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2004 Tahoe LM7 with original harness, underhood fuse block and harness. TAC module is a replacement, but should work with the combo. Swapped into a 68 Suburban C10.
Last night I started it for the first time. It was rough and died and had no throttle response. I found a bad head-body ground and fixed that. Truck started up and idled great after that fix. But no throttle response. I was only getting 8 and change in voltage to the TAC module, so I found 12V and tried to wire it to the TAC module direct.
Now the truck won't start. If I pull the ignition relay, I can get the crank in the attached video. (here's the link:
) As you can see, if I pull that relay, I get that not quite but sorta kinda crank.
I also have lost my 12V for the fuel pump (at the fuse block, so it isn't a bad ground, I get nothing from the terminal of the fuse block to the negative of the battery thru my multi meter with the truck in run. This is way beyond my abilities, so I'm hoping you guys can help. I don't have an OBD2 port wired yet (that's tomorrow's project, if the parts get here) so I can't tell what codes if any it is throwing.
OK, chased down some bad grounds and a buggered relay, and everything is gravy EXCEPT the throttle, I have no pedal. I have 12 volts across the pink to black on the TAC module. Do I need to ground the pedal?
I had a problem with a 2004 LM7 in my '39. Turned out it was a mismatched tune to throttle pedal. Once I changed to an '05 pedal everything was golden.
Pedal isn't from the Tahoe, but is the same part number and came matched with the pedal (Roadrunner managed to drop my power unit in shipping and it showed up with a busted tailshaft on the trans, no TAC module or pedal to be seen, so they were replaced by the seller.
I got the obd port wired and now I'm getting the following that relate:
P2136 Throttle / Pedal position sensor switch a/b voltage correlation
P0120 Throttle / Pedal position sensor / switch A circuit
P2135 Throttle / Pedal Switch A/B correlation
So it seems to me that the pedal is the problem. So, two questions before I order a new pedal position sensor:
1. Should I have a ground just for the pedal?
2. There are different codes for the TAC module, right?
IIRC I once got those codes on a swap project when the pedal wasn't connected correctly to the TAC.
You do not (cannot) ground the pedal; however it is recommended you ground the case of the TAC module.
I also think you have a pedal problem, perhaps the wrong one.
This diagram may help you understand how the DBW works:
This pedal has two potentiometers. Each is connected to +5V and a ground reference. As you press the pedal, the voltage of one goes up and the other goes down. In other words, at e.g. idle, one signal will be at 0.5V while the other is at 4.5V. At WOT their voltages are reversed. The TAC module monitors their signal voltage to ensure that as one goes up the other goes down. This is an important safety feature; otherwise if there were only one potentiometer and it failed, the engine could go WOT without any throttle.
When the TAC senses any potentiometer problem, it keeps the engine at idle and sets those codes.
Thanks for the detailed response. I have the TAC case grounded. I'll check the pedal connection again, but the new pedal position sensor (which houses the potentiometers, I think) gets here today, so I'll add that to the mix as well. For $28 it seemed worth the money.
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LM7 that WAS running 
