Engine/Transmission (1994 - 1998) TPS myth buster

[Ridesamule]

looking for opinions from some with a little deeper knowledge than my own... I have been running 50K without a working TPS and want to trow out these observations for all of the myth busters out there.

GENERAL KNOWLEDGE:

1) TPS was first used in a high RPM gasser trany.

2) Gassers have more natural engine breaking capability than a diesel.

3) It is more fuel efficient to unlock the converter when letting off the throttle.

4) The ECM monitors engine rotation and signal voltage from the TPS for gear selection decisions.

5) The brake signal will unlock the converter.

6) The TV cable can force a down shift or delay up shift.



MY OBSERVATIONS:

1) While being fed a constant voltage within the range of 1. 2-2. 8 volts the transmission up shifts and down shifts according to the speed sensor.

2) letting off the throttle with the converter still locked up does not rapidly decelerate a diesel.

3) The only harm from too much/little signal voltage is the converter will not lock up and there is a hidden high TPS voltage code saved to the register.

4) Converter lock up can be hand manipulated with in the range of the speed sensor with no impact to transmission.

5) Especially when towing heavy I like to DRIVE my truck.

6) A TPS (2-3K potentiometer) mounted in the dash is not subject to sloppy linkages, engine heat, stray signal, and only costs a few bucks.

7) Unless forcing unlock is important to a driver a TPS in a diesel should not be necessary at all except the driver who is constantly forcing downshifts with the throttle for racing or passing. ( which is a fruitless event with a stock converter anyway).

8) Since placing a pot in my dash I never use my lockup switch any more.



MY question is: Does a diesel transmission need a TPS or can a resistor in the line with a set voltage to the signal wire be installed and the TPS bypassed for ever with out any negetive impact on the transmission?



I am open to any flaming or constructive thougts out there. I plan on replacing the old worn out 47RE with an aftermarket soon.

Sorry so long. https://www.turbodieselregister.com/ubb/icons/icon6.gif

Cool

 

[Willys]

Great observations. It definitely puts things into perspective for me. I'm battling a tps/something issue with my truck so every little bit of understanding helps.



1. Do you ever adjust your pot? While driving(to adjust lockup speed) or just to experiment?



2. I've seen two other guys(from TDR) with pots wired in. Howard Durand has one inline with the TPS, another fellow completely replaced his TPS with a rheostat in the cab. Is yours inline with, or replacing your TPS?



3. This subject has come up before on previous posts with these guys, but haven't seen anyone else who tried a specific resistance for this. Willing to be a guinea pig?



I'm thinking about wiring a pot in place of the TPS to see how I like it- set it for a output of 1. 2V. I think Howard Durand has his set for 1. 2 or 1. 8, can't recall...





(edit: Just realized you're the other fellow that has the pot in the cab. Ooops... guess most of my post is moot now. )

 

[Howard Durand]

Good morning folks,

I have a potentiometer in the ground line from the TPS. Yesterday I removed the TPS from it's mounting but left it in place electrically. I started out with the TPS voltage reading about 1. 2 volts. I drove a ways then changed the setting to 1. 4 and continued doing this until I had it set at the maximum I could get with my pot which is 1. 8 volts. From the lowest setting to the highest, the shift into OD varies between less than 35 mph at 1. 2 to about 45 mph at 1. 8. I like the 45 mph setting and the greatest advantage to this process is that shifting is predictable. My next step will be to remove the TPS completely and use Ridesamule's idea.

I wish I could find a male plug that fits the connector on the TPS cable. It would be handier for experimenting with modifications.

 

[Ridesamule]

Willys

I seldom touch my pot. If I do it's because, I for some reason don't want my TC to lockup. I no longer pay any attention to where I turn it to ( just turn it CW/CCW to disengage the TC, and then reach down and turn it until the TC locks up again when I want it to).

 

[Ridesamule]

Howard

When i started my experimenting I got 3 pins from Radioshack and wrapped them indevidually and then as a group taped them to the plug going to the TPS. I now have everything shrinkwrapped to the ECM plug and all wrapped up.



For anyone who is tired of their transmission coming out of lockup every time they let off the throttle I think this is a great bomb. But I was hoping for someone knowledgeable to jump in and present a case where this could be detrimental to the transmission before I ever suggest someone else try it.

 

[DConrad]

I've made the jump to this thread from the lockup switch thread I started. I'm thinking now that I should scrap the switch idea and go for the dial. Am I reading this right. Is that what you guys are doing? My issue with lockup is that when the fifth wheel is behind me and I'm entering the interstate the TC still locks up around 45-50 MPH and usually I'm not ready for it yet. My revs drop and I lose acceleration. What do you think my best avenue is?

 

[Dieselnerd]

Derek, the answer to your problem of locking up at 45-50 MPH when towing is to do what I used to do when towing with my previous 2nd gen truck. When starting out, lock out the overdrive with the button and keep it floored until you get to about 55 or so (which is near max speed with 4:10 axles) and then hit the OD button again and it will lock up at about 60.



As far as a variable pot on the dash, I doubt it will hurt anything and is probably a cheap solution to a bad TPS or instead of a lockup switch. The reason they unlock so quick is because Dodge was trying to save the transmission. Unless the 47RE is modified, they wear out faster when lockup is forced. The 48RE in the 3rd gens stays locked up when slowing down and will even shift locked up in Tow/Haul mode. They can do this now because the newer autos have been beefed up enough to handle the additional stress.

 

[Howard Durand]

The difference between the 94 that I have and the 96 and above is probably enough to make them react a little differently to the TPS replacement scheme. I want to try it long enough to know for sure though.

I can definitely relate to what Big C is saying. Erratic OD shifts have been my biggest problem while towing.

I made a controller that prevents OD before 45 mph and TC lockup before 55 mph. It helps a lot and seems to be a good combination.

It appears Ridesamule has come up with a much simpler solution though. I tried his fixed TPS voltage application and it appears that it provides reliable OD control and is simple and easy to set up.

For normal, unloaded driving, I like to have OD at around 40 mph but for towing, 45 or even 50 is better. Especially with the 3. 54 gears.

My biggest problem has been the tendency for the thing to shift into OD at 30 or 35 mph when towing in traffic at varying speeds and then lock up the TC. I am hoping the fixed pot. scheme will eliminate that.

 

[BBliss]

ridesamule, the problem with leaving the converter locked up when towing and no throttle is that the governor pressure drops significantly because the throttle valve cable is relaxed. This can cause the torque converter clutch to slip because of low apply pressure, this also can be hard on the already weak od clutches.

 

[Ridesamule]

BBlis, that is the reason I have stayed with this system because I can just turn the signal up to unlock the converter under these situations without any need to change the actual "foot feed" position. One thing I should point out is that unlike a lockup switch the VSS will unlock the converter naturally if the speed decreases enough. An example of my shifts with overdrive on are 1-2 12mph 2-3 by 31 mph depending upon the amount of throttle. . 3-OD 42 mph lockup always 48-52 depending upon preset signal. When decreasing speed the VSS will unlock the converter at 44mph. If I am in the lower range of lockup and need to accelerate rapidly I turn up the signal to unlock and turn it down to lockup again.



Thanks for the repy, I think I know what you were getting at but no throttle = no load on the clutches, or am I missing something?

 

[Joe G.]

Yeah, there is some load on the clutches, especially if you have an exhaust brake. If the TCC is locked then the trans is turning the engine faster than idle. The question is if the load is enough to cause a problem.

 

[SGehrke]

What is the best rheostat or pot to use for this set up and how do you wire it? I would like to try this for my erratic shift problem I'm having. Up to this point I have replaced the batteries and cables,replaced tps,cleaned all grounds I could find, unplugged the tss, checked the tv cable throttle linkage, and checked the brake switch.

 

[Ridesamule]

Just go to a neighborhood radio shop and pick up a used rheostat 2-3K2-5 bucks with a knob. I suggest a small shop instead of a radioshack type because old pre digital systems used pots that would do a near 360 deg. sweep. They are more durable and many have male connectors you can slide a female wire connector on.



Then hook it up to the current TPS wires in the same configuration that the TPS is wired, the only wire you truely will need to keep correct is the center wire. Where you decide to tap in is your decision.

Run the wires through the firewall hook up the pot turn the pot roughly to center and drive. When you are up to speed 50+ turn the pot slowly if you are already locked up at a certain point it will unlock. If you are unlocked at a certain point it will lockup if all is working correctly. It doesn't matter for function which way you turn the pot however I try to turn mine so that I am placing the voltage reducing resistance between the center wire 5v and signal wire to ECM (Clockwise if you are wired the same as the TPS). This will insure the main current drain is going to ground.



I can't tell you wire colors because it has been 3 years since I have had a TPS and I dont want to dive into my ECM plug unnecessarily.

 

[Howard Durand]

This is a drawing of my scheme with the potentiometer in the ground leg of the TPS. It is different from Ridesamule's idea in that the TPS is still in place and functional. As with the scheme where the TPS is removed completely, the TC also remains locked with the accelerator released.

I assume the reason DC prohibited use of exhaust brakes with the auto trannys was due to the concerns posted here. DTT installed a beefed up input shaft in mine so I could use the exhaust brake but said nothing about these concerns. How valid are they?

 

[SGehrke]

Thanks for the information I will try it, hopefully it will cure my problem.

 

[Howard Durand]

Ridesamule, did you put a resistor in the signal circuit to avoid totally grounding it on one end or applying full 5 volts on the other?

By measuring the resistance of the TPS from each end to the signal pin and then measuring the fixed resistance end to end, I believe there is about 300 ohms, more or less, in the signal lead.

 

[Ridesamule]

No, I did try a few different pots and noticed a small change in the lock up speed but never did try to clip either end of the pot. If you were thinking of circuit protection I felt the ECM is designed with the digital capability of seeing 1-0 or "on off" so I didn't feel there would be an isue with turning the pot from one extreme to the other. I have had no problems so far.

 

[Willys]

Any updates on these?

 

[Howard Durand]

An update on my experiment is that I used a potentiometer in place of the TPS, similar to what Ridesamule has done.

It is in the dash so I can adjust it as I please. The difference is that I have other resistors in series with the potentiometer, arranged so that the maximum signal voltage is about 2. 5 volts and minimum is 0. 8 volt.



With this setup, the shift point for OD is constant for each setting with shift into TC lockup shortly after that.

The knob on the potentiometer is graduated to read between zero and ten.



The settings and OD shift points are as follows:

Zero on the knob equals a signal voltage of 0. 8 and the shift into OD occurs at 32 mph.

Five on the knob equals 1. 65 volts and OD shifts at 34 mph.

Ten on the knob equals 2. 5 volts and OD shifts at 47 mph.



The TC shifts normally except there is no downshift for throttle kickdown nor is there a downshift under load. I use the switch for that.

I have not tried advancing the pot for that. I just leave it where I want it for OD lockup control.

 

[Willys]

Thanks for the update.



So, to make sure I understand this right, you have no automatic kickdown, AND the torque converter clutch(TCC) continues to stay locked when you let off the accelerator pedal? Does this seem to help slow the truck down? Have you noticed any adverse effects on the transmission? Will the TCC unlock when you apply the brake petal?



Lots of questions, I just want to make sure I'm understanding everthing correctly before I attempt this myself. I still have an issue with my truck's overdrive unlocking at random, so I'd like to bypass the TPS completely, at least temporarily.



How have you spliced into the harness? Soldered connections or have you found a female plug that fits the stock harness?



Thanks again!