Improving auto trans (4 speed)

[RobbieH]

I think I read somewhere that the 4 speed auto on the 91. 5 can be improved for better shifting and longer life by doing something fairly simple. I think it had to do with pump pressure, but I don't know for sure. I was unable to find anything by searching.



Is the 4 speed auto in the 91. 5 a 47RH or what?

 

[CDegner]

The 4 spd auto in the 91. 5 is the A518 or 46RH, different names for the same transmission. There are a number of adjustments that can be done. Do a search for post by HDM48. He has some good descriptions on what needs to be done. The bands can be adjusted adn the line pressure can be increased a little.



It has been done on my transmission and it does help.



Carl

 

[cerberusiam]

Like Carl said, A518 or 46RH. Its essentially a 727 with an OD bolted on the back. The base trans is pretty tough and given good care will take a lot of use and abuse. The OD is the weak spot with the hard parts being susceptible to high torque accelerated wear. If it has a lot of miles adjusting the bands will help some but if there are other problems it will just hasten the inevitable. You can increase the line pressures and OD shift pressure by way of screws on the left front corner of the valve body. If you keep the adjustments within 10 lbs of stock it makes a nicer shift and won't cause other issues. Cranking pressures up too much will cause issues with direct clutch pack apply piston and the 2nd gear band release. Higher pressures have a tendency to roll the piston seal and cause excessive leakage when applied and leads to issues with a 2-3 bindup or cut loose depending on how things shake out. The OD clutch pack was a problem in the earlier models. The OD direct is 10 clutches backed up by a one way sprag and a 800 psi apply spring and is spring apply hydrualic release. It is sturdy and given everything else hangs together works well. The OD clutch pack was only 4 disks on the early models and the factory clearances were too tight when towing or really putting power thru it. They tended to apply before the directs released and would eat the OD unit up. Letting the upshift to OD happen at low rpm's in the torque band and/or loaded was a good way to kill it fairly soon. Keeping the trans in drive until at least 60 mph so pressures and rpm's are up will minimize the wear and extend the life. Youc an drop the OD unit for maintenance and adjustment if so desired. Just make sure you find somebody with enough experience to know how to do it. The single best way to preserve the trans is change the fluid regularly and keep the max temps under 190 to 200 degrees. If you use it hard change the transmission fluid every third or fourth oil change and get a trans temp gauge. I have the same trans in a 92 and have 280k with most of those miles towing a 5'er or 4 horse slant. Shifts are still good but have finally wore the OD planetary into submission. It still works but it howls at me. You can push up to around 250 HP and then the converter starts to be way too loose. The A518 has taken a lot of abuse but it is a good piece and given a little care and judicious use will give good service. If need or want to rebuild it most of the heavier 47RH and some of the 48RH hard parts will fit. Just make sure you find somebody willing and able to work on the trans knowing it is behind a Cummins. Too many shops either don't know or won't learn it ain't the same as the gassers they work on every day.

 

[BushWakr - RIP]

Well... I guess all that is left to say is... ... . BULLSEYE!!. . cerberusiam.

Excellent post and summation. .



pb.

 

[RobbieH]

Is there a rule of thumb when adjusting the screws?



The trans seems OK right now. When he first bought the truck it was shifting funny, but the new owner and the previous driver babied the heck out of it, I got in and pushed it hard for a few shifts and it seems fine now.



The truck currently has 200,000 miles. The previous maintenance is unknown, but given what I've seen so far it was probably minimal at best.



We've built a list of what needs to be done. Trans fluid and filter change is certainly on the list. I figured while we were in there we could readjust the bands and crank the pressure up a little.



The goal here is to make it last as long as possible.



If the transmission is rebuilt, can the later model OD parts be put into it or will the entire OD unit have to be replaced?

 

[cerberusiam]

If I remeber correctly its about 5 psi per turn on the line pressure screw given no other issues. Check and make sure the TV cable is not set too loose also. The norm for the babied trucks was a loose setting so the shifts were light and and early. By setting the TV cable tighter it will increase the line pressure at shift points and make a firmer shift and reduce the amount of slippage. If it will be used for towing you must get the OD shift point up if your are going to use the power. The OD will not live long towing if you are "driving it like you stole it". If the truck is driven normally and not much towing and maintained the trans could go another 100k before needing work or it could die tomorrow. At 200k you are a bit beyond what Dodge ever figured the life expectancy to be on that engine trans combo.



If you service the trans I would highly reccomend flushing the cooler lines very well. If the maintenance schedule hasn't been good all the crud will hang there and be flushed out when new fluid is introduced. Be careful with your choice of replacement fluid also. I think it originally used Dexron II and if you want to keep things as long as possible use it again. While ATF III and ATF IV are a better fluid and are recommended by a lot of people and DC they can definitely cause seal issues plus cutting thru all the accumulated crud in the trans. I used ATF III in mine without issues but others have not been as fortunate.



Some of the OD parts will interchange but the unit as whole is not the same critter. The later OD's are a bit longer and a little different design to handle the increased TQ and HP. I am told it is possible to adapt a 94 to early 96 47RH OD to the 518 with some mods. That gives you more OD direct and OD clutches, heavier parts, and better cooling but you have to modify drive shaft lenghts and trans mounts. It is possible to upgrade to the A618 and gain a lot of the benefits plus a lockup converter but there are modifications to make and new parts to but to get it to fit. A good trans shop can build a 518 to much better than a stock unit and for a lot of people and their use it is more than adequate.



Does that help? Hinder? Confuse? I have basically hit the high points on the trans issues. There is a lot more info available in the threads and just asking will get you a broad range of responses and experiences. I apologize for the treatise but there is a lot of info available. Good luck.

 

[RobbieH]

It will be used to tow approx. 7,000 lbs on a regular basis.



Thanks for all the tips, keep 'em coming!



Hmm, I would have certainly suggested that he use ATF+3, I figured that is what Dodge put in everything back then.



Modifying the shaft length is no big deal tu us, we do this sort of thing all the time.

 

[RobbieH]

Here's what he quoted to me from his owner's manual:



"It is important that proper luricant is used in the transmission. MOPAR ATF PLUS (AUTOMATIC TRANSMISSION FLUID TYPE 7176) or equivalent should be used to aid in assuring optimum transmission performance. Fluids of the type labeled DEXTRON II Automatic Transmission Fluid should only be used if the recommended fluid is not available. "

 

[cerberusiam]

Ouch! Dated nyself on the Dexron II. I was off a couple of years there, ATF +3 it is. JMO, switch to the newer 47RH with a big triple disk converter and the upgraded parts since modifications are not an issue. The later cases were stiffer and while you are towing the ability to lock the converter and keep things cool will be a big help. The 518 will work for towing and all the big names can build you one that will be great but the fluid connection is just not effective as a lockup converter and when you get a tighter converter the temps can be a issue depending on ambient temperature and terrain. In addition, the OD takes a beating towing and the later models have a better, not great, setup to handle it.

 

[JStraw]

The down side to the stock transmission is the aluminum carrier assy and non-hd clutch packs. Towing will take a serious toll on these parts. I totally gutted the transmission by pulling a 5'ver. And not by lead footing. It just all went south. Now its all HD and steel.