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77 Johnson 70hp(70EL77S) Drawing Voltage from Battery while running

A

Adam Taylor

New member
Correct me if I'm wrong, but, shouldn't this motor put out at least a small charge while running? With volt meter on battery terminals and the engine connected to the battery and not running, it was showing 12.9 VDC. While still having the meter connected to the terminals, I start the engine, which of course drops voltage to 12.4 VDC, which is normal because the starter is pulling a load. But once engine is running the voltage should go back up. It does not. It does however keep dropping over time. The other day I had the boat on the river and after running a long distance using the outboard only(no electronics or trolling motor), I switched the engine off to take a phone call and when I got ready to start back up the battery didnt have enought juice to start the engine. Good thing I had the optional pull cord with me and I could pull-start he motor to get back to the landing.
 
(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

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(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.

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The engine itself should not draw down the battery while running.

But, you do have the key on, and that provides power to the purple wire, which feeds accessories.

I once had a cigarette lighter/power jack that had begun to rust internally. The rusting caused a battery drain when the key was on.

Even tho it sounds as if the reg/rect is bad, I cannot see how the motor would draw on the battery. The motor generates it's own power for running the ignition, which is completely separate form the charging system.

I guess it is possible in an oddly shorted regulator........and, I have to admit....that is one large draw to load the battery down from 12.9 to 12.4 volts....that is .5 volts drop.....hard to imagine that much load without some kinda heat or burning wires, or something like that. It is also possible that your battery is ageing.....

But no doubt, without having to shotgun the reg/rect, you can simply remove the large red wire that comes from the reg/rect and connects to the starter solenoid. This would remove all possibility that the engine was pulling from the reg/rect. Remove it, run it, and check for drain as you have done.

But, I would still look elsewhere for a drain. You can put an ammeter in line with the purple wire to check for current flow with the key on, and all accessories off, both with the engine running and with it off.
The purple wire current flow BEGINS at the key, and runs numerous places in the harness....tach, horn, etc.
 
The mounting bolts that hold the rectifier to the block were loose. I tightened them and now with engine running and a volt meter on the battery, the reading goes up by .01 VDC about every second. I'll test further when I can return to the river and see what happens. Thanks again for sharing your knowledge. :)
 
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