OMC Cobra Voltage Fluctuations

[Marine Sparky]

I have a 1992 Four Winns with a 5.0L OMC Cobra. Runs decent but the trim and oil gauges fluctuate to the extreme. I replaced the oil sending unit and it didn't really help the situation. The trim has the most trouble when trimmed all the way down and settles in after its trimmed up a bit once on plane.

The voltage gauge bounces around about between 10.5 and 11.5 while idling at 650rpm. It settles down to 12v at 800rpm and goes to 14v anywhere above 1000rmp. The tach and fuel gauges flutter a bit below 1000rpm but smooth out above that.

Could this be a faulty voltage regulator creating erratic/noisy reference voltage needed by the gauges? Or something else?

 

[bruceb58]

Could be a bad reg or a bad alternator. Measure the voltage at the alternator. Also put your meter on Ac and see how much AC ripple voltage is present. You may have some blown diodes.

 

[Marine Sparky]

Thanks Bruce. Didn’t see how to access the alternator wires to measure. But measured the next best thing at the battery and at the gauge cluster buss.

RPM Batt DC Batt AC Gauge DC Gauge AC
Off 12.99 V 0.10 V 12.99 V 0.10 V
650 rpm 14.60 V 0.17 V 14.10 V 0.40 V
800 rpm 14.75 V 0.17 V 14.30 V 0.50 V
1000 rpm 15.10 V 0.17 V 14.60 V 0.50 V
1500 rpm 15.50 V 0.17 V 15.00 V 0.50 V

It's my understanding these should charge at 13.6Vdc and regulator output above 14.0 Vdc or is not really regulating. But I'm not certain of that.

My last battery only lasted two years. The current one is 1 year old. I'm thinking the regulator is running over voltage which is tough on the system. But maybe I'm being hopeful because its cheaper and easier to replace than sending units and gauges. Any thoughts on this?

 

[bruceb58]

Yes. Regulator is not working properly.

You haven't ever done the trick where you disconnect the battery while the engine is running have you? That's a sure fire way of frying the regulator.

 

[Marine Sparky]

Not sure why anyone would do that. I didn't.

I'll get the part this week and let you know how it works out. Thanks for the help.

 

[bruceb58]

Not sure why anyone would do that. I didn't.

You would be surprised!