Mercury 850 thunderbolt 1974 gearbox check

[Legend850]

That LU looks serviceable but apply lots of penetrating oil first. Another piece that needs to be looked at is the alternator stator under the flywheel, the wires might be corroded they are yellow and run down the stbd. side of the block.

I will rewire all of it, its pretty easy, and I can add a few improvements over factory design. Heres how I think I will do it.

 

[Chris1956]

It is a bad idea to put a fuse on the charging wire. If it blows or has a bad connection with the engine running it will destroy the rectifier.

I am also not sure what would happen to the switchbox, if the fuse blew with the engine running, as the battery regulates the voltage. You might get a high voltage spike at the switchbox, which might be bad.

Put the fuse on the power wire to the control, instead.

 

[Legend850]

Good one. I was trying to have as much of the wiring fused as possible, but a fuse failure as you point out could easily result in over voltage from the alternator. Thanks for that.

 

[merc850]

That's why they used nuts and bolts on the terminals and the ignition switch uses soldered connections; because any disconnection of the wiring while the motor is running can result in destruction of the rectifier or switchbox or both, make sure the battery connections are clean and tight.

 

[dingbat]

That's why they used nuts and bolts on the terminals and the ignition switch uses soldered connections;

Have never seen soldered connection on an OE installation

ABCY rules are quite clear when it comes to soldering.

11.16.3.7. Solder shall not be the sole means of
mechanical connection in any circuit. If soldered, the
connection shall be so located or supported as to
minimize flexing of the conductor where the solder
changes the flexible conductor into a solid conductor

NOTE: When a stranded conductor is soldered, the
soldered portion of the conductor becomes a solid
strand conductor, and flexing can cause the
conductor to break at the end of the solder joint
unless adequate additional support is provided.

http://www.elettronavigare.it/files/E-11%20norme%20americane.pdf

 

[Legend850]

I must say I am surprised its not fused out of the factory. Putting the fuse in the hot lead to the control protects the most likely locations for a problem, and a fuse failure would simply remove power, essentially the same as turning the key off. But a short when you did not have a fuse anywhere in circuit almost guarantees you a fire. Especially if the wire does its its melting, arcing and sparking within a few inches of the carbs and fuel lines.

 

[Legend850]

Those engine harnesses are available on the web. Of course you could make your own with 7 different color wires and some ring terminals.

Check the wires that go to the distributor. They also go bad. The rotor is non-removeable, fragile and $240+USD to buy. You need to install it then. The trigger is also about that cost. The distributor cap is also about the price, but they last a real long time.

The rectifier can be replaced by any 10A or more full wave bridge rectifier.

I checked the wires to the distributor. They are acceptable. The wire is OK, some minor cracking of the insulation, I will heat shrink new insulation on.

But the wires to the alternator stator are just powder.

This model has a distributor. I assume I can simply whip the alternator flywheel off and not upset the timing ?
I am expecting the rot to continue right up to the connector to the actual copper on the stator.

Does anyone know what resistance the stator should measure ? Think you again, Dave

 

[Chris1956]

You can remove the six 5/16 bolts that ring the flywheel nut. That removes the outer flywheel. Set the motor to TDC before removing the outer flywheel. The stator will come off with a allen wrench. Expect the wiring to be bad up to the stator.

Dingbat, despite what your regulation says, Mercury ign switches in the MerControls of the 70's are soldered. Not sure if that changed over the years.

 

[Legend850]

Thank you Chris. I knew I had a job on my hands, but this project just seems to keep on growing. Nonetheless I dont mind, I am learning a lot, and when I do finally get on the water I will have a good boat and motor that owes me very little money.

 

[merc850]

Make a mark on the hub and the flexplate on the flywheel that line up,
remove the stator with a 3/16" Allen wrench; the wires go into a rubber grommet that you pull off. Use a Moto tool to carefully remove the plastic potting compound until you see the connections to the windings. Make your connections with 2 14 gage yellow wires and reinstall with blue Loctite on the screws (don't tork them too tight).
Reassemble the flywheel with Loctite on the bolts I don't know the tork spec because that's not the way to re and re a flywheel.
I did this and used silicone as a potting/insulating material.
Solder is used in the starter motor, power trim switch group, and early 50's rectifiers.

 

[merc850]

If you can find another motor 74-75 that has a damaged powerhead buy it for spare parts starter etc or even power trim.

 

[Legend850]

If you can find another motor 74-75 that has a damaged powerhead buy it for spare parts starter etc or even power trim.

I will keep an eye out. Parts are definitely starting to get scarce, and power trim would be great. But progress is OK. I have got the stator off, soldered some new wires on OK, and put epoxy on to hold it all together. The engine end of the boat is almost all rewired, but the new cable to the control has not been done at the control end. Tomorrow if it rains I will do the control end of the cable, reassemble the flywheel etc, and the electrics will be sorted. Stator measured at 0.6 ohm continuity, infinite to ground. I dont have a manual, but thats probably not too far off OK for a nine amp stator. Rectifier tested OK. I managed to extend the original distributor leads, they were acceptable at the distributor end. Lots of unknown wiring all through the boat, I will spend another day tearing it out, and do proper new wiring for the Nav lights, Bilge Pump and VHF. I will add a volt meter so I can see if the stator shits itself the easy way rather than the hard way. Then I will pressure test the gearbox cos with the exception of a missing fibre washer I cant see how the water got in. Then I will do the impeller, replace the fuel pump (just to be on the safe side - its not got a known fault), new plugs and I should be ready to get on the water.