CAT 3116 AT545 to MT643/653

[DtotheAmbrose]

Hello,

I have spent the last several months trying to come up with an answer on what bellhouse to use to get an MT643 into my GMC Topkick based bus chassis.

A bit of history on it for reference. The chassis is a C5500 "bus not a school bus" chassis with a 215hp Cat 3116 and the AT545. I use it to haul a trailer and we had a problem with the AT545 slipping going over passes in Oregon. I figured it was from milage and the trans jsut needed a rebuild. We rebuilt the AT545 and installed the largest external transcooler i could find which is rated for 90000btu and had -10 lines. I also installed a 180 thermo switched fan on the cooler as well as a just in case. 200 miles into the last trip and the trans would slip on the 2-3 shift which is what it did before.

Doing a bit of homework after the fact the torque rating on the AT545 is pretty low for what I am trying to accomplish.

Looking at the MT643 it is rated for 70k GVW which will do the trick for what I am towing (a 24k racecar trailer). I have spent time here on the forum lurking reading up on people swapping in the MT643/653s but the documented swaps have been behind Intl and Cummins powerplants. I haven't found a post on the Cat 3116 swap being done. I am fine with the Cat power plant, I am going to use a 20 year newer BW SX200S turbo along with a small adjustment to the fun screw to get a little more power out of the unit.

My hang up to getting started has been the bellhouse. I know the AT545 is a SAE3 and the MT643 is an SAE2 from all the reading I have been doing.

The snag is the wreckers i have been shopping for the 3116 bell housing are listing the AT545 trans bellhouse as an SAE2 which isn't correct. Does someone have the 3116 Cat part# for the correct SAE2 bellhouse? Can you use a 3116 manual transmission SAE 2 bellhouse for this?

I appreciate any insight I can get on this and I am more than happy to share the swap with pictures/tips/etc when I get it done.

Thank you in advance

[DtotheAmbrose]

Just a quick update, I found a heavy truck wrecker who had a transmission, supposedly the correct bellhouse, and necessary brackets for $1000. Parts ETA is next week. I will verify the bellhouse part# when it arrives and make sure it is the SAE 2 variety.

[jimburke77502]

Didn't have time to find a specific thread on the Allison 545, but I ran across this the other day. Don't know if it's been put here before but it might help many of those with the 545. I happen to come across it because I am one of those with a 545. Only advantage I have is that it was completely rebuilt 25-30K ago before I bought the bus. Thing I found odd was the part about using engine oil instead of tranny fluid. Input on that from anyone?

I recently bought a 1995 Ford F800 with the Cummins 5.9 engine and
Allison AT545 transmission. The truck has about 40,000 miles and has 210 HP and 485 ft-ib.s @1600 rpm. The total weight of the vehicle plus the load is about 20,000 Ibs.

1)
The truck gets a lot of local and highway driving. When I drive in
traffic, the temperature stays within 140 F to 160 F. But the thing that bugs me is when the truck comes to a stop. I don't know if it is the transmission or the brakes, but after downshifting to first (this happens around 15mph), Ifeel this big jerk at around 13 mph. I guess it is some type of stopping mechanism but am not sure what it is. Do you know what this could be? Is this normal?

Ans:
It sounds like your modulator is on. You will either have a cable
mechanical modulator or an Electric modulator and what the modulator
does is raise and lower shift points from full throttle (high) shift
points to off throttle (lower) shift points. If you modulator is staying on, all your shift points are at there highest setting and when you downshift - bang- too high of shiftpoint.

2)
Also, when I drive on the freeway, it barely stays within 60 mph. The
top speed is around 65 mph on downhills. The engine rpm is at around 2600 rpm. The temperature stays around 150 F. Is this all the AT could handle? Could I push it more? I am afraid it might decrease the transmission life if I drive over 60 mph constantly.

Ans:
This transmission does not have lockup which would give you more road
speed. The only thing you can do is change the differential gearing.
This truck was probably set up for local deliveries only.

3)
When shifting from first to second, I feel this big shock when I do not press the throttle too much (40% throttle). It seems like I get better shifts when I push the throttle about 75%. Is it better to kind of floor the truck? How much throttle am I suppose to use when driving? During traffic, is it better to let the transmission change gears and keep the revs low or rather stay in one gear with the revs high?

Ans:
You are correct. The best shifts occur at full throttle, but it
should never be harsh even at part throttle. Most people drive at about 60% throttle.

4)
I want to change the tranny oil and the filter myself. I have no
previous service record, but the truck was used by the city. So, I am thinking it has had some maintenance. Looking through the forurm, I see that there is an internal and external (spin on) filter. It seems like their are mixed reviews on whether to change the internal filter or not. Some people say it is not worth the money and labor and is not going to affect the life of the tranny. Others say otherwise. What is the real deal? Should I change the internal filter and
drop the pan? I also read that there is a new polyster filter for the AT545 that replaces the brass filter. How does this compare to the paper filter? If the internal filter change is necessary, what filter do you recommend? Could you specify the part number and the average cost?

Ans:
I recommend the poly filter. YES! Indeed change the internal filter
every 12 mos - 25,000 or 1000 hrs. service when using Dexron III. You
also should flush out your cooler at this time and of course change the external filter. The transmission you were reading about is the world transmission which an AT is not one of. You will have to call your local Allison distributor or dealer for a part number and cost.
Transynde synthetic is great oil but cost more. If you look at
allisontransmissions.com website, there is a list of oils that are
approved. As long as Allison has it on there approval list, it is good
fluid. We actually use Chevron Dexron III. With an AT545, Delo 400 15-40 engine oil actually makes the shifts better and stays cooler and this oil needs the Poly filter.

5)
For external spin on filters, I was thinking about using Wix filters
instead of the original Allison filter. Do you think this filter is safe to use? There was some talk about Transynd synthetic transmission oil on the forum. I see the added benefits are increased service interval and lower transmission temperature. I remember you saying that it is only worth it if the transmission has low miles (~20,000 miles). My truck has about 40,000 miles, do you recommend that I stay with the Dextron or switch to the Transynd? If you recommend the Dextron what oil is reliable? I was thinking
about using the regular Cheveron Dextron oil they sell at Costco for
around $13 for 12 quarts. I also see that some people are using C4 motor oil as transmission fluid, what is the difference? Does this have any added benefits?

Ans:
This is normal with an AT 545. This model does not have lock-up in
the converter which means it is all moved hydraulically with the torque convertor. Hydralics cause more friction and heat. You are doing everything correctly by pulling over and reving to 1300 until cooled. The only way to keep it cooler is by adding an additional air to oil cooler in front of your radiator.

6)
I occassionally tow a Bobcat 863 behind my truck. I see that the
temperature increases a bit, 180 F when driving and sometimes 200 F when during a stop. When the temperature hits around 200 F, I stop the truck and put the transmission to neutral. Then, I rev the engine to about 1300 rpm. This lowers the transmission temperature. Is there anything else I should do when towing a trailier? How should I drive when towing?

Ans:
This transmission is only rated at 30,000 lbs. With the trailer, what
is the combined weight? When towing a trailer, it would have been better to have a model MT643 but the AT will be ok.


Thanks for the questions. If you can get into the forum at RVUSA, please could you post that I have been booted out of going in? I think they thought I was promoting my company which as you can tell, I didn't even mention. I was there just to answer peoples questions, but someone must have complained.

Thank you - AllisonMan - Rick

[Booyah45828]

Not sure how cat did it. I know a lot of manufacturers, when they used the at545, used an sae2 flywheel housing with an sae2-sae3 adapter ring to mate to the at545 trans. I'm sure you found that out when you were looking up information.

All of the cats I've worked on were the newer 3126/c7 variety and used electronic transmissions with an sae2 housing, so I have no first hand experience there to help you with.

[Booyah45828]

Originally Posted by jimburke77502

Didn't have time to find a specific thread on the Allison 545, but I ran across this the other day. Don't know if it's been put here before but it might help many of those with the 545. I happen to come across it because I am one of those with a 545. Only advantage I have is that it was completely rebuilt 25-30K ago before I bought the bus. Thing I found odd was the part about using engine oil instead of tranny fluid. Input on that from anyone?

The at545 isn't really picky about fluid used, as long as it's clean and isn't burnt.

School districts would use engine oil in them so they could stock one less fluid and then they would change them more often, which isn't a bad thing IMO.

You can also use certain hydraulic fluids in an at545 successfully as well.

As far as the rest of your post, I didn't make it through all of it, it gives me a headache. I'd suggest you link the url next time vs copying and pasting. Or maybe select bullet points from it and then include the link as well for those that want to read the whole thing.

[Booyah45828]

Originally Posted by DtotheAmbrose

I am going to use a 20 year newer BW SX200S turbo along with a small adjustment to the fun screw to get a little more power out of the unit.

Great choice on upgrading the turbo. The modern designs are more efficient then the stock pieces. Some of them have ball bearing center sections, and that makes for a pretty responsive turbo. Whatever you do, make sure it's sized correctly, or you'll be disappointed. Don't be afraid to ask a turbo guru for help.

I imagine you're intercooled, I've never seen a cat without one, but if you're not, look into doing that as well.

[EastCoastCB]

Originally Posted by Booyah45828

The at545 isn't really picky about fluid used, as long as it's clean and isn't burnt.

School districts would use engine oil in them so they could stock one less fluid and then they would change them more often, which isn't a bad thing IMO.

You can also use certain hydraulic fluids in an at545 successfully as well.

As far as the rest of your post, I didn't make it through all of it, it gives me a headache. I'd suggest you link the url next time vs copying and pasting. Or maybe select bullet points from it and then include the link as well for those that want to read the whole thing.

They may not be picky, but they seem to do a lot better on TES-295 aka Tansynd. Florida actually wrote Transynd use into their statutes, so all our buses run it with 545's. Also Allison fills all their reman 545's with it.

[DtotheAmbrose]

Originally Posted by Booyah45828

Great choice on upgrading the turbo. The modern designs are more efficient then the stock pieces. Some of them have ball bearing center sections, and that makes for a pretty responsive turbo. Whatever you do, make sure it's sized correctly, or you'll be disappointed. Don't be afraid to ask a turbo guru for help.

I imagine you're intercooled, I've never seen a cat without one, but if you're not, look into doing that as well.

I did my home work on the turbo and compressor maps on the old vs new so we should be a ~400rpm more response which I hope will line up with the drop in RPM from the new trans. The BW SX200 is a sleeve bearing but from Borg Warner performance line which runs the extended reach compressor.

Yup it is intercooled, I have been looking at the higher HP intake piping from the 235-250hp 3116 engines and they run a dual pipe setup which splits the front and back runners after the intercooler to give a little more even airflow between all the cylinders. If I get time I may install one of those setups as well once the trans is in and see what they does to EGTs and performance.

[DtotheAmbrose]

Originally Posted by Booyah45828

Not sure how cat did it. I know a lot of manufacturers, when they used the at545, used an sae2 flywheel housing with an sae2-sae3 adapter ring to mate to the at545 trans. I'm sure you found that out when you were looking up information.

All of the cats I've worked on were the newer 3126/c7 variety and used electronic transmissions with an sae2 housing, so I have no first hand experience there to help you with.

I am kind of wondering if the trans will show up with a 3126 bellhouse for the swap, I didn't ask them specifically which model of bellhouse, it just needed to bolt to the 3116 and the MT643.

I was hoping for this but when we had the trans out before there was no adapter that I remember. I get to support the engine when we swap the bellhouse, the Cat 3116 uses rear engine mounts which bolt to the bellhouse. Not looking forward to supporting 1500lbs of engine as I don't have a gantry at home.

[DtotheAmbrose]

Moderators:

Is there anyway to move the AT545 fluid post out on this one to its own post? Fluid wasn't the problem for why I am changing out the trans, it was sheer capacity for towing.

Thanks

[EastCoastCB]

It was two comments, man.

[DtotheAmbrose]

Originally Posted by EastCoastCB

It was two comments, man.

I think it is more like a epic saga of a post which is why I was suggesting it get made into its own thread, not buried away on the cat drivetrain subforum

[DtotheAmbrose]

Update - Trans arrived:

The MT643 arrived and looks super clean for a used unit. I was given the correct flexplate, spacer, and torque convertor adapters to bolt it in. I got thrown for a loop though, I thought I would need to swap the bellhouse since I have an AT545 in the truck.

I checked the truck and the CAT bellhouse actually has 2 bolt patterns in it, a narrower SAE 3, 16.857" spacing which is currently occupied with the AT545 and the outer which I think I am safe to assume is the SAE 2 18.375" spacing. After looking at this I remember thinking it was odd when we had the AT545 out for a rebuild but it had slipped my mind.

I am really happy as this means no needing to build/borrow/steal a gantry to hold up the engine to swap the bellhouse.

If the weather holds up I will get started on the trans swap this weekend. I just need to chase up a 55gallon drum so I can pump out the passenger side full saddle tank to get access to everything.

[DtotheAmbrose]

I found a picture online of the AT545 bellhouse, it looks like it is an adapter ring on there. I will confirm when I get it pulled apart.

[DtotheAmbrose]

I got the passenger saddle tank pulled to have space to swap the transmissions.



The bottle is catching diesel from the drivers side tank, the cross over valve needs to be replaced but the tank is full of 50 gallons of diesel and I only have one 55 gallon barrel for draining a tank.

I got the AT545 out pretty easy as I have done this before.



With the transmission out you can see the SAE3-SAE2 adapter ring on the bellhouse. Mine was held on with a Torx-55 bit.



Adapter ring removed which shows the SAE2 pattern.



None of the AT545 flexplate/flywheel can be used so I swapped in the parts I received with the transmission. I got the MT643 parts installed. A quick note about what model the flexplate/spacer/ring gear came out of. I have seen references in GM parts books for C6500/C7500 chassis trucks having the MT643 bolted to the Cat 3116. I have also seen the same thing for the cat 3126. I wasnt able to get an answer for which one I received but everything bolted up.

The installation of the MT643 went pretty smooth. Word of warning, I used one of the harbor freight motorcycle jacks which says it has a max load of 1500lbs. The MT643 on there didn't look or feel safe and I made sure to be clear in case the jack failed. I only moved the jack in a direction that the wheels were already facing to not twist the jack. Sketchy is being nice on how it looked.

That being said I didn't die or loose any limbs/fingers and the transmission is installed.



I had a few concerns about exhaust clearance as the cat topkick exhaust covers over the tail of the transmission to get to the drivers side. With the trans bolted up there is plenty of space.



Sticking points on the swap.

#1 My trans came from a Cummins powered truck (ugly tan/white paint gave it away) so none of the sensors had the same plugs or wiring. Everything from the AT545 swapped to the MT643.

#2 Dipstick tube. The dipstick tube on the MT643 sits at the bottom of the pan, on the AT545 it is closer to the top, ~1.5 higher. I am going to figure out a way to get the original MT643 dipstick installed.

#3 All of the brackets/fixtures which bolted to the bellhouse will not line up with the MT643. An example is the exhaust bracket shown below. I am going to make a simple adapter or slot the holes on each end to get it secure with 2 points.



Next steps:

My truck is based on a C5500 chassis which means it came with Hydraulic brakes and my AT545 had the parking brake on it. I was able to get lucky and find a truck wrecker which had a MT643 with a parking brake on it and they were willing to sell me just the brake mechanism. Looking at the GMC parts book it shows the same brake cable for the AT545 and MT643 so I dont have to swap that out. Getting the parking brake is important so I can get the driveshaft shortened and get the correct flange for the new parking brake as it looks different from the original.

I also need to get the shift selector sorted out as well, the original bolted to the bell housing which wont work due to the bolt spacing. I am going to see if I can get the cummins bracket to work as it bolted to the transmission side not the top like the old trans.

If anyone has any questions feel free to ask.

[Booyah45828]

I figured the clutch housing would have been adapted down to sae3.


Was the flywheel visibly different that you had to swap them? I'd be surprised if they had different ones. Typically they only have different adapters and spacers for the two.

[DtotheAmbrose]

Originally Posted by Booyah45828

I figured the clutch housing would have been adapted down to sae3.


Was the flywheel visibly different that you had to swap them? I'd be surprised if they had different ones. Typically they only have different adapters and spacers for the two.

The offset of everything was different with how far the mt643 sits inside the bellhouse. The at545 also had a real flywheel behind the flexplate in the stack. The mt643 was more of a conventional setup aka pretty light weight.

[DtotheAmbrose]

I had to get a new line made as one of the angles wasn't quite right on the top port for the mt643. I should be able to fire it up this weekend and make sure everything works.

The parking brake is due in on Wednesday and the driveshaft can he shortened in a few days.

[DtotheAmbrose]

The parking brake arrived and looks great. There is a little bit of some blue paint so this probably came off an intl powered truck originally. It is much bigger than the AT545 brake, looks to be 3-4 inches in diameter and about 1 inch wider.

[DtotheAmbrose]

I got the the parking brake installed and was able to re-use the original cable by picking up a new clevis from Mcmaster carr. The cable on the GMC topkick is a 3/8-24 with a 3/8 pin. The new parking brake uses a 1/2 pin and needs 1/2 clearance for the lever on the drum.

The clevis i used is part#6071K16 which was a perfect fit.