Fishfinder says overvoltage and undervoltage

[Merc90HPnewbie]

Mercury 90 HP inline 6 outboard, 1985.<br /><br />I was on the water this weekend when I noticed the fishfinder kept going off. I don't fish- in fact most of the time I am going over 25 MPH, but since it was in the boat when I bought it, I leave it on since it tells me MPH, water temperature, and depth. Anyway I kept turning it back on and it would be off within a minute or so. I put it in diagnostic mode and every test passed but it put up two warnings: Overvoltage Detected and Undervoltage Detected. The voltage readings it gave varied between 16.4 and 17.6 volts. This was at 3100 RPM but I checked again when I was going slower and it was the same thing. (For comparison, I think a running 12V system in a car is supposed to be around 13.8V at idle. I don't know if an outboard should be the same.) 17.6V sounds high to me but that only explains the Overvoltage Warning not the Undervoltage one.<br />The fishfinder has turned off in the past but never this often. I am guessing I have something wrong in the charging system. I guess the first step is to put a voltmeter on the battery and make sure the voltage really is going too high and too low. Assuming there's no loose wires or other obvious problems, what components should I be focusing on here and does anyone know what part is a likely culprit? <br /><br />Thanks!!

 

[KCLOST]

Re: Fishfinder says overvoltage and undervoltage

Is this a new problem, or has it always been this way?<br /><br />I had a "low voltage" warning on my finder a while back. It turned out to be a bad battery... Replaced it, and no problems.... However, I'm not sure if a bad battery can result in "high voltage".

 

[Merc90HPnewbie]

Re: Fishfinder says overvoltage and undervoltage

Hi Kclost<br />Well, the fish finder has turned off by itself in the past, but only a handfull of times, so I never looked into it. This past weekend is when it really started happening a lot and when I discovered the voltage errors.<br /><br />I think the battery is probably OK but I can swap out another one from my car or something... <br /><br />And as you say, I don't think anything in the battery can cause a high voltage problem. Is there a voltage regulator on these motors? My guess is that there's some kind of solid state circuit past the alternator that's crapped out and is not regulating AC voltage (which would explain the Hi and Low voltage errors).<br /><br />Oh if it helps the diagnosis, I just had to replace a switchbox that had gone bad. I don't think this is involved though.

 

[KCLOST]

Re: Fishfinder says overvoltage and undervoltage

I need the serial# for your motor... To check the regulator...

 

[Merc90HPnewbie]

Re: Fishfinder says overvoltage and undervoltage

Hi Kclost;<br />Serial # is 6624434. It's a 90 horse 1985 inline 6 with 3 dual carbs...one coil for each cylinder... no distributor...

 

[emckelvy]

Re: Fishfinder says overvoltage and undervoltage

Your Merc does not have a voltage regulator. Its maximum charge rate of approx. 10 amps is dependent upon engine speed.<br /><br />So, if you run for quite a while with light electrical load, the battery terminal voltage is going to rise.<br /><br />One very simple way to counteract this would be to leave your running lights on all the time the motor's running at speed. This may put enough of a load on the system to keep the voltage down to a reasonable level.<br /><br />I think I've seen posts in the past which indicate that the Gel type of batteries don't have such a drastic voltage rise, but don't quote me on that. Maybe some of the other folks have some experience in that area.<br /><br />Another suggestion that was posted on the news.kraits.org Inline 6 Forum was to wire in one of the newer-style voltage regulator/rectifier units that are found on some 80's-vintage and 90's-and-up vintage Merc's, such as the 3-cyl 90 HP, 4-cyl 100-115-125, and V6's. <br /><br />One of these units could be retrofitted to the front cowling support, then connected to the stator wires coming from under the flywheel, which are presently hooked to the old square rectifier assy. You'd also need to run the red +12V wire to the new regulator/rectifier and move the tach wire if it's connected to the existing rectifier.<br /><br />While the newer voltage reg's are capable of handling 15-amp (or more depending on the model) output, the older Inline stator will never develop more than its full rated output. However, the regulator will prevent the voltage from rising to an overly-high level. The regulator will allow full output up to a certain level, then clamp any excessive voltage and route it to ground.<br /><br />Anyway, hope this sheds some light on the problem and Good Luck............ed

 

[dogsdad]

Re: Fishfinder says overvoltage and undervoltage

I think you've gotten good advice so far, but I thought I would throw this in:<br /><br />I think it MAY be possible for a bad battery to cause this. At first I didn't think so, but Ed's post made me think...<br /><br />A battery with an open cell might cause an unregulated charging system to appear to be putting out too much voltage. If the battery does have an open cell, it should be fairly easy to verify with a voltmeter. <br /><br />Each cell is rated at a nominal two volts. Six 2-volt cells in series will yield twelve volts. We all know, however, that a twelve volt battery charges up to around 13 or 13.5 volts. So, if your battery reads 5/6 of this voltage or around 11 volts, it's a safe bet that a cell is bad and tghis might explain the higher voltage present while the engine is running.<br /><br />Now, another thing just popped into my head, and that is how does the motor start with only 5/6 of a battery? I guess it might. <br /><br />A voltage reading on that battery with the motor "off" might be informative though.<br /><br /><br />-dd-

 

[Merc90HPnewbie]

Re: Fishfinder says overvoltage and undervoltage

Ed Mc: Basically what I gleaned from your post is that there's nothing really wrong with my motor/electrical system. I guess the concern is with the electronics/gauges. I don't really have much installed, the gauges that came with the boat I assume can handle some voltage fluctuations. I can protect the fishfinder by wiring a voltage regulator to it. I don't have a radio or anything so that should be fine. The only outstanding issue is why this just started happening a lot this past weekend.<br /><br />Dogsdad: It couldn't hurt to check, this weekend I'll do some tests on the battery with the motor running and off and see what happens...

 

[KCLOST]

Re: Fishfinder says overvoltage and undervoltage

Sounds good....

 

[emckelvy]

Re: Fishfinder says overvoltage and undervoltage

Battery is a good thought, you could put a full charge on it then check specific gravity with a hygrometer. Also take it to an auto parts store after you've charged it fully and have them do a load test on it.<br /><br />Another thing that pops into my head would be a 'dirty' rectifier that's passing AC. But, I would think on that, the DC voltage output would be decreased, not overvoltage'd. Maybe the fishfinder is sensitive to AC component.<br /><br />You can do resistance checks on the rectum-fire to make sure it's doing its job, check the battery, if all is well turn on the running lites and let it be!<br /><br />Let us know if you find anything strange, it's an interesting tech subject..........ed