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Weird Transmission Issue Dropping Out Of OD

T

TravelingMan0491

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Picked up my truck in Norfolk, VA after shipping it out about 40 days before hand. When I drove it, I noticed that it liked to drop out of OD constantly and of course my MPG suffered. I think I only had about 95 miles on the trip meter when the fuel level needle hit the full line. The shifting issue has since stopped but I wonder what caused it. Anyone else experience this?
 
Many things can cause this. The coolant temp sensor for the engine could be bad or have a bad connection and cause it. You could also have a bad thermostat allowing the engine to stay cold. The transmission fluid temp sensor could be reading to cold. Could be a speed sensor, or even a shift solenoid getting flaky down to a bad brake switch. Need to get a scanner on it and see what trans temps are, also engine temps, and check it for codes.
 
If the truck is OBD-II and if it the thermostat, the motor will sound extra 'clattery' when the temps drop under 170F; the more under 170F, the more the extra clatter.
 
JayTheCPA said:
If the truck is OBD-II and if it the thermostat, the motor will sound extra 'clattery' when the temps drop under 170F; the more under 170F, the more the extra clatter.
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All the DS4 trucks use the same control system for temp to allow better warmup. OBD2 was better at recognizing a stuck thermostat and not locking out OD, or causing it to kick in and out whereas OBD1 was a bit dumber in it's algorythims involving lockup. Theres so many variables that can cause this, that it is tough to say where to start other than to get a scanner on it, and see what the ECM is seeing and commanding.
 
TravelingMan0491 said:
Picked up my truck in Norfolk, VA after shipping it out about 40 days before hand. When I drove it, I noticed that it liked to drop out of OD constantly and of course my MPG suffered. I think I only had about 95 miles on the trip meter when the fuel level needle hit the full line. The shifting issue has since stopped but I wonder what caused it. Anyone else experience this?
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overdrive clutch slipping...its suppose to do that. Worn from abuse.
 
Our ecm protects the transmission from running before a certain temp. If your sensor is bad then it tells them ecm some crazy outside temp like -30 or something. It's like a 20 buck part and takes 2 minutes to swap out. Ibhad a tranny issue or so I thought, and it was my CTS. Do a search on it. May or may not help you but when the local shop said my tranny was bad, it was an easy fix for me that was offered by so many people on this forum. Good luck

Sent from my XT1254 using Tapatalk
 
Get a scanner on it before wasting money on parts thrown at it. All that does is cost money and rarely fixes things on these. If the inputs to the ECM, lacking codes, are ok then look hard at the TCC slipping from a worn valve body or worn TCC clutch. If the TCC or other trans part slips the ECM will unlock the converter. Any other engine performance issue like miss detected it unlocks the converter. Air in fuel, bad injectors, the list of parts to guess is very long.
 
the computer senses a slip in the overdrive clutch and downshifts the trans. to save it. The coolant sensor is for the engine. The trans oil can get so hot...and the trans will just burn up. If what you say is true how come there's no temp sensor for that? To save a tranny. It will give a code for that. Mine did and it was for what I've described.
 
Are you asking why the GM engineers did something? Don't know. Kind of like why did they make it so you have to take off the injectors and intake manifold just to take off the valve covers. Or why do you have to strip the whole front if the motor just to change a water pump. Who knows? All I know there is plenty of documented cases on CTS and tranny issues on our trucks. Warwagen was right, a scanner helps out so much.
 
nealpellecchia said:
the computer senses a slip in the overdrive clutch and downshifts the trans. to save it. The coolant sensor is for the engine. The trans oil can get so hot...and the trans will just burn up. If what you say is true how come there's no temp sensor for that? To save a tranny. It will give a code for that. Mine did and it was for what I've described.
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I do hope you're not serious about the CTS not affecting the trans shifting. This is done for engine warmup. When the engine is cold lockup and OD are disabled to allow the engine to warm up quicker as well as to warm the trans quicker. And yes, there is a TFT as well. If the trans is to cold, it will also disable lockup and OD. Not sure on the 4L80E behind a 6.5L what happens if the trans gets hot. I know on later trucks utilizing the T42 or late model PCM's, it will go into a tow/haul type mode to keep the converter locked and help to reduce trans temps.

The OP needs to get a scanner on it and see whats going on, otherwise he might as well just start lighting $20 bills on money. He'll get about the same result.
 
For what it's worth, I did plugin my GMTDScan tech and it wouldn't read the trouble codes. It kept giving me INVALID PCM RESPONSE. I had this happen before so I did the paperclip method with the accelerator and brake pedals pressed to clear the codes. I didn't bother to wait for what codes were present. Ever since then, which was about a month ago now, I've had no problem. There was a time before shipping out the truck that it would throw the code for ENGINE COOLANT TOO LOW or something to that effect. It turned out to be a bad connection in the coolant temp sensor that gives the ECM its signal (crossover?) I unplugged it, cleaned it up and plugged it back in and didn't have a problem until the first day of driving after taking receipt of the truck which was in transit and or storage for a month and probably only started at the most 3x. GMTDScan Tech can now read the trouble codes which of course there aren't any.

My thinking behind the ECM not allowing OD is to give the motor a chance to get to the correct operating temperature. Higher RPMs typically generates more heat right? A faulty sensor/bad connection would definitely give erratic signals to the ECM whether the coolant is actually at the correct temperature or not. just my opinion/observation though.
 
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Glad to see that the OD drop out is not recurring. On cold starts (20's - 30's F) it only takes me about three miles (or less) for TFT to warm enough to lock the TCC. Seeing as the mid-Atlantic area is not getting that cold at the moment, am sensing there was something else that caused the OD drop-outs.

Toward the INVALID PCM RESPONSE, consider just un-plugging and re-connecting the ECM to see if it is a mild corrosion issue. In the computing world, there is more cause-and-effect than coincidence, so it is possible that this issue is more related to the OD drop-outs than temps.


Toward the general discussion, here is what I know from an OBD II perspective:
> Ferm is correct that the ECM also gets TFT signal. OE programming is to not lock the TCC under 70F and lock it more (not always) at or above a temp (270F comes to mind, but not 100% sure) based on the shift tables.

> When TFT is in the 'Hot' range, the TCC will still lock & unlock, but the OE lock & unlock speeds are (generally) much lower than when TFT is in the normal range.

> Have read about a TFT 'Cold' setting (much lower than 70F), but have yet to see this in the few tables that I am aware of. Also, do not recall how it affects shift / TCC behavior.
 
I can wind up my 1993 in 3rd gear, leave it there towing a grade, and after a time the TCC will lock even at WOT. Apparently this is due to heating from the unlocked converter. So there is some logic to reduce heating or save the trans from high temp burnup. IMO the TCC clutch isn't strong enough to do this for very long, but, GM does it. (50K is what I get out of factory TCC clutches.) Of course extra power doesn't help...

Changing the trans oil and filter every 25K does help reduce TCC valve body wear.
 
Funny thing happened last Thursday . . . At highway speed, the tranny started to flutter the TCC (unlock - lock) and 4 - 3 - 4 shifts. Looked over at the Edge and noticed that the ECM's coolant temperature was all over the place (dash gague was rock-steady). When it dropped into the 150's, the motor sounded like a machine gun. When it dropped under 130 F, the tranny started to unlock the TCC and shift down to 3'rd.

Changed the ECT sending unit in the thermostat housing last weekend and now the ECM's temperature is rock-steady and no more machine gun or fluttering tranny.
 
Over the years I have dropped big bucks into the 4L80e what I found is that it is fully functional at 28 deg f. however pure synthetic should be used to protect it at extreme low temps dino fluids don't offer the low temp protection. Using conventional trans fluids in extreme old you must warm up the vehicle longer for longevity of transmission.

Look at the temp tables you find 157 deg f. is max recommended operating temp for the 4L80e I'm below that on the hottest of days I run 3 coolers the radiator, the large fin plate B&M super cooler w/fan and a large low pressure drop fin plate cooler.

There are those who claim running the trans so cool allows condensation to add lots of water to the mix this is not true because the converter always gets hot enough to boil off any water resulting from condensation so it vents.



 
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