I have been observing my starter motor slowly failing over the last 5-6 months. Works great when the engine is cold, but really struggles when the engine is hot. Seems counter-intuitive as a warm engine should be easier to start and with warm oil, be easier to spin.

There are at least 3 reasons a starter motor will turn slowly - failing battery, corrosion/high ohmic loss in the wiring/and gummed up internals. The battery checked out, and I figured I'd look at the wiring when I went in to remove the starter.

On the DB9, after disconnecting the battery, you (of course) have to get the car in the air, remove the right front wheel/fender liner and then remove the lower two heat shields from the cat & header. You also have to remove the two downstream O2 sensors. All of this so you can jam your hand into remove the solenoid and main starter power leads and the two bolts that hold the starter in place. The nuts for the two bolts are captive on a carrier, so you don't need a second wrench. Once you have removed the starter bolts you can drop it down and slide it between the header and the block. Just fits!

I then found a number of videos on the web that show how to take apart a Ford PMGR starter (Permanent Magnet Gear Reduction). Pretty straightforward to follow the video. Mine didn't need new brushes or bushings, but did need to be cleaned out and regreased the rotors/steel ball (don't lose the ball!) with white lithium grease. There are 4 bolts, two circlips and one half moon holding it together. The circlips have 1mm holes which required me to find a tool with tiny tips. But all in all pretty simple to get it apart, back together and reinstalled.

I also cleaned the leads where they attach to the starter and the combination seems to have cured my lazy starter. Took 3 hours to get it out (so little room...), 2 to rebuild it and a couple of hours to get it back. I used anti seize on the bolts and dieletric grease on the connections. Hopefully it will be easier if I ever need to do it again (meaning, I really hope I got this right!)

Some pictures for the curious.

solenoid and primary shell.

brushes, end cap and one of the 4 gears

The rotor that holds the 4 gears rotating inside the plastic hub. These gears engage the bendix.

Armature and hub that has a gear that engages the flywheel / ring gear. I also polished/cleaned up the armature.

 

Pretty cool, thanks for sharing!

 

Back when I still had my Vantage I had a similar issue which I thought was a problem with the starter. I bought one and was about to replace it when I found a terminal junction where the nut had completely fallen off and the terminals were barely making contact. It's on the passenger side, behind the front wheel arch. You have to remove the aft portion of the RH inner fender liner to see the stud. I don't know if the DB9 has the same stud but if it does, make sure you check it.

Replacing a $4 nut saved me a LOT of time and hassle replacing a starter that wasn't bad.

 

I love this! Another example of the fact that these cars are just ... cars! Not some fancy mystical object that needs to be taken to fancy mystical wizards at exclusive Aston Martin dealerships where you pay eye watering prices for the privilege of them sprinkling holy dust on your car!

Thanks for posting and congratulations on the repair!