When I turn the key all lights go on as usual when I turn it all the way I get one loud click and thats all. I thought the starter was a rapid series of clicks. Any ideas. 1985 350sd

On gassers I know that is most likely a bad starter. one click. Series of clicks may be a relay?

got a hammer?

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It could also mean that your wires and cables are not getting enough juice to the starter due to poor connections/high resistance in them (clean all connections and grounds), a weak battery (charge the battery and have it load tested for free at the local chain type autoparts). An easy way to test for a weak battery is to hook up some jumber cables from your battery to another good battery before you try to start. If you have a good normal start with the jumper cables on this is a resonable indication that your battery was not fully charged or if fully charged is weak.
If you remove the starter that can also be tested for free
If it is the starter itself it could be just a problem with the solenoid or the brushes in the starter could be worn down and are too small to make good contact. The brushes do not cost too much but the solenoids can be a little expensive and of course you have to take the starter apart to see what is wrong/ fix it.

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84 300D, 82 Volvo 244Gl Diesel

1. how "Clean and Tight" are your Electrical connections?
(if ALL your connections are GOOD ,the one "click" may beead Battery or
an unhappy "Bendix" ["Throw-out" device on the Starter gear shaft].)

Disassembly,cleaning and brush replacement will usually "ReNew" these Bosch
units for another hundred thousand miles.Unless the "Bendix" has to be replaced
(Which is really no big deal).

EDIT: Diesel911 beat me to it.

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'84 300SD sold
124.128

my 81 was having trouble starting (turning slow not the click like yours) and all it took to fix mine was a new set of cables, I also added an additional ground going to the engine connecting to the stock ground location. for right now, hammer, if that fails maybe check the brushes on the starter.

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'81 300D - swapped to a 617.951 turbo motor with a 150 amp alt.

I had the same problem and it turned out to be a dead spot on the winding of the starter armature. I would first check all the wires for condition and to make sure they are connected tight. Also as some one else said, try jumping it to make sure it's not a weak battery. On my 83 changing it was not a bad project. the hardest part was getting the rubber boot back over the connector lugs on the back of the bendix.

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1983 300SD
200000miles

Check your ground connection from the battery to the frame. Make sure it's tight and clean. Mine was loose when I bought the car.