2004 Tranny Gone Mad

Mudddawg · May 19, 2008

Hi All

I have a buddy that has a 2004 chevy 2500hd extended cab 4x4 with a 6.0 v8 with the 4l80e transmission. The problem that he is having is the service engine light comes on and when this happens the transmission shifts really hard. He has had a scanner on it by his mechanic. They still can not locate the source of the problem. When the code is cleared the tranny shifts fine until the light comes back on. Ususally about 150-200 miles. The tranny does not slip, but the code (can't remember the #) says transmission slipping. Is there any sensors that have been problematic that might cause this issue? He took the truck to the dealer and of course since the truck is out of warranty(110,000 miles) they say a new transmission is in order. If his trans is toast then he has no problem spending the money for a new one. But he is concerned if it is an electrical problem that he will still have the same issue with the new transmission. He never tows anything and always keeps his tranny serviced at the dealer every 40,000 miles. Any ideas?

markw8 · May 19, 2008

He either needs to listen to the dealer or find a mechanic that can understand and diagnose what the scan tool is telling him. Mark.

C6Racer · May 20, 2008

Well, if the code says it's slipping then you may very well need a new transmission. The hard shifting is kind of a fail safe mode it goes into when it slips(if it's really bad the trans will go into, and stay in, the gear that it is in when all shift solenoids are off which is almost always 2nd gear). The PCM will increase main line pressure to prevent slipping and that causes harsh shifting. I'm not familiar with the 4L80 but, if it has an input shaft speed sensor then that is what would be telling the PCM that the trans is slipping. The gear ratios are programmed into the PCM. When it commands an upshift it compares the input shaft to output shaft speeds to make sure the trans has made the shift. Does the speedometer act strange when the MIL(malfunction indicator lamp) is on? Does the engine flare up when it shifts?

Since you mentioned that you cannot feel it slip, and if the speedometer does NOT act strange when the MIL is on then a wacko input shaft speed sensor may be the culprit. If you have an ocilloscope then you can easily test it. Otherwise, it's off to the dealer. If the input sensor is not the problem then you simply have a defective transmission that wore out prematurely. End of story.

Good luck!

Mudddawg · May 20, 2008

Well, if the code says it's slipping then you may very well need a new transmission. The hard shifting is kind of a fail safe mode it goes into when it slips(if it's really bad the trans will go into, and stay in, the gear that it is in when all shift solenoids are off which is almost always 2nd gear). The PCM will increase main line pressure to prevent slipping and that causes harsh shifting. I'm not familiar with the 4L80 but, if it has an input shaft speed sensor then that is what would be telling the PCM that the trans is slipping. The gear ratios are programmed into the PCM. When it commands an upshift it compares the input shaft to output shaft speeds to make sure the trans has made the shift. Does the speedometer act strange when the MIL(malfunction indicator lamp) is on? Does the engine flare up when it shifts?
Since you mentioned that you cannot feel it slip, and if the speedometer does NOT act strange when the MIL is on then a wacko input shaft speed sensor may be the culprit. If you have an ocilloscope then you can easily test it. Otherwise, it's off to the dealer. If the input sensor is not the problem then you simply have a defective transmission that wore out prematurely. End of story.

Good luck!

Thanks for the info. His speedo has not been working for some time now. About a year or so. Could this have an affect on it? The engine does not flare or rev up when it shifts. How do you check the input shaft speed sensor? As I said earlier, If the tranny is toast then he does not mind replacing it. It is just that when the service engine lamp is off the tranny works great. It only goes nuts when the light comes on.

cammiller · May 20, 2008

I would find a good tranny tech and let him/her look at it. The 4l80e's can usually be rebuilt cheaper than replaced if there are no big components gone.

C6Racer · May 20, 2008

As I said before, I'm not familiar with the 4L80-E so I'm not 100% certain it will have an input speed sensor. If it does, and you have an ocsilloscope, then you can easily test it using specifications from a service manual. It's pretty complicated to try and explain by typing so do some research online and find some instructions. Otherwise, it's off to the dealer. If your not familiar with digital meters or ocsilloscopes then I would not recommend you attempt any work yourself. That's the best advice I have.

As for the non-functioning speedometer, there is a high chance that has something to do with your problem. That is because the PCM uses the signal from the OUTPUT speed sensor to determine vehicle speed. If that sensor isn't working then there's no way it can determine how fast your going, hence the dead speedo. If the PCM can't compare the speed signal from the input sensor to that of the output then the PCM might think the trans is slipping and will illuminate the MIL, during which the trans will shift much harder. Harsh shifting the the computer's way of protecting the transmission and will ONLY happen when the MIL is on.

I would just go to AAMCO since I hear from many people that it's a great transmission shop.

waltv · May 20, 2008

C6Racer is pretty much dead on. Would be nice to know the code though. Some food for thought:

"Both of the automatic transmission input shaft speed (AT ISS) and the automatic transmission output shaft speed (AT OSS) sensors are magnetic induction sensors. The input and the output sensors are accessible from the left hand side of the transmission. The AT ISS sensor is located just forward of center and the AT OSS sensor is located near the rear. A voltage signal is induced in the AT ISS sensor by serrations, which are cut in the outside diameter of the forward clutch housing. Voltage is induced in the output sensor by gear teeth, which are pressed on the outside diameter of the rear carrier assembly.

The PCM uses speed information from these sensors in order to determine the following:

• Whether the engine is running

• Vehicle speed

• Calculation of the gear ratio

• Calculation of TCC slip

• Calculation of turbine speed

Code P0502 and P0503 set if a fault exists in the AT OSS sensor circuit, and the PCM calculates a default value using the AT ISS sensor values. As long as the fault remains, and the code is set, the PCM also commands maximum line pressure, freeze shift adapts, and the MIL illuminates. If the fault is removed, normal operation resumes after the next ignition cycle."

P0502 - When the PCM detects a low vehicle speed when there is a high engine speed in a drive gear range, then DTC P0502 sets. Conditions for setting the code: The transmission output speed is less than 150 RPM for 3 seconds.

P0503 - When the PCM detects an unrealistically large drop in vehicle speed, then DTC P0503 sets. Conditions for setting the code: The transmission output speed drop is greater than 1,300 RPM for 3 seconds when the transmission is not in PARK or NEUTRAL.

Even if you don't have an ocsilloscope, you can still test the sensor circuits with a DMM and isolate the problem to being a faulted sensor or another electrical concern in the sensor circuits. Don't forget there are a slew of other codes that could have been the one that set beside those two OSS codes.

Mudddawg · May 21, 2008

C6Racer is pretty much dead on. Would be nice to know the code though. Some food for thought:

"Both of the automatic transmission input shaft speed (AT ISS) and the automatic transmission output shaft speed (AT OSS) sensors are magnetic induction sensors. The input and the output sensors are accessible from the left hand side of the transmission. The AT ISS sensor is located just forward of center and the AT OSS sensor is located near the rear. A voltage signal is induced in the AT ISS sensor by serrations, which are cut in the outside diameter of the forward clutch housing. Voltage is induced in the output sensor by gear teeth, which are pressed on the outside diameter of the rear carrier assembly.

The PCM uses speed information from these sensors in order to determine the following:

• Whether the engine is running

• Vehicle speed

• Calculation of the gear ratio

• Calculation of TCC slip

• Calculation of turbine speed

Code P0502 and P0503 set if a fault exists in the AT OSS sensor circuit, and the PCM calculates a default value using the AT ISS sensor values. As long as the fault remains, and the code is set, the PCM also commands maximum line pressure, freeze shift adapts, and the MIL illuminates. If the fault is removed, normal operation resumes after the next ignition cycle."

P0502 - When the PCM detects a low vehicle speed when there is a high engine speed in a drive gear range, then DTC P0502 sets. Conditions for setting the code: The transmission output speed is less than 150 RPM for 3 seconds.

P0503 - When the PCM detects an unrealistically large drop in vehicle speed, then DTC P0503 sets. Conditions for setting the code: The transmission output speed drop is greater than 1,300 RPM for 3 seconds when the transmission is not in PARK or NEUTRAL.

Even if you don't have an ocsilloscope, you can still test the sensor circuits with a DMM and isolate the problem to being a faulted sensor or another electrical concern in the sensor circuits. Don't forget there are a slew of other codes that could have been the one that set beside those two OSS codes.

Okay guys. Thanks for the help so far!!! The code # is PO849. Like I said it works great after turning off ses light until the light comes back on. I understand why it shifts hard when the light is on. But if the tranny was toast, wouldn't it shift funny all the time or slip without the light on? It will run about 200 miles before resetting the ses light.

waltv · May 21, 2008

Ok, I'm guessing you mean P0894. If that's the case, then the PCM is seeing excessive TCC slip of 100 to 500 rpm when the TCC should be engaged. It could be your torque converter, a damaged solenoid valve, or about half a dozen other internal transmission components. Of course like was said earlier, the PCM bases its readings off the input speed sensor and output speed sensor. These sensors should set codes of their own though, which would prevent the P0894 from setting. Options are check your speed sensor circuits or dig into your transmission/pay someone to dig into your transmission. I'd go with the cheap route first and pray.