U0107 Only on Startup

[Callaway Crow]

Thanks to everyone on this forum. I?m working on my first LS swap. From this site, and a few others, I have been able to get my car running smoothly. For the most part I?ve been able to find a lot of info on my problems just from searching and reading posts.

So here?s my issue. I have a DBW 6.0 LQ4 out of a 2007 Chevy 3500. Often, on engine start, I get U0107 and the car tries to stall until I can clear the codes on the scanner and get the throttle working again. Once I get past the startup, the DBW works perfectly fine. This only seems to happen when I have the scanner running, which is just about every time, so it?s hard to make the correlation. And it seems to happen most when I?m up at the track and it?s a pain to try and clear the code.

Based on what I?ve read, I?ve checked all the grounds and even ran a 10ga ground wire directly to the TAC module. I tried 3 different TAC modules, with different part numbers, all behave the same way. I ran a separate switched power line from a relay to eliminate positive supply issues. I?ve tried wiggling the wires on the throttle body with no result. I checked the resistance sweep of the two pedals I used, and I?ve done the same for the TPS internal to the throttle body. I don?t really want to switch to DBC, but I feel like I?m approaching that point. Anyone else have this problem? Is there a way I can disable U0107? How about for the first few seconds? Is it possible the scanner interferes and shouldn?t be connected until after initial startup? Any other ideas would be appreciated.

The ECU is 12589162. The TACs I?ve used are 12573059, 12588923, and 1259008.

[blindsquirrel]

Is some of what you've read the official GM diagnostics page for U0107?

Circuit Description
The throttle actuator control (TAC) module and the powertrain control module (PCM) communicate via a dedicated serial data circuit. This serial data circuit is separate from any other serial data circuit on the vehicle. Accurate transmitting and receiving of serial data requires not only good circuit integrity, but also adequate system voltage. This diagnostic test monitors the accuracy of the serial data transmitted between the TAC module and the PCM. If the PCM detects a loss of data or invalid data, DTC U0107 sets.

Translation: Check the dedicated serial line from PCM C1 14/15 to TAC 12/13 very carefully - is it a proper twisted pair? Shielded? Does it run near anything that could cause interference while cranking?

Conditions for Setting the DTC
The PCM detects invalid or missing serial data messages for more than 0.83 second.

.83s is not a very long time.

? DTC U0107 sets if the battery voltage is low. If the customer concern is slow cranking or no crank because battery voltage is low, ignore DTC U0107. Clear any DTCs from memory that may have set from the low battery voltage condition.

Also the inverse: If the only customer concern is U0107, ensure cranking voltage is up to snuff.

? DTC U0107 sets when there is a short to B+ on the TAC module ground circuit. Inspect the fuses for the circuits that are in the TAC module harness, i.e. cruise, brake. An inspection of the fuses may lead you to the circuit that is shorted to the TAC module ground circuit.
? DTC U0107 sets if the TAC module ignition feed circuit is shorted to a B+ supply circuit. The TAC module stays powered-up when the ignition switch is turned OFF. When the ignition switch is turned ON, the TAC module is powered-up before the PCM. DTC U0107 sets because no communication is detected by the TAC module from the PCM. Inspect related circuits for being shorted to a B+ supply circuit.
? Inspect the TAC module power and ground circuits and the TAC module/PCM serial data circuits for intermittent connections.
? Inspect the TAC module connectors for signs of water intrusion. If water intrusion occurs, multiple DTCs may set without any circuit or component conditions found during diagnostic testing.
? When the TAC module detects a problem within the TAC system, more than one TAC system related DTC may set. This is due to the many redundant tests run continuously on this system. Locating and repairing an individual condition may correct more than one DTC. Remember this if you review the stored information in Capture Info.

You can rule out constant shorts, instead look for possible intermittent shorts to voltage on the described circuits only while cranking.

[Callaway Crow]

Thanks for the ideas! The signal wires are just plain wires. I do have some shielded mil-w and I'll give that a try for the signal. I'll also check the voltage when starting. There is a little longer cranking when the engine is hot, like at the track, so that could potentially explain the issue as well.

[Callaway Crow]

I believe I found my issue. Rookie error for sure, but I'll share in the event that my mistake helps someone else later down the road.

My ECU and my TAC were being supplied power from different fused circuits. I only checked that they were both hot with "key on" and off when the ignition was off. What I found is that the circuit supplying power to the TAC was not providing power while cranking. Any start cycle under 0.83 seconds and the TAC would work. Starts with cranking time over 0.83 seconds and I would get code U0107 on start-up.

blindsquirrel, thanks for the help!