The contacts in the junction in the engine compartment block on the passenger's side also need to be clean and tight. I access there for the remote start because it's easier to reach than the starter itself. The cranking system needs to be in good shape so if in doubt, go for the starter.

I changed to full synthetic oil because it makes cold starting easy. Anything below 15*F was a problem on dino oil especially when the battery wasn't perfect. Also note that the glow plugs can pull voltage measured at the battery posts down so low that you may condemn the charging system.

Finally especially now that it is becoming colder, these alternators have a regulator that is easy to change with the alt installed. Buy quality ie Bosch in this case. Brushes are another item that causes charging issues but doesn't require replacing the alternator.

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.