AT545 and 15w40...?

Mr4btTahoe · 09-15-2022, 10:39 AM

Hey alls..

So in the process of replacing cooler lines on my hauler build and was planning on servicing the transmission before heading out next month.

With that said, I've been doing a lot of reading and many of the old timers and fleet mechanics say the 545 works really well with C4 rated 15w40 instead of DexIII.

Supposedly, it helps tighten up the "slip" in the converter at speed as well as provides better wear and heat protection.

It was also recommended by Allison up until a few years ago for anything in "severe" service. That changed due to the need to "warm up" the fluid when cold.. so idle time... and emissions standards. It is still the recommended fluid for their "off-highway" transmissions.

So... is anyone using C4 15w40 in their AT545?

Any noticed benefits?

Per gallon cost is about the same between Dex3 and 15w40... so that doesn't really matter.

Thanks

magnakansas · 09-15-2022, 10:49 AM

So the viscosity of dexIII I would think is lower than the 15w40. So a thicker oil, so to say, would decrease the converter slippage. Makes sense to me. I bet you see 1mpg increase from less converter slip. That is going out on a limb, but that is my prediction.

Down side—- I wonder if the shear strength is as good as dexIII? If not, means you will be replacing oil more often.

William

Mr4btTahoe · 09-15-2022, 11:10 AM

Quote:

Originally Posted by magnakansas

So the viscosity of dexIII I would think is lower than the 15w40. So a thicker oil, so to say, would decrease the converter slippage. Makes sense to me. I bet you see 1mpg increase from less converter slip. That is going out on a limb, but that is my prediction.

Down side—- I wonder if the shear strength is as good as dexIII? If not, means you will be replacing oil more often.

William

Based on what Allison used to say (and currently says for their off-highway lines), service intervals are the same.

Also... I've attached some pages from the Allison manual for the 500 series. I'd say my hauler trying to pull grades when loaded would be considered "severe" duty. Lol

https://www.allisontransmission.com/...highway-fluids

cadillackid · 09-15-2022, 12:09 PM

I didnt notice much difference in driveability but I do know allison recommended 15w40 for what they dee mas severe duty. (which seemed to be anything that a bus would do....)

I had 15W40 in my DEV bus.. and i will say before that trans Nuked, it was a good running 545.. but not as good as my original 540 thats in my 1978 Superior.. the worst was the 545 in my redbyrd.. that was the loosest worst shifting 500 series ever.. partly because of the crappy on / off modulator...

Mr4btTahoe · 09-15-2022, 12:14 PM

Quote:

Originally Posted by cadillackid

I didnt notice much difference in driveability but I do know allison recommended 15w40 for what they dee mas severe duty. (which seemed to be anything that a bus would do....)

I had 15W40 in my DEV bus.. and i will say before that trans Nuked, it was a good running 545.. but not as good as my original 540 thats in my 1978 Superior.. the worst was the 545 in my redbyrd.. that was the loosest worst shifting 500 series ever.. partly because of the crappy on / off modulator...

I've noticed a difference in driving my bus with the jeep on board vs unloaded and there is a ~5mph difference in top speed yet rpm isn't different which makes me think the converter is slipping more under the weight.

The fluid that came out of the cooler when I pulled the hoses this morning was still red but dirty and due to be changed so I don't think it's getting too hot.

Just curious if the heavier weight of the 15w40 will reduce the slip a bit under heavy load at highway speed and get me that few mph back.

Mr4btTahoe · 09-15-2022, 12:29 PM

I also find it interesting that in another manual I just found that talks about fluids for the 643R model, they ONLY spec 30wt or 15w40.. No atf / dex... They claim it is due to the extra stress the input retarder puts on the unit.

turf · 09-15-2022, 12:34 PM

my 2 cents is that its temperature related.

if you're in an area that doesn't freeze hard..... maybe usda zone 7 and up, you can use that year round.

if you are in a colder climate, you'll want dex 3 for year round use.

if you only run in the summer it doesn't matter.

Mr4btTahoe · 09-15-2022, 12:39 PM

Attached pic of the manual for the 643R

cadillackid · 09-16-2022, 07:22 AM

Quote:

Originally Posted by turf

my 2 cents is that its temperature related.

if you're in an area that doesn't freeze hard..... maybe usda zone 7 and up, you can use that year round.

if you are in a colder climate, you'll want dex 3 for year round use.

if you only run in the summer it doesn't matter.

when I ran 15W40 in my DEV bus 545.. I **HAD** to let it warm up for a few minutes before driving it (or have it plugged in).. whuich in that engine if its 0 or below she going to give you a hard time unless the webasto has been pre run or its been plugged in.. keeping it plugged in transferred a decent amount of heat to the transmission ..

plus that bus has air shutters to when the Tstat opened the radiator stayed warm and actually warmed the transmission...

I still remember as a teen.. my dad.. "you MUST let the scout warm up for awhile before you drive it in really cold weather.. its super hard on the engine otherwise"... the truth was you had to let it warm up anyway because you couldnt move the Stick into gear! (85W90 is stiff at 0f)

-Christopher

turf · 09-16-2022, 08:17 AM

as my 545 trans failed, i considered using the 15w40 as a fluid.

being in Colorado, i opted for transynd instead.
transynd didnt save my transmission. on the replacement, i've run both dex 3 and dex 6 (synthetic dex 3).

since dex 3 was original to the vehicle and at545's, i settled on dex 3 over the long term.

if i were in texas or florida, i'd run 15w40

i like dex 3 because its cheap. i think clean fluid is better than thick fluid. if you overheat the trans, change the fluid.

Booyah45828 · 09-16-2022, 10:20 AM

100% depends on temp.

What temp is your trans fluid normally at? Around 212°F, a 40w is twice as thick as atf, but the gap closes pretty fast over that. Around 240 they're nearly the same thickness. At 100°F 40w is 5x as thick, and at freezing it's nearly grease. 15w40 fairs a lot better, but it's viscosity index, or the fluid's ability to resist thinning with temperature, isn't anywhere near what dex3/merc is.

The reason fleets used 15w40, IMO of course, is because it was cheaper and allowed them to stock one less fluid.

I don't think I'd personally run 15w40, for the simple fact of how thick it is when it's below freezing, and in Ohio, it's below freezing several months in the winter.

I would consider running a 5w40, as it has a viscosity index a lot closer to atf then what 15w40 is, and would be the same 2x as thick at temp. But a good 5w40 isn't necessarily cheap compared to either 15w40 or a common dex3/merc.

Truthfully, if you're looking at actually fixing this, you can install a tighter converter, like this one here. Or swap in a different trans that has a locking converter.

Mr4btTahoe · 09-16-2022, 10:33 AM

Quote:

Originally Posted by Booyah45828

100% depends on temp.

What temp is your trans fluid normally at? Around 212°F, a 40w is twice as thick as atf, but the gap closes pretty fast over that. Around 240 they're nearly the same thickness. At 100°F 40w is 5x as thick, and at freezing it's nearly grease. 15w40 fairs a lot better, but it's viscosity index, or the fluid's ability to resist thinning with temperature, isn't anywhere near what dex3/merc is.

The reason fleets used 15w40, IMO of course, is because it was cheaper and allowed them to stock one less fluid.

I don't think I'd personally run 15w40, for the simple fact of how thick it is when it's below freezing, and in Ohio, it's below freezing several months in the winter.

I would consider running a 5w40, as it has a viscosity index a lot closer to atf then what 15w40 is, and would be the same 2x as thick at temp. But a good 5w40 isn't necessarily cheap compared to either 15w40 or a common dex3/merc.

Truthfully, if you're looking at actually fixing this, you can install a tighter converter, like this one here. Or swap in a different trans that has a locking converter.

Already have a different trans that is going in it after this trip but I have to fix a few things before making this run and I'll be changing the fluid anyways. Figured if I could gain a little mpg/mph out of it, I might as well.

Not worried about temp as it wont be driven with this transmission in the cold.

I'll see if I can find an approved lighter oil though. Most of the oils that meet the Allison spec are 15w40. Looks like Rotella T6 meets the spec so maybe I'll go that route.

Booyah45828 · 09-16-2022, 10:48 AM

C4 has been dropped in favor of tes439 in off highway stuff, and C4 hasn't been the on-highway spec for new transmissions for over 20 years.

A lot of fluids will meet the c4 designation, they're just not going to pay for the certification as it's old and obsolete.

If I was in your shoes, and wanted to truly try something, rural king had a 5 gallon bucket of 5w40 for 100 bucks the last time I was there. It very likely won't be allison certified, but I doubt you'll have issues with it. At 5 bucks a quart, you'll be paying near that for a good 15w40 or dex3/merc regardless, and on a trans that's getting pulled anyways, I'd say it's worth trying out.

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