Transmission Swap C6 to AT545

Topic: Transmission Swap C6 to AT545 (Read 2046 times) previous topic - next topic

Transmission Swap C6 to AT545

June 12, 2017, 04:42:10 pm

Has anyone done this swap? The C6 in my 86 Grand Villa is tired, and I was thinking maybe it might be a better idea to put a sturdier transmission. The Allison is also a 4 speed, which couldn't hurt.
I've got the OREG with the V-814 chassis and a Ford 7.3 IDI engine. I sort of doubt anyone else has such a chimera of a vehicle. I think the only thing left from the original drivetrain on this beast is the C6. I'm guessing since the 545 was used by international on the 7.3IDI, it would just bolt up?

Or maybe there is a better option? This is the first vehicle I've ever owned with an auto, so I have a lot to learn about them.

Re: Transmission Swap C6 to AT545

Reply #1 – June 12, 2017, 04:50:16 pm

MT643 is almost identical to the AT-545 but locks up in fourth gear. I believe it is about 1/2"-1" longer.
Might want to check with James Holder the Service Manager at Tennessee RV.

Re: Transmission Swap C6 to AT545

Reply #2 – June 12, 2017, 05:12:39 pm

I believe the 643 locks the torque converter in both 3rd and 4th.

Yes, a lot more robust than the 545.

But, I believe their "bolt up pattern" to the engine is different. Do not know which has same pattern as C6.

Re: Transmission Swap C6 to AT545

Reply #3 – June 12, 2017, 05:27:17 pm

Here are the two specification sheets for the two trannys.
Brett is correct. The AT545 mounts to a SAE #2 bell housing and the MT643 mounts to a SAE #3 bell housing.
Seems more comparisons are required.+
I got my info/recommendation from James Holder so hopefully someone else can do some more research on this swap.

Re: Transmission Swap C6 to AT545

Reply #4 – June 12, 2017, 05:47:36 pm

I was worried that the 643 needed a computer, but then I know very little about them. No Computers on the old 7.3s.

Re: Transmission Swap C6 to AT545

Reply #5 – June 12, 2017, 05:54:19 pm

Another dumb question- Is there any difference between transmissions for diesel pushers and front engine vehicles?

Re: Transmission Swap C6 to AT545

Reply #6 – June 12, 2017, 06:13:03 pm

Quote from: Flopsie – June 12, 2017, 05:47:36 pm

I was worried that the 643 needed a computer, but then I know very little about them. No Computers on the old 7.3s.

It is a hydraulic transmission, not electronic.

Re: Transmission Swap C6 to AT545

Reply #7 – June 12, 2017, 06:39:57 pm

Quote from: wolfe10 – June 12, 2017, 06:13:03 pm

It is a hydraulic transmission, not electronic.

I guess the 643 would be the best choice, if it fits. I was told the 7.3idi fits a sae #2 housing.

Re: Transmission Swap C6 to AT545

Reply #8 – June 12, 2017, 06:48:57 pm

Yup, if it bolts up, the 643 is a much better choice. Rated for higher HP and Torque and very importantly, torque converter locks up in 3rd and 4th gear.

So, better MPG and less transmission fluid heating.

Re: Transmission Swap C6 to AT545

Reply #9 – June 14, 2017, 03:04:37 pm

Here's a consideration. The C6 in there now is 175 pounds. The AT542 is 253 pounds. the MT643 is a great big juicy 510 pounds; 638 with the retarder. This probably isn't a big deal with the 917 chassis, but i have the 814 which already is 200 pounds heavier with the diesel swap.

Re: Transmission Swap C6 to AT545

Reply #10 – June 14, 2017, 03:16:44 pm

The C 6 is a pretty good trans as is . Does it have a bigfat cooler and shift kit ? it should do what you ask of it.

Re: Transmission Swap C6 to AT545

Reply #11 – June 14, 2017, 03:20:25 pm

When you say the c-6 is tired, is it slipping or does it just have "morning sickness" when cold doesn't up shift?

Re: Transmission Swap C6 to AT545

Reply #12 – June 14, 2017, 03:51:14 pm

What transmission does Ford use in back of the Powerstroke or Binder diesel.? Seems to me that would be the quick fix, maybe with a shorter tail shaft.
The C-6 is a stout trans. enough to hold on to a Ford big block so rebuild and shift kit ?

Re: Transmission Swap C6 to AT545

Reply #13 – June 14, 2017, 05:42:28 pm

My C6 slips badly. its fine in reverse, but from a stop its really bad. I wasn't slipping so badly i couldn't drive it, so I thought maybe it was just the burnt looking fluid, but when I changed that it barely moves at all in forward, so I assume the PO dumped a bunch of that stop slip into it. My assumption with that is that it needs a rebuild, but this is the first vehicle I've owned with an auto, so I am far from an expert.

Then adding to the fact that the C6 is only a 3 speed, it just seems if i do have to swap it out, I should consider something more robust and with at least 4 speeds.

Re: Transmission Swap C6 to AT545

Reply #14 – June 14, 2017, 06:00:48 pm

Whenever you put new fluid in an auto trans that has slipping problems, the detergent in the new fluid cleans the remaining clutch material off and they slip worse. I don't believe you were tricked by the seller with a cocktail. If the fluid was brown and smelly it was bad. I agree if you have to rebuild something use the heavier duty one.

Re: Transmission Swap C6 to AT545

Reply #15 – June 14, 2017, 06:15:48 pm

Quote from: craneman – June 14, 2017, 06:00:48 pm

Whenever you put new fluid in an auto trans that has slipping problems, the detergent in the new fluid cleans the remaining clutch material off and they slip worse. I don't believe you were tricked by the seller with a cocktail. If the fluid was brown and smelly it was bad. I agree if you have to rebuild something use the heavier duty one.

Ah. I had no idea of that. I had read if a transmission is slipping and the fluid was burnt, changing it out might fix the problem.
I also wouldn't say i was tricked. Its a long story, but I give the PO a pass on a lot of the questionable stuff he did to this coach.

Re: Transmission Swap C6 to AT545

Reply #16 – June 14, 2017, 06:18:37 pm

Quote from: Olde English – June 14, 2017, 03:51:14 pm

What transmission does Ford use in back of the Powerstroke or Binder diesel.? Seems to me that would be the quick fix, maybe with a shorter tail shaft.
The C-6 is a stout trans. enough to hold on to a Ford big block so rebuild and shift kit ?

I think they moved to the E40D with the Powerstroke. I have heard with the C6, you can beef up some of the parts on rebuild and make it a lot sturdier, so that is an option too.

Re: Transmission Swap C6 to AT545

Reply #17 – June 14, 2017, 06:21:12 pm

Quote from: Protech Racing – June 14, 2017, 03:16:44 pm

The C 6 is a pretty good trans as is . Does it have a bigfat cooler and shift kit ? it should do what you ask of it.

I does have a transmission cooler in front of the radiator, and I wouldn't know a shift kit if it came up and bit me.

Re: Transmission Swap C6 to AT545

Reply #18 – June 14, 2017, 07:16:47 pm

The e4od would be a benefit. They are strong and common. That being said nothing wrong with a good c6

Re: Transmission Swap C6 to AT545

Reply #19 – June 14, 2017, 09:49:14 pm

What is the final drive ratio? What is the cruise RPM? If the Cruise RPM is just under max torque, you may not see any fuel reduction with more overdrive.
In your case with a conversion from gas to diesel, the RPMs may indeed like to be reduced a little . 22-2400 Is a nice RPM for those Fords. If it is under 2500, I would rebuild the C6, if over I would look for the big OD Ford trans. Both will be a lot less hassle - money than the heavy Allisons. The 643 is direct in high gear, not sure on the smaller one. Plus the rotating stuff weighs a lot more and will result is less performance .

Re: Transmission Swap C6 to AT545

Reply #20 – June 15, 2017, 12:28:58 pm

Quote from: Protech Racing – June 14, 2017, 09:49:14 pm

What is the final drive ratio? What is the cruise RPM? If the Cruise RPM is just under max torque, you may not see any fuel reduction with more overdrive.
In your case with a conversion from gas to diesel, the RPMs may indeed like to be reduced a little . 22-2400 Is a nice RPM for those Fords. If it is under 2500, I would rebuild the C6, if over I would look for the big OD Ford trans. Both will be a lot less hassle - money than the heavy Allisons. The 643 is direct in high gear, not sure on the smaller one. Plus the rotating stuff weighs a lot more and will result is less performance .

I wish i could answer these questions. I've only had it a few months and I just got it running, and only drove it enough to tell the transmission was bad. Its got a 2 speed Spicer rear end which i also just got working, and have no Idea what the ratios are for either.

The E4OD like It's Toby mentioned is the big OD? the only real issue I see there is that its electronic, so would I would have to get a transmission controller?

Re: Transmission Swap C6 to AT545

Reply #21 – June 15, 2017, 01:13:46 pm

The diff may have a tag on it .or you can jack it up and count the turns. Look at U tube, you only need to spin one side . My guess is 4.10 But I have no idea what diff your coach came with .
Most 2 speed rears are underdrives as installed . If you have true overdrive and come up with a total gear ratio under 4/1 (3.9, 3.7 etc) than the direct top gear trans should be fine.
Figure out how/if the OD /2 speed works .
Figure the diff gear
ask around for optimal cruise RPM for your engine.
Post what you find and I'l be happy to do some math research.

Gear Up! this shows how to figure gearsets and tire rollout vs speed.
The easiest way to determine your gear needs is to coax it down the road one more time with a GPS and tack working.
IMHO ;Short story and most cost effective is to get the C6 rebuilt with a HD shift kit and clutch packs and deal with the resulting cruise speed .
****Editttt. If the 2 speed is a gear vender aftermarket OD than you should be good on the overall gear ratio.
MY racing partner tows with a 7.3and says that 2300 RPM works the best. He has towed @ 2800 with his C6 @ 70MPH .
For the AEOD Ford trans , you need a controller and some are using a Microsquirt ECU. The same thing that I sell and install . The parts are about 400$ ECU and Harness.
He has the Gear vender for sale at 1000$ with controller . Corrected, 1500$ sorry.
The OD Ford can be built for about 1500 $ here inFL/Tampa area.

Re: Transmission Swap C6 to AT545

Reply #22 – June 15, 2017, 02:34:23 pm

Rebuild the c6 IMO

Re: Transmission Swap C6 to AT545

Reply #23 – June 15, 2017, 03:40:56 pm

What Mike is saying makes the most sense to me ........HD rebuild on the C-6 and count the turns on the rear axle.

We had a feed truck on the ranch ( 1981 ) that had the Spicer split shift and it worked as an under drive, but it had a Clark 5 speed stick so split shifts were easy to accomplish, not sure how it works with an automatic, would need the shift kit just to control trans gear selection.

Mick

Re: Transmission Swap C6 to AT545

Reply #24 – June 16, 2017, 04:16:13 pm

Yeah, keeping the C6 and beefing it up a bit is probably the best answer. not the most fun one though.

Affiliate Notice