So I have an 86 D100 that had a slant six and a 4 speed. I swapped out the 6 for an 88 318 and swapped it over to an old school carb setup.
My problem is when I try to turn the truck over the starter just grinds and makes a hell of a racket, like it wont mesh with the flywheel.

I have replaced the wiring going to the solenoid, the starter relay and even the starter. It has a brand new flywheel also.
I was under there today checking to see if maybe the bendix doesn't mesh properly with the flywheel, when I touched the 2 terminals on the solenoid with a screwdiver it worked no problem.

Once I crawl out from under the truck and try the key it grinds again. It will eventually catch, mostly once the truck has been running. This has me baffled.
I am not the best man at wiring, but I have been tracing wires all day trying to see if maybe there is a short or SOMETHING that is causing this.
This may sound stupid but is my ignition switch not giving it enough power or something? I have no idea at this point.

 

If the switch was OK before the swap I wouldn't think that is your problem. Are you using a start relay in the circuit? If the relay has dirty contacts it could cause this or if the relay got knocked loose during the swap.

 

Well it started fine for the first week, then I had problems with the starter relay. So I replaced it, ran good for about a month. I cleaned all connections I could see in the starter circuit and replaced the connectors, and used all new wire. The weird thing is when the truck has been running for about an hour or so I don't have this problem, unless I shut it off for more than an hour. BUT for the first week when it was in the shop my buddy had one of those starter boxes hooked up to it so he could start it from in the engine bay. I did have somewhat of the same problem with the slant six too. Would the wiring for ignition up in the cab have anything to do with this? Everything seems to be good in the engine bay.

 

Any thing is possible. When I do electrical trouble shooting I go to the middle of the circuit in question to isolate the problem in one direction while eliminating the other side of the circuit as the problem. There are 2 circuits inside the relay. There is an energizing coil with 2 contacts and a points side with 2 contacts. The easiest way to check that is to energize the coil side of the relay with a jumper from the battery. If the engine turns over the problem is in the energizing side which includes the dash wiring and switch. If it doesn't turn over it is in the contact side that includes heavy wiring and the starter. This assumes the new relay is good.

 

Thanks for the help SEAL, it turned out to be the relay. Brand new part and it doesn't work, gotta love NAPA.

 

Searching around on the internet and having the same exact problem with my 88 W250. I am on my way to get a new relay right now. Gota live these forums!!