Cant get voltage from Alternators / Isolaters

J

Jeff6566

Member
Oct 12, 2016
81
Central Texas
Boat Info
370 Sundancer, 1995
Engines
7.4 Mercruisers
1995 Searay 370 / 7.4 carbs
same problem on both engines
Alternators checked good by 2 shops

I thought the isolater was the problem, but bypassed w/ no diff
AC charger disconnected
Battery 12.5 v
rpm 1500
Reading from bat; no change
Reading form main post on alt. : 0v
Ran temp. wire from battery to same post: 0v

Help
 
Wire on main post of alt. connects directly to battery. Should read battery voltage whether engine is on or off.
 
Not when using an isolator, this is another reason why one engine should be left in its factory configuration Direct to battery, no isolators in the circuit.
I`m guessing there are those 90A fuse blocks at the starter that somehow got blown.
 
Something is not making sense. He eliminates isolaters and no go. Runs a wire directly from battery to alternator and gets a 0 volt reading. How is that possible ?? Alternator will be grounded just by the brackets. Maybe sketchy test equipment ?
 
Are you sure the exciter wire on the alternators are in properly?


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unless these are single wire alt, the newer ones wont charge unless they see voltage from the battery, its why they dont charge if a isolator is in the circuit but show output when tested on a bench
 
Have you confirmed keyed ignition power to the alternator for excitation?
Where are you putting your ground lead from your volt meter?
You can use your volt meter to check for bad or open connections by connecting it parallel to the wire/circuit in question. It should not show any voltage if the circuit is good. If the circuit is open (disconnected) it will show full battery/charge voltage. If just a bad (weak connection) it will show the voltage loss across that connection. Remember, you want your meter to show 0 volts for this test. Any voltage registered is a problem.
Confirm you have keyed ignition power at alternator for excitation. With engine running, put the black lead from your volt meter on the large + battery post. Positive lead from meter on alternator B+ terminal (while running). You should show 0 volts. If you show full output then there is an open circuit in between the two leads. In that event, by moving the ground lead (meter) and working your way back to the starter + post and alternator, you should be able to find your problem. As long as your meter is showing voltage, the problem is between your test leads.
(edit) You can test the ground side of charging system as well. Put the negative lead on alternator housing and positive lead on - battery post and work your way back. Again, you should show 0 volts if circuit is good.
This works great for identifying problems in ground circuits as well. Just have to keep in mind the direction the current is flowing and place your leads accordingly.
I would not rule out that your alternators are not being tested correctly but the likelihood of losing both at the same time is slim.
 
Last edited:
New boat, and they never have worked.
Shop one said they were OK
Shop two said one was marginal, so having it rebuilt
I do have switched power to back of alt. and Im testing from grnd terminal on alt. or block.
Havent had a chance to do Thorton69s test but found continunity back to the isolater but not going thru it back to the starter.
could this be the problem? Not familiar with how a Isolater functions..

Thornton69 -. Thanks
 
I do have switched power to back of alt. on one small terminal, I have three in use . The Alt. shop told me which two terminals should have power and one does and the large one does not
 
When I bypassed isolater,
Is it possible that not having voltage below 12.5v , alt. is not charging?
 
Are you showing the same 12.5v with engine off and key on?
The alternators should be looking to balance the system at the 14v level.
Have you had your batteries tested to rule them for contributing problems?
You may have to bump up engine rpm above 1000 to initiate charging. (I do with mine)
Do you have any other accessories on during testing.
Performing the voltage drop tests I mentioned earlier should identify connection issues.
Test the ground side as well. As mentioned by others, when you jumped direct from battery to main output post on alternator, you should not lose it.
Where are you placing the ground lead from your meter when checking that?
Any chance you can post a pic of the back of your alternator. There was a few styles used and I would not rule out that they are hooked up wrong.
You said this is a new boat to you. Are we right to assume that this problem came with it?
 
Are you using the indash gauge or a separate voltmeter to view the voltage?
 
rpm 1500
don't remember where I ground when using jumper.
going to boat today to try all above
thanks
 
I am not familiar with what you have for an isolator. When you bypassed it though, you would have taken it out of the equation. Using your multimeter for continuity checks on a higher amperage circuit is not the norm as it will give you false readings. Those circuits have to be loaded. That is where testing for voltage loss comes in handy for identifying problems. When you jumped direct from B+ on the alt to + post on battery you should not have lost batt power unless the ground was lost at the housing (you probed) under load opening up the circuit. All the mercs I have looked at have a dedicated ground wire to one of the housing studs. Sounds like yours does not. I would not rule out a bad ground. Did you check the large fuses mentioned previously by another poster? Until you do the earlier tests mentioned, not sure what else to offer.
 
Lets go back to step 1
The alt were checked and found good by a shop, that says there is output
There should be 4 wires , 2 small gauge (red, red/purple, black,blue ) those are sense and excite.
They must be connected for the alt to work. Then a larger Orange or Red output wire and a large Black wire going to a case bolt or stud. https://tse2.mm.bing.net/th?id=OIP.J3XVdKj-Pi7HSHCoRAStMAEsEh&w=229&h=220&c=7&qlt=90&o=4&pid=1.7.
If you have this type, the center stud goes to the alt and the 2 outboard studs go to the batteries.
with meter connected to the starting battery , record voltage
start motor set rpm @1500 record voltage
Go to center stud of isolator, record voltage
Go to output stud on alt, record voltage
Post the results
 
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