At this point I'd lean towards a toasted starter. Usually the solenoid is stuck, I've whacked the starter with a hammer in a pinch to get it working temporarily. Go ahead do the ground thing.

If it's bench testing ok, why would the solenoid stick when it's in the car?

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

Ah I missed that part.

Jumping Terminal 1 to Terminal 3 makes a direct connection from Circuit 30 (Unswitched 12VDC) to Circuit 50 (Cranking). This should energize the glow plug relay through the ignition switch wiring.

__________________
mjk

'84 300SD 119KMi (Liesl der Diesel)
'84 300D 326KMi when the oil left (former parts car)
'82 300SD 253KMi (new parts car)

Offering hope/support.

Don't freak out if the engine does not want to crank with a ratchet, consider the following.

I had two last year.

#1. Turned out to be a broken flywheel that dug into the bell-housing jamming the starter.
A local shop had quoted $7,000.00 repair.

#2. Was a locked A/C compressor and alternator, very strange they both failed at the same time.
A local shop quoted $2,500.00 to install a good used engine.


Both owners had given up, and I convinced them to let me diagnose the problem = fix it under $500.00 or part it out and split 50/50 with the owner..

Owner #1 stuffed money in my pocket when I showed him the problem, and DIY replaced the flywheel with a spare.

Owner #2 had me cut the A/C belt and replace the alternator + stuffed enough money in my wifes purse to make her grin for two weeks.

Offering hope/support.

I tried jumping the car (thinking the battery might have been drained from trying to start it so many times) and still nothing! Arg!

Ok, so connections to the starter... there is one small power wire that connects to the starter solenoid with a small screw. There are two ground wires that loop over a bolt on the starter and are fastened down with a nut. Is this correct? Where do these these two ground wires run to? Maybe one of them is faulty and that's why the starter wont engage.

I'm starting to think maybe I connected the wires incorrectly to the starter. Can someone tell me how the connections should be set up?

Those aren't ground wires, they're power wires. One of them goes direct to battery (always live...). Ground is via the engine body, there's a braided harness on the other side of the engine to the chassis.

Ok, so one goes to the battery and one goes to the terminal in front of the battery (that connects to the glow plug relay)? What about the wire from the starter solenoid? What should that connect to?

Anyone? OR can anyone point me to a diagram that may help? I've tried searching but cannot fond the answer.

Both large cables go on the top starter solenoid stud.
The small wire is attached with a screw.

To test cranking directly on the starter.
Jump from the small wire to the top starter solenoid stud.


If you are asking what controls the circuit.

Ignition Cylinder Tumbler replacement + Key issues; mixed diesel/gas
http://www.peachparts.com/shopforum/diesel-discussion/142232-ignition-cylinder-tumbler-replacement%3B-mixed-diesel-gas.html#post1065596

Broken Neutral Safety Switch and replacing shifter bushings
http://www.peachparts.com/shopforum/diesel-discussion/96048-broken-neutral-safety-switch-replacing-shifter-bushings.html#post646647

Replacing all shifter bushings on W126 - p.1
Replacing all shifter bushings on W126 - p.1

I tried jumping directly from the small wire to the stud as whunter said and still nothing. This must mean a bad ground correct (since the starter was bench tested and worked fine)?

Correct.

The factory ground goes (under the car) from the (17 MM bolt) transmission bell housing to the (13 MM bolt) body = if it is loose or damaged = no cranking.

As a fast diagnostic TEST, take jumper cables:

#1. Attach both clamps at one end to the engine METAL.

#2. Attach both end clamps to the Negative battery terminal, you may need someone to hold them in contact.

#3. Try jumping the starter again..

If it cranks..

Here is a cheap fix + extra insurance that I use due to Michigan corrosion:

Get an aftermarket ground cable, long enough to go from:

Point "A". The battery ground cable 13 MM body bolt.

Point "B". The "T" shaped air cleaner mounting bracket, top rear 13 MM bolt = remove the air cleaner housing.
Note: On naturally aspirated engines you will need to choose an alternative engine attachment location.

At worst it does nothing but double your ground = reduce resistance.
Generally it improves cranking ability.
In your case, it should get you running again..

Thanks whunter.I've now tried about everything. I tried grounding with jumper cables (they are a nice thick gauge too) with no results. So I went about it methodically. I:

- ran a cable directly from the positive battery terminal to the solenoid stud. Then I jumped the stud and screw. Nothing happened so the problem isn't power, must be ground (or starter itself).

- Then I clamped a jumper end to the ground stud on the starter and the other end to the chassis, still nothing.

- Then I added an additional ground strap as you suggested. I ran it from the 13mm bolt used to ground the battery to the air filter T-bracket. Still nothing.

What the hell is going on?! I've tested every logical possibility! I'm about to take out the starter and have it bench tested again. Maybe something has happened to it since it was tested 3 weeks ago. Either that or despite all my grounding attempts I'm STILL not getting enough ground.

:/

The basic circuit is in picture #1.

If it does not function it could be a mechanical issue:

#1. Manually crank the engine roughly 45° listening for a snap/clunk, (Example: Degree wheel), this should release the starter gear from the flywheel if it has jammed = possible bad starter gear.

#2. Apply power to the small screw and use a hammer to whack the side of the solenoid/rear of starter = possible bad starter.

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