Engine/Transmission (1994 - 1998) 1997 Automatic Downshift Under Load

[DonTX]

I just got back from thousands of miles towing my fifth wheel, I weighed in at 19,800 lbs total weight.

My only complaint was the downshift of the automatic under load. When approaching a hill with the cruise engaged, it will just pull until the EGT gets to redline and then shift down, usually I just back off the cruise and manually feed thottle waiting for a downshift that should come sooner I feel. I thought I could just hit the OD button for an instant downshift, but that seems to be a rough "double shift" which I think is the OD shifting down as well as the torque converter unlocking.

It was this way ever since I bought it I think, can anyone give me some feedback what is happening and how to make it act nicer?

When is the OD supposed to shift down, and when does the torque converter unlock, and more importantly just what determines when each event happens?

(I don't think this has anything to do with my other shifty problem below)

 

[Howard Durand]

Not sure about the 97 but in my case, the "downshift" under load is the TC unlocking. If you feel you need it sooner, pressing the accelerator hard should unlock it too. If you are pulling hard and still losing ground, the OD will disengage.

If you manually lock out OD with the TC locked, the TC will unlock first then it will shift out of OD. Following that the TC may or may not lockup. The PCM controls this and seems to have a mind of it's own at times.

 

[DonTX]

Thanks a bunch, you are describing what I am speaking of. Apparently mine is working as it should I guess, I just need to modify it so I can control it better it seems.

I am not a screamer with rpms, and pressing down tight to the floor, well I am not sure I ever tried that! I would give it more accel beyond where it was pulling, but it still just don't seem to shift down, I prefer NOT to use WOT for downshifts. Somewhere I saw something about putting the torque converter lockup on a switch, perhaps that is what I need to do. I suppose the TC unlock cannot be adjusted?

Not sure about the 97 but in my case, the "downshift" under load is the TC unlocking. If you feel you need it sooner, pressing the accelerator hard should unlock it too. If you are pulling hard and still losing ground, the OD will disengage.

If you manually lock out OD with the TC locked, the TC will unlock first then it will shift out of OD. Following that the TC may or may not lockup. The PCM controls this and seems to have a mind of it's own at times.

 

[Joe G.]

Don,



You might want to read this thread to get a better understanding of what is going on: https://www.turbodieselregister.com...mission-1994-1998/156901-tps-myth-buster.html

 

[DonTX]

Wow thanks, that is good stuff. I have read it twice and still have smoke coming out my ears from the strain. This is getting more interesting post to post.

Don,



You might want to read this thread to get a better understanding of what is going on: https://www.turbodieselregister.com...mission-1994-1998/156901-tps-myth-buster.html

 

[Howard Durand]

Since that thread I have completely replaced the TPS with a potentiometer and after several trips back to Nebraska, towing a 7000 lb travel trailer, I am fully satisfied with the results. I also use a speedometer circuit to give me more control over the TC.

You will find that the potentiometer will allow you to set the shift point for overdrive but TC lockup follows too soon in most cases. That is OK when not towing but the TC locking so quickly doesn't give the engine time to get under the load and causes unnecessary lugging.

With the speedomter circuit, I can hold out TC lockup until the vehicle speed is up.



I find that there are only two settings that I use for the potentiometer.

When not towing, the speedometer circuit is disabled and the pot is set so OD shifts between 35 and 40 mph. TC lockup follows shortly after OD.



When towing, the pot is set so OD shifts at 45 mph and the speedometer circuit is set to allow TC lockup at 55 mph.



One drawback to this system is that you no longer have "kickdown" capability to unlock the TC. It will remain locked until you tap the brake pedal or engine speed drops below 1200 rpm. This is a benefit when using an exhaust brake but can be troublesome when you want to unlock the TC without dropping out of OD.

I bought an old fashioned dimmer switch and use it to unlock the TC if needed.

There are other ways as well.

 

[DonTX]

My previous owner had some issues with the TPS, I think he said it needed a shot of lube once in a while.

With my gearing and use, all I really want to do is make it shift down (out of TC lockup apparently) sooner, all other shifts seem very nice with my gearing. UP shifting is perfect, don't want to change that either. Usually when I let off the accel, it goes back to idle, I like that too since I have no exhaust brake, loafing engines use less fuel.

 

[biggy238]

What TDR issue had the POT wiring diagram?

 

[Howard Durand]

The only diagram I have seen in the TDR was one of the potentiometer in the ground leg of the TPS. That scheme was designed to give the operator a means of adjusting the TPS signal voltage. It was successful but didn't do as much good as the total TPS replacement has. I don't remember which issue it was in though. If you are interested I can e-mail you a copy.

 

[biggy238]

No i'd rather have the function that you have described. I want my OD only at 60mph and have no way to do that.

 

[Blue Coyote]

My 97 did the same thing. Did every diagnostic check on the system and TPS and it was all good. A DC tech friend of mine said that is normal on these



Finally just by passed the TPS completely and wired in a potentiometer (same thinge that Rides A Mule did). It works great. You can control if and when the TC locks up in 3rd or OD. No lock up for heavy traffic, locked up in 3rd for hard mountain pulls, locked up in cruise, etc. The TC will unlock regardless of pot position at around 25mph



Only down side is no transmission down shift and it you do have to spin the dial sometimes.



I tow a 38' 12k GN trailer - and would not tow with out this set up.

 

[Howard Durand]

This is what I am using. The fixed resistors establish the maximum and minimum signal voltages that can be applied to the PCM.

I don't think they are really necessary and can most likely be eliminated. You could just use a common 2K to 5K potentiometer connected in the same configuration.