Engine RPM is different at the same hwy speeds. Not an O.D. issue! (2 Viewers)

[alia176]

This issue is recent and I need to figure it if one of my solenoid valves is sticking or something.

Conditions:

pics taken during highway speeds.
Over Drive is active, meaning the 80 is hitting 4th gear on the hwy. Turning the OD button off/on make the RPMs jump another 500rpm.
Engine is fully warmed up.
Air temp is in the 80-90 deg F (plus/minus)
ATF cooler is NOT bypassed under the radiator.
ATF oil change - drain/refill at every oil change.
Engine is NOT on boost.

Question:

Please compare the two pics and let me know why the engine spins just a tad higher at the same hwy speeds (around 72ish or so). This has not been a consistent thing but it is a new thing as of two months ago.

 

[cahill]

Same driving conditions? Flat road, same headwind, same tire pressures, same weight/load on the vehicle?

 

[Malleus]

I wouldn't trust the tach to accurately distinguish 50 rpms; if it was 200, then you might (might) have something to worry about. I'd ignore it. Until it gets worse.

 

[sbman]

I'd say torque converter is locked up in the lower RPM one, and unlocked in the higher RPM one.

Also, your temp in the left picture looks excessively high. The factory temp gauge is horribly bad at showing high temps, and normally if it is showing high, it's REALLY high.

 

[smritte]

+1 on converter lockup.

 

[alia176]

Same driving conditions? Flat road, same headwind, same tire pressures, same weight/load on the vehicle?

That's a great question, yes, same stretch of highway, same weight, etc.

 

[alia176]

+1 on converter lockup.

I specifically mentioned in my post that overdrive is not the issue here, as I can physically make the RPM jump 500 RPM by pushing the OD button on and off

 

[alia176]

I'd say torque converter is locked up in the lower RPM one, and unlocked in the higher RPM one.

Also, your temp in the left picture looks excessively high. The factory temp gauge is horribly bad at showing high temps, and normally if it is showing high, it's REALLY high.

The factory gauge has been modified to reflect real life temperature.

Torque converter lockup is not the issue here because I can make the RPM jump 500 RPM by pushing on the OD button at any highway speeds.

 

[ppc]

The factory gauge has been modified to reflect real life temperature.

Torque converter lockup is not the issue here because I can make the RPM jump 500 RPM by pushing on the OD button at any highway speeds.

Torque converter lockup can happen in Drive or Overdrive. You are trying to equate the two which is not the case.

 

[sbman]

The factory gauge has been modified to reflect real life temperature.

Torque converter lockup is not the issue here because I can make the RPM jump 500 RPM by pushing on the OD button at any highway speeds.

I remember that mod, never did that on mine.

I don't think you understand what the O/D button does. O/D button is locking 4th gear out. Shift sequence is like this: 1-2-3 with O/D disabled. 1-2-3-4 with O/D enabled. Once in 4th, if above 45 mph, and throttle at certain position, the torque converter will lock up and reduce RPMs slightly. Press the go pedal a bit more and it will unlock. Press it some more and it will shift down to third.

 

[scottryana]

The factory gauge has been modified to reflect real life temperature.

Torque converter lockup is not the issue here because I can make the RPM jump 500 RPM by pushing on the OD button at any highway speeds.

OD and torque converter lockup are not the same.

 

[smritte]

Diagnostics is not about finding the issue as it is proving what is not the issue.
You proved O/D now prove converter clutch.
Get the rpms to drop at speed and touch the brake enough to turn on the stop lights. That disengages the lock up and the RPM should jump about 300 rpm's.
If it is the lock up, it can be as simple as a worn or out of adjustment stoplight switch.
You can have a bad stoplight switch and still have stoplights. Your switch is actually several in one housing.

 

[alia176]

Diagnostics is not about finding the issue as it is proving what is not the issue.
You proved O/D now prove converter clutch.
Get the rpms to drop at speed and touch the brake enough to turn on the stop lights. That disengages the lock up and the RPM should jump about 300 rpm's.
If it is the lock up, it can be as simple as a worn or out of adjustment stoplight switch.
You can have a bad stoplight switch and still have stoplights. Your switch is actually several in one housing.

This is good info, I'm learning about converter clutch lockup. So, how do I prove to myself if the converter clutch is the issue?

So, in order to eliminate the stop light switch theory, I can say that I'm able to use the CC at any time w/o issues. It doesn't cancel on me unexpectedly so I'm ruling out the stop light switch, unless I'm told otherwise.

300 rpm delta is in fact what's happening here. Let me do what you're suggesting on the hwy.

Thx.

 

[alia176]

I remember that mod, never did that on mine.

I don't think you understand what the O/D button does. O/D button is locking 4th gear out. Shift sequence is like this: 1-2-3 with O/D disabled. 1-2-3-4 with O/D enabled. Once in 4th, if above 45 mph, and throttle at certain position, the torque converter will lock up and reduce RPMs slightly. Press the go pedal a bit more and it will unlock. Press it some more and it will shift down to third.

Good explanation and I feel the TC clutch lockup is the issue. It's slipping in one case, and locked up in the other case.

Additionally, a slipping TC does contribute the increased temp of the ATF, which isn't a good thing either when the radiator temp is also very high. I have a turbo, so I routinely see 210+ deg while on boost.

Thanks.

 

[alia176]

Torque converter lockup can happen in Drive or Overdrive. You are trying to equate the two which is not the case.

You're correct, I'm confusing the two. Thx.

 

[smritte]

The difference in rpm from lock to unlock is 300 ish. I took a peek at the schematic and I was wrong, our 96's share the stop light wire. The cruise however uses a diffrent part of the stop switch.
Here's a snip it of the diagram. I'm at work so I only have PDF.
The computer will turn it off because of stoplight input and engine load. Pull a grade, load increases and it can kick out. "If" the stop light wiring to the computer is good, then start looking into why the PCM is disabling it. Now you need a scan tool to view PID data.

 

[Malleus]

Good explanation and I feel the TC clutch lockup is the issue. It's slipping in one case, and locked up in the other case.

Additionally, a slipping TC does contribute the increased temp of the ATF, which isn't a good thing either when the radiator temp is also very high. I have a turbo, so I routinely see 210+ deg while on boost.

Thanks.

Unfortunately, if your problem is related to the torque converter, there's very little you can do. Even testing is rudimentary. All the information available is in the transmission section of the FSM. If you have a torque converter lock up failure, you should get a fault code P0770 or P0773:

 

[Malleus]

The only physical test for the torque converter requires removing it, which requires removing the transmission (or engine, depending on how much you hate yourself):

 

[Malleus]

So you see, unlike other more controlled drivetrains, the ECM sends a signal to the valve body to increase pressure in one particular circuit (the lock-up line). Once that happens, it looks for an rpm signal in a range that matches the A/T speed sensor/engine rpm map. If it sees what it wants, nothing happens; if the two are mismatched, it records a fault code. That's all you get. Well, that, and a malfunctioning drivetrain.

The problem could be a) a bad solenoid of electrical circuit, b) a bad torque converter, or c) a bad ECM. I know I wouldn't try to check b) first...

 

[Malleus]

FWIW:

HTH