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

[biggy238]

If it didn't start shifting differently, the trans speed sensor is bad... It should have made a noticable difference in th shifts and lockup.



Mine lowered the rpm on all of those when it was out. (unplugged)

 

[MrDiesel]

Is the TSS in the trans? I'll try it again (un-plugging it) but if it is bad how do I go about a fix?

 

[biggy238]

The TSS is on the driver side of the trans just ahead of the shift linkage for the transfer case. It has a two wire plug and replacement is simple. . it's got a hex head cast onto it so a socket will fit.



I can't say with 100% Certainty that this is the solution... But it makes sence to me. The PCM can use either the Trans speed sensor or the VSS (vehicle speed sensor) to control the shift points. IF the TSS is giving a faulty reading, the PCM may think that the trans is turning more slowly than it really is, and be trying to downshift and limit overdrive because it thinks it's not up to speed.



I broke my sensor while installing the front driveshaft in january and ran without it for almost a month. It lowered the shift points and lockup a noticable amount. I think it's strange that yours didn't.

 

[MrDiesel]

Is this it? When I first un-plug it it seems to shift late, with no change in OD. Then after slowing down and again speeding up it seems to make no difference. Maybe I'll try taking the one out of my fresh trans and trying it?

 

[biggy238]

Yes. . That's it.



The PCM can operate off either that signal or the VSS... ... It may not help, but it's worth a try at this point.

 

[Willys]

My '95 4wd auto 4. 10 geared truck was acting in a similar manner yesterday. It would unlock/lock at random, and doing so more and more frequently. Then, along with the unlocking/locking problem, it would slip down to unlocked 3rd gear, then back to 4th and lockup. It kept getting worse until it would almost constantly stay in 3rd gear the last few miles. (which is SLOW with 4. 10's!) I checked the fluid- a little low, so I put some in and took it for a test drive. NO change. Ok, so I took the TPS off and cleaned it with electrical contact cleaner- making sure to get it down in there as much as possible. Reinstalled and took it for a test drive- and the truck is back to normal. I wish your luck were the same...



This has happened on my '96- and cost me a vacation and hundreds of dollars in troubleshooting from different shops with no help- the TDR provided the answer. I noticed on the old worn out TPS off my '96 that the spring that returns the internal parts back to idle position was worn slap out. The '95 was sticking a little until I cleaned it. Is yours doing the same?

 

[Howard Durand]

Have you checked the voltage on the TPS signal wire? In the 47RH, the higher the signal voltage is the later it shifts. I don't know if that is true with the 47RE that you have.

Right now I have mine set at 1. 8 volts with the throttle at idle and it does not shift into OD until 65 or so. This is OK with the 3. 54 gears but I wouldn't like it with 4. 10's.

 

[Willys]

Well its been a few weeks later, and the truck was driving fine with only the faintest hint of unlocking(just a few times over the last few days) So, I figure I should swap the spare TPS back on to see if it helps. It worked fine for a while, but while driving home, the truck started acting just as I described in my last post. I swapped back to the other TPS- same symptoms. I tried clocking it, disassembling and cleaning, etc. Same symptoms... So I swapped in a perfectly good TPS from my '96, which is crippled in the shop with a busted u-joint at the moment... to be put back together tonight. :argh: The same problem of the trans. unlocking, then slipping into 3rd, continues. Now its also flashing on the trans. temp warning lamp for a few seconds as it shifts into third.



What could I have done in the removal of the TPS that would cause this to come back(with a vengeance)?



I'm about to go try a few things to see if there is just a coincidence of the TPS swap/unlocking/shifting problem.



1. Remove wires from loom(electrical noise? Still searching on that one but haven't found much info yet. )



2. Unplug TSS and see if it stays locked correctly. If not, plug it back in...



3. Go to radioshack and get a 1K ohm resistor and put in the TTS plug.



4. Test voltage at the TPS. Would do this first but my voltmeter is in the '96 20 miles from me. I'd like to eliminate a few other things before I have to drive it that far in the condition its in. Is a digital voltmeter adequate for showing a smooth progression of the voltage(as in, not jumping around erratically)?



5. Permanently remove the TPS and install rheostat in cab. Oo.



Am I on the right track with all this? About to go start testing, wish me luck.

 

[Howard Durand]

The flashing temperature light is puzzling. The resistor trick may not work for you unless there is a temperature sensor that you can get at.

The early trucks like mine have one in the cooler line but I don't think yours does. What your truck is doing could possibly be the result of an erratic temperature sensor. The transmission should shift out of OD if a high temperature was being reported to the PCM. If you are getting a temperature light that would indicate the transmission is severely overheating. Well over 250* If you don't have a temperature gauge pull the dip stick and see if the oil feels excessively hot.

I know some computers can be become confused after a period of time and turning them off and unplugging them will let them reset to their default programming.

You might try disconnecting both batteries for a short time to see if this has any affect. See what you can find out about the temperature sensor in your transmission. A flaky sensor could be causing the problem or an internal problem could actually be causing high heat.

 

[Willys]

It indeed does have a temp. sensor in the line. I haven't made it to radioshack yet. Will try the resistor next.



I'm confused now about the VSS(vehicle speed sensor or transmission speed sensor?)- I don't see one(or what I think it should be). Is that only on the 47RE?



The trans. temp light is kind of scaring me a little- so I made sure to stop every few miles and let it cool(in Neutral). When I stopped to check it, the fluid was hot, but not HOT. I could hold it between my fingers, but it was somewhat uncomfortable. Not hot enough to burn, nor hot enough to scream, "owwiee".



I'll try disconnecting/reconnecting the batteries in a few minutes and see if that helps. This transmission was supposedly rebuilt 30k miles ago- I have only driven it about 1500 miles since November. Again, this is the '95 I mentioned a few posts up, not the '96 in my sig.



Thanks Howard!

 

[Willys]

argh!



Ok, tried a 1W, 1K resistor. What wattage should I be using? They had 1/4W, 1/2W, and 1W. I like the looks of the big 1W, so in it went.



It didn't work so far though. Time to go pull the battery cables and see what happens.

 

[Howard Durand]

I was not paying attention. Yes, the 94 & 95 models do have the temperature sensor in the line and as far as I know, they do not have a speed sensor in the transmission. Only the VSS.

Acording to the manual, if transmission temperature reaches or exceeds 280 degrees F, the temperature light will come on and the transmission will downshift out of overdrive. When the temperature drops below 280 degrees it will then allow an upshift into overdrive. As oil temperature increases, the resistance of the sensor is reduced.

At about 240 degrees, the sensor measures about 400 ohms, give or take a few.

A 1000 ohm resistor used to replace the sensor would tell the PCM that transmission temperature is around 190 degrees.

 

[Willys]

Ok, I'm going to have to do some more reading- I thought the VSS is the speed sensor in the transmission. By VSS, do you mean, TPS? The differences between the '96 and '95 are still confusing me. I need to get a manual for the '95, the '96 is close enough to my dad's '98 that I can borrow his shop manual when I need it.



Your explanation of the the TTS is excellent. I'm beginning to understand where some of the pieces of the puzzle fit. I don't understand why Dodge would want the trans. to downshift when it is overheating. I don't see how that would cool it significantly if it causes the RPM to jump 800 or so with no warning.



I do have some good news on the truck... when I test drove the truck earlier today, it was only for a few miles- and it was still shifting/unlocking erratically. On my way to the shop this afternoon to work on the '96, it only unlocked a few times, no downshifts. On the way home tonight, so far as I could tell, it performed flawlessly. I'm hoping this trend continues...



Thanks again for your help!