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Twin Mercruiser 485s, Regulator/Rectifier + Isolator wiring - problems

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OneEyedWillyIsMyCopilot

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Hi everyone, I'm working on a friend's 1982 SeaRay 255 SRV with twin Mercruiser 485's. I'm trying to wire up the charging system, and I've run into a curious anomaly. I'm wiring the two charging systems in parallel, connected to a single isolator (Schottky diode type), which feeds two batteries (one house battery, one 'engines' battery). I know there are dual alternator isolators available, but this one is functional and came with the boat, and I'd like to make it work. Also in parallel with the two alternator outputs is an Intelli-Power PD9130 AC-to-DC converter.

The problem I'm having is, when wiring the system according to the Mercruiser #8 manual's instructions, I get approximately 10 volts on the input terminal of the isolator, meaning that somehow the isolator's diodes are being circumvented. When I modify the wiring slightly, the voltage is no longer present. I believe there is something internal to the regulator/rectifier units that would explain this anomaly, but without a schematic of the regulator/rectifer's internal workings, I can't pin it down. If anyone has such a schematic, or can at least verify that the latter wiring is acceptable, I would very much appreciate it.

This is how the Mercruiser #8 manual recommends wiring the regulator with an isolator:

155.jpg

According to that diagram, the metal jumper between "Output" and what I assume is "Regulator" terminals is to be removed, with the "Sense" and "Regulator" terminals to instead be connected to one of the isolator output terminals (presumably to account for the voltage drop across the Schottky diode).

Following these instructions, this is how I have the system wired:

Current Setup.jpg

With this setup, however, I measure roughly 10 volts at terminal "A" of the isolator - when no input is present. When I remove the "Sense" wires from the output side of the isolator, the voltage disappears. This problem exists even when only one system is wired, so the issue is not related to running two systems in parallel.

I decided to try wiring it back "stock", only moving the sense wires to the output of the isolator, but leaving the jumpers between "O" and "R" terminals in place. Here is a diagram describing this arrangement:

Functional-Setup.jpg

With this arrangement, everything *appears* to function as it should. The quiescent voltage at terminal "A" is no longer present.

I'm rather curious if the Mercruiser manual is just completely wrong, or if something is amiss. If anyone has any information, schematics, or advice; I would appreciate it tremendously. If nothing else, I would very much like to know if the last wiring method I described is acceptable.

Thanks in advance.

Edit: Here are links to larger versions of the images. The forum uploader appears to have resized the originals.

Mercruiser Wiring Recommendation

Problematic Wiring Arrangement

Presumably Functional Wiring Arrangement
 
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470 systems are finicky enough as they are let alone playing with the regulators.the power box needs 105-120 AC input but your trying to charge 2 batteries thru an isolator.
My advice is either leave them be or use an isolator on the output of the power box to charge the batteries.Those reg`s are NLA .
Another option is to convert 1 to a alt kit and use the isolator with that .
 
Thanks for the response, Bt Doctur. I'm aware of the AC-to-DC converter's functional requirements, and it is indeed fed 120VAC when connected to shore power. To be honest I'm quite certain it has nothing to do with the issue I'm having, I only included it in the diagrams for the sake of completeness. Electrically there is nothing wrong with charging two batteries through an isolator. They're simply batteries in parallel isolated by Schottky diodes (which are rated for the current and properly heat-sinked).

The problem is solely with the instructions Mercruiser has published for connecting these regulators to an isolator. Simply following their instructions to the letter, without the systems in parallel and without the AC-to-DC converter, somehow internally circumvents the isolator itself, as evidenced by the potential measured at the input terminal of the isolator in a quiescent state.

I understand these regulators are problematic and prone to failure, however these appear to be functioning properly. An alternator conversion is likely in the event of a failure, but until then I have to work with what I have (again, this is not my boat).

Currently, there are only two explanations that I can think of for the problem I'm seeing. Either both regulators have failed in such an unusual way that they still function as intended (as regulator/rectifiers), but somehow circumvent the isolator - or the method for wiring them to an isolator provided by Mercruiser is incorrect. I have only two goals with this inquiry. 1) To find a schematic of the internal design of the regulator rectifiers, so that I may find out for myself what could be causing the issue; and 2) To find out if my proposed alternate wiring method is viable.

Again, thanks for your reply.
 
Hello,
I have an ‘85 SRV255 with twin 470’s for 7 years now (thanks to everyone on this forum for all their help). Do you have the full manual section on the charging system, it goes through the procedure to check the regulator and from that you should be able to get the diode arrangement from the test method. I can send you the section if you if you want.

I know this doesn’t directly answer your question but BtDr is right; convince your friend to do at least a one-wire marine alternator conversion as it is about 1/3rd of the price of finding a regulator these days. Of course that unit will be used and of questionable status and could fail the day you put it in and then your friend is really out $$. If the VR is questionable, don’t waste your time and do a conversion, there is only upside and no down side. (Your time has value too) There are low and high mount kits depending if one engine has a power steering pump.

The other thing to be concerned with is the age of the motors and the adhesive that holds magnets on to the damper. If you lose those you lose charging capabilities as well.

I have house & start batteries on starboard and start on port from the factory. I converted my starboard to alternator, leaving the regulator in place as a spare for the port when it fails. Additionally converting to alternators lets you physically remove the VR’s and splice the coolant loop that runs through them which adds some capacity to the engine cooling system which is always a good thing on these motors.

If he hasn’t already done so, convert the points over to Petronix electronic ignition & coil, probably the single best expenditures he can do other than an alternator.
 
Your "second diagram isn't consistent with the instructions in Manual #8...I have found many errors in many manuals over the years but don't believe this is one of them. Think of the R terminal on the regulator as a remote input. This feature is as you suspect - to allow the output voltage to be compensated for the isolator's voltage drop. Your third diagram eliminates this function and will likely result in the batteries never being fully charged.

2X on the alternator mods...

Also, BT's suggestion of just using the isolator on the converter's output and keeping the charging systems in a 'stock' configuration has merit...also uses the KISS principle...

Finally, don't put a lot of stock in measurements that aren't referenced in the manual....unless you are trying to reverse engineer known good but obsolete parts. The factory has always been good about limiting info on their "proprietary" components...
 
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