Failing starter

I've been having some difficulty getting my engine to turn over in the cold temps here close to 0 F. I have a frost heater and if I plug it in for an hr, the car will start fine. It also starts fine if it has any amount of residual heat. When its Ice cold, it may only turn over for half a second and them stop. On subsequent attempts, the same thing happens or it wont turn over at all.

The battery was almost 5 years old, So I replaced it, but there was no change. At this point, I'm thinking the starter is tired and needs to be replaced.

Does anyone have any other suggestions?

The bearings in the starter are probably worn OR the main +12V cable from the battery is corroded. See if you can put a jumper cable on the battery and the starter, then when the engine is stone cold, try starting it. This will eliminate one of the possibilities.

For what its worth, I took VW at their word and shut down at stop lights ala Europe if its going to be more than 30 seconds or so.

Starter is still going fine (knock on the DOC)

Make sure to do a voltage drop test, neg and pos sides and if you have access to a clamp on amp meter check amperage draw while cranking if the drop test shows OK. I'm assuming anything between 250 to 400 amps on the higher side is about normal. To do the voltage drop test all you need is a volt meter and anything under 1 volt drop during cranking should be OK on either Neg or Pos. You can google how to do it, easier than me explaining it

Just went through this with my Jetta. New battery improved cranking but still too slow for comfort in winter. I replaced my starter (I think around 5 years on this Bosch rebuild) with another Bosch rebuild and she's spinning nice and fast now.

Yup. My TDI 6 speed has that issue. Been getting slower and slower the past few winters. Good thing I've retired it recently.

As Brian's96TDIPASSAT said, do a volt drop test. Use a DVM alligator clips. Red lead on Bat positive black lead on starter solenoid terminal that has the starter motor lead connected to it. Make sure the leads are out of the way, prop the DVM where you can see it, and try to start it. Do the same thing with the red lead on the starter case and the black lead on the Bat negative. If the total is over 2 volts break it down into smaller pieces like battery to solenoid, then across solenoid, etc. to find the problem. First you could easily check the volts across the battery while cranking in case the battery is just not up to it.

If you have very little or no drop at all it is likely the brushes in the starter.

It's usually the bearings/bushings in the starter that wear out moving the rotor too far to one side and reducing the strength of the starter. If the brushes are worn so much that contact is limited or lost then, the starter would spin very slow or not at all..... pretty much all the time.

Had to use the TDI this morning for a dump run...(Haven't moved the hitch over yet) battery was charged yesterday and coolant heater plugged in for 2 hours. Barely turned over and not fast enough to start.

Only posted my personal experience. Starter started acting weird. Sometimes not spinning at all; sometimes spinning slowly; other times working fine. Did volt drop test and had good voltage and ground at the actual motor. Pulled the starter and the brushes were shot. Bearings were fine. Needed the car so put an AutoZone reman in it to get it going. That one never started it very well at all. Put new brushes and cleaned up the commutator on the original one and it's as good as it ever was. Starts fine even in the recent near 0F temps without a preheater. Could be a single data point, but might be useful nonetheless.