Hey everyone,

My 2004 Ion 2 just developed a transmission problem.
Symptoms:
- VERY sluggish acceleration in drive - engine RPM flares up with throttle, but car accelerates slowly
- normal behavior in reverse
- MIL illuminated (has been on for some time due to unrelated EVAP issue)
- "Service Vehicle" message on info screen
- rapid onset: shifted through all gears and moved just fine... shut off vehicle... next start it was suddenly sluggish
- I did notice sporadic hard shifting in the days prior to the problem.

Here are some preliminary findings I got with the help of an old Tech2:
- The commanded gear is (almost) always stuck in 5th, so the transmission is not even attempting to shift. This explains the sluggish acceleration.
- I observed no indication of slipping when viewing data on the Tech 2. All the "slipping" seems to be in the torque converter.
- No indication of excessive heat.
- No scary mechanical noises/vibration/clunks...
- I can use the Tech2 to manually force the transmission to shift through all the gears while driving and it has no issues shifting.
- The car will go into 2nd gear if I put the selector in low range.
- Sometimes the car will briefly blip 4th gear on the Tech 2 while shifting between park & drive, but only for a second before it goes back to 5th.
- The car does get up to speed eventually and the torque converter clutch seems to engage properly once cruising.

I think it's safe to rule out slipping clutches or any other mechanical problem. The base transmission seems to be fine. The issue seems to be electronic/electrical.

DTCs:
P1779 Torque Delivered Signal
P1781 Engine Torque Signal Circuit

There are also P0700 and P1700 codes, but they just indicate the Engine computer received an alert from the Trans computer that there's a problem.
Some other unrelated air/fuel/evap codes as well.
P1779 and P1781 indicate some sort of failure of the ECM and TCM to communicate info about engine torque which is used to execute proper gear shifts.
According to the manual, one of the actions taken when P1781 is set is the TCM will "Maintain current gear". That might explain why it's stuck in 5th now.

So in summary, the mechanical/hydraulic side of the transmission seems to be fine. It CAN shift through all the gears, but due to some (likely) electrical/electronic error, the TCM is not commanding normal shifts and is just commanding the car to be in 5th all the time.

Has anyone on the forum seen this before? Any hunches on a likely cause?
(I'm hoping it's not a control module, but that'd still be better than a transmission teardown or replacement.)

 

From my limited research, the following seems consistent with what you have already found but also lists other possible causes for the codes.

• P1779 generally indicates an issue with the transmission pressure control solenoid or the engine torque delivered to the transmission control module (TCM) signal. This can cause symptoms like delayed shifting, slipping, or the transmission going into "limp mod
• P1781 often relates to a problem with the transmission temperature sensor or transmission shift solenoids. Issues here can lead to transmission overheating warnings or incorrect gear shifts.

Have you tried using the diag to manually operate the pressure control solenoid and each of the transmission shift solenoids? I know you stated that it can shift through all of the gears, but you have the opportunity absolutely remove these from consideration as part of the problem.

It really does seem to be related to the engine torque signal not getting to the TCM, along with a suspect transmission temperature sensor, which, based on its reading, may also play a role in inhibiting certain shifts.

I would
Check your transmission fluid level. Many a misbehavior can be caused by an insufficient level or deteriorated transmission fluid.

Check for 5 volts on the high side of the transmission temperature sensor. Also ensure the ground for the circuit is intact. If it is not present, figure out why/repair the ground. If it is present, replace the transmission temp sensor and clear and rescan
I am assuming you have a traditional A/T. Please confirm.

The engine torque data is broadcast over the canbus serial Network in the vehicle. The ECM, BCM, TCM, and other modules are connected in parallel on the digital canbus Network. The TCM should be reading this signal when it is broadcast. The fact that it does not appear to be reading the signal may be due to a problem with the network connection for the TCM, an internal problem with the TCM, an issue with a malformed or distorted message sent .....

Others with more experience with troubleshooting canbus networks may be able to assist you if it comes to that. I would first try the above with investigating the transmission temperature sensor.

 

Sound like it is in emergency drive 5th gear mode.

 

These look like troublesome transmissions. Saturn only used them for a couple years. They take a special mineral oil, so beware. Not as bad as the VCT but they went to the 4T40 in 2005 and later. The L always used the 4t40 and 4t45 with the v-6.

 

These are factory manuals online, a little difficult to find things sometimes but most of it is here. Charm.li

 

Nice find, sir.

 

Thanks for your input guys. With relief and some embarrassment I can announce that the problem is fixed.

I had today wide open, so this afternoon I set about troubleshooting the problem. My plan was to check the transmission fluid and then start going through the diagnostic flow in the service manual.
Over the past couple days, I had a thought that some of the "unrelated" trouble codes might not be so unrelated. In particular, there was a P0113 - intake air temperature - code. I had begun to wonder if the IAT sensor data was used in the ECM engine torque calculation, and suspected that a faulty IAT sensor/circuit might be causing the torque-signal-related codes. Well, it turns out that was basically what was going on, but neither sensor nor circuit were faulty...

This transmission problem began immediately after I replaced the front hubs. Sometime during that procedure, I also investigated a rattle in the engine bay. I thought it might be coming from an idler pulley and at some point unplugged the connector to the IAT sensor to get a clearer view of the pulley. I forgot to reconnect it and only realized it this afternoon when I opened the hood to begin checking the ATF.

DIAGNOSIS: FAULTY MECHANIC.

I plugged the sensor in, cleared the codes, and took it for a drive. All gears were back again. None of the transmission codes came back. (Two pending O2 sensor codes came back, but haven't set the check-engine light yet.)

I did go ahead and check the fluid. Before I had this problem, I was mistakenly under the impression that this transmission was one of those "filled for life" deals. I knew there was a dipstick, but I thought that the differential must have its own chamber with its own fluid and that's what the dipstick was for. I never checked the ATF before today, because I didn't think I could. The fluid level was fine. The color was a little dark, but not scary dark, IMO. Picture below. I suspect it's about due for a change, but could probably wait until spring.

Agree/disagree?



Anyway, thanks again for your help, and sorry to waste your time on a goose chase.

 

Since I don't know anything about what transmission is in your car, the only comment I'm going to make is that if my fluid looked like that, I'd go ahead and change the fluid and filter NOW.

 

It takes a special mineral oil not regular ATF, so I am unfamiliar with what the color is supposed to be. Glad it shifts again!

 

Oh well, leave it to GM to do something different to a car line that was selling well until they stuck the corporate noses into it.