This issue began last summer some time and has progressively gotten worse.

I can turn the key and everything lights up normally as it should. When they key is turned to the start position, nothing happens- no clicking from the starter, no attempt to turn over, just nothing. It would present these symptoms every once in a while, and after a few turns of the key it would start normally. Now, it is not working at all. When it does start and run it will start up again immediately with seemingly no issues.

It seems to have gotten worse in the colder weather, but that could be coincidence.


***This truck was converted to run on 12v shortly after I bought it by a shop that I know and trust, and has run with no related issues for years, but that could be a point to consider***

Things I know:

I replaced the ignition switch (it cranked once after I replaced the switch, so I'm reasonably certain I did that right)
The batteries are less than a year old
The starter spins enthusiastically when it does start
The battery terminals are tight and seem to be making good connection
There is power getting to the starter
All wires at the junction points are tight and seem to be in good condition
All fuses appear to be good

I am suspicious of a neutral safety switch, as I halfway recall that being a possible cause of this issue, but I don't know where it is located or even if it exists on this variant of the vehicle. It'll come up when I search on auto parts websites, but that hasn't been a reliable source of information in the past. Is it possible that its a starter solenoid issue? I'm having a really hard time actually getting to the starter to mess with it, since its tucked up and in, and the transfer case is in the way.

Any help diagnosing this is greatly appreciated.

 

I know you said its hard to get to but does the little wire in the solenoid have power when key is in the start position - if so neutral switch should be good - if 12 volts is there and main lead has 12 volts to the starter you prob have a bad solenoid or bad armature or dead spot in the windings -
As the key is bumped to start the starter will move internally just slightly moving past the bad spot - cooler weather sets in metal shrinks some and the bad spot is not bypassed.
just my

 

That is definitely not what I want to hear, but it makes sense to me. I'll have to crawl around tomorrow and see if I can really get to the starter. I was thinking to myself "it can't be the starter, because it's new..." but when I actually think about it, the starter is like 8 years old.... haha sucks to get old...

 

I can relate to the getting old. Walk across the shop for something , get there and with a stupid look wonder just what it was I wanted.
I've had it happen with new starters - installed a month or two and poof.
I do my best to install life time warranty stuff now - I have a file on each vehicle I own and all needed receipts are saved - even tho it costs me time the parts are paid for.

 

Update for posterity-

I pulled the starter out today to go have it tested. I took it to autozone and it failed the first time and passed twice after that. The guy testing it was very puzzled. To me it makes sense, and would totally explain the intermittent nature of this problem.

There is a guy locally who rebuilds starters, so I took the starter to him. He looked at it and said with disdain, "Do you want that one back?" It caught me by surprise. He mumbled something about having a starter he could dig up and would be lighter and a better starter because those ones only last "about six months." Okay dude, whatever. How long will it take? Long pause.... "well.. when do you need it?" I need it yesterday, man, and if you've got it sitting around I want it now, but I guess on your schedule is going to be fine, since autozone starters have been risky for me in the past. (I didn't actually say that) He's cheaper than the chain stores too, so I guess I can put up with it.

I think I might have gotten taken for a ride, but we'll see.

 

Here are some more updates for posterity, since this issue is persistent and appears to be puzzling to people. If anyone has any definitive ideas, I would love to hear them. I will keep updating this thread as I have more information.

I put in the new rebuilt starter, and it is still being difficult.

The new starter spins and tests fine outside the vehicle. BUT...

Symptoms:

-More than half the time, the starter won't turn when its installed. It will just click (like they sometimes do when the batteries are dead or weak) but after clicking five or six times, it'll suddenly fire up and spin.

-When the starter does turn, it seems like its not spinning fast enough to turn the motor over. This is much worse when the engine is cold. If the motor has been running, it starts immediately with no drama. If it sits for a while, it goes back to either just clicking or spinning slowly. (this says to me that something inside is expanding with heat, allowing it to turn more freely)

-It starts (hard) with a jump... sometimes. My buddy was able to get the starter to spin and the truck started fine by whacking the starter a few times while hooked up for a jump.
*A weird other symptom is that the starter gets kind of hot and actually smoked a tiny bit once. I wasn't cranking on it for a super lengthy period of time, (about the same amount of time I would need to crank it on a pretty cold day) but I've never felt a starter that is 100% operational, so I don't know how hot it should get normally.

-It sounds funny... kind of "hollow" and like its working pretty hard when its cranking, I know this isn't a super good indication of how it is operating, but I immediately noticed it, and feel its worth mentioning.

Things I know now:

-Both batteries have been tested twice. Once at Walmart and once at Autozone. They tested as good both times, with 100% charge. The guy at autozone tested one of the batteries twice to make sure, and it still passed.

-All wiring connections appear to be clean and good. Battery posts, positive cable to the starter, etc. I still have to clean the main ground on the engine block, but it was tight and looked to be free of corrosion but a bit dirty on first inspection

-Glow plugs all test good with a test light, they are getting voltage, and the relay works as it should. (as I mention later, I'm not prepared to rule them out of a no start problem, but I feel they are a separate issue)

-The ignition switch is brand new

-When the truck has been plugged in over night, the exact same symptoms are present, but the starter spins a little bit faster.

-It has been cold for a week, but not that cold. Average highs in the mid 30s to mid 50s, lows down to high 20s at the most extreme. When the truck is in working order I've never had issues getting it started in this temperature, and it doesn't really even need to be plugged in usually at this temp. When highs are in the 30s and below, I always plug it in.

Conclusions:

-The guy who rebuilt the starter has a stellar reputation around town, and I trust his work, but I can't help but think that he missed something along the way. He is convinced it isn't his starter, since it spins fine on his bench.

-I'm not prepared to rule out glow plugs from the whole no-start scenario, but I feel that is a separate issue from what is happening with the starter. It does not make sense to me that burnt out glow plugs would cause the starter to malfunction in the way that it is currently.

-The batteries in it are wal-mart batteries and they are under warranty, and about 6 months old. Obviously they won't warranty batteries that test good, and I am not about to shell out $200 on new batteries just to check that variable out when they have been tested multiple times.

So here's my current theory, and could someone please weigh in and tell me if I'm full of buloney:

I think the starter motor is requiring a larger than normal amount of voltage to turn under load, and something internally is causing undue friction or stress. This, in turn, takes voltage away from the glow plugs, causing them not to heat up enough to fire the engine even if the starter is turning just fast enough to start the motor. This, in my mind, would explain why it will start with a jump- the jump vehicle is sending a lot more juice through the system, allowing both components to work.

A great big advance thank you to anyone who can weigh in. This truck is my daily driver and I am beyond frustrated since I've had the starter in and out a half dozen times over the last week for different reasons, and the guy still insists that its not the problem.

 

Two things come to my mind:

Engine ground is the first one. Should be easy to figure that one. May take a jumper cable with the pliers and go from some good spot on the block directly to the battery negative.
I've seen ground cables being broken inside the insulation or oxydized black at the connectors. At some point no large current can flow.

Relay is the second. I would diagnose the voltage to the relay itself (from key to relay) then to the starter as Odat has suggested. May need wiring diagrams to figure that one correctly.

 

It is my understanding that a relay is not used in this system, since it has been converted to 12v. I asked the mechanic who converted it over

as far as the main ground goes, I checked all of the ones I could get to and was aware of, and they are all clean and appear to be in good condition. Now, that doesn't mean that there isn't some internal problems in the wires, but they are all big suckers and it would take a lot to have internal breaks. However, I will have to try the jumper cable thing, as soon as the guy who rebuilt the starter gets back to me and I can put it back in for the tenth time.

 

I'm pretty sure your right A 86 won't have a start relay. That was usually a closer to the 90S and after thing.
Glow plugs will not cause a no crank - a no start yes.
Having it plugged in will warm up the cylinders for start but will not warm up the block, starter and such enough to close a cold open.
Internal corrosion in battery cables reducing voltage.
Worn bushings at either or both ends of the starter.
Worn brushes or brushes that hang up some so they don't make positive contact to the armature.
A intermitent cold open in the winding's to the brass contacts on the armature or a intermitent cold open in the main winding's.
Nose cone crushed by over tighting the bolts that install the starter { Don't ask me how I know this one }
Grease - dirt between the starter and block reducing the ground between them.
Even tho your rebuilder comes well recommended and I have no doubt he does good work I still wonder if he missed something. I had the dead priest of a org starter in my 83 with a 6.2 rebuilt and it never was 100% reliable. I went to the new style 1/2 the weight and size starter found on the newer style Chevy Diesels.
A diesel takes A lot of emph to spin it over esp when it's cold - the starter may spin on the bench but be weak on a cold hard needed spin .....