LB7: Trans Limp and P1781 / P0700 under load acceleration

[db131994]

It's baaaaaaccccckkkk.

We made about 3 trips without issues, but it started up again last weekend... I took apart the two connectors to the TCM looking for pushed out pins and wiring issues there, but didn't find anything. I also read somewhere about checking continuity across pins 1 and 5 of the TCM on the bottom (clear) connector. If it's open, the TCM is bad. I had continuity, so at least that check says its ok.

I put an injection pump on it around Christmas, so that is the one thing that has changed from before. It's making a lot more power, since it's actually getting rail pressure...

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.

I'm a little bit of a newbie on wiring, so I could use some help here. I don't have any wiring pinouts of the connectors. The only thing I could find were newer TCM's, or the trans connector. Can anyone point me to a pin out diagram? I'm assuming C1-2 is referring to the (clear) connector on the TCM and the 2nd pin? In this case C1-2 and 4 should be hot when the key is in the on position with 12V, and C1-3 should be hot with the key off, correct? So, with the key on, try to jiggle the switch to see if the voltage drops out? What about continuity checking C1-1 and 5? Are there any Ohm specs for this? I know the solenoid circuits are 22.5 Ohms.

Appreciate the help.

 

[dndj]

TCM connector pinouts from the 03 FSM attached.

You're correct on the +12v pins.

An intermittent voltage supply may or may not show up while you wiggle the ignition switch. The dropouts may be only a few 10's of milliseconds which may be enough to upset the TCM but fast enough that you couldn't see with a test lamp or digital voltmeter.

Grounds should be continuity checked to chassis ground and be less than a couple ohms.

Intermittent electrical issues are pretty tough to track down. If it was my truck I would probably be swapping the ignition switch or the TCM at this point, whatever was cheaper.

 

[db131994]

Thanks for the info Dale. I'll do some probing on the electrical stuff tonight.

Do you think there are any connections to the trans that could cause this, or based on the P1781, it's strictly a communication issue between the TCM / ECM? Or, just a power supply issue to the TCM?

The guy who rebuilt the trans scanned the ECM / TCM and the only code was the P1781. I was a little surprised that there weren't any other trans related codes. He said the P1781 was in the ECM, and there wasn't anything in the TCM. I would have expected the TCM to have a code, since it went into limp mode... That's assuming the TCM would have commanded the limp mode condition, and not the ECM.

What about the alternator? I saw some stuff on P0700 that the alternator could cause voltage issues that messed with the TCM.

Thanks,

 

[Mike L.]

Have you looked under the fuse box at all the pink wires?

 

[db131994]

Have you looked under the fuse box at all the pink wires?

I'll add that to my to do list tonight.

Thanks again guys.

 

[db131994]

Well, here is what I found... C1-2 and 4 had 12 volts when the key was on, C1-1 and 5 had 0.5 ohms resistance, so far so good. C1-3 had nothing with the key on, or off... I spoke with Mike and he directed me back to the ECM-B fuse. And........ It was blown. Changed the fuse, and voltage was back to C1-3! Not sure if this will solve my Transmission limp issue when pulling the trailer. That test will come next weekend.

Big thanks to Mike and Dale for the help!

I'll report back after next weekend to see how it goes.

 

[dndj]

The ECM B fuse (20A) does feed 12v to the TCM, but it also supplies the control side of the FICM relay. I don't see how your truck would even start with this fuse blown? Possible that the fuse got blown during your testing process? Hopefully you have it sorted, but if it comes back, more info.

I did some digging and the P0700 and P1781 are both reported by the ECM. P0700 just means the TCM is telling the ECM to turn on the MIL light or there is a short to ground on the MIL signal between the TCM and ECM. Here's more specifics on the P1781.

• Parameter: Transmission Control Module (TCM) MIL Request Circuit
• Acceptable Operating Range: IGN 0 on time > 1 second but < 7 seconds
• Primary Malfunction Detection Parameters: MIL Request line inactive
• Monitoring Time Length and Frequency of check: Monitored for 6 seconds after IGN 0 transitions from OFF to ON (every 125 ms)

This tells me that the ECM is monitoring for the MIL circuit to be active at key on during the time window from 1 sec to 6 sec and sets the code if it's inactive during this time. This is a kind of power-on self test that runs only at key on to make sure the MIL circuit is functioning between the TCM and ECM. It makes sense when you read the detailed conditions for P0700, which is set when the TCM drives the MIL circuit is active any time after 7 seconds or more after key on (and P1781 is not set).

I can't remember if I asked yet, but are you sure the TCM codes have been read? Both codes you've referenced are read from the ECM, not TCM. P0700 means the TCM has a code stored.

 

[db131994]

I don't think I would have done anything to blow the fuse during testing. I checked the voltage from the pin to ground post. I don't discount anything at this point...

The only time the SES light has come on is when the Trans goes into limp mode. Otherwise, there has not been a light. I have seen the P1781 code stored, even though it never sets the SES. Then, it will clear itself after some time. I haven't seen the P0700 the last several times I've had the limp mode issue, just the P1781.

There has never been a trans code stored. After this past weekend, I took the truck to the trans guy, and there wasn't any stored trans code, just the P1781 in the ECM. On the way home the SES light turned off, before I got to the trans guy.

If we assume the 12V hot feed to the TCM has been blown, would that potentially wipe codes from the TCM when you shut the truck off? Similar to unhooking the battery for an extended time? Not sure how the system stores codes...

I'll check the fuse when I get home to see if it's still good.

It still doesn't add up to me yet, but hopefully with all your insights and help I can get it lined out.

Any advice on an affordable scan tool that would read trans codes? I think it might be a good idea to be able to pull / monitor some of this on my own.

Thanks again guys.

 

[db131994]

Back to the drawing board...

Well, when I got home tonight I decided to pull the ECM-B fuse that was blown to see what would happen... Just like Dale thought the truck would not start. It must have blown somehow when I was checking things out the night before... I'm back to square 1... I'm assuming that the issue may lie in the 2 and 4 power pins (maybe a short somewhere), the TCM, or the ignition switch. Tomorrow I'll hook up the multimeter and try to move the wiring harness around to see if I can get it to drop out. Then probably replace the ignition switch. Can you replace just the switch, or can you only get the wiring harness that is shows on Rockauto?

How often are the TCM's found to be bad in an LB7?

 

[dndj]

Great idea, thanks for confirming, I suspected that might have been the case with the ECM B fuse.

More info that might help. The FSM says that the P1781 (changed to P0802 in 03 & up) only turns on the MIL (SES) if the power-on diagnostic for the MIL line between the TCM and ECM fails on 2 consecutive key on cycles. Otherwise it just stores a code. MIL is turned off by 3 consecutive key on cycles without a failure. The current P1781 code clears when the test passes but codes remain in history until 40 consecutive warm up cycles without a failure.

According to the GM docs, P0700 sets any time the key has been on 7 seconds or more when:

• The TCM is requesting MIL illumination active for > 4 seconds
• The MIL request circuit is shorted to ground

Turning off the MIL and current/history DTC is cleared same as P1781.

Thinking through these conditions logically from the info in your latest post. If the GM docs are accurate (which is not always the case) the P1781 test only happens when the ECM is powered up. This condition could be just a normal key-on, or the result of a momentary power glitch or loss of ground on the ECM while in operation.

In contrast, I'm interpreting the descriptions as P0700 being a continous runtime test, 7 seconds or more after ECM power on.

P0700 diagnostics follow this procedure

1. Check for TCM codes.
2. If no TCM codes, and while ignition is OFF, the MIL signal (C1-25) should be 12-14v.
3. If C1-25 is not 12-14v, check for MIL signal for short to ground.
4. If C1-25 is 12-14v, replace the TCM.

The interesting thing here with this procedure is that the MIL signal should have 12v on it with key off, meaning that the ECM expects to see it active at key on until the TCM boots up and drives it inactive. It also means the MIL signal power must be coming through the TCM always hot C1-3 connection that led you to the burnt fuse. It is still possible you have an intermittent short to ground on this circuit - momentary enough to not blow the fuse.

If the MIL signal is ground during the first 6 seconds after key on, P1781 will set. If P1781 is set, P0700 won't set. Also neither of these are supposed to result in a trans limp. So all the evidence here is not really pointing clearly at one thing except possibly a bad TCM because of the trans limps.

I have one of the chinese clone Tech2 scan tool units - they run about $200-$300. There are other threads on DD discussing these. Best to keep it in your truck and scan it right on the spot next time it happens. You can also discern information from the freeze frame data and historical codes that help with troubleshooting as well.

 

[db131994]

If the MIL signal is ground during the first 6 seconds after key on, P1781 will set. If P1781 is set, P0700 won't set. Also neither of these are supposed to result in a trans limp. So all the evidence here is not really pointing clearly at one thing except possibly a bad TCM because of the trans limps.

That's kinda where my thought process has went as well. If there was a short, or other issue somewhere, I would assume I would get codes for the system that's shorting out, say "line pressure out of range" for example.

I hate throwing parts at it, but it looks like we are at that point. Are there any diagnostics that could be done on the TCM, maybe try having it re-flashed? I've heard if it won't flash, that's a sure sign it's bad. Now the next question, anyone have a source for a TCM???

Appreciate all the great help on trying to run this issue down.

 

[db131994]

Found a TCM at Hicksville Auto Recyclers for $265. They specialize in Duramax parts, so they will have anything you need. I'll get that hopefully before next Friday to see if that does the trick...

 

[dndj]

I would be doing the same thing - eliminate the TCM as a cause and rule that out.

I bought a used TCM from an ebay seller ("deeringssales" 100% positive feedback) for $259, and it came VIN programmed for my truck. Make sure yours comes programmed as part of the price or you may have to pay someone else or the dealer.

 

[db131994]

Does the TCM need to have the VIN to "unlock" it? When I asked them about it, they said as long as it was the same model year it would work. Did they feed me a line?

 

[dndj]

Does the TCM need to have the VIN to "unlock" it? When I asked them about it, they said as long as it was the same model year it would work. Did they feed me a line?

Actually I'm not sure about this, I just assumed because VIN programming was advertised on the unit I purchased. I'm confident others on DD would know if the TCM VIN needs to match.

 

[db131994]

So this weekend, it tripped the P0700 code. It seems like it happens when the Trans gets to 190F. Below there, it doesn't happen when I would expect it to. No other codes that my SuperChips tuner could read.

The new TCM is supposed to arrive today, so I'll check the MIL circuit voltage when I swap them out. I'm going to have the ECM and TCM re-flashed to the latest update as well. We will see where we go from there...

 

[Chevy1925]

it really wouldnt hurt to get a scanner on the truck that can read the actual codes in the TCM. might help you further on this venture.

 

[Mike L.]

it really wouldnt hurt to get a scanner on the truck that can read the actual codes in the TCM. might help you further on this venture.

I agree. Codes displayed by tuners are not accurate many times.

 

[db131994]

Hmmm. Checking the C1-25 pin with key off I had 0.3 volts. With the key on, I get 10.5 volts... this was checked with both C1 and C2 connectors disconnected from the TCM. Not sure if C2 was supposed to remain connected during the testing... I'm going to hold off on installing the "new" TCM, until I can understand if there's an issue with the MIL signal or not. Does this line just run from the TCM to the ECM? I don't have a chance to track it down for a few days. Let me know your thoughts.

This is over my head at this point...

On the scanner, I'll see if the trans guy can try and see if there are any stored codes again. There weren't any last time in the TCM.

Appreciate the help guys.

 

[dndj]

Hmmm. Checking the C1-25 pin with key off I had 0.3 volts. With the key on, I get 10.5 volts... this was checked with both C1 and C2 connectors disconnected from the TCM. Not sure if C2 was supposed to remain connected during the testing... I'm going to hold off on installing the "new" TCM, until I can understand if there's an issue with the MIL signal or not. Does this line just run from the TCM to the ECM? I don't have a chance to track it down for a few days. Let me know your thoughts.

This is over my head at this point...

On the scanner, I'll see if the trans guy can try and see if there are any stored codes again. There weren't any last time in the TCM.

Appreciate the help guys.

If you have a P0700, the TCM has codes set. They need to be read with a tool like a GM Tech2 or Allison DOC capable of reading the TCM. Mike and James are giving good advice here, report back when you have read out the TCM codes.

Not advised, but if you still want to probe around on wiring, the MIL C1-25 test I referred to as part of P0700 diag in post #30 above is done with the ECM and TCM connected. You need to backprobe or otherwise T into the circuit.