[docwatson]

I heard about a mechanical overdrive (four speed) automatic tranny made in the mid 90's which might work in my 1990 Cummins 5.9 250. It currently has a 3-speed automatic. I have also been told there is a mechanical overdrive tailshaft I could use on my current tranny. Do you folks know which model numbers I should be looking for?

 

[ghostman]

None of the 727/618/47RH series have a mechanical OD. The 47RH has TCC and OD, but both are electrical. That transmission came in the 94-95 year models. The rest of the transmission is hydraulically controlled via govenor housing.

 

[docwatson]

Thanks

Thanks for your reply. it sounds like I can't use any available overdrive tranny without some adapting. Is it feasible to use an electric switch of some sort to engage the overdrive? I had a Triumph GT6 with a manual tranny and electric switch for the overdrive.

 

[ghostman]

Yes, on a 47RH there are only three wires that run to it. One is a 12v +(hot) and the other two are the grounding circuit for the TCC and OD. You can do something as simple as putting a switch connected to ground and connect to transmission. When you flip the switch, it engages.

 

[docwatson]

Thanks again,

Are you aware of any fitment problems? The truck has the Cummins adapter for the 3-speed. I suspect I will have to use another driveshaft or shorten the one I have. What about the torque converter and pilot shaft?

 

[ghostman]

Thanks again,

Are you aware of any fitment problems? The truck has the Cummins adapter for the 3-speed. I suspect I will have to use another driveshaft or shorten the one I have. What about the torque converter and pilot shaft?

If your current trans has lock up TC then it all should interchange. Ofcourse driveshaft changes.


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[docwatson]

That is all good news.
Thanks

 

[1070]

gearvenders makes a od that goes on the back of your current trans they are 2500-3000 bucks if i remember right.there was one for sale on here not to long ago you might look back in the forsale section and see if its still there.

 

[ghostman]

gearvenders makes a od that goes on the back of your current trans they are 2500-3000 bucks if i remember right.there was one for sale on here not to long ago you might look back in the forsale section and see if its still there.

Whoa! For that price he could buy and build himself a nice 47RH.


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[docwatson]

Well guys,

I think I saw one on eBay for a lot less. I am still sorting all this out. I would have to drive a LOT of miles to pay for a $3000 overdrive. If I saved 10c a mile, I would have to drive 30,000 miles. Not a likely scenario for a 24 year old truck.

 

[1070]

i tried putting 307 gears in my 91.5 with a 518 my fuel mileage went down.if your after mileage a lockup might work or a 5sp.i think there is to much loss in a 727-518 converter.as said above a 47rh is the way to go.

 

[docwatson]

I have a 3.58 Dana 60 rear end from a 1999 2500 I plan to install under the truck. It is a converted airport tug on a 1990 Dodge 250 chassis. It is a "bobtruck". I will pull a trailer with it, but need a taller ratio for running empty. That is why I would like an overdrive. I am looking around for the 47rh you recommend. Not sure what you mean about the TC. Are you suggesting I not use the one from my 3-speed?

 

[1070]

the 3sp 727 and 4sp518 transmissions are not lockup transmissions.the converters are always slipping a small amount a 47rh converter locks so it pulls like a clutch.you might be ok with those 3.54 gears and a 518 thats what i have now.i am still not happy with mine,i am getting ready to put a 5sp in mine,i am a clutch guy i guess.

 

[docwatson]

Wow, so many factors. I will look for a 47rh with locking TC. My truck has only 28,000 miles on it, so someone might want the 3-speed tranny and TC. I can drive a 5-speed, so I will consider that. I started driving in 1959 and bought my first automatic in 1988. Right now, I am looking for a simple conversion to get it on the road, and installing a clutch is just one more factor to slow the process.

 

[ghostman]

Check on the classifieds here and CompD as often a 47RH will pop up. All 47RH are locking TC.


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[docwatson]

I had a thought. Since I will be engaging the OD with a manual switch, could I use a transmission from a gas model, like a V10 or 360?

 

[docwatson]

Guess I will have to upgrade my membership before I can see the classifides. What is CompD?

 

[ghostman]

I had a thought. Since I will be engaging the OD with a manual switch, could I use a transmission from a gas model, like a V10 or 360?

No you want one from a diesel. The govenor weights , VB ,and internals are different.


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[docwatson]

Like I said, just a thought. Gas models are more common, but I want it right or might as well stay with the 3-speed. I found CompD.