Voltage regulator failure?

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CharlieO.

CharlieO.

Guru
Joined
Sep 21, 2020
Messages
1,821
Location
Lake Champlain Vermont, USA
Vessel Name
Luna C.
Vessel Make
1977 Marine Trader 34DC
Cruising back to our slip today I noticed the amp meter reading close to what my alternator is rated for(51a).
My USB plugs that also read voltage were at 17.5 volts. My DC2DC charger was reporting 17.5 volts feeding it.
I backed my RPMs down for the final 20 minutes to the slip and the voltage dropped to 17.

I immediately went to the engine room to check my batteries, they were not Hot and not showing any sign of distortion.

I am thinking it is a failed voltage regulator, does that sound plausible?
 
Shopping for a replacement regulator, how do I determine if I have a short keyhole style case?

Voltage Regulator 105-64, 8RG2010A, R7-1 For Motorola / Prestolite MR/8MR Series with Short Keyhole Case Style - 8010197​


Can anyone point me to a replacement regulator they have used, or an economical direct replacement alternator?
I have the original style Motorola MR2018K (51amp) alternator on my Ford Lehman.
 
Charlie, where did you find "Motorola MR2018K (51amp) alternator".
I wanted to see it but google cannot find MR2018K
 

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Which wire would i disconnect to prevent my alternator from coming on? I am assuming it could be one of the wires coming off the oil pressure switch? I only need to move my boat from the slip to pumpout and back. Then from slip to haul out. I think I can get away with just running on the battery without the alternator to prevent any damage.
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Cannot say for sure but the wire colors are very confusing. I would guess if you disconnect the yellow and red that are on the alternator luggage you will not be sending any power from the alternator.
 
READY said:
Cannot say for sure but the wire colors are very confusing. I would guess if you disconnect the yellow and red that are on the alternator luggage you will not be sending any power from the alternator.
Click to expand...
I just don’t want to damage my alternator because I’m pretty sure it still works and that regulator fried allowing the full 50ish amp and 17.5 volts to discharge to my batteries.
 
The line from the pressure switch turns the alt on when pressure comes up.

Could this be your replacement?
www.maniacelectricmotors.com

Voltage Regulator 105-64, 8RG2010A, R7-1 For Motorola / Prestolite MR/8MR Series with Short Keyhole Case Style - 8010197

<h3>Regulator Specs</h3> • For Motorola / Prestolite MR/8MR Series with Short Keyhole Case Style, Alternators w/ Isolation Diode • Plug Type: K, Wiring Code: 3, 12 Volt, B-Circuit, 15.0 Voltage Set Point, w/ Isolation Diode • Terminal ID: Red Isolation Diode (+), Black Ground, Green Field...
www.maniacelectricmotors.com
 

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Lepke said:
The line from the pressure switch turns the alt on when pressure comes up.

Could this be your replacement?
www.maniacelectricmotors.com

Voltage Regulator 105-64, 8RG2010A, R7-1 For Motorola / Prestolite MR/8MR Series with Short Keyhole Case Style - 8010197

<h3>Regulator Specs</h3> • For Motorola / Prestolite MR/8MR Series with Short Keyhole Case Style, Alternators w/ Isolation Diode • Plug Type: K, Wiring Code: 3, 12 Volt, B-Circuit, 15.0 Voltage Set Point, w/ Isolation Diode • Terminal ID: Red Isolation Diode (+), Black Ground, Green Field...
www.maniacelectricmotors.com
Click to expand...
I have looked at that one may try ordering one, but my current voltage regulator seems to have more space on that backside because some of the guts of the alternator protrude out and kind of intrude into the space behind the regulator.

That is why I was trying to determine what the definition of a short keyhole alternator is.
 
At this point, why not go with an external reg? Just a thought, but not sure it could be done on that Alt.
 
Iggy said:
At this point, why not go with an external reg? Just a thought, but not sure it could be done on that Alt.
Click to expand...
That is still a possible solution, and may consider it but I need to do more research on the external regulators, I really am not familiar with them at all.

I think if I did that I would switch the alternator to charge my 100a lifepo bank at say 25 amps and use the 18 amp DC2DC charger for the start bank. Or maybe an ACR to charge my start batteries first then the house. I’ll have learn a bit more.

But ideally I’d like a direct replacement, that would be easiest right now.
 
You had questions about this alternator back in mid August, as shown here.

Another Alternator Question

So monitor my alternator output thru the Bluetooth connection on my Victron DC to DC charger shows my alternator voltage fluctuates between 12.4v and 13.7 and sometimes a bit higher. So my DC to DC charger will cycle on and off quite a bit due to low voltage. Pretty sure this is not a new...
www.trawlerforum.com www.trawlerforum.com

Any idea what the green wire that is shown projecting from the alternator at about the 9 o'clock position in the latest photos actually does? Why is it hanging out there?

My advice today is to remove the alternator and take it to an alternator repair shop. They will likely give you a quote to fix it within half an hour.
 
Your recall is good my friend.
Once I get hauled if I haven’t a replacement regulator or drop in alternator I will pull it off for a shop to bring it back and n shape or replacement.

That is the signal for the tachometer.
 
Just loosen the bolts and take the belt off. Then you can know for sure it cannot be further damaged. Assuming the belt only drives the alternator.
 
Looks like the large yellow is the positive to the battery, the small red goes to the oil switch, the other small red when switch is closed goes to the small yellow to excite the ALT. disconnect the small yellow from small red.
 
Don’t take the belt off, it drives the coolant pump and operating without it would probably increase your problems.
 
SteveK said:
Looks like the large yellow is the positive to the battery, the small red goes to the oil switch, the other small red when switch is closed goes to the small yellow to excite the ALT. disconnect the small yellow from small red.
Click to expand...
That was what I was thinking when I looked at it, I just wanted to hear what other people thought also. Thank you Steve.
 
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