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2005 Cadillac CTS-V
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Discussion Starter #1 (Edited)
Anyone have tips or a how to on removing the main engine harness? Main thing I'm wondering about is disconnecting it where it goes up under the steering rack. Everything else seems to be pretty straight forward.

(I'm trying to track down a short in my harness to what I think is the main 02 sensors. I'm going to try testing the wires for resistance but, ultimately, I think I'll have to find the place in the harness where the short is located and I think having the harness out will make it easier than trying to fix it w/ the harness in the car.)
 

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'05 CTS-V
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8,299 Posts
It's been a few years since I pulled mine, but I don't remember it going around the steering rack. My recollection is that there's pretty much a continuous loop around the top of the engine, and then there are branches that go down from each top, back corner and come forward along the sides of the block (to get to the starter, oil level/temp, A/C compressor, etc). And there are two legs that go back along either side of the transmission.

I never actually pulled mine completely. I disconnected the various tranny sensors, pulled those two harness legs off the transmission, pulled up the two legs that run down the back of the engine on either side, and flopped the whole thing forward over the radiator support. So I guess I never really dealt with whatever would've been down around the rack.

 

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Premium Member
2005 CTS-V
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8,363 Posts
Based on the wiring diagram (see attached) the O2 sensors have two sets of wires. One set runs to the underhood fuse block and a second set runs to the ECM/PCM. I think you can test resistance from the ECM/PCM pins (see attached) to the O2 sensor and then again from the O2 sensor to the fuse block. The wiring harness would presumably remain in the car unless the test revealed high resistance or a short to ground.
 

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2005 Cadillac CTS-V
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420 Posts
Discussion Starter #4
yeah, I'll test continuity and the resistance of all four wires on both sides back to the PCM this evening. I'm expecting I'll find an issue in one of the (-) wires since I'm still getting voltage readings off both of them. If I do have an issue on those wires, I'm just trying to figure out how to pinpoint/correct the issue - that's why I'm asking about removing the harness. (What makes me think I've got a problem? I've got no long term fuel trims but I do have hz readings for the 02 sensors. 02 fuses are good.)
 

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2005 CTS-V
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yeah, I'll test continuity and the resistance of all four wires on both sides back to the PCM this evening. I'm expecting I'll find an issue in one of the (-) wires since I'm still getting voltage readings off both of them. If I do have an issue on those wires, I'm just trying to figure out how to pinpoint/correct the issue - that's why I'm asking about removing the harness. (What makes me think I've got a problem? I've got no long term fuel trims but I do have hz readings for the 02 sensors. 02 fuses are good.)
If I understand what you are saying (and I may not) - the PCM is seeing the O2 sensors, is not throwing any O2 sensor-related codes, but is not responding to the O2 sensors. If that is the case, I would suspect the PCM has an issue. I say this, because there are variety of O2 sensor-related codes the PCM can throw if it sees the O2 sensors, but the does "like" what it is seeing.
 

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2005 Cadillac CTS-V
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420 Posts
Discussion Starter #6
Yeah, I'm at a loss. I swapped to another PCM/ECU & applied my current tune. No LTFTs, no codes. I don't think any of the applicable codes are turned off.

I put in 2 new AC Delco O2 sensors just for the heck of it. No LTFTs.

O2 wiring tests: I tested both continuity and resistance of all four wires (from the O2 end of the extensions back to the PCM and, for the heater (+) wire, from the O2 back to the fuse box) and all tested continuous and .2 or .3 Ohms resistance. I also tested the extensions themselves and they showed continuous and .2 or .3 Ohms resistance.

Still no LTFT values when the engine is over 115 degrees or otherwise... STFTs are at like -1.8 and O2 hz are around 825 each (rich).
 

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2005 CTS-V
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Correct me if I am wrong, but I thought LTFTS are driven by STFTS, and STFTS are driven by the responses to the O2 sensors. If that is the case, the fact that you have STFTS means the O2 sensors and circuits are okay.
 

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2005 Cadillac CTS-V
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Discussion Starter #8
just an update. It was so strange having STFTs, no O2 codes and all wires from the O2s to the PCM testing fine that I took the car to a GM dealer and had them re-flash the computer back to the latest OS. Problem solved. I've got LTFTs again and can start tuning in earnest.
 
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