Engine/Transmission (1994 - 1998) TPS myth buster

[Howard Durand]

Ridesamule,



Interesting observations. I am using a combination of fixed resistors along with the pot. The signal voltage varies from a low of 0. 8 to a maximum of 2. 5.

At the lowest setting, the OD shifts at 32 mph and the TC will not lock up.

At the high setting, OD engages at 47 and TC locks at 51.

I have not tried it with OD locked out.

I may change one of the resistors to get a little higher shift point for the OD.



The only drawback to this setup is the loss of kickdown or automatic downshift but that is not much of a problem. I am using a foot operated dimmer switch to accomplish that.

 

[Willys]

I can't give too many observations since I've only been testing a few days, and had to have the truck towed yesterday since I had a trans. line leak pretty bad(compression fittings, not the plastic).



I don't think there is really a speed difference when the truck shifts from 3rd to OD and then locks- say it shifts at your high setting, Howard, at 47 mph to OD, then TC locks at 51. In mine, under similar settings, it will shift, then lock, dependant sometimes on throttle, other times on time, so it seems. If it shifted to OD at 47 mph, TC lockup may occur at 47-48 if I back off on the throttle and stop accelerating. If I'm at WOT, TC lockup may occur at 53-55. That's without adjusting the pot. Leastwise, that's how its working in my '95.



On the lower end, I can keep it unlocked, though I wouldn't see a point in that? The lowest I've locked it was around 40-41 mph, highest was around 56-58, but I haven't played with the upper ranges much. This is in OD, with 4. 10 gears and 285/75/16 tires. IIRC it will lockup in 3rd at 27 mph the lowest, and 40 something the highest.



Howard, the 10 turn pot you mention- I'm new with the electronics stuff, but is that 10 turns of the dial, or 10 distinct positions- not infinitely variable like what we're using now? That would be interesting to try... If someone else drove the truck more often, it might be a lot better- just leave at a setting and forget about it. I like to be able to adjust the gearing and such to fit the load- or be the 'computer' as I like to think of it. I don't like vehicles that are designed to 'dumb down' everything about operating the equipment. I guess I should be driving a stick then... :-laf



Ridesamule- interesting how the RE must be working a little different. I might need to do a little side by side testing with my '96 and '95 to see how they operate differently. It might be hard to tell a whole lot since they feel so dramatically different- the '95 has a much tighter trans/TC, 4. 10 gears, and a more solid 'truck feel'(also feels like more power ).

 

[Howard Durand]

Willys,

The 10 turn pot requires 10 revolutions of the shaft to cover it's full range of resistance. It requires a 10 turn knob that advances one number on the dial for each revolution of the knob. Most of these knobs have a small lever that locks the pot at the desired setting. There are other choices, such as 5 turn pots, which might be better for these applications. Then a 5 turn knob would be required.

 

[DMC]

Well fellas, I just got through removing my TPS plug and installed the Potentiometer in the place of the TPS. I am really happy you guys figured out how to solve this problem. I had been lucky with my TPS it lasted 178,000. I have the Potentiometer under the hood for now. I will move it into the cab in a couple of days. I left my notes at home and bought a 10K spdt switch . 05 watts,<47mv,3amp 30vdc, cost $2. 69. :-laf I'm sure I didn't buy the exact one you guys are using but it works just fine. The transmission shifts great. Oo.



I haven't been on the forum much lately, and realized my membership had expired. I figured I would splurge and renew, considering my huge savings.



Merry Christmas Everyone



Don

 

[Howard Durand]

It is good to hear the reports from the readers that have tried this modification. So far, I don't think anyone has been disappointed.

There is obviously a number of ways this can be done and still achieve satisfactory results.

Whether you use a multi-turn or a single turn pot is not really important. I dont think that the resistance range is too critical either. I use one that has a total resistance of 3000 ohms. About the same as the original TPS.

With the dimmer switch available to open the TC circuit, I have a really handy way of getting a "kick down". I haven't tried it towing yet but really believe that this is the answer to my previous problems.

 

[JGrover]

WOW!... ... ...

After reading all them Auto trans post.

Glad I have a 5 sp :-laf

Hmmmm... ... . Do I turn my speedo cable to the left on my 5sp to unhook it

Yes , I do :-laf

 

[DMC]

You should be very happy that you have a manual.



I think I will go that route next time. I say that now but who knows.



To me a good manual would be made by Fuller. At least a 13 speed.



So far I'm really happy with the Potentiometer set up.

 

[Misenheimer]

Thanks all for the info

 

[Willys]

WOW!... ... ...

After reading all them Auto trans post.

Glad I have a 5 sp :-laf

Hmmmm... ... . Do I turn my speedo cable to the left on my 5sp to unhook it

Yes , I do :-laf

LOL... not to make this a stick vs. auto debate... but... of the 6 CTD trucks my family has:



'94 5 speed- had to drop the transmission for a 3rd gear synchro repair, clutch was just replaced as well.

'95 auto- TPS acting up, replaced with $3 pot. It was rebuilt before I bought it.

'96 auto- TPS replaced several years back. Trans is original, though could use some R&R at 287k miles.

'96 5 speed- just dropped trans. for complete rebuild, 5th gear nut had backed off, clutch was slipping(#11 plate :-laf )

'98 5 speed- 5th gear nut backed off at 41k miles, over 2k miles from home on a camping trip...

'98 auto- original, working flawlessly.



I love the 5 speeds(except in traffic), but a good auto is not a terrible thing. They've been about the same as far as reliability.



Update on the pot- it is still working well. I've put maybe 8-10k miles on it so far.

 

[SFrey]

Please excuse my electronics stupidity, can I run a pot & a t/c lock up switch ? I already have a t/c lock up switch wired like this rocker switch in the dash,(3-way on-off-on) middle position is no lock up at all, down it uses the ecm to lock up the t/c, and up position goes to a dimmer switch on the floor so I can lock up at any time. So I think I could keep this set up, and also add the pot to control when O/D comes in ????



thanks, scot

 

[Joe G.]

Yes. The circuits are independant. In fact, I think Howard has his set up like that.

 

[Howard Durand]

You can replace the TPS with a potentiometer and still use the TC lockup switch. The potentiometer just gives you better control over the OD shift points.

 

[DieselDoghouse]

I'm gonna jump in the TPS discussion with a quick idea. Looking at all the schematics for pot substitutions, has anyone tried this:



a 500Ω trimpot above the 2500Ω control pot and a 500Ω trimpot below to ground.



This would allow adjustment of the 'top end' voltage and the 'low end' voltage separately and expand the range of voltages that the 'sweep' of the control pot would express. Sounds complicated but its not. In model railroading before computers we would do this on transistorized throttles to compensate for motors that needed higher starting voltages, had lower top ends, or simply needed less/more voltage range compared to another. Sounds like it might help here. Just my 2¢

 

[Howard Durand]

Interesting experiment. There is no obvious reason that you can't do it. Once it is installed, you will need to do a test run and log the vehicle speed at which OD engages and TC locks up for each setting of the pot.

With the pot set so the signal is much less than 1 volt, OD will engage but the TC will not lock up. You need a sig of

+0. 9 volt or more for the TC to lock. After that TC lockup occurs soon after OD engages.

Being able to reliably control OD engagement is the primary purpose of this scheme and it has proven to be very effective. I use an electronic speed sensing circuit to control TC lockup which makes towing a real pleasure.

 

[Willys]

Just a quick update- I've put at least 25-30k miles on the truck since I first installed the 'manual TPS' and the transmission is still holding up and the dial works well. I have been reading about the homebuilt APPS units over in the 24 valve section and it's gotten me to thinking about rigging up a linear pot, maybe on the accelerator pedal so it stays out of the weather. Has anyone else tried this? It would restore some of the advantages of having a TPS such as kickdown under load.

 

[BILLVO]

Is there any adjustment or calibrations that can be done to a tps on a 97 or are they just good/bad? I have a friend having the unlock lock issues now and transmission shop Goerends said it sounds like a tps issue.

 

[EBramwell]

Back to the top.

 

[JackStone]

This seems to be a good description of how eliminate the TPS.



You need to get the following:

1)A 5K potentiometer, 1 watt rated. It should have three terminals on the back. I used a one turn pot, you can use a 10 turn if you want finer control.



2)6'-8' of 3 conductor 18 gauge shielded wire. The colors are red, black, and white. Don't use solid wire as it will tend to break easier. I couldn't find any of this so I used a red, black, and green 18 gauge automotive wire and made up a harness. The green was substituted for the white.



I mounted the pot in a small project box and secured this to the dash just to the right of the cargo light switch but you can put it any place you can get it to fit.



Run the wires from the TPS up to the dash. Cut the wires off the TPS leaving a few inches on the sensor. I used insulated push together connectors and on the end of the TPS installed the opposite type that were installed on the TPS wires. This allows the TPS to be easily reconnected should the need ever arise.



The Red wire will connect to the violet with white stripe, the white to the orange with dark blue stripe, and black to black. the violet is the 5V going to the TPS, the orange is the voltage that is leaving the TPS and the black is the ground.



Connect the white to the center connector on the pot and the black and red to the end connectors. If the pot works backwards, reverse the red and black leads. The black should be on the #1 connector and the red on the #3 if the leads are numbered on the pot you have. Seal the connections with dielectric grease to prevent corrosion.



After hooking up, set the voltage out of the pot to about 1 volt. On the pot I used, all the way off was at the 7 o'clock position and full on was at the 5 o'clock position. Setting the dial at 9 o'clock gave me one volt. That caused the transmission to shift at about 35 MPH with normal driving. Pulling the trailer it was a little early and after playing around, I am now set at the 12 noon position and the transmission shifts at 45 MPH. This is good when pulling the trailer.



I noticed that the transmission downshift can be felt much better and there is a little more engine braking this way. All in all, I am pleased with the set up. It never once gave a problem with not shifting or shifting out of OD when going down the road if not at the set point. The heavier the throttle, the higher the shiftpoint. one drawback is the transmission doesn't shift out of OD when passing and you need to manually take it out of OD ( push the button ) if this is desired or needed to pass.



I tapped into the sensor wiring harness and also removed the mount from the top of the pump. Now it is easy to get into the top end of the pump for adjustments.

 

[EBramwell]

Is there any adjustment or calibrations that can be done to a tps on a 97 or are they just good/bad? I have a friend having the unlock lock issues now and transmission shop Goerends said it sounds like a tps issue.

I'm in the same boat, does this work on a 97?

 

[WilsonF]

I ran across this in a diesel mag. The guy has a kit w/pot and wiring for $30 fora 1st gen. But I contacted him and was told that he is testing them on some 2nd gens. For $30 buck I would jump at it in a military minuet.

-- Specializing In Diesel Performance and Custom Fabrication --

Floyd