Just bought my 06 CTS (3.6L) two weeks ago. This past week when I would turn the ignition switch to the start position I would hear a solid "thud" but the starter motor would not energize. Cycling the key switch a few times would finally give a good start. I checked battery terminal voltage while the starter was engaged noting that it never dropped below 10.5 volts indicating the battery was not the problem. The battery is also only a few months old and fully charged. I checked the electrical connections at the battery and the starter; both were in good shape. I next swapped the starter relay with another relay in the power distribution box finding I still had the engagement problem. Having completed these steps it was now clear the most likely cause for the problem was the starter solenoid. The solenoid acts as a large electrical switch that electrically connects the battery cable to the starter motor power terminal. Inrush current for an engine starter can easily exceed 600 amps with steady state current usually around 120 amps. This high inrush current causes heavy arcing causing the solenoid contacts to erode over time. Once the surfaces of the contacts become heavily eroded they can no longer consistently complete the electrical circuit that supplies power to the starter motor.
I found very little information online or the forums regarding steps for changing the starter. I live in a descent size city with multiple store locations for each of the big name auto parts suppliers; no one had the starter in stock. Local dealer did not have it in stock either. I am guessing this means the starters are usually pretty healthy and hold up well. Given that I could not find a step by step DIY instruction for this task I decided to post one myself. Hope someone finds it helpful.
As this job must be done from the bottom side the first step is to lift the front end. I used a floor jack to raise each side and then placed ramps under each wheel. This raised each wheel around 12" and provided plenty of room for the work. Here is a shot of the car on the ramps.
Lighting is pretty important for this task. If you have a cylindrical bodied shop light (fluorescent) you can slip it down between the dip stick and oil filter housing and have it pretty much in the perfect position to light the entire work area underneath. Perhaps the most important step for this job is DISCONNECTING THE POSITIVE BATTERY LEAD. The battery cable that connects to the starter is hard-wired to the battery lead. There does not appear to be any type of master power relay that isolates the battery from the starter power lead.
I had read a few comments on the forums indicating the exhaust manifold had to be removed to replace the starter. Definitely not the case on the 06 CTS with the 3.6. While the space is not wide open its plenty for doing the job. Here is a shot of what you will see when you crawl under the driver's side and look up and inboard behind the exhaust manifold. In the left-had area of the photo you will see what I believe is a knock sensor. There is an identical one on the right-hand side of the engine as well. This sensor may not need to be removed but it would be a tight fit removing the starter if it were to remain in place. There are three electrical connections to address for this job; 1) knock sensor connector 2) starter solenoid connector 3) battery cable connection to starter.
Remove the connector from the knock sensor. BTW, in my opinion the hardest part of this job was disconnecting the electrical connectors. The lock tab should be depressed while pulling the connector body.
Remove the knock sensor mounting bolt using a 13mm socket and 11" ratchet. I used a ratchet with a pivot head which is usually easier to maneuver in tight places but I don't think it's a requirement. The knock sensor bolt is not torqued very tight so its pretty easy to break and back out.
Next remove the solenoid connector. This one was a bit stubborn to release. The lock tab points down. I used a long shank common head screw driver to depress the lock tab (pushing up gently) while prying to the rear. Once I figured out the locking mechanism for the connector removal was pretty clean.
WARNING: MAKE SURE YOU HAVE DISCONNECTED THE BATTERY BEFORE PROCEEDING.
Next disconnect the battery cable from the starter power stud. Use a boxed end wrench to break the nut and then remove the nut with your fingers. The cable terminal slides off the starter power stud easily once the nut is removed. Use side cutters to cut the nylon tie-wraps anchoring the battery cable to the engine block. Bend the battery cable out of the way. You can now see the the upper and lower starter mounting bolts.
Remove the two 15mm bolts. I used a deep socket with the 11" pivoting head ratchet with no problem. Slide the starter forward. As the nose of the starter clears the crank case let it drop down and clear of the vehicle. Install the new starter by reversing each of the steps taken during removal.
Due to the size of the battery cable its important to re-anchor it properly to the engine block. I prefer using two tie-wraps arranged in the "butterfly" technique for anchoring electrical leads/cable. Tie-wrapping a cable directly to a rigid object can cause the cable to chaff at the point where it contacts the object. The butterfly technique provides a stand-off that keeps the cable anchored while also keeping is separated from the rigid object. The following two photos demonstrate this technique. First, wrap one tie-wrap around the cable and the anchor point. Next, wrap the first tie-wrap with a second tie-wrap at a point between the cable and the anchor point. Don't fully tighten either at this point. Progressively tighten the two tie-wraps until both are snug. Clip the excess length.
As a side note, while under the car I noted that the two transmission fluid lines that carry fluid to the cooler and back were touching one another. This contact has existed for some time and has abraded some material from the OD of the lines. If your lines are making contact like mine were they should be separated. Firmly grasp one of the lines and gently pull it away from the other. Don't be too aggressive with these lines as they seem to be moderate wall aluminum tubing. I am lucky to have caught mine before a breach developed.
Start to finish this is around a 3 hour job. Good luck!