Hey folks, hoping to confirm that what I have is indeed a dead starter motor on my '84 300TD Benz. I tried starting it today for the first time in two years and it was turning over (and clicking), but VERY slowly. I was jumping it while attempting to start it, so I assume the battery was fully charged and not a limiting factor.

I connected a 12.4v battery directly to the starter motor to test it, and it did not run. All I got was a clicking sound (I think the solenoid?) but no motor whir. You can see/hear it in this video: https://youtu.be/dyztJ4lOZk0

The car has been sitting for about 2.5 years. Before I parked it in Dec 2014 it was running/starting well, although the day I finally parked it it wouldn't start (I thought because of the cold - in Taos NM it was below freezing). I had one issue earlier that year (2014) where it wouldn't turn over at all and I jumped it across the solenoid by touching a screwdriver to two of those bolts. It started right up and no problems until the cold that December when I left it parked. I tried starting it once since then (probably winter of 2015) and it would turn over slowly but not crank fast enough to get going. Battery was fully charged and I plugged in the engine block heater before trying... still I was hoping the winter/cold was the problem.

Now it is summer so cold shouldn't be a factor. I'm back at it - this time I need it to run. Two questions:
1) based on the video do I definitely have a dead starter motor? Or is there some other reason the motor wouldn't run when connected directly to 12v? I'm also confused as to how the car was turning over this morning if the starter motor is indeed dead - is that possible? Do dying starter motors turn over slowly or does that indicate a battery problem?
2) if starter motor is dead, any thoughts on replacing vs rebuilding? I know there are threads/DIY guides for both options on this site, just curious about the benefits/costs of each option. I'm not a super experienced mechanic and it seems like most people replace.

many thanks!

One other question that may be related. I often get this loud whining sound when I turn the key to the first on position (to cycle the glow plugs). It seems to be coming from behind the driver's side front headlight, under the hood (I think near the glow plug timer relay). I'm not sure if this could be related to starting problems but worth a mention. You can hear that (terrible) sound here: https://youtu.be/XIrYiKvOUnI

The second "Sound" is almost exactly what a starter whose "Bendix"
won't engage sounds like.
[Starter spins with NO LOAD,since it doesn't engage]
The fact that it happens during the "Glow" portion of the ignition sequence
says Zomething is amiss with the Wiring,since that's an Impossibility.
Either the ELECTRICAL portion of the Ignition Switch is at fault.
OR ZOMEBODY'S Buggered up your Starter's Wiring.

OR Mayhaps BOTH ?

"Shorting" a Starter has risks and I hope you've not found a different
type of one !

__________________
'84 300SD sold
124.128

It certainly helps to troubleshoot the right way. The terminal you were touching with the battery lead was the solenoid terminal, not the main terminal to the starter motor. All you've proven is that the starter solenoid works. The battery + has to be connected to the main terminal on the solenoid for it to pull in the starter motor.

When you're turning your key, do you hear the same clunking you got from that video? If so, you either have a bad connection, a bad starter motor (unlikely), or the contacts in the solenoid have failed (replace the solenoid).

As to the buzzing noise, who knows? You were 15 feet away from the car when you made the video, it could be anything. Get under the hood and find out what's making the noise! It shouldn't be hard to find.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

If there is a shortage of current to the start (ie just using jumper cables) there will be a click but no crank (with the starter still bolted on the engine), but I can't tell if the click is the solenoid moving all the way or just part way, I'm more familiar with starters that make a louder clunk when engaging. But the fact that it did once turn over slowly leaves me thinking you've got a shortage of current to the starter rather than a solenoid problem

__________________
1978 300D, 373,000km 617.912, 711.113 5 speed, 7.5mm superpump, HX30W turbo...many, many years in the making....
1977 280> 300D - 500,000km+ (to be sold...)
1984 240TD>300TD 121,000 miles, *gone*
1977 250 parts car
1988 Toyota Corona 2.0D *gone*
1975 FJ45>HJ45
1981 200>240D (to be sold...)
1999 Hyundai Lantra 1.6 *gone*
1980s Lansing Bagnall FOER 5.2 Forklift (the Mk2 engine hoist)
2001 Holden Rodeo 4JB1T 2WD

Hey folks, thanks for the helpful responses. I cleaned all the ground wire connections and bought a new battery and it started right up.

I thought the battery was not the limiting factor because I had tried jumping the car AND had tried starting it off my brother's van battery (nearly identical specs), but I've learned I don't understand batteries very well.

Diseasal 300 - thanks for pointing out that I was testing the starter motor wrong. This made me hesitate (and not spend all day pulling my starter motor out of the vehicle), which is a big help. I was wondering if I was doing it wrong, which is why I posted the vehicle. So thanks for the response.

NZ Scott - you were totally right that I had a shortage of current problem. I'm still not sure why it wouldn't start with the borrowed battery I tried....

Anyway, thanks for the help. Started it a few times since then and it starts more easily than it ever used to. The battery has probably been weak for a long time.

Batteries can be a source of great frustration. Just because they show voltage doesn't mean they can deliver adequate current. If the battery is old and the plates worn or sulfated, it will struggle to start a diesel.

Testing the starter motor correctly is important as it can cause you to pull the motor thinking it is bad when it really isn't. The solenoid does 2 functions: it throws the bendix assembly into the flywheel to allow the motor to turn the engine and it also closes a set of contacts to cause the motor to run. A bad solenoid can still operate the bendix, but fail to close the contacts, usually indicated by clicking when turning the key, but no starter engagement, but eventual starter operation after a few tries.

Jumper cables aren't always a reliable test either. A good set of jumper cables will be a minimum of #6 wire, but preferably #4 or larger with a high quality (heavy) clamp. Good jumper cables aren't cheap and cheap jumper cables aren't good. The pathetic flimsy #8 cables they sell at Wal-Mart and most people carry in their cars will struggle to start a Corolla, you can forget about them starting a diesel of any variety.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Never fault the starter motor unless you measure >8 V between main supply stud and case, right at the starter, when cranking. Even then, could be due to engine drag, so insure you can turn it over by hand (use p.s.pump bolt).

Glad you're sorted.

Something to also bear in mind on the jumper cable topic is the clamps, some of the cheaper ones are like trying to clamp on with your fingernail ends. Which barely any of it touches the terminals so bugger all current flows

__________________
1978 300D, 373,000km 617.912, 711.113 5 speed, 7.5mm superpump, HX30W turbo...many, many years in the making....
1977 280> 300D - 500,000km+ (to be sold...)
1984 240TD>300TD 121,000 miles, *gone*
1977 250 parts car
1988 Toyota Corona 2.0D *gone*
1975 FJ45>HJ45
1981 200>240D (to be sold...)
1999 Hyundai Lantra 1.6 *gone*
1980s Lansing Bagnall FOER 5.2 Forklift (the Mk2 engine hoist)
2001 Holden Rodeo 4JB1T 2WD