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Diagnosing auto shift timing problems- info available from the ECU

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G56 Transmission - 273D Transfer Case - Rebuild

P0192

Hello,

First time poster- thanks to all you guys who answer questions and build out a reference for all the owners out there.

I have a question about troubleshooting auto trans shift issues. My father has an '04 DR 5.9 automatic that I work on for him when needed. I have replaced the pressure control solenoid and sensor in the trans a long time ago, so I didn't think too much of it when it started acting a bit goofy again. I replaced the pressure control solenoid and sensor and did the fluid/filter. All was well- for a little while. Now occasionally the upshifts are delayed, or it gets stuck in low gear. Sometimes it is fine. When it shifts while misbehaving, the mechanical shift seems to be OK- the shiftpoint vs. vehicle speed for a given throttle is just way wrong. For what it is worth, I replaced the TPS (out on the engine) a while back because of a DTC and an occasionally glitchy pedal off idle.

It would seem something is breaking down with the pressure control loop or the governor pressure and I need to do some more diagnosing. I thought, since the ECU is obviously reading the sensor and PWM'ing the solenoid, that I should be able to see these, but my little handheld OBD reader does not give me any information. Is there an inexpensive tool out there that would let me see/log the pressure target/setpoint, the sensor feedback, the solenoid control duty, or the solenoid current (if measured)? And as a side note, is there not a DTC when the pressure fails to be controlled?

And in case I am focusing on the wrong thing- more generally, what can help me narrow down the problem? Any advice is appreciated!

Thanks,
Tim
 
Welcome to the forums.

I'm not an auto guy, but to start off what trans are you working with?

As far as scan tools you're going to need something abit more beefy, Bi-directional feed back and controls would be a bare minimum.
 
Thank you for the welcome.

This would be a 48RE in an '04 if I am not terribly mistaken.

I guess that is the heart of my question- what tools are out there? I don't need to tweak, tune, or flash anything. I am just looking for something to be able to see some of the operating parameters of the transmission to home in on the root of the problem.
 
what tools are out there?

Million dollar question right there, I just spent about an hour searching and reading reviews for a low priced scan tool able to do it and it feels like I've been lead down a rabbit hole. One major caveat/criteria is one that works with diesel trucks, and on top of that a pre-canbus diesel truck. There are alot of hit or miss cheaper options out there but the majority of people looking at them are intending to use them on gas engines and its hard to verify a straight answer without putting your money down.

I use blue driver for alot of small quick stuff on other vehicles, but it has little to no support for my 03 6MT.

On the other hand, you're looking at a professional garage tool that will most certainly be able to do it i.e. Autel, Launch, etc. Either you find a friend with one, know a guy, or buy it which could exceed the cost of the repair you're trying to do potentially.

Wish I had a better answer for you, its something i've needed to look into for a while to upgrade my tools, I do alot of mobile mechanic work on the side and I missed an opportunity for a rear brake job on my neighbors' daughter's 16' Audi A3, had to have software to disable the rear electronic parking brakes to change the pads and rotors.

If anyone else has any input im all ears as well for a good scan tool that has garage and dealership functions without the price or subscription fees.
 
If it refuses upshifting or delayed to high rpm often the Trans speed sensor is compromised.
It is magnetic and collects debris over the years and finally fails.
Cheap as dirt.

There isn't a lot you can do with a scanner on these stone-age trannys.
It's more or less old school mechanics work to diagnose them.
 
Thank you for the suggestion. I don't recall the speedo dipping, but I will have to specifically watch for that. Is the output shaft speed the source for the vehicle speed in the instrument panel? In other words- can I assume that if the speedometer reading remains plausible during a delayed shift the VSS is functioning properly?
 
But to your point about old-school techniques- if I tap and measure the gov signal, will it be directly proportional to vehicle speed, or does the ECU just discrete step across the pressures at which the valve body would switch in a fully mechanical transmission?
 
But to your point about old-school techniques- if I tap and measure the gov signal, will it be directly proportional to vehicle speed, or does the ECU just discrete step across the pressures at which the valve body would switch in a fully mechanical transmission?

ECU changes gov pressure according to speed, load and gear. It fluctuates a lot.
Signal from ECM is PWM for the gov solenoid.

I've a gauge in my truck that shows real time line pressure and it's all over the place.
 
Also, how many miles on the transmission?

A worn out valvebody is probably your next step if the basic electronics are all good.
And these VBs are well known to wear out. But this is a total guess from my side now.
 
Ah. Good to know. All of this is helpful.

This is certainly a high mileage, original transmission and may very well be on borrowed time. He doesn’t haul anything with it anymore because he’s in bad health. I’m just trying to nail down what is going on for him. Everyone else is just going to tell him to buy a new transmission. And they may be right. I just hate to jump to that if the quality of the shifts is still good when it is behaving. I don’t want to waste anyone’s time, but if it is a sensor or solenoid thing- it has to be figureoutable.

It wouldn’t surprise me at all to find a ferrous mustache on the output speed sensor. I am assuming it is a VR sensor with an internal magnet, right? I should check that, since a falsely low or zero VSS would probably result in a very low gov pressure setpoint.

I did get the position of the TPS sensor pretty close when I replaced it. The best I remember it was within about 50mV of the target- which I *think* was 0.650V. No codes, no problems, so I did not try to adjust it in closer. I’m assuming that there is some kind of redundancy with the signals so they “self check” within a certain range. Is the transmission control known to be pickier than the engine with respect to the TPS/APP?

But it sounds like there is a real possibility it could be a valve body problem from normal wear. If that is the case, could I positively identify that with a gauge on the gov pressure tap? Say if it is over X psi and it is still in first, that means the ECU is successfully controlling pressure but the valve body is not taking the right action based on that hydraulic control signal?

Thanks again for your input.
 
Programming the TPS means; switch on ignition, don't start engine, push the go pedal slowly all the way to the ground and release it also slow, then switch Off the ignition. That way the range off the TPS is saved to the ECM.
 
But i think the best shoot now is the TSS as I had that problem, same symptoms as you, with more then one 727 transmission.

There is a simple test for that too, switch your TC to low range and drive, if it shifts only at the red line, you got it.
 
As an update: More odd behavior noticed- getting stuck in third for instance. While that could still happen if the governor pressure control is out of control, we found part of what we assume is the OD snap ring in the pan. Whether that is the right snap ring or not I'm guessing there could be debris in the valve body by this point. With the mileage on the trans, everything points to a full rebuild. So that pretty much answers the question. Thanks for the help.
 
For Reference:
IMG_6125 (2).jpg
IMG_6126 (2).jpg
 
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