Not an MB question but still diesel. On my Fuso FG, about every tenth time I try to start it I just get a click. The starter almost always engages if I try it again. A few times it has taken 2 attempts. This began after I wired in a 12 volt connection from the battery to my truck camper, using a relay that engages when the key is turned on, to charge the truck camper battery.
It first happened after the truck had been sitting for about six weeks and I changed the batteries since I was unsure of their age. I've also since cleaned and tightened the connections at the battery and have not found any other loose connections anywhere in the starter circuit.
Does anyone know any definitive diagnostic techniques that can be applied in circumstances like this to determine if it is the starter solenoid that is at fault?
New starters for an FG are not cheap (about $450 I think) so I don't want to just replace the starter particularly since the symptoms seem identical to having a bad connection somewhere that I have not yet been able to find.

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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

When it happens again you might try turning the starter a little by hand if possable and then try starting to see if you have a dead spot on the armature of the starter. I wrote up this test procedure for another site so see if it helps.

I decided to write this up after having a problem with a starter an a non MB car last weekend.

FIRST- a simple tool to have around and is easy to make- get an old 12v light socket and add a wire to the bottom connection (mine uses a 5 foot wire) and put an alligator clip on the other end, next attach a wire to the housing of the socket with and alligator on the other end (I used 5 feet of wire on this also). Put a bulb in the socket and your tool is ready.

SECOND - put the tester where you can see it from inside the car and then find the smaller wire, control wire, on the back of the starter solenoid, attach one wire/alligator clip of the tester to the terminal for this control wire on the solenoid and the other wire/alligator clip to any ground point on the car.
NOW get in and turn on the key - if the light lights up you have a start signal to the starter.

THIRD - go back to the starter and remove the wire/alligator clip from the solenoid and touch it to the large wire going to the back of the solenoid (this wire is usually red) - if the test light lights up you have power to the starter.

IF you got light on the tester during both tests then your starter is probably bad.

IF you got light on the tester only when testing the control wire then try testing the large power wire again but this time if during your original test you used a point on the engine for ground then try using a point on the body for ground. If the light lights up this way then you have a bad ground strap to the engine and you can try using a jumper cable from the negative terminal of the battery to the block and try to start the car, if this works the it confirms the bad ground strap.
If you don't get a test light either way check the power wire for bad connections or a broken wire.

IF you don't get a test light when testing the control wire then you need to start tracing that lead for breaks, fuses, relays and connections.
If this is the situation I would first check fuses then try to test with the test light a some point closer to the ignition switch.
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1983 300SD
200000miles

Don't forget to be sure you have a good connection at your Groud strap. Unbolt the Ground Strap and clean the area to make a good contact.

Keep in mind I have never seen what type of Starter you have so this is a generic starter pic.
The area labled "Contacts" is the place on the Solenoid that burns away over time. It is hard to see in the pic but there is a Disc that brdges (touches those 2 contacts).
So what happens if you electricity gets to the Solenoid and it moves (you hear the click) but when that plate hits the Contact Bolts it does not really make contact due to it being burned (oxidized; creating a lot of resistance) so you starter does not rotate.
What makes it work sometimes is that sometime the area gets hot enough to expose some copper and it makes good contact in a small area until it burns again.

What to do about it:
You can buy an new Solenoid and replace the no good one.

One of the members claimed he rotates those contact bolts to a different position so there is a fresh spot for that Plate inside to make contact. He did this on a Mercedes Starter.
However, when I suggested this to another member he said he could not get the plastic end cover off (Mercedes) so I am not sure how that member did it.
I am guessing that he removed the retaining nuts on the Copper Contact Bolts; holding on to the tip of the Contact Bolt pushed in inside of the cover just enough to rotate it to a different position. Pulled it back out and installed the retaining nut.

I have have removed that End Cap on General Motors Starters and cleaned up the Contatct areas or rotated the contact bolts myself. But, in my cas the End Cap was held on with screws.

As far as Starter test proceedures go most people just take it to have a free bench test done at an auto parts store. However, intermittent problems some times do not show up and of course there electrical connections are usually better also.

The best resource I can think of for Starter Testing is that if you City has a main Library. Locate the Automotive Book section. Also back in the 60s and 70s the Motors Manuals used to have a Starter Section in them that might have some test details.

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84 300D, 82 Volvo 244Gl Diesel

As you said your starter is too expensive to fool around with. At least you should be able to source just a solinoid if thats what turns out to be defective. Or brushes if indicated.

I have experienced many solinoids that do not get enough trigger voltage. Since your situation is intermittent I would put a separate line from the positive terminal of the battery to a good pushbutton and terminate the output wire at the starter solinoid activation connection.

If the problem persists them you have a solinoid or starter problem. The next sequence to separate the two is pretty easy.

Monitor the very short line from the solinoid to the starter with a remote light bulb. If you activate the solinoid and the light lights when the starter does not turn it is a dead spot or worn brushes in the starter If the light does not light it of course is the solinoid. Make sure the battery terminals are basically pristine as they can cause a lot of difficulties.

No other possibilities I can think of if the primary supply wire at the battery and starter connections are really clean. This avoids guessing with an intermittent condition. It is wise to clean and treat any ground circuit connections as mentioned already as well of course. I usually remove the wire terminal ends, clean well and apply a little grease or specific designed paste and reinstall. That seems to eliminate any future problems at those junctions. I do this with all terminations I remove as a habit anyways.

This approach is of course because the starter is expensive. It also tends to be pretty conclusive. Finally this problem should respond well to a decent alternator/starter repair shop once pinned down if required. Much cheaper than a so called rebuilt or new starter. You can even have them install new bearings,brushes and dress the commutator if the need is indicated. I too preffer cheap.

The intermittent character of the problem makes it difficult to trace down. I had been thinking about using a remote start switch to bypass the key switch for diagnostic purposes.
Let me see if I have Barry's suggestion correct:
The heavy starter cable comes from the battery to the back of the solenoid. There is another cable going from the back of the solenoid down to the starter motor. I need to put a test light on this 'output' cable and monitor the current when I use the remote start switch. If when it 'clicks' and doesn't turn and the light indicates power in the output cable, the problem is internal to the starter motor. If when it 'clicks' and doesn't turn and the light does not indicate power, then the problem is either in the solenoid or in the connections coming from the battery to the solenoid (but not in the key circuit if I'm using a remote start swithc).

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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

You have it but first clean and treat all terminations in the circuit. Grounds and power feeders. Especially those battery terminals. Chances are strong the solinoid is just getting a little tired. You want to know for certain though.

This approach makes dealing with an intermitent problem a walk in the park. Chances are the starter would pass a bench test with this intermittent situation. Now if the intermittent condition dissapears with the remote starter button then hook up a ford solinoid to provide better start voltage to the solinoid. Your key switch activates the ford solinoid and provides higher voltage to the solinoid activation circuit.

Many older cars have too much line voltage drop or the older solinoids want a little more voltage to function reliably. I suspect the cause is the wires in the solinoid have heated too many times over time. Their resistance is slightly higher. Or the solinoids internal contacts have developed a little more resistance with age and use.

If you would rather just hook a light you can see in the cab to the wire going between the solinoid and the starter. It will separate the starter or solinoid difficulty. Avoids a lot of prework if it is inside the starter. Then if you establish it seems more solinoid related do all the other things.

Since this problem began after you added the relay for charging the camper battery, you might want to temporarily disconnect your modification, just in case it is loading down the battery and preventing the starter from getting enough current. I consider this an unlikely scenario, think you probably just have a "tired" solenoid as others have said, but it's worth eliminating that possibility while you're checking all the grounds and such.

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

I like this idea. I'll give it a try. It could just be an accidental correlation of events but it's worth an experiment. Don't know what I'll do if it fixes the problem.

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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

Disconnecting the relay makes no difference so it's pointing more and more to the solenoid or starter since I rechecked all the connections at battery again.

__________________
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

More data:
After turning the key numerous times to get the problem to repeat itself, I've noticed that when it clicks and the starter does not spin, the idiot lights dim. I believe this symptom points in the direction of a bad starter because it seems to indicate that there is a strong current draw but no starter action. I think this means that current is getting to the starter motor but not turning it. I don't think the solenoid current draw would be heavy enough to dim the idiot lights.
I haven't yet assembled a long test light lead that will enable me to put Barry's diagnostic ideas to work.

__________________
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

My ford truck wouldn't start the other day, I jumpered the battery same thing just a click. After disconnecting the power wire going to the starter I soaked it in Coca-Cola over night. works like a champ now. The nasty green corrosion that is hidden inside the wiring could be to blame, but since your idiot lights are dimming sounds like she's drawing too much current which would not be the case with corrosion, so never mind,,,

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Closing the store
Benzbonz.biz on your smart phone or tablet.

The starter needs a ground in order to engage the solenoid and turn over. The brushes provide the ground in the circuit, if they are worn out they can mimic the symptoms Kerry is having.

If you have a local starter/alternator shop in your area they could probably fix the starter for a fraction of the price of a new/reman unit.

Before you yank the starter out, do a voltage drop test in order to verify your wiring is ok on the positive side.

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"I was a dirty bird, Carol's not grungey - she's *****in" John Milner....American Graffiti

Can you describe a voltage drop test?
My current plan, if the testing confirms a bad starter motor, is to buy a used starter, swap it in and have the existing one rebuilt since I want to have a replacement starter available if and when I start traveling to remote places with the truck.

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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

Voltage drop test: Because the wire from the battery to the starter is big and heavy, it has almost zero resistance. Electrically, measuring the voltage at the starter should be the same as measuring it at the battery itself -- you should get the same results. This test is to make sure that you do. If not, you have a problem in that big wire.

With the vehicle power "off," measure the voltage between the negative battery terminal and the positive battery terminal. Use a digital meter, the best one you can beg, borrow, steal, or buy. Write down the voltage. Now measure the voltage between the negative battery terminal and the big wire at the starter, Write down this number. With the vehicle power off, the two numbers should be identical since no current is being drawn.

Now repeat these measurements with an assistant holding the key in the "start" position. The measurements should be lower because of the load placed on the battery by things in the car that are now drawing current and because of the starter (if it is drawing any current at all). If the voltage at the starter is significantly (more than a volt or so, probably) lower than the voltage at the battery, you have a voltage drop in the big wire from the battery to the starter. That voltage drop, probably due to dirty/corroded connections, could be preventing the starter from drawing enough current to start.

Jeremy

__________________

"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

The voltage drop test is very simple. Use a digital DC voltmeter. You can get a pretty good one at Sears for $20 or less.

When you are doing a voltage drop test, you are looking at the voltage across each link in the chain. The total of all these will be the voltage across the entire circuit. For the starter circuit, start with one lead at the battery post and the other at the other end of the cable on the starter solenoid and have someone hit the key. If you have a fraction of a volt drop while the circuit is active then that means there is low resistance between the battery post and the solenoid post.

Next place one lead on the solenoid post and the other on the other big wire from the solenoid to the starter motor. Again there should be a very small voltage across this point when the key is turned. It is only a switch so if it is dropping voltage it has resistance, not a good thing for a switch.

Now put one lead on the big terminal of the starter motor and the other to ground and again hit the key. In this case you should see a high voltage (9 or more) because the motor is the load in the circuit. The cables and the solenoid were supposed to simply transfer current with low resistance, while the motor is supposed to use the energy, thus it will have high resistance thus a higher voltage drop.

Hope this helps.