2006 3500 6.0L overdrive issue

[arborman]

 

 

Bought a truck a couple months ago and having issues with it. As far as history previous owner had new tranny put in by dealer 4 months ago. The problem I’m having now the truck won’t go into overdrive. Brought it to mechanic and have no transmission code. It did have bad knock sensors which I got replaced and that made the engine run smoother. 

 

I still have a few codes which are a couple o2 sensor, fuel level sensor low voltage which makes the gas gauge not working.

 

here is the kick, if I’m driving along say 80km\r in 4th gear and I clear the codes with the scan tool hooked up, it will shift into overdrive just fine and the gas gauge will work for about 30sec at which point it will read empty again and most time the transmission will go back to 4th.

 

another thing is the check engine light flashes at highway speed which gives me a random misfire. I’m at a lost here and would like it if someone could point me in the right direction. 

[davester]

Are you sure it's not in OD (as in 1,2,3 & OD)?  Or maybe the torque converter doesn't stay locked?  And did the mechanic have a code reader that can read TCM codes?

 

And how/when the transmission shifts/locks the tc also depends on how well the engine is running, so you might want to get the engine right before delving into the transmission.

[arborman]

53 minutes ago, davester said:

Are you sure it's not in OD (as in 1,2,3 & OD)?  Or maybe the torque converter doesn't stay locked?  And did the mechanic have a code reader that can read TCM codes?

 

And how/when the transmission shifts/locks the tc also depends on how well the engine is running, so you might want to get the engine right before delving into the transmission.

Yes I’m sure it doesn’t go into overdrive. It will the odd time when I first get up to highway speed after I restart the truck  or when I clear the cel when driving but it kicks out of overdrive fairly quickly, easy to notice as the rpm goes from 1900 to 2500 at 100km/h. My mechanic does have the reader that will read the text as well. 

 

Before I replaced the knock sensor my cel would illuminate right away when erased. Now it stays off but it start flashing at highway speed and it gives me a multiple misfire code. Sometimes I notice it kicking out of overdrive when the cel flashes, sometime it does it without the cel flashing...

 

the engine runs runs very smooth and the transmission shift perfectly other than this problem

[davester]

If there's no TCM code, and given that the transmission is basically new, I would still look at getting the engine working so there's no CEL, and seeing if the transmission still does it.

 

In particular, the upstream O2 sensors are very important to how the engine operates, and if the computer thinks the engine can't/shouldn't produce X amount of power, then it will downshift to reduce the amount of power needed...

[arborman]

5 hours ago, davester said:

If there's no TCM code, and given that the transmission is basically new, I would still look at getting the engine working so there's no CEL, and seeing if the transmission still does it.

 

In particular, the upstream O2 sensors are very important to how the engine operates, and if the computer thinks the engine can't/shouldn't produce X amount of power, then it will downshift to reduce the amount of power needed...

I agree with you that the transmission downshift to get more power, but in this case it downshift even if I go downhill...it only does this once after the codes are cleared...it will not shift back into overdrive until codes are clear again...it’s like the tranny goes into limp mode.

[arborman]

Little update. I went for a drive the other day without hauling anything, cleared the codes before I left. I had no issues the transmission shifted to overdrive everything was great and no cel...

 

i found out also also that the flashing cel isn’t triggered by speed but rpm,,,it will flash if I rev the truck up to 2400rpm and I stopped at the shop and he said cyl 1 6 and 8 has misfire  counts. I went ahead and chanced the plugs and wires and it’s still doing it. 

 

A lot of reading online and I’ve seen quite a few people had luck with a crankshaft sensor relearn when they had similar issues 

[SierraHD17]

Have a CASE relearn performed with a scan tool.  A Snap On scanner, GMs own, EFI live or HP tuners, etc can all perform it.  It just resyncs the crank and cam position sensors with one another.  1 and 6 are the most common.. my one truck was like yours with misfires on 1, 6 and 8.  

 

Should remove all the false misfire counts and likely will fix your issue with the trans at the same time.  It's a normal thing with the LQ4 trucks especially.  The misfires will prevent the torque converter from locking up.  

Edited September 30, 2018 by SierraHD17

[arborman]

1 hour ago, SierraHD17 said:

Have a CASE relearn performed with a scan tool.  A Snap On scanner, GMs own, EFI live or HP tuners, etc can all perform it.  It just resyncs the crank and cam position sensors with one another.  1 and 6 are the most common.. my one truck was like yours with misfires on 1, 6 and 8.  

 

Should remove all the false misfire counts and likely will fix your issue with the trans at the same time.  It's a normal thing with the LQ4 trucks especially.  The misfires will prevent the torque converter from locking up.  

I will do that tomorrow! Fingers crossed 

[arborman]

Little update. Took the truck to my mechanic and asked him to do a crankshaft relearn. He was sceptical at first because he said he had never done one before and he didn’t have much faith. I asked him to just give it a try and it fixed the problem. No more flashing check engine light and the torque converter locks up just like it should. Thanks guys 

[SierraHD17]

Yup.  Funny how guys swap parts forever and not even "mechanics" know how to do this lol.  You should be good for a very long time!