Alternator Woes
 

Had to replace the orig alternator on starboard eng after 30+ years of service. Replacement was a 55A as opposed to orig being 35A. Got it from local alternator shop. Ran it for 1 season, then one day noticed voltmeter in the red on high side while running hard. Then it dropped back to normal voltage. A few mins later I feel power loss. Look at panel V meter and it's just below 12V. Stop and raise eng cover. A small puff of smoke came out near alternator. Got home and opened alt to find a charred stator wire. Tried to disassemble and discovered stator wiring is aluminum!! Got 2 of the stator wires un-soldered from the diode tree but the 3rd one broke. Chunked that alternator and got one online from DB Electrical. Installed it and it looked good on the hose. Went to the lake and discovered it's putting out 16.5V with eng at 3000 RPM. Verified with Fluke hand-held meter. Notified seller and after waiting close to a month, finally got an RMA#. Send it back for replacement. The new one is doing the exact same thing except it's putting out 16.7V.

I measured the same voltage at the batt and at the SENSE terminal on back-side of alt.

Anyone seen this issue before??
Is there an adjustable voltage regulator for a Mondo alternator available (like it had from factory)??

Check both small wires on back of ALT, one is sense and other id power, see if they both read the same

I've verified that the SENSE wire is connected to batt and the other small wire (EXCITE) goes thru ign switch. They read close to same V (with key ON) but not exactly (within 0.2V).

Just got a reply back from seller saying, in this case, the alternator does have an adjustable voltage regulator. I'll have to remove the voltage regulator and confirm it is actually an adjustable type since the adjustment is on the alt side of regulator. This will be great news if true. I've been wanting to get both alternators set to same exact same voltage in case I run with batt switches in BOTH. I don't normally do that, but it has happened.:picard1:

I have gotten a bunch of alt's from DB, but they all have been the Delco marine style. No issues with those. Some are at 15+yrs old ? More ?
Anyway, subscribing to see more about these adj reg's and how it works out for you. :thumbs

Bad news. Good news. More confusion
 

Pulled the voltage regulator/brush holder. Couldn't find any adjustment pots. Took to bench and verified its regulating the field current right at 14.6V. Noticed brush voltage is abt 0.2V lower than EXCITE wire when SENSE voltage is less than 14.6V. When EXCITE wire is 14.7V or above, brush voltage drops to abt 30mV. Don't see any issues with the regulator operation.

Checked alt wiring without alt connected and found essentially zero ohms from alt output to pos batt cable (with batt switch ON). Same for ground wire to neg batt post. Re-checked EXCITE and SENSE wires. EXCITE at 12.2V and SENSE wire is right at batt voltage of 13.0V. Everything looks normal. Humm,, now what??

Have you checked the 90 amp fuse

https://cimg8.ibsrv.net/gimg/www.off...76cf8d66c3.jpg

Don't see how an open fuse could cause high voltage at the batt.
Assuming that's the one down on the starter? Would probably need to pull the eng to get at it.

You said
The excite and sense wires are intact
The ALT tested good
There was smoke coming from the first ALT

Only thing left is there is a break in connection between ALT output and the Bat, so that would be the fuse

Could connect a 10 AWG wire from ALT output to Bat Positive and see if it acts the same

The regulator tested OK on the bench. Haven't tested the alt on a test stand yet. Maybe it's self-exciting somehow?? Hopefully I can run it up to the parts store for a test this weekend.

I don't have a schematic handy but believe the alt output connects to the starter via the 90?? amp fuse, then into the starboard power side of harness, then thru the batt switch to the batt.
Before starting, batt is at 13V, after starting it's at 14.5V at idle and 16.7V at 3000 RPM.. Plus, I ohm'ed-out the alt output to the pos terminal of batt and got less than 1 ohm. Seems like the fuse would have to be good..

I'll try a jumper for $hits and giggles. At this point I'll try pretty much anything.. Starting to think this thing is haunted...:rolleyes:

Grounds all good?
I know on the automotive side of things, battery,frame, body,engine, a bad ground between these can cause an overcharge condition.
Is your voltmeter right?
13v on a battery at rest?