1979 70 HP not charging? Tach not working

[banjosam]

The last time we (successfully) went boating I noticed that the tach had quit working. A friend suggested that it was probably a broken wire, which sounded possible, so I paid no more attention. The next time we tried to go the engine wouldn't start - dead battery. I then began to think that the dead battery and dead tach were related. I tested the alternator windings as instructed in the repair manual and they tested okay. I tested the rectifier and the yellow and yellow/gray leads tested okay, but when I tested the red to yellow and red to yellow/gray I got a high reading in both directions. I got a new Mallory rectifier and installed it. I noticed that there was no yellow/gray wire on it. When I first connected started the motor it didn't seem to run right, so I shut it off and reversed the two yellow wires and started it again. It ran fine, but no tach. I assumed that the tach registering would indicate that it was charging fine.
>Did the tach die when the rectifiers did?
>Did I not fix the problem?
>How can I tell if it's charging?

 

[Lion hunter]

Re: 1979 70 HP not charging? Tach not working

May be totally off base here as I am more familar with old Mercs but,

Tach has to work to charge

Battery should read somewhere above 12V if its charging. Measure voltage not running, then running. Any increase most likely means it's charging, and if it's above 12 its good. Most older motors don't charge so much as they maintain a battery. You almost have to have a fully charged battery to start with.

Crossing the terminals or a short will fry a rectifier in short order.

Don't know if these will work on your motor, but they work great on old mercs and they are cheap.
http://www.radioshack.com/product/index.jsp?productId=2062584

I also chased similar problem around only to find that I had a bad tach.

 

[Joe Reeves]

Re: 1979 70 HP not charging? Tach not working

(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.

 

[wvmedic]

Re: 1979 70 HP not charging? Tach not working

banjosam, I would listen to Mr. Reeves. But I to just changed my rectifier, I don?t remember if it was a Mallory or a Sierra but both wires where yellow and the paper that came with it said that they were interchangeable. If I remember correctly on my bar it was yellow, yellow, and red from top to bottom.

Jeff.

 

[jumanjii1]

Re: 1979 70 HP not charging? Tach not working

The rectifier is bad. The tach not working and the system not charging, those are the signs. Just change it out. Most rectifiers have 2 yellow wires and 1 red wire. Yellow wires are AC in from the alternator and the red wire is DC out. They do make a 3 phase rectifier for those 3 phase alternators and in that case, there would be 3 yellow wires and 1 red wire.

And sometimes they'll add that black wire, that would be the ground, but the rectifier gets grounded when you screw it down on newer models so there's no black wire.

Of course the alternator could have shorted out but these are known to last forever, or until the wires get old and the insulation comes off. Well maybe not forever but my alternator on my Evinrude has been going strong for 38 years now. Holy cow it may be time for a new one, lol

Good Luck

 

[vahunter]

Re: 1979 70 HP not charging? Tach not working

Will this change the performance at all?

 

[F_R]

Re: 1979 70 HP not charging? Tach not working

Will this change the performance at all?

The alternator and rectifier are for charging the battery and nothing else. Well, it does drive the tachometer also. But the motor does not run off the battery and the charging system doesn't have a darn thing to do with performance. In fact the motor will run without any battery at all----but DON'T try it because it will destroy the rectifier.