How to remove gear case on 15hp johnson out board motor and change impeller

you have removed the leg to replace the water pump impeller
the leg has the selector rod for the forward and reverse action
as the direction selection is a simple dog that moves forward or back to connect the spur gears and it was working before you pulled the leg off , then I am thinking that when the leg was reassembled that the selector was in a wrong position and now you are only moving it in one direction neutral or reverse instead of the 3 positions forward neutral reverse
nothing else can change except where the rod connects in the leg

Some models charged through a Magneto, but see if you can find the 1979 Year Manual. I have a 1979 engine myself in a 15Hp which is a Manual start(no battery) and I have a Sailhorse Johnson 8HP which has an electric port and is Manual start. I believe the 8HP would charge a battery as on a Sailboat the running lights needed power. The 8HP could power the lights directly.

Johnson offered options for electric start in lower HP units and options in charging systems to contain costs. You might get a better answer from someone by listing the HP of your motor. I was the original owner of the 15HP and remember the 9.9HP and others in the same Sales brochure. The Sales brochure may advertise more about the charging systems.

Remove Linkage Under Carburetor
Remove Bolts Securing Gear Box Remove Gear Box
Do Not Turn Shift Linkage
Remove Water Pump Housing
Fit New Impeller
Re Fit Gearbox Follow Procedure In Reverse

It sounds like you need to change prop shaft seal. While at it check water pump impeller (in sure it is still playable). Also change seal under pump wear plate.

Sounds like a bearing gone in the drive shaft or a worn gear

There is no special tricks.Unbolt the gearshift linkage that has access through the side of the leg.Usually a small plate.Unbolt the lower g/box.4 bolts I think.The box should then drop.If not,use a rubber hammer and be patient.Put the lower skeg of the g/box in a vice to make things easier.NOW.When you lift the housing of the w/pump take a note of which way the impellar pump lobes are curled.DON'T put the new impellar in the wrong way.It's probably worth putting in a new seal whilst it's apart.You spin the housing or imp. as you put it together.Use a small bit of grease for the initiall dry start-up.Put some very good grease on the top splines on the driveshaft.It goes straight into the crankshaft.If it corrodes,nothing comes apart.As you put the g/box back on,spin the prop so the splines go back in.There is no special alignment.Gearchange linkage and you go fishing.(take the prop off and regrease the splines on the shaft of it)Hope this helps

There is also the gear change rod to undo.Look on the side of the leg,for a small plate.Inside it,is where the connecting bolts will be.Be in nuetral when you undo it and put it back together.On the water pump.Pay special attention to which way the lobes on the rubber impellar are facing.Wrong way around and you will damage the new impellar.Hope this helps

What year model? Tools you will need. (1)gear case pressure tester from any outboard dealer. why buy and not build. matching the threads on the drain plugs.(1) detergent and water in a spray bottle. troubleshooting step (1) Remove lower unit from engine as if you were changing the impeller.(2) secure lower unit in a holder so you can drain oil and do visual checks.(3)remove drain plug and vent plug, drain oil(4) reinstall lower drain plug and put pressure tester into vent plug hole(5) pressurize gear case(6)with spray bottle spray gear case at all seals and joints just like you would spray a tire looking for a leak. detergent will bubble at leak.(7) if you find leak at a seal were you will need to remove drive shaft or propeller shaft you will have to buy removal tools and a Clymer repair manuel from your local dealer.(8) hopefully the leak is at the shift shaft or the drain and vent plugs so you wont have to buy special tools to replace seals.(9)if you find a crack in the case have it welded at you local outboard dealer

first go to www.crowleymarine.com to get a picture breakdown of your water pump. website instruction, click on part catalog tab, click on omc Johnson and evinrude tab. choose product line, year tab, engine size, choose model. click on gearcase. the picture showes all parts you have and the correct order they go. ok now we are ready to disasemble the lower gearcase from the motor.(1) trim the outboard down using the tim and tilt system (2) shift remote control into forward gear, this lines up the shift rod so you can reach it. a picture of the linkage is refernce number 50 on the picture from crowleymarine.(3) Remove engine coul, under the carburators you will be able to see the shift rod. if you can't see it follow the shift cable to the shift linkage to the shift rod(4) the shift rod has a hole in the top, that a long screw goes through, connecting shift rod to shift linkage. remove this screw(refer to picture downloaded)(5) again refer to the downloaded picture remove these bolts that i list. number 66 there is 4. #77 there is 1. pull off #76 trim tab.#68 there is 1. #70 there is 1. lower unit will drop after you remove all bolts. remove the impeller housing. look at impeller you will see the fins are bent in one direction. put new impeller in with fins bent in the same direction. reinstall housing and gear case following my dirction in reverse order.

I have a 1979 Johnson 15hp Long shaft, and as a single motor just followed the rule of a shortshaft motor for a short transom and a longshaft for a deep transom. You have a little adjustment in the mount clamp. You want enough shaft not to cavitate and want the trim fin below the hull. If this is a kicker motor you want it deep enough for water pickup but less deep for drag on fixed mount.