Engine/Transmission (1994 - 1998) TPS Replacement Revisited

[Howard Durand]

For those who have replaced their TPS with a potentiometer or are considering it here is what I am using and the results I get on the road.



The stock TPS is still in the truck but completely disconnected.

I ran a set of three wires from the PCM to the dash mounted pot which is a 1000 ohm, ten turn potentiometer with a 500 ohm fixed resister at each end.

I use a turn counting dial so I know exactly where the pot is set.



With the pot set at zero, a minimum signal voltage of 1. 2 volts is applied to the PCM. That results in a shift into OD at 30 mph and the TC will not lockup no matter how fast I go.

At the next setting which is one full turn on the pot, OD shifts at 32 mph and TC locks at 44.

At the other end of the potentiometer, turn #10, it shifts into OD at 62 mph and TC at 68.

I plan to run with it set at position #5 which shifts into OD at 45mph and TC lockup at 50. If I am towing a load, I use a vehicle speed controlled relay to delay the TC lockup.



The one drawback to this setup is that it prevents manual kickdown or downshift under load. I have overcome that by using a foot operated dimmer switch to open the TC circuit.

At the end of our summer "vacation" I'll have a better idea as to how I like it.

The biggest gain with this setup is that, unlike the stock TPS, it is reliable. If it is set for OD shift at 45 mph, that is what it does.

The only variable I have seen is a minor change as the vehicle comes up to operating temperature.

 

[Ridesamule]

I have to hand it to you Howard your research and reporting has been awsome. Just might be a market for a TPS replacement Smart Controller in your future.



Again, great work.

 

[Howard Durand]

Ridesamule, thanks for the nice comments. You started this, you know.

I enjoy doing this stuff but wouldn't want to spoil it by trying to make money at it.

I have been a gun nut all my life and, at one time, thought I wanted to do stock making and be a gunsmith.

I soon gave up that notion.

I found there is a big difference between doing something for your own enjoyment and having to get it done for an impatient customer.

 

[Willys]

I'm happy with mine as well. It doesn't kickdown under load, but turning the dial up or down past the lockup voltages briefly causes it to unlock. So far I've only had to do that a few times. The truck shifts into overdrive and lockup depending on throttle pressure it seems. So if I'm driving lightly, it might lockup at 42mph, if heavy throttle, 50+. So with a trailer behind me I tend to give it a little heavier throttle and it locks up at a decent speed. I only have a few thousand miles on it since I've done this. I'm swapping my gear from the '96 to the '95 soon so I'll get to test the longevity of it more this summer. Thanks for posting how to do this.

I just happened to find this thread, since I don't scan the TDR on a regular basis, so if you replied to the original posts perhaps more folks that have done the same mod might reply? I tend to subscribe to threads when I post... others might do the same?

 

[Ridesamule]

Willys what you see for shift and lockup is the same as me. I figure it is the nature of the RE vs RH Howard has.

I never bothered to investigate it because if I am just putting around I like the early shift. When towing or trying to accelerate rapidly the shifts naturally come later.

My guess is an RE has a heavier reliance on the TV cable than the RH with this setup.

 

[Willys]

Ridesamule... sorry for being unclear. It's on my '95 with the RH. I haven't had a need to install anything on the '96 yet as the TPS is working fine on it and the transmission is really slushy. Whenever I get around to rebuilding the transmission I'll likely install the potentiometer in that too. I really like how the '95 works though. I think if I buy another auto/cummins truck, it will be a '94 or '95.

 

[Howard Durand]

I haven't tried advancing the pot momentarily to see if that would cause it to downshift. If so, that would open up the experiment to designing a TPS cable operated pot. I would like to get ahold of a couple of spawned out TPS bodies to see what could be done along that line.

I do believe that the success of this experiment has been due to the fact that the PCM receives a set signal voltage and is able to operate accordingly.

 

[Willys]

Howard, I think the reason mine will unlock when adjusted up or down is I used a 5k ohm pot. It allows me to adjust above or below the voltages. Apparently its not enough to damage the PCM, at least not yet... I didn't intend on doing this- it was the only pot within an acceptable range I could find at the local radio shack. I don't use this feature often though, since usually its easier for me to tap the brakes to force an unlock. The floor mounted dimmer switch is a good idea and I might give that a shot at some point- perhaps with a dash mounted LED to determine whether the switch is still engaged.

 

[Howard Durand]

Willys, I tried advancing the pot today in an attempt to get a downshift but it doesn't work. I am guessing that, with the 10 turn pot, I can't raise the voltage quick enough. I like the dimmer switch though so don't think I am going to make any more changes except to add the LED you mentioned. Our annual road trip this summer will tell me how well everything is going to work.



On a different tangent, I have made up another little controller that will lock out overdrive if the EGT gets above a set temperature. It was necessary to put in a second EGT probe to make it work but that isn't really a drawback.

I think it is going to be worthwhile when I finally get it set.

There is a commercially made device that does the same thing but it is a little more expensive.