LB7: Trans Limp and P1781 / P0700 under load acceleration

[db131994]

Hi Folks. Hoping someone out there can help with my issue... Haven't found much info out there on the interwebs...

Here's the scoop: Truck is a 2002 Duramax with Allison and 293K on the clock. Trans was rebuilt in May (maybe 20k on it since). I have a SuperChips DashPaq tuner on it for economy and tow tunes. Cold air intake, and no other mods. It's my daily driver, and I do not beat on it... We pull a 34' Work & Play Toy Hauler on the weekends (kids race go karts and quarter midgets). I've had 3 times pulling the TH when we were accelerating from a stop and the Trans goes into Limp Mode, then the SES pops up. I'm getting P0700 and P1781 trans codes. Pull over and shut it off, start back up, clear codes, and it goes on it's way, but I need to get this corrected...

Everything I've read is this usually occurs on startup with the TCM / ECM handshake. I haven't found anything on it popping up during driving... I'm going to go through and check batteries / cables, and connections tonight hopefully.

Hoping someone has come across this before and has some insights.

Appreciate the help.

 

[Mike L.]

Voltage drop out during initial engine cranking.

 

[db131994]

Hi Mike,

That's the majority of what I've been able to find, but I do not see the issue at startup. It has occurred when the trans is in 3rd gear, pulling, then kicks down into 2nd (I think) where it locks for trans limp mode. This Has happened after 20+ miles into a trip. Sunday night it happened 150 miles into the trip home.

It shifts into 2nd smooth, not like it did when it was slipping and would slam into limp mode...

 

[Mike L.]

I'd be checking voltage to the trans.

 

[dndj]

P1781 for the 01-02s is the same as P0802 from the 03-05s, if that helps with your research.

Ever changed out your ignition switch? Sure sounds like a momentary voltage interruption to the TCM to me. Try unplugging the connectors at the TCM and checking for corrosion, etc.

 

[db131994]

Hi dndj,

I have not changed the ignition switch. I checked when I got home yesterday, and there was a stored P1781 code (didn't trip the SES light...). I check when I got to work this morning and it wasn't there (I cleared it last night). So, I'm assuming the P1781 could be tripped on startup, like all the info suggests. The P0700 must trip when it goes into limp mode? Is there any detail on what P0700 could mean?

Thanks,

 

[db131994]

Oh, I did unplug and re-plug the TCM connections last night, and looked over all the connections I could get to. Didn't really see anything out of sorts...

 

[zakkb787]

P0700 is a generalized transmission fault code. Nothing specific about it.

First step is to continue what you already have started. Make sure your connections are good and tight. Double check batteries and connections there. Pull apart your tcm connectors again, make sure they’re clean, and maybe put a dab of dialectic grease into them. Same as your connections on the trans itself. These trucks do strange things when there’s voltage fluctuations or if it momentarily looses its signal to the tcm or trans. Code is different, but sounds similar to the 06+ issue when the tcm looses internal ground and randomly looses communication. My truck will run fine till I get off the interstate, then I’m stuck in limp mode because the ecm isn’t communicating to the tcm. This is random for me as well. But, the early TCMs are different and cannot be remedied the same way as newer trucks can. If all your connections are tight, make sure you have a steady voltage going to your trans as mentioned. Double check wiring harnesses and chaffing of wires. Make sure there is no mouse damage under your fuse block as well. Not saying you’ll find your issue with these, but triple checking everything first sure beats dumping money into something that winds up being a worn wire or loose connection.

 

[db131994]

Does anyone know where the TCM wiring harness is grounded, or where I can test the voltage going to it? Any good points of the wiring harness to inspect for damage?

Would you expect the loss of voltage to send it into limp mode when pulling the trailer?
Appreciate all the input.

 

[dndj]

Hi dndj,

I have not changed the ignition switch. I checked when I got home yesterday, and there was a stored P1781 code (didn't trip the SES light...). I check when I got to work this morning and it wasn't there (I cleared it last night). So, I'm assuming the P1781 could be tripped on startup, like all the info suggests. The P0700 must trip when it goes into limp mode? Is there any detail on what P0700 could mean?

Thanks,

This P1781 code is specific to the 1 wire circuit what exists between the ECM and TCM on which the TCM signals to the ECM to turn on the SES/check engine light. Since the TCM cannot drive the SES/check engine light directly, when the TCM has a trouble code it signals to the ECM to turn the light on its behalf.

I strongly suspect this communication link is checked by the TCM any time it's powered on. So if the TCM loses power momentarily while the truck is running due to a bad connection, poor ground, or flaky ignition switch and the ECM doesn't respond in the expected way the TCM may set the code. To the TCM it looks just like any other regular startup/key on event, only in the case of a momentary TCM power loss while running, the ECM is already alive.

As Zakk said, P0700 is a generic code which indicates the TCM thinks something is amiss. Focus on the P1781.

Referencing the 03 FSM, assuming 02 is same: there are 2 +12v power pins on the TCM that come from the ignition switch, C1-2 and C1-4 (pink wires) and one +12v always hot from the battery at C1-3. C1-1 and C1-5 are grounds (black wire with white trace). Manual is not super clear here, but it appears that the TCM ground pins tie into the ground point G102, which is on the right side of the engine block, just in front of the starter.

If all connections on the TCM look OK, and you've cleaned up the TCM ground, I would recommend changing your ignition switch out. Other possibility is maybe just a bad TCM.

 

[db131994]

Any thoughts on why it only sends the trans into limp mode when pulling the trailer during accelerating?

For a bad wire, or connection, I wouldn't think it would be as predictable. In other words, I would expect it to happen randomly.

Appreciate the discussion and info.

 

[dndj]

Any thoughts on why it only sends the trans into limp mode when pulling the trailer during accelerating?

For a bad wire, or connection, I wouldn't think it would be as predictable. In other words, I would expect it to happen randomly.

Appreciate the discussion and info.

Not sure. Possible heat sensitivity - engine bay hotter with the trailer load behind you?

Is it possible the code reader you're using is displaying the wrong code, or only capable of displaying a few of the possible TCM codes and there's another one besides the P0700 and P1781 you're not actually seeing? If you have access to a Tech2, try that to make sure there's no other codes hiding.

 

[Mike L.]

Any thoughts on why it only sends the trans into limp mode when pulling the trailer during accelerating?

For a bad wire, or connection, I wouldn't think it would be as predictable. In other words, I would expect it to happen randomly.

Appreciate the discussion and info.

If there is a damaged wire, lets say there are only one ore two strans; 12V's will show on a meter. Problem is there are no amps to work the unit even though you see 12V's.

 

[dndj]

If there is a damaged wire, lets say there are only one ore two strans; 12V's will show on a meter. Problem is there are no amps to work the unit even though you see 12V's.

Spot on, Mike. The TCM, or any embedded controller module for that matter is not a constant current (amps) drain. The current required will vary based on what the firmware programming is doing and what outputs the TCM is driving at any moment in time. I can only speculate, but the conditions at which the OP is seeing the failure may be a voltage/current droop having increased probability associated with the specific sequence of TCM operation you've seen failures with.

 

[db131994]

Well, I may have found the issue... Long story, but it's needed for the background. Last weekend we were camping and had an issue with our converter not charging the battery. When we got home, I went to start troubleshooting the converter issue, and it was magically running (it was not working before and drained the trailer battery). Digging into it, I found a cracked solder joint on the board in the converter. I re-soldered it and all is well with the converter.

We made a trip this weekend, and didn't have the transmission limp issue. I won't say it's "solved" yet, but this may lend to your advice about an issue in the 12V system. My theory is the short in the converter could have been causing volt / amp fluctuations and the communication issue. My trans guy said has seen trailer electrical issues back feeding to the truck causing similar issues.

Until we get a few trips under our belts without issues, I won't say all is well. I did lean on it pretty hard to push the conditions where it caused the trans limp issue before, and it survived...

 

[DAVe3283]

If that doesn't solve it, one thing I thought of while reading through might be worth investigating. You say it only happens while accelerating while towing. That would mean a very high engine load, which could cause the engine to torque to the side if motor mounts are getting weak/torn. This could in turn pull on the wire harness and cause an intermittent fault. So it may be worth checking if your motor mounts are bad.

 

[dndj]

Well, I may have found the issue... Long story, but it's needed for the background. Last weekend we were camping and had an issue with our converter not charging the battery. When we got home, I went to start troubleshooting the converter issue, and it was magically running (it was not working before and drained the trailer battery). Digging into it, I found a cracked solder joint on the board in the converter. I re-soldered it and all is well with the converter.

We made a trip this weekend, and didn't have the transmission limp issue. I won't say it's "solved" yet, but this may lend to your advice about an issue in the 12V system. My theory is the short in the converter could have been causing volt / amp fluctuations and the communication issue. My trans guy said has seen trailer electrical issues back feeding to the truck causing similar issues.

Until we get a few trips under our belts without issues, I won't say all is well. I did lean on it pretty hard to push the conditions where it caused the trans limp issue before, and it survived...

Hoping you got it sorted out, but I'm left wondering. Assuming you're talking about a normal RV converter which converts AC line voltage to 12v DC to run the house lights, 12v appliances, and charge the house batteries? If so, this unit wouldn't be powered up while you're driving down the road so I can't really think of how that would be a cause.

With '03 OEM wiring, the 12v circuit that feeds the 7 way trailer connector comes from the 40A "Stud 1" fuse which connects directly to the truck battery. Standard setup is that this pin in the trailer connector will be wired directly to the house batteries on RVs, providing ability to charge them while traveling.

Hope it stays fixed. :thumb:

 

[dndj]

If that doesn't solve it, one thing I thought of while reading through might be worth investigating. You say it only happens while accelerating while towing. That would mean a very high engine load, which could cause the engine to torque to the side if motor mounts are getting weak/torn. This could in turn pull on the wire harness and cause an intermittent fault. So it may be worth checking if your motor mounts are bad.

Really good idea, something to check out.

 

[db131994]

I don't disagree, like I said, until we get a few more trips under our belt I'm not convinced...

Going down the road, the frige is still drawing 12V power from the battery, and the converter is still in that loop. Could capacitors on the converter board cause power fluctuations back through the truck, due to a short?

 

[dndj]

I don't disagree, like I said, until we get a few more trips under our belt I'm not convinced...

Going down the road, the fridge is still drawing 12V power from the battery, and the converter is still in that loop. Could capacitors on the converter board cause power fluctuations back through the truck, due to a short?

True, the fridge will draw on the 12v going down the road. Most modern Dometic or Norcold units only operate the main cooling function on 120v AC or propane, and the 12v is only needed for the fridge control circuitry and it's a pretty low draw though.

The RV converters I'm familiar with bus the 12v RV battery (or batteries), the 12v charge circuit from the tow vehicle 7-way connector, and the RV converter 12-14v charging outputs all together. Then the 12v fused branch circuits for appliances inside the RV are fed from that. Since you mentioned you had a cracked solder joint, without knowing specifics it could be possible that a continuously flaky connect/disconnect situation (road vibration) internal to the RV converter, to a high current load like depleted RV batteries could cause electrical noise on the 12v charge wire from the truck, yes.