1978 240D won't start...starter?

[apsaulters]

Folks,

Just when I thought my leaky trunk was my biggest problem, the car refused to start last night. Here's some information on what's going on (apologies for length; trying to be thorough).

Situation: Last night I drove the car for about thirty miles then parked it. I came back to it after fifteen minutes and it wouldn't start. The first time I tried to start it, I ran the starter for about ten seconds and gave it some fuel; no go. The second time the starter went for five seconds; third time, two or so seconds. The fourth time the starter would not move at all. The motor sounded weaker each time. When I tried it again this morning, the engine still wasn't turning.

History: The car's been taking longer than usual to fire of late, but I thought that was the cooler weather we've been having recently (Greensboro, NC). The previous owner says the starter was replaced some time ago, perhaps 10 years ago, before his father handed it down to him. The earliest repair order he has on the car says the mileage was 517,531 on 8 August 2000. The present mileage is 540,198. Engine was rebuilt around 400,000, in case anyone was wondering.

Starter sound: I think the solenoid is clicking, but I can't tell for sure; there's a click-like sound when I try to turn it over, but I can't tell if it's coming from the ignition lock or the solenoid.

Voltage: Took a reading of battery voltage after the first failed crank last night; it was 12.5 volts at that time. I've tried to crank it maybe six times since; my last voltage reading was 12.25 volts. (I'd give a current reading, but my multimeter just died. Alas.) Lights and the lovely 'door ajar' buzzer still work without fail.

Fuel pump/fuel level: I haven't checked this yet; the tank has about 13 gallons in, and, like I said, I drove it for thirty miles without a problem beforehand.

Glow plugs: The glow plug light comes on for the customary length of time when I turn the key. I check the resistance of the glow plug circuit from the relay under the dashboard, and it looks like one glow plug is bad. The car has been starting roughly recently, so I suspect I'll have some work to do on the glow plugs when I figure this out. Don't think that's the culprit, but I figured someone might want to know.

So, I guess my question is...what's up with this? I've read on the forum that one can expect a decent starter to last 7 to 15 years--by that metric, it sounds like the starter could be dead. But at that same time, the car was driven an average of 3,400 a year over the past seven years, which doesn't sound like a very heavy duty usage history (for instance: assuming ten miles driven per crank, a minimum of two cranks per trip, the car would have been driven less than one day out of two each year for the past seven years). Over the past few months I've driven more than the previous owner, on schedule to do about 10,000 a year, but that was mostly on long (out of town) trips.

I haven't tested voltage to the solenoid/starter yet because I am not really sure what to poke down there (any ideas?). I have not yet plunked it with a hammer--does that sound like something that would be useful here? (Again, it sounds as if the solenoid could be working properly.) What's the sound of a starter dying? (I expect it would be the sort of affair where the starter motor just stops, not the slow fading that I experienced.)

I expect to replace the starter, but before I put down the money for a replacement, I'd like to make sure I'm replacing the right things. Interpretations of this information and suggestions on what to inspect are much appreciated.

Thanks,
Andrew