1983 OMC tach operation

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johnrude
Posts: 8
Joined: Sat Apr 28, 2018 5:18 pm

1983 OMC tach operation

Postby johnrude » Mon Sep 10, 2018 3:50 pm

Gentlemen,
My OMC tach is beginning to indicate erratically on my 1983 Johnson 70. Just old age or poor connection somewhere? The plug and play connector into the throttle/shift control looks OK for a 35 year old connector.It does not effect the engine running at all. Thanks for your suggestions.
Jim

FrankR
Posts: 3720
Joined: Tue Mar 10, 2015 6:32 pm
Location: Florida

Re: 1983 OMC tach operation

Postby FrankR » Mon Sep 10, 2018 3:53 pm

Probably poor connection. Clean your battery terminals as a first step.

fleetwin
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Joined: Mon Mar 09, 2015 4:27 pm
Location: Rhode Island

Re: 1983 OMC tach operation

Postby fleetwin » Mon Sep 10, 2018 4:57 pm

Could be a bad connection, or perhaps a problem with the alternator...The tach gets its signal from the alternator windings. So, if the alternator is messed up, the tach could act erractically.
The simplest thing to do would borrow another tach and plug it into the connector on your control box...I realize it might be tough to find a donor tach, but possible.
Those rubber amphenol connectors can be troublesome also. Pull the tach plug off and look inside, the three pins should be straight, and make sure none are pushed back in the connector. Try to peek at the socket end of the connector on the control box as well, perhaps one of the sockets has pushed back in its connector.
Please do not cut the leads or attempt to lube the rubber connector with grease or WD40, this could create a bit of a short. Soak the two halves of the connector with rubbing alcohol before reassembly, the alcohol will lube the rubber and evaporate quickly.
I suppose I should have began my reply with checking the "pulse setting" on the tach. It may be very hard to see buried behind the dash, but the OMC tachs should be set to position "6" for your engine.
Like Frank says, check simple things first, like battery connections/cables. You could try wobbling the rubber tach connector while the tach is misbehaving to see if it straightens out. How is the tach misbehaving?

johnrude
Posts: 8
Joined: Sat Apr 28, 2018 5:18 pm

Re: 1983 OMC tach operation

Postby johnrude » Tue Sep 11, 2018 5:32 pm

The tach is operating erratically. At dead idle it's bouncing low while at mid throttle it's fluctuating perhaps 1000RPM from a constant throttle setting. I do a bit of trolling usually about 1000 so it's fluctuation is more obvious at idle. It is erratic because at times, it seems to be just fine. I will dig a little deeper in the connection. Thanks!

fleetwin
Posts: 2414
Joined: Mon Mar 09, 2015 4:27 pm
Location: Rhode Island

Re: 1983 OMC tach operation

Postby fleetwin » Wed Sep 12, 2018 9:46 am

Sounds like the tach is probably just worn out...Try wobbling the connector on the control box while the tach is fluctuating to see if that makes a difference....Having the tach pole setting incorrectly adjusted would not cause the tach to bounce/fluctuate, it would just cause an inaccurate RPM reading. A loose wiring connection would probably cause the tach to bounce up and down from zero RPM....
But, as with anything electrical, anything is always possible....It is best to start with the simplest/easiest solutions though....
The simplest thing to try would be to plug in another tach to your control box....I realize it might be tough to find a plug in tach to "borrow" for a few minutes, but maybe your local dealer can help you out....Again, your tach gets its signal from the alternator....So, if the alternator is not working properly, this may affect tach operation. There are some relatively simple ohm meter tests for the alternator windings and rectifier diodes though.

BillW
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Joined: Sat Apr 11, 2015 6:35 pm
Location: Cape Cod MA

Re: 1983 OMC tach operation

Postby BillW » Wed Sep 12, 2018 7:50 pm

Believe it or not, I have had some luck, in multiple boats, taking a screw driver and turning the pole adjustment switch back and forth a few times, then putting it back on the correct setting. It's like the switch contacts were dirty. Your results may vary; but it's worked for me quite a few times....

fleetwin
Posts: 2414
Joined: Mon Mar 09, 2015 4:27 pm
Location: Rhode Island

Re: 1983 OMC tach operation

Postby fleetwin » Thu Sep 13, 2018 10:34 am

BillW wrote:Believe it or not, I have had some luck, in multiple boats, taking a screw driver and turning the pole adjustment switch back and forth a few times, then putting it back on the correct setting. It's like the switch contacts were dirty. Your results may vary; but it's worked for me quite a few times....


Sure seems like an easy solution, will only take a few minutes....Worth a try...

johnrude
Posts: 8
Joined: Sat Apr 28, 2018 5:18 pm

Re: 1983 OMC tach operation

Postby johnrude » Thu Sep 13, 2018 4:45 pm

Excellent thought. I would not have thought about that. I will give it a try. Thanks!
Jim


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