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tohatsu md70 charging issues ??

diablotoona

New member
Hello new to outboarders here and have a question.
I have a 4yr old tohatsu md70 and recently my battery went bad so i replaced it and on my depth finder volts i always see around 12.3-12.1 as voltage when operating the boat, i also measured the same volts at the battery when running the motor.
Is this too low ? My father in law says it is but i am not sure as i do not know , any input here would greatly be appreciated.
 
Way too low. You need over 13 when running; preferably over 13.5 when charging. 12.1 is almost dead. But cheap voltmeters may not be accurate, so test it with a good one. TLDI motors require a 1000-amp battery, BTW. If you measure battery voltage before starting, I would like to see at least 13 for a fully-charged battery. Then it will drop some when cranking. Then it should go up to maybe 14v initially as the alternator recharges right after starting. Then settle down to about 13.5.
 
Way too low. You need over 13 when running; preferably over 13.5 when charging. 12.1 is almost dead. But cheap voltmeters may not be accurate, so test it with a good one. TLDI motors require a 1000-amp battery, BTW. If you measure battery voltage before starting, I would like to see at least 13 for a fully-charged battery. Then it will drop some when cranking. Then it should go up to maybe 14v initially as the alternator recharges right after starting. Then settle down to about 13.5.
I did verify with volt meter so it is indeed that low, i got a type 27 battery but forgot the specs on it but should be proper for that motor, now any advice as to what i should check for here? Total noob when it comes to marine stuff, handy with cars but not marine.
 
Ok easy enough, i meant to ask, is this a common issue with these motors?
Also, would a short in the system cause anything like this? Reason i ask is i bought the boat used and there has been some "rigging" to the electrical system. I am going to disconnect the leads going to that from the battery and verify thou.
Thank you for your help so far.
 
There is a distinct possibility that you have boat issues (not just motor issues -- maybe no motor issue at all). I would connect the motor directly to the battery (separating out the rest of the boat as you suggested) and test.
The MD70 charging system is pretty robust, so no, charging is seldom a problem with these; a short in the boat could overwhelm it, and could also drain the battery. For that matter, connecting the battery cable backwards (it has been done) can blow a fuse or cook some electronics in the motor.
Do you have an ammeter for testing? That would help -- to see if the motor consumes or makes amps.
 
ok some more info, did not have enough time to what i wanted yesterday so will try the disconnect of all other hookups today. Yesterday i fired the motor up and watched voltage, it charged at 14.2ish for a good 5 minutes at just idle, then started to slowly creep down to the low 12's, i then turned the motor off and let it sit for a good 10 minutes and fired it back up and it did the same thing a again, charged at 14.2 for 5 minutes until it started to creep back down.
Maybe this will give someone some insight.
If i understand the alternator and its respective parts are located under the flywheel which appears to me needs a special tool to be removed?
Thanks for any input here.
 
What voltmeter are you using? It sounds like you are charging; just the numbers are funky by a volt or so.
OR, there could be a load on the system, soaking up a fair amount of power. If that were the case, the battery should go stone dead in a day or so.
 
Its a fairly cheap one, but the volts on it coincided with the volt reading on the depth finder so i would imagine it is pretty accurate.
I do have a kill switch on the battery which i turn off and i also have a solar panel that is hooked up charging (sunny Florida here so), the solar panel is hooked up so kill switch will not affect it.
Before i started motor the battery voltage with everything still off was 12.8
 
12.8 at rest is OK. Odd that it dropped below that while the boat was on. Are you actually having an issue with the battery, or was it just that you had a bad battery originally?
 
Well the battery is brand new as the previous went bad, then after that i paid attention to the volts noticed it going down like described above. The new battery is a type 29 deep cycle from west marine.
 
So you are thinking what i am seeing is normal operation then? I will take it out some more a pay attention to the volts and report back, thank you for all your input.
 
OK, update, took it out last night for about an hour, first 5 minutes it charged at 14-15 volts, then after that went to 12.2 and hovered around there for the rest of the time. This was as read by the depth finder volt reading.
Should i be concerned still ?
 
The motor's rectifier is regulated, so you should be OK... unless the regulation is off-spec. If really concerned, grab a battery hydrometer (assuming you have a flooded-type battery that has access to the individual cells), and test the specific gravity after the battery rests for an hour with nothing connected. That will confirm the state of charge.
 
I just looked and the rectifier on this motor seems pretty simple to replace, part # 3Z5-76060-0 , i thought i saw that you sold parts also, would you have a price on this ?
 
I just looked and the rectifier on this motor seems pretty simple to replace, part # 3Z5-76060-0 , i thought i saw that you sold parts also, would you have a price on this ?
Diagnose it first. I think it's working, based on your readings that show charging. If you do indeed need a rectifier/regulator (I would test it first -- it's a simple bridge rectifier with regulator), you can order
3Z5760602MRECTIFIER (NEW)$105.39

 
ok thanks, i measured its output yesterday directly on the motor and it showed the same as i have been reporting and did the same thing, charged up in the 14's in the beginning then trickled down to the 12.2, it did get very hot, to the point i could not touch it.
 
If you want to test the rectifier, whip out your Factory service manual and a good ANALOG ohm meter and test it. Odds are it's fine. They do get quite warm; hence the huge aluminum heat sink. One thing to do is isolate the motor and battery (remove all other parts of the electrical system) and see if it still reads that low. Use a GOOD meter.
 
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ok, so i ordered the rectifier and installed last night, went for a 45min ride and charging volts was 13.9-14 the whole time so i would say the rectifier was the issue here, thanks for all the input.
 
Its a fairly cheap one, but the volts on it coincided with the volt reading on the depth finder so i would imagine it is pretty accurate.
I do have a kill switch on the battery which i turn off and i also have a solar panel that is hooked up charging (sunny Florida here so), the solar panel is hooked up so kill switch will not affect it.
Before i started motor the battery voltage with everything still off was 12.8
I think you are right.. Even I have used kill switch and it did not worked at all..
 
Its a fairly cheap one, but the volts on it coincided with the volt reading on the depth finder so i would imagine it is pretty accurate.
I do have a kill switch on the battery which i turn off and i also have
solar kits that is hooked up charging (sunny Florida here so), the solar panel is hooked up so kill switch will not affect it.
Before i started motor the battery voltage with everything still off was 12.8
I think you are right.. Even I have used kill switch and it did not worked at all.
.
Anybody else made similar mistake?
 
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