Insulated return alternator regulator

MikeBz

MikeBz

Well-Known Member
Joined
22 Aug 2005
Messages
1,877
Location
East Anglia
Visit site
I need an insulated return (earth) regulator for Prestolite alternator, part number P00370 (https://www.peivinlookup.com/Category/P00370).

I have spoken with Prestolite - they don’t have any and there aren’t any on the next shipment from China. Optimistically the soonest they MIGHT have one is late May. It doesn’t appear to be a part that anyone stocks (I’ve spoken with Parts4Engines who supply the alternator but don’t sell the regulator separately).

A knowledgeable auto electrician tells me it’s a modified version of the non-insulated version - there are insulating sleeves in the bolt holes, and the 0V connection to the alternator is made by a contact inside one of the bolt holes to a boss in the alternator.

IMG_2025-04-17-115437.jpeg

IMG_2025-04-17-115527.jpeg

It seems that my options are to either buy a new alternator (Parts4Engines had stock yesterday, but not today), wait indefinitely (not an option) for a regulator, or… I’m wondering if it’s worth spend twenty quid on a non-insulated one and seeing if I can modify it. I’m guessing the plastic insulating sleeves will just press in but the issue might be the 0V connection - it depends on whether the body of the alternator itself is different where that connection is made. Has anyone looked into this?
 

Attachments

  • IMG_2025-04-17-115527.jpeg
    IMG_2025-04-17-115527.jpeg
    86.2 KB · Views: 1
We have a Sterling Pro Reg which piggybacks off the existing regulator. Maybe it could be made to work with the internals of the existing regulator removed. Interesting thought, thanks!
 
From memory the Balmar doesn't so if it's a case of you can't replace or get a new alternator that could be an option. You would have to convert the alternator to external field control.
 
Last edited:
The Sterling overrides the built-in regulator’s field control. The instructions allude to it working without another regulator: “This regulator can be used with alts up to 150A without a regulator or 300A with an existing regulator”. Obviously I would need to keep the body of the existing reg with its brushes, one brush connected to the alternator output and the other to the field control on the Sterling, which it already is. I’ve emailed Sterling for confirmation.

Thanks for the contact Boater Sam, I will try them.
 
Is the brush assembly intact ?

If so then an external regulator and diode assembly might work but you would also need access to the three phase field coil output wires.

Then this eBay item 276160129060 for the regulator and diode assembly.

I have this on my motorbike and the same principle will work for your alternator. This stuff is good for 60 amps or a bit more.

I can provide a wiring diagram to show how to hook it all up, but it isn't rocket science and it does work.
 
I would get a new alternator and repair the old one as back up.

The Sterling will work without a regulator but I am not sure if a broken regulator will allow it to work and "fail safe" as designed. Check all your wiring and connections are good - I have had some "fail" modes with my Sterling due to poor connections - took a while to discover.
 
superheat6k said:
Is the brush assembly intact ?

If so then an external regulator and diode assembly might work but you would also need access to the three phase field coil output wires.
Click to expand...
Yes the brush assembly is intact. The internal regulator only connects to ground and the alternator output (B+ - I think, with a multimeter it measures the same as B+ anyway), plus via brushes the field winding, nothing else. One of the brushes is connected to B+ (the ‘I think’ bit repeated), the other is modulated by the regulator to control the output. The Sterling piggybacks on this with a wire soldered to the brush that is modulated.

Obviously having only the Sterling means no fallback if that fails, but it would be a good enough interim solution until I can find an insulated regulator just to reinstate the fallback. I’ll try it this afternoon, if that doesn’t work then I probably will go down the new alternator and repair the old one as a backup route.
 
Well best laid plans and all that. Arrived at the boat this afternoon, refitted the old regulator with its electronics disconnected, switched the engine breaker on, ignition on & panel lights up, press Start button and…. not even a click from the engine earth relay. Spent the rest of the time checking connections & continuity but ended up none the wiser. Now have circuit diagrams so will have more chance next time, but that won’t be for over a week from now.
 
Sterling confirmed that the Pro Reg will work without a functioning alternator regulator. But the way our boat is wired it needs D+ to be connected to 0V when the alternator is not producing output in order for the start button to energise the start relay. As soon as the alternator produces voltage on D+ then this de-energises the start relay even if the button is held in. Google’s AI Overview on the topic of D+ being at OV when the engine is not running is wrong. My gutted/defunct regulator was not doing this.

With nothing (well, £12) to lose I bought a standard A127 (non insulated return) regulator. It turns out that to convert it to insulated return all you have to do is carefully increase the size of 2 holes in the metal part of the casing and press in the 2 plastic insulating sleeves from the defunct insulated version.

Old on the left, new on the right with sleeves fitted:

IMG_2025-04-28-220512.jpeg

The hole top right bolts to the earthing boss inside the alternator and is not electrically connected to the alternator casing. So the upper (metal) part of the regulator case is earthed but the 2 sleeved bolts which thread into the alternator case are isolated.

So if anyone needs to replace an insulated return regulator there is a cheap and simple option.
 
Last edited:
You must log in or register to reply here.

Share this page

Top