OK, so this is ancient thread, and I really should have figured this out by now. I finally broke down and bought a Craftsman non-contact DC current meter (the kind with the doughnut pincers). I have measurements!

So, when the key is turned to the start position with the meter installed around the solenoid wire it reads 33 amps while (non-)cranking, and that holds steady for several seconds while the starter, of course, is sitting there dead. On an earlier occasion I managed to get a reading on a successful start, and it appeared that the solenoid wire was reading about 10 amps for the few seconds that the starter was engaged and turning. I think what I'm seeing there is the two different coil impedances: there is a high current coil in the solenoid, and a low current coil.

There is definitely a solid click that I sense coming from the starter/solenoid pair while I'm under the hood with the meter. I put the meter in voltage mode, and measured the voltage on the output terminal of the solenoid (the thick wire that goes into the starter brushes). While the key is turned to start, and after the click, this voltage reads about 0.7 volts. This is with the black lead of the meter on the battery negative post, and the red lead on that solenoid output nut.

Do these measurements point toward brushes that are stuck in some way? It certainly seems like the 33 amps ought to be enough to activate a solenoid and slam it against the contacts (the click) to send juice to the starter motor proper.

Thanks,

Kurt

__________________
- '79 240D - engine swap complete! Engine broken in! 28-31 mpg! Lovin' the ride!
- '86 190D (W201-126) - 2.5 NA engine, 5 speed, cloth interior, manual climate controls, 33-34 mpg (sold to forum member).

33 amp draw for the solenoid is too much, should be around 8 amps. There's a fault somewhere in your car or your meter.

0.7v is almost a dead short. Again, a fault some where in your car or your meter.

__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

No, I'm very sure of my measurements.

I finally took the car in to the auto electric shop that rebuilt the starter. I was loathe to do that at first because they were snobby about "we'll look at it, but if we determine the problem is something else on your car we're going to charge you for the diagnosis." However, I had come to my own conclusion, and was confident that they would come to the same one. And they did: bad rebuild. The mechanic wasn't 100% sure, but he thought it was the solenoid. I, on the other hand, am pretty confident that it IS the solenoid, confident enough to go and buy a new $55 Bosch solenoid.

Here is why I think it is the solenoid. There are two coils in the solenoid, the pull-in coil and the hold coil. The manager at the auto electric place confirmed that the pull-in current is about 30 amps. The way it works is like this: when you initially turn the key to Start, the circuit goes

Battery -> Keyswitch -> Solenoid_screw_terminal -> Pull-In_Winding* -> Starter_Motor_Supply_Terminal -> Starter_Motor_Windings -> Ground

I put an asterisk above because current also flows through the hold winding directly to ground, but that current is less.

So, what happens when the solenoid operates properly is that the pull-in winding slams the bar across the contacts inside the solenoid to send battery voltage (and many amps) to the starter motor supply terminal. This raises the voltage at bottom end of the pull-in winding from near ground (which is what it is before any solenoid movement, because of the low impedance of the starter windings) to battery voltage. Now the pull-in coil has battery voltage on both ends, and current ceases to flow through it, leaving only the hold coil current, which is the afore-mentioned 8-10 amps.

The web page that led me to this understanding is here:

Bosch Starter Solenoid Rebuild - Inside a Dual Coil, Pull-in and Hold-In Starter Solenoid

So, how does this fit with my symptoms? Well, since I see in 33 amp pull-in current all the time, and the starter motor does not energize, the bridging bar is not reaching or not properly contacting the contacts that bridge the battery input terminal on the solenoid to the starter motor supply terminal. Ergo, bad solenoid. Or they replaced the solenoid winding/housing, and re-used the old plunger (and it doesn't match well).

So, now we get to my question. The starter is back out of the car, sitting on the dining room table. The new Bosch solenoid is sitting next to it. How much of the starter do I have to disassemble to be able to put both the new Bosch solenoid and the plunger that came with it onto the starter? I've seen the writeup Stretch did on disassembling the whole starter; what I could not tell from his posting was whether it was possible to replace the plunger without pulling the whole casing to get access to the engaging fork. I'm thinking that the end of the engaging fork goes in the slot of the plunger shaft, with the spring there to keep it against one end of the slot. I could be wrong, but I don't think I replace that plunger without some disassembly of the electric motor portion of the assembly.

Thanks for your help.

Kurt

Edit: I'm sure someone will suggest to take back to the place that did the rebuild... I may still do that, but if it is relatively straightforward to replace the entire solenoid myself I would just as soon avoid those clowns.

__________________
- '79 240D - engine swap complete! Engine broken in! 28-31 mpg! Lovin' the ride!
- '86 190D (W201-126) - 2.5 NA engine, 5 speed, cloth interior, manual climate controls, 33-34 mpg (sold to forum member).

Kurt, you are exactly correct in your testing and results. You described the system to a tee. Best description I have seen. The solenoid is bad.
It is a shame that you had to buy equipment just to diagnose someone else's problem (the re builder) but the amp meter will come in handy for many other test, like alternator. How ever now that you know how it works, a $10 test light would have worked just as well and given you the answer much quicker.
I am old (71) and set in my ways and have been using test lights all my life and once you understand how the system works the test light will tell the story.
I am not saying that you shouldn't have bought the amp meter as it can do things the simple test light can't do.

Good work on your diagnostics. Better than most Pros.

PaulM

PS: If we still had our shop (now retired) I would offer you a job.

__________________
84 500 SEL (307,xxx miles)

I had same issue on my Kubota tractor...SAME! changed everything, BUT the ignition switch, cause it was changed a few years back...here is a common issue..NOT easy to think through....what happens SOMETIMES, rare though. IF starter isn't sitting exactly right against the bell housing, when you go to start...solenoid will NOT FULLY engage, and drive gear will NOT hit teeth in ring gear, when this happens, the solenoid CAN'T CLOSE, and then you don't get the correct AMPS to start the motor, it will NEVER work right. You can try and remove starter, put a washer / spacer in between bell housing and starter mount, this will shim it back a tad and MAY help the driver gear to correctly align with the ring gear. You can also grind each tooth of the drive gear on starter, to more of an angle, this will help it to engage better with the ring gear when you turn key on. This is a very weird troublesome problem to have...this is a bad issue, that sometimes NEVER gets fixed, even by the greatest mechanics. good luck!

I am currently having the same problem with my 300 sd it seems to start fine when cold and sometimes will not start when warm. If I bang on starter it starts. When I was looking closely how to get this animal off I noticed the bottom bolt which the ground strap is attached to was a tad lose that I could wiggle ground strap not good and when I looked for top mounting bolt discovered there was none there so went and purchased another bolt flat and lock washer for $5 and will install today hoping that this was my whole problem and there is nothing wrong with my bosh starter and does not have to come off. My starter was held on by 1 bottom bolt and front brace not good. Maybe po got discouraged trying to put top bolt in or did not have it tight enough. The ground is very important will not start w/o it and yes proper alignment is important also and could be overlooked while troubleshooting.

Glad I saw this post just now, as I am experiencing the same thing happening. Some times it starts right up and then try again and nothing. All I might hear is a click when I turn the key off.

Some history: I have been pouring tons of time into this girl

New glow plugs
Rebuilt injectors
IP seals replaced at same time
Recent new battery
Replaced ground strap on motor
Harness wire replaced as it shorted out and I almost had a barbaque
New harness housing
New 50 AMP GP fuse
Removed and cleaned fuel tank and new strainer
All new filters
Resealed Vacuum pump
New radiator
New alternator brushes
New copper fuses

I purchase lots of stuff from Kent Bergsma (Moly, rebuild kits, hoses, connectors, tools. etc.) I know a little pricey at times but usually worth it in what you get the added knowledge you receive with the parts.

I purchased a new ignition switch. and electrical switch connector for it. But cannot seem to see how it could be even remotely possible to replace them. At this point I am totally stumped. Maybe one of you could throw me a bone here?

The 2 terminals in the fender to jump starter idea would be a help for me to diagnose the starter at least. Have to work by myself on this! Let me know if anyone knows about this. Thanks

So I follow your ideas regarding NSS and go thru the gears then put it park and pow she starts right up. No glow plugs waiting or anything. Pretty sure it is my NSS in this case but am going to try in the morning same thing and see if I can start it without playing with shifter.

We'll see about this. I will post back