1978 70 hp charging question/problem...

i have a 1963 17' skagit skimaster with a 1978 evinrude 70. it is the venerable 3 cylinder and runs like a sewing machine...but this weekend while i was out boating i killed the engine to talk to some friends and when i went to start the engine it went 'ruur' and wouldn't turn over. luckily, it is a great outboard and we pull started it (on the first try). the depth sounder had drained my battery - i checked the output of the engine at the battery and there was nothing. i looked under the cowling to trace out the generator and i didn't see anything obvious - please help! does the magneto power the generator via a rectifier? where would i find it? are there any fuses i should check first? thanks.

as a side note, a local marine repair shop has a 79 johnson 70 hp with a thrown rod...but it has a new ignition setup , good prop and lower end, and a newer starter. he wants $200 - i think i might pick it up.

Re: 1978 70 hp charging question/problem...

That engine incorporates a stator (alternator) under the flywheel (that large ring thingy) which emits AC voltage to a rectifier that converts the AC to DC voltage in order to charge the battery. I assume that the rectifier is faulty.

(Small Rectifier Description & Location)
(J. Reeves)

On most 2,3,4,6 cylinder engines, the small rectifier is located on the starboard (right) side of the engine just in front of the engines electrical wiring strip. There are a few older V4 engines that have the wiring strip on the rear portion of the engine and the rectifier would be located just under that terminal strip. The smaller horsepower engines usually have the rectifier located on the starboard side of the powerhead close to the carburetor area.

The rectifier appears to be a round object approximately one inch (1") in diameter and also about one inch (1") high. The base of it is sort of triangular in appearance and is attached to the engine with two (2) screws/bolts..... usually one screw/bolt is larger than the other. The rectifier, depending on which one your engine uses, will have either:

One Red wire, one Yellow wire, and one Yellow/Gray wire, or One Red wire, and two Yellow wires.

Note that either of the above rectifiers could have a fourth wire which would be Yellow/Blue

(Small Rectifier Test)
(J. Reeves)

Remove the rectifier wires from the terminal block. Using a ohm meter, connect the black lead of the ohm meter to the rectifier base (ground), then one by one, connect the red lead of the ohm meter to the yellow, yellow/gray, then the red wire (some rectifiers may also have a fourth yellow/blue wire. If so connect to that also). Now, reverse the ohm meter leads and check those same wires again. You should get a reading in one direction, and none at all in the other direction.

Now, connect the black lead of the ohm meter to the red wire. One by one, connect the red lead of the ohm meter to the yellow, yellow/gray, and if present, the yellow/blue wire. Then reverse the leads, checking the wires again. Once more, you should get a reading in one direction and none in the other.

Note that the reading obtained from the red rectifier wire will be lower then what is obtained from the other wires.

Any deviation from the "Reading", "No Reading" as above indicates a faulty rectifier. Note that a rectifier will not tolerate reverse polarity. Simply touching the battery with the cables in the reverse order or hooking up a battery charger backwards will blow the diodes in the rectifier assy immediately.

Re: 1978 70 hp charging question/problem...

thanks! i am somewhat engine savvy, but i need to buy an owner's manual for this engine...i am headed out to test it right now.

Re: 1978 70 hp charging question/problem...

i pulled the rectifier and tested it - there was a dead ground from the red to the base (0.3 ohms) and the other two (yellow and yellow/gray) showed nothing. there was no continuity across any of the leads either...looks like a trip to the store. i don't see what difference swapping the meter leads makes....it is just checking for a current path and resistance, it doesn't care which way the leads are attached. thanks for the help in identifying my problem!

Re: 1978 70 hp charging question/problem...

In your reply, you state "i don't see what difference swapping the meter leads makes....it is just checking for a current path and resistance, it doesn't care which way the leads are attached."

A diode reads in one direction and not the other. Why is it that some simply want to argue against the proven test procedures?

Re: 1978 70 hp charging question/problem...

i wasn't trying to step on anyone's toes. i was simply asking a question. i understand the concept of a diode and a wheatstone bridge rectifier...is it because the small test current the meter uses to detect continuity won't go backwards through the diode? on my dead one i think it made no difference because it was a dead ground. the new one read across all three leads and had Mohm resistance between red and the housing.

in any case, i went and bought a 70 hp johnson for parts, three years older than mine and has a thrown rod, and i installed the other rectifier. thanks for the help!

edit: i tested the replacement rectifier and i am getting 15.5 volts at the battery...13 when the engine is off. i swapped my chewed up my prop for the pristine one that came with the parts motor.