Engine/Transmission (1994 - 1998) auto trans driving habits

[dray]

While driving my auto Cummins in the city, would it help the trans life to turn off the O/D? It seems to shift into O/D at a low speed and I wonder if this will cause the trans to heat up? Without O/D and with the higher RPM's, would'nt this help the transmission? Seems like I've read that the auto has low line pressure, which is one of it's faults.

Thanks

 

[Bob Wagner]

dray, good observation, Lock out OD when traveling under 55MPH is a very good habit to get into.



Keep the RPM's up in the 1600+ range



Good luck to you

 

[Jim Fulmer]

Dray, nice to see another Oklahoma CTD driver around, I've programed my mind to turn the O/D off everytime and do just what Bob is saying.



Jim

 

[redramnc]

Stop and Go = OD off

You get used to hearing the motor run 2K rpm. Now it bothers me if it gets below about 1800 rpm. If I am hauling I don't turn on OD until I am out on open road where I can run 65+. I will point out that I have 3. 54 gears so 65 is 2500 rpm with my stock size tires.



My next step will probably be a smart controller so I can tailor the OD lockup to my liking. The tendency for it to go OD at <45 mph is annoying me - hence the manual OD switching.

 

[Pit Bull]

I understand it is a good idea. I just figured this out myself. Trucks got 50K miles on it and I hope that this practice helps me get over 150K out of the stock unit.

 

[SlyBones]

Re: Stop and Go = OD off

Originally posted by redramnc

My next step will probably be a smart controller so I can tailor the OD lockup to my liking. The tendency for it to go OD at <45 mph is annoying me - hence the manual OD switching.

My experience is this is partly correct. The smartcontroller does not control when lockup occurs. The PCM does this. The smartcontroller HOLDS lockup for engine/exhaust braking purposes. It also overrides when the transmission shifts from 3-4 and 4-3 downshift.



On my truck, with the OD button off, lockup still occurs between 48 and about 65 or so, according to what the PCM thinks. With the OD button on lockup occurs at 30-40 depending on what the PCM things.



Anyways if I set the SC so that the 3-4 shift is at 55MPH ( for example ) and accelerate thru the gears at light throttle what happens is ( OD button off ):



1. When the PMC wants to shift to OD ( about 35 ) the SC intercepts that and overrides it keeping you in 3rd.



2. The PCM see's that I have reached 48mph and says go into lockup. The transmission goes into lockup. But its in 3rd gear lockup not OD like the PCM thinks.



3. The truck reaches 55mph, my selected speed, and the SC instructs the transmission to shift to OD. It does so. Now I am in OD lockup.



4. I let off the throttle. The SC holds lockup and the truck decelerates like a manual transmission would. RPMs lower as the speed lowers and does not just drop to idle.



5. The truck slows to 55. The SC instructs the transmission to shift into 3rd. The transmission does so. Lockup is also held. The truck continues to decelerate like a manual. Only the RPMs have risen as a result of the downshift and the ebrake ( if enabled ) is working much better.



6. The truck slows to 30mph ( adjustable ) and the SC removes its hold on lockup.



7. Lets say that I drive 70. Decelerate to 40mph. The downshift occurs at 55. But since I did not reach the kickout speed, I am driving around in 3rd locked. If re-acclerate above 55 into OD lockup it goes.



8. The OD button on the SC holds lockup as always, but no 3-4, 4-3 shifting takes place. The transmission stays in 3rd like normal.

 

[redramnc]

Thanks for the explanation

Great info! I have not got around to looking deeply into how a SC works so your explanation is much appreciated. I know somebody who has one so I will now be able to discuss the way his works (or doesn't work) with a little more understanding.



I wonder if there is a way to fool the PCM into not locking up so early? Might be worth looking into... ... (like I have any spare time right now).