seeking a down-to-earth answer
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