seek­ing a down-to-earth an­swer

Practical Boat Owner - - Ask The Experts -

Q i have pur­chased a second-hand Volvo Penta 2003 which came with a con­trol panel and wiring loom. The al­te­na­tor is an iskra, but there is no iden­ti­fi­ca­tion plate on it so i don't know the type or the charg­ing amps, etc. The al­ter­na­tor has the fol­low­ing ter­mi­nals: B+, B-, D+ and W. The volt­age reg­u­la­tor is marked ‘AeR 1720 14V’ and has a sin­gle blue wire, but i don’t know where this wire should con­nect to: i think it might con­nect to the B+ or B- ter­mi­nal, de­pend­ing on whether the volt­age reg­u­la­tor is a pos­i­tive or neg­a­tive ma­chine-sens­ing type. if so, is there a mu­time­ter test to find this out? Also, would con­nect­ing the wire the wrong way round dam­age the al­te­na­tor or reg­u­la­tor, or stop the reg­u­la­tor work­ing?

As the al­ter­na­tor has a B- ter­mi­nal, the al­ter­na­tor and starter mo­tor are earthed by their own wires di­rectly to the bat­tery neg­a­tive ter­mi­nal, but i don’t think the sys­tem has a ded­i­cated in­su­lated earth re­turn as the al­ter­na­tor is bolted di­rectly to the en­gine block and isn’t in­su­lated from it. This might be be­cause the al­ter­na­tor is not the orig­i­nal one for the en­gine and has just been bolted on. i don’t know if the starter mo­tor is in­su­lated or not. My ques­tion is, is it al­right to have the starter mo­tor and al­ter­na­tor earthed to the en­gine block and also earthed with their own ded­i­cated wire to the bat­tery neg­a­tive ter­mi­nal? if the earth re­turn sys­tem has to be fully in­su­lated, should the bat­tery neg­a­tive ter­mi­nal also be con­nected to the en­gine block with a heavy-duty earth wire, and should this also be the case for the sys­tem i have de­scribed? Andrew Greaves, by email

STU DAVIES REPLIES:

Some of these en­gines were iso­lated neg­a­tive earth, so the block wasn’t earthed: this was al­legedly to pro­tect saildrives etc from elec­trol­y­sis. I am pretty cer­tain that yours is iso­lated re­turn, be­cause you have an­swered your own ques­tion by telling us that the starter mo­tor has its own earth wire, as does the al­ter­na­tor, and they go to a bat­tery ter­mi­nal. They are in­su­lated in­ter­nally, so bolt­ing them to the block di­rectly does not sig­nify any­thing from the point of view of elec­tri­cal con­nec­tion. The two B ter­mi­nals are self-ex­plana­tory: one goes to the bat­tery pos­i­tive and the other the bat­tery neg­a­tive. They may, like on mine, go to the starter mo­tor con­nec­tions first, but that is only a mech­a­nism to get them to the bat­ter­ies by pig­gy­back­ing them onto the main bat­tery ca­bles. The W con­nec­tion is for the rev counter and the D is for the warn­ing light.

On Va­leo al­ter­na­tors, which are com­monly used on VP en­gines, the wire com­ing from the built-in reg­u­la­tor is yellow and goes to the B+ ter­mi­nal: this is for the reg­u­la­tor volt­age sens­ing. In the Ster­ling al­ter­na­tor ‘booster’ kits (one of which I have fit­ted to my MD22), the in­struc­tions show how to add an ex­tra wire to the brushes so the Ster­ling can ‘over­ride’ the stan­dard reg­u­la­tor. Make sure the blue wire is com­ing/go­ing to the reg­u­la­tor and not the brushes: ie, make sure that no­body has been adding wires. My own view is that if the blue wire is the stan­dard volt­age-sens­ing wire, it could be worth tak­ing the wire to the bat­ter­ies di­rectly. This would then al­low the reg­u­la­tor to sense the volt­age at the bat­ter­ies, which in some sys­tems can be slightly lower as a re­sult of volt­age drop due to the dis­tance from the al­ter­na­tor. Be care­ful if do­ing this, how­ever!

As to the fi­nal ques­tion, should you use the block as an earth? Some of the VP en­gines, such as mine, also have a re­lay sys­tem to al­low the stop so­le­noid and glow plug sys­tems to work. (If I re­call cor­rectly, yours doesn’t use glow plugs.) So, to start earth­ing the block may, if yours is an iso­lated earth sys­tem, cause the law of un­in­tended con­se­quences to come into ac­tion! Leave it as is.

I've looked at the al­ter­na­tor man­u­fac­turer web­site and they seem to have made them to the same de­sign, so ev­ery­thing should work as the orig­i­nal. If you are bencht­est­ing the al­ter­na­tor, make sure you feed the D ter­mi­nal with 12V through a pea bulb.

Reader Andrew Greaves has wiring queries re­gard­ing the al­ter­na­tor on his boat

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