would not warranty a unit unless it had an oil to coolant exchanger. I think the worst ones ever made were the Nissan frontier and xtera units. Ruined a lot of transmissions. Back to the Allison mt 643. Allison wants a minimum of 100 degrees and would prefer that you use oil to coolant for that reason. I did install an oil to air cooler with fan and thermostat fan control after the oil to coolant exchanger and a gauge on the out line to the transmission and it did drop the oil temperature about 20-30 degrees, so I think it is worth it. At 100 degrees outside 4th lock level road 200 degrees converter out 180 degrees in. Before the added cooler it was about 220 degrees converter out. We are in the process of changing to a MTB643 retarder transmission at the moment and will have to add a second oil cooler just for the hydraulic retarder. That will be a new challenge in fluid cooling.