Engine/Transmission (1994 - 1998) OD only works at 55mph + ???

[MrDiesel]

98 12V I have only drivein it a hundred miles or so. At first it drove fine then on my way home from town it decided it didn't want to go into OD. It will ONLY shift in to OD if you get the speedo up around 60mph. If it falls below 55mph it immeadiately drops back into 3rd. It dosen't seem to matter what the RPMs are or where the throttle is either..... other wise shifts are good and firm and the converter will lock if you get it into OD. Any ideas???

 

[biggy238]

Are you sure it's a downshift you are feeling? Mine has always unlocked the TC at just under 55mph. The loose TC makes it feel like a downshift. The downshift only happens closer to 45mph for me.



Lock up and overdrive rely on the TPS signal as well as speed... You might try cleaning the TPS and clocking it just a bit to get better results.





I was gonna ask you to fill out your sig, but I guess that would take all day...

 

[MrDiesel]

That would take all day... right now I have 5 CTDs... . I am sure it's down shifting. Slight jerk and the RPS hit 1800 or so. What do you mean by "clocking" the TPS?

 

[BigEasy]

1800 sounds like it's just unlocking... mine does the same thing. In 3rd at 1800 locked up with 3. 54s, I'm going about 45. I can't wait to get a real transmission in this thing!



Is there any way that I can control the TC lockup with a switch, instead of relying on the stock calibration?

 

[MrDiesel]

1800 w/o any throttle... just coasting. But when you step down again it jumps to 2300 or so, run it back up to 60mph and it shifts into OD, then locks up. I have a spanking new transmission and a BD converter, but I'm kicking around the idea of selling the truck so I'm not sure I wanna dump another 3K into it... ... . I think I'll try a different TPS first... .

 

[Howard Durand]

One of the other members posted lately that he had removed the TPS completely and replaced it with a potentiometer that he sets manually. He runs it with about 1. 8 volts on the line. Says he likes it.

I put a potentiometer in the ground line to my TPS and like what it does. But it would sure get rid of a common problem if the TPS can be completely removed.

 

[biggy238]

Clocking refers to turning it. You would loosen the two screws holding it on the pump and then turn the TPS body just slightly clockwise or counterclockwise to change the shift points.





From what you describe, the truck unlocks the TC when you let off the throttle almost immediately, and then when you come back into the throttle, it has shifted to third (hense the 2300rpm rev)





Sounds like maybe your transmission speed sensor is acting up, making the computer think it isn't turning as fast as it is. That's just a guess.



You could test it by unplugging it, it's just forward of the 4wd shift linkage and points down slightly from the transmission body. The truck should shift just a little bit differently.

 

[Joe G.]

Twisting the TPS really does not work. It changes things just a little bit, not nearly enough to deal with a problem. The pot Howard is using to turn his TPS does work.

 

[biggy238]

Am I right in the fact that the TPS itself shouldn't cause the downshift? Something else is causing the PCM to think the trans it turning more slowly that it is. . Vss or TSS.

 

[BigEasy]

Do these trucks have a TV cable like the gassers?

 

[Joe G.]

The TPS could cause a down shift.



These trucks have a TV cable. It would be a good idea to check the TV cable. Also check the spring on the lever that the TV cable pulls. It may be missing.

 

[Ridesamule]

I am the person Howard was refering to. I do not at this time have a modified VB but I have experienced the conditions you describe. I checked out my TPS, it was bad but tested OK after a clean. No great improvement so I read the threads on wiring issues, I prefer to consider it an overall impededance issue rather than just stray signal.

Anyway, while in the process of rewiring to the TPS with grounded cable I figured what the ... . went to the radio shop picked up a 270 deg. 1. 2K rheostat put it in the cab and set it to an output of 1. 8 volts, by-passing the TPS. I figured that this was enough voltage that the ECM would recognize the signal as strong enough to shift and lock up in OD around 48-50 MPH.

I have since installed the POT in my knee bolster and just leave it there. The transmission shifts up and down according to the speed sensor. If I let off it stays locked up untill ~48 mph, works for down hill on a slight grade while towing. And I don't know why but on a long down hill if I slightly ride the brake long enough the ecm will resend the signal to the TC to lock up for the rest of the ride down. With the miles I have on my transmission 220k+ I do not kick it down to pass or speed up too often, but the TV cable will take over and force the kick down. I also turn the voltage up if I want to unlock, or stay unlocked. Such as a long slow incline where I do not want to over throttle but also don't want the TQ to lock up too early while towing.

I don't know if this will help but it may be a much cheaper method of trouble shooting than just throwing parts at it... . Good Luck

 

[MrDiesel]

I'm confused... did you by-pass the TPS all together, or is it still in use? I've also heard of wiring a capacitor into (connecting) both lines together, but A) I don't know any one who's actually done it, and B) I don't know which wire is the TPS Signal wire and which one is the Return wire... .

 

[Joe G.]

The TPS has a three wire connector. There is five volts on one, ground on another, and TPS output on the remainding wire. I think it is the middle one. Ridesamule replaces the TPS with his in cab controller pot. Using a cap to control a bad signal may or may not work. Even if it does it's just a bandaid.

 

[Howard Durand]

The center wire (orange with a dark blue tracer) at the TPS is the signal wire.

The ground wire is black with light blue tracer and the 5 volt wire is violet with white tracer. At least that is the color coding used on my 94.

Capacitors have been used to attenuate voltage spikes and noise by connecting them between the signal wire and ground as well as between the 5 volt wire and ground.

Their success is debatable. I think most of the problems associated with the TPS is due to wear and contamination in the TPS itself which is nothing more than a potentiometer rotated by the throttle.

The pot I put in the ground leg works fine but still requires that the TPS be in good condition. I am really interested in Ridesamule's idea.

 

[MrDiesel]

Last night I cleand the TPS-which resulted in nothing, and checked the TV cable and spring-they appear to be good. There is a slight amount of slackm moves freely and the spring is all well. One thing to remember is that it will do this at any amount of throttle. It shifts exactly the same when you mash it to the floor as it does if you let it take 5 mins. to get to 60ph??? At least I get to drive my 6 speed to work and forget about that *&%$ auto!

 

[biggy238]

Unplug the TSS trans speed sensor and see how it shifts... ... ... .

 

[MrDiesel]

OK tried that didn't notice any real difference. Tried un-hooking the - battery for a few mins. nothing either... .

 

[MrDiesel]

WELL... ... . I must have made it mad with that manual trans. comment, b/c now I only have 3rd gear! What do you suppose the chances are that I bought a truck with "a perfect shifting transmission" that just lost OD first, now it's on it's way down the crapper? I just hate to throw my fresh transmission/converter in there to find out I have some electrical problem...

 

[MrDiesel]

I drove it today about 30 miles... seems to be shifting fine again except the OD thing. fluid is full & clean. I'm thinking it has to be electrical??? Any more suggestions? I'm gonna be real ticked if I throw a new transmission in and still don't have OD under 55mph...