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 > *UPDATE* Just installed auxiliary trans in my 1992 OFED

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50pascals

rochester, ny

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Posted: 04/13/09 09:11pm Link  |  Print  |  Notify Moderator

This is a long post - sorry!

I just finished installing a Spicer 7231 auxiliary transmission in my motorhome. I wanted to share this with all of you, but some of the information will not necessarily be pertinent because I am writing this for posting on 3 different websites. Many people have talked about doing this, but I have never seen a write-up of someone that did. I did all this work with my own hands, with the exception of machining some spacers and final welding the cradle. My MIG welder is not big enough.

I have a 1992 motrohome with a stock 160 hp version Cummins 5.9, Allison AT542 transmission, 3.73:1 Dana HD70 and 225-70-19.5 tires. Chassis is an Oshkosh MC-16-FD, manufactured in 1991. It weighs 16k as we roll. I am not dinghy towing a car with this-yet. This is identical in every way to the 18k chassis with the exception of horsepower. The 18k chassis is 180 hp.

Our initial trips with this rig were fine, but slow. Top speed was roughly 62 mph on the governor at 2600rpm. On hills I would slow to roughly 50mph, no matter how big the hill. We got a solid 10mpg. Hills or flats, didn’t matter 10mpg was the norm. The torque curve is basically flat, but the 50mph on hills was the clue as to where the “sweet spot” was in the torque curve.

After talking with my friend who owns a large diesel truck repair shop, I decided to make some changes. I investigated the logistics of changing transmissions, or using a modern day aftermarket auxiliary transmission. Changing the tranny was VERY expensive, and permanent. None of the currently available auxiliary OD transmissions would hold up behind this combination. So he suggested getting an old Spicer auxiliary gearbox.

He believes using the OD will get the Cummins in a better RPM range where we can actually go faster AND get more mpg. He believes I’m looking at 2 to 3mpg more. He also gave me a large a intercooler from an OTR truck which ought to get me 1 to 2 more.

For those that don’t know, this is a manual transmission, that is mounted after the main transmission. It is otherwise identical to a normal manual transmission except that it has a driveshaft in and out.

Pictures of the auxiliary project are here:
Spicer Auxiliary

Whole motorhome project here:
Whole Restoration

Spicer made variations of these units until the mid ‘70s. Mine is a 7231-D. “D” is the gearset for the box. Mine has a 2.14:1 under drive, 1:1 direct, and a .86:1 overdrive. I currently have no plans to use the under drive, which would yield a top speed of 37 mph. It can be shifted into low manually, but you would have to crawl under the rig to do it. The casting date on my case is 12-2-69. It was in a late ‘70s F-800 when I bought it. These are also called "brownie boxes," "brownies," or "brownlipes" as the company Spicer originally bought out was Brownlipe. Apparently "Brownie Box" was a synonym for auxiliary transmission.

This series of box was used in tandem axle dump trucks and some OTR trucks. It hauled much heavier loads, behind much bigger engines than I will. I am told it is not possible for my rig to break this gearbox. One nice thing about this unit is that it has a neutral position. So if something goes wrong, you can shift to neutral and do not have to worry about your driveline parking brake.

This is a non-synchro unit. It must be shifted at very low speeds, or parked. I used an electric linear actuator from ebay and a custom linkage to shift the D-N-OD rail. Provisions are there, to use lawn tractor seat switches to turn on indicator lights on the dash to show what gear the box is currently in.

As far as mounting this thing. I did not want it to be the lowest point under the chassis. It turns out the stairs and the muffler are just a tad lower. I made a T shaped cradle that sits on top of the bottom web of the framerail in the rear, and has two legs going forward to catch the crossmember just behind the main tranny. I used rubber mounts from McMaster-Carr 6309k3 - $6 each. The majority of the weight is on the rear of the cradle, thus it sits on top of the bottom web. The front of the cradle bolts to brackets on the bottom of the crossmember. I used metal conduit reducer washers as shims to tweak the pitch of the gearbox for better driveline angles. They matched the OD and ID of the bushings perfectly. Plus the mounts are designed for 7/16” plate. I used 3/8”. I cut the existing ford rear mount and welded it to the cradle. I used the existing aluminum clamp mount up front.

This thing weighs close to 200 pounds. I can probably just barely bear hug it to move it from truck to stand. I stopped lifting it manually after I had the countershaft installed in the case. I parked the motorhome on 6” blocks. I wrapped the gearbox in a blanket and shrink-wrapped it. I used the engine hoist to set it onto a hand truck to get it to my wooden ramp. I used the ramp to shimmy the gearbox up onto my floor jack. I then jacked it into place and tightened and loctit’ed the bolts. The gearbox mounts rigidly to the cradle, the cradle uses rubber mounts to bolt to the frame.

It took 4 hours on a work stand to get the tranny perfectly level and aligned with the cradle. Then I tack welded everything and had the fab shop do the final welds. After it was all welded and painted I found out that the stock tranny pitches downward at a 6 degree angle. I then had to lower and pitch the gearbox. This was done by cutting the cradle down, and shimming the bushings, and also shimming the gearbox to the cradle. Luckily the rear flange had tweaked after final welding, which pitched the box perfectly. Five washers were used between the front mount and the cradle.

On our first trip, I left the box fixed in OD and noticed almost no difference in off-the-line acceleration. Highway driving was absolutely magnificent. Words cannot fully describe how happy we were with that box. Cruising on the governor was 70 to 72 mph. At one point on a slight downhill I hit almost 80, and immediately lifted! Uphills were about 53mph. It makes no noise inside – even when pressing your ear to the wood floor.

Top speed wasn’t our main goal, so I backed WAY out of the throttle and cruised between 65 and 67 for most of the trip. It takes quite a bit more pedal to rev the Cummins up to run on the governor. Interior noise was much lower – oddly lower. I kept thinking we were going slow.


As far as costs go. I bought the box for $300 off Ebay. I paid $150 to ship it. It needed to be rebuilt, that kit was $420. I then found out that the mechanics style yokes were trashed and no longer desirable for a highway vehicle. New companion flanges that would allow use of a 1410 or 1480 U-joint were going to be $300 apiece! I called a few friends and managed to scrounge up an acceptable companion flange for the input and a 1480 yoke for the output from a nearby junkyard. I also grabbed a few 1410 flange yokes and some other bits for a total of $37! The driveline shop was not as happy about this as I was!

I had the yokes sandblasted and needed to get custom spacers and machinery shims to get the thru-bore lengths right and get the new yokes in proper position for the seals. I needed to get two new seals for the input and output as well.

I painted everything myself, relocated the carrier bearing, and spent $420 for a new yoke shaft on the input side and to shorten my two rear shafts. I also added a heat shield for the exhaust. The yoke shaft on the input is 10 5/8” long. The rear shafts are roughly 48” and 41” long. The carrier bearing was moved back and mounted right to the crossmember and dropped ¾”.

One of the new yokes needed a new nut and a hardened washer. The various machining for spacers and the final welding (I just tacked everything) cost me $175. The steel to make the cradle (with plenty left over) cost $70.

Total cost was about $1600.

I can’t comment on fuel usage as we never filled it up, our first trip was too short. We will definitely do this on a later trip.

Before our next trip I will likely have the intercooler installed and have turned up the pump pressure a bit. I don’t care to take it right to 230hp, but a little more power to improve drivability while towing a car.

I did not hook up the speedometer to this gearbox. I used our GPS to figure the relevant speeds on our existing speedometer. I will someday.

* This post was last edited 09/02/09 09:21am by 50pascals *   View edit history

Trailer Trash 2

Santa Fe Springs, CA

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Posted: 04/13/09 09:25pm Link  |  Print  |  Notify Moderator

Very nice discription of the Spicer 7231 and the install. your pictures of the restoration were great too, you did a fabulus job.


Don & Georgia
AKA: Trailer Trash 2
Real trucks don't have spark plugs.
2009, Dodge, 3500, Q Cab, Cum/Diesel, D.R.W.
Pulling a Super Fine Montana 2980RL


Corkey05

Washington State

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Posted: 04/14/09 04:21am Link  |  Print  |  Notify Moderator

Enjoyed every word, nice write-up. Splitting the pic's into 2 groups helped understand your project.


2008 HR Endeavor PDQ - Ford Edge 4 Down
FMCA F374292


50pascals

rochester, ny

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Posted: 04/14/09 06:51am Link  |  Print  |  Notify Moderator

Sorry bldrbuck - didn't write it for entertaining. I researched adding these for 6 months and never found anything like this. Someone contemplating adding one of these will find it someday. I use forums like this as an online archive. This will be here as long as the internet can find it.

Stim

NE Florida

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Posted: 04/14/09 10:39am Link  |  Print  |  Notify Moderator

Back in the mid 70's I had a Boom Truck and hauled concrete block and building supplies. The truck was a Ford with a Cat engine and had a 5 speed main trans. and a 4 spd auxiliary, two shift levers. Which gave me 20 spds forward and 4 reverse! Once I learned how to drive that truck it was awesome! I lived in Cincinnati at the time which has many hills.
I'm not sure what trans the aux was but I routinely hauled 35-40,000lbs. It was fully synchronized though. I used it as an over/under (3rd-4th) when a full shift with the main was too much.
You didn't mention how much profanity was involved?

50pascals

rochester, ny

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Posted: 04/14/09 11:14am Link  |  Print  |  Notify Moderator

Stim wrote:

You didn't mention how much profanity was involved?


Actually, very little. I tend to overthink everything. Plan it all out to the last detail, then execute. I drew the whole chassis and components in AutoCAD. Then it's just a matter of click and drag. I drew 6 different mounting configurations before I settled on this one. Between the driveline brake hydraulic cylinder and the muffer, there wasn't alot of room.

For example, when I discovered the tranny was tilted downward, I just came inside and started moving the gearbox/cradle down. This told me how much to cut off the rear. Of course the first cradle was already perfectly fabricated and painted!

I do this stuff for therapy. Therapy ought not be stressful!

I am planning to shift it into OD as I am getting on the highway. Just let off the throttle and hit the button.

On another forum they tell me if I want to go up and down in the Rockies I should get a lever on that underdrive gear!

By that point, I would probably add another hydraulic disc brake to the output shaft and Tee into the rear brake line with a proportioning valve - That's stopping power!

50pascals

rochester, ny

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Posted: 04/14/09 12:16pm Link  |  Print  |  Notify Moderator

Daveinet wrote:

I have a general question. Why don't more RVer's go for a 2 speed rear end? I remember the school bus I rode in for many years had a 2 speed rear end. Seemed to work fine, we would hit the freeway, let off and pull the lever.

BTY: I always like seeing process pictures. Thanks for sharing. Its a pain to stop and take photos while you are working, especially when your hands are greasy.


I looked into a 2-speed. What I found (for me) was that they basically had a low gear and then an even lower gear. My rear has 3.73 which was the tallest available in a two-speed. So I guess if I had 22" rubber it would be different. I investigated getting a different brand entirely, but there are lots of variables there, like too many! Plus, my axle is fairly light duty (10k) as two-speeds go. So finding a light duty 2 speed with the ability to use wheels somewhat resembling mine got to be a bigger task than it was worth!

If you happened accross a truck with the exact axle you needed, it would be fairly easy. Otherwise, there are too many brands and configurations to investigate. Most salvage yards don't know the details, they know the weight ratings and that's about it.

don/lou wrote:

Great Job & excellent post!!
Did a 93 1/2 dodge cummins and auto trans swap into a 83 28' Pace arrow and changed diff to 3.83 ratio -with the .81 over in the trans it was great
sold it to brother and he put lots of miles on it
See it now and then -still going strong
Good luck on future use


I considered this, but many people told me the Allison I have was a better choice than the Dodge trannys (New Process I believe). These are also "Dodge only" tranny's - as I was told. If it was a lighter rig, this would have been more viable. Also they have no provision for the driveline parking brake. Controlling it and electronics was also a question.

We are about to do your swap into a 90's era F-Super Duty wrecker though!

don/lou

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Posted: 04/14/09 12:53pm Link  |  Print  |  Notify Moderator

Mine was a AUTOMATIC OD dodge trans
the parking brakes were cable to the brake drums
He has put over 150,000 miles with no trans trouble
JFYI
don


Don & Louise
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don/lou

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Posted: 04/14/09 11:44am Link  |  Print  |  Notify Moderator

Great Job & excellent post!!
Did a 93 1/2 dodge cummins and auto trans swap into a 83 28' Pace arrow and changed diff to 3.83 ratio -with the .81 over in the trans it was great
sold it to brother and he put lots of miles on it
See it now and then -still going strong
Good luck on future use

Daveinet

il

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Posted: 04/14/09 11:45am Link  |  Print  |  Notify Moderator

I have a general question. Why don't more RVer's go for a 2 speed rear end? I remember the school bus I rode in for many years had a 2 speed rear end. Seemed to work fine, we would hit the freeway, let off and pull the lever.

BTY: I always like seeing process pictures. Thanks for sharing. Its a pain to stop and take photos while you are working, especially when your hands are greasy.


Dave

The Flying Fortress
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502 w/Howell/GM 16197427 ECM/Edelbrock MPFI,Thorley's & Magnaflows,
4L85E 4 speed, KoniFSD,
6% grade = wanna drag?
MISC photos
Revcon Forum


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