Testing an alternator output

[lydiamight]

I am not getting any charge to the batteries when running the engine---Yanmar 3GM30F, fitted with a standard Hitachi alternator and an elderley TWC regulator. I intend to start the fault finding at the alternator "end" and try to check the output with a multimeter. Access down the back of the alternator is vey limited and I really don't want to be fiddling about down there with a meter whilst the engine is running and the belts and pulleys,etc are whirring round in very close proximity to various of my body parts. I propose to remove the alternator wiring and fix a couple of extra wiring "tails" that I could bring out of the engine box and connect to my meter in safety.
Question is--which terminals on the back of the alternator should I connect the meter to? I haven't taken the alternator off yet (scheduled for tomorrow morning) so any advice as to how the terminals look would be most welcome.

 

[halcyon]

Before removing wires, does the ign / warning light go out? if it does you may well be ok
Trace the output wire, goes to startermotor, blocking diode etc, the voltge will be the same as the alternator , and may well be easier to check. check if the voltage rises should be around a nominal 14.0 volt. If you have no volts or battery voltage, get the alternator checked, if high move on down the line to next connection.

Brian

 

[Guest]

My red lkight went out - but still I had duff regulator diodds at back of my alternator ... I found the fault in the end using a Draper Batt / Alternator tester .... £9.99 in Halfords ... has a row of LED's that light up to show alternator charge to batts ... if engine not running it shows batt state. OK it's not best tool - but has saved me a few pennies over the years. I took alternator to service guy - he confirmed and replaced diodes ... £20 later - all well and refitted to boat and has been great ... that was about 5 year ago ...

To original poster - the tester can be connected to battery being charged - not necessary for extra leads etc.

 

[William_H]

The larger bolt terminal is the connection to the battery ie the output.
I don't know the TWC regulator. Most alternators have a regulator mounted into the end plate of the alternator. It is attached by 2 screws and has the 2 brushes attached to the regulaor and sme contacts made when it is screwed in. However some have an external regulator so there will be additioanl screw terminals (smaller). Some people have external smart regulators which are wired into the alternator.

If the alternator has failed as proven by no voltage rise on the batteries when you expect it to charge. The most likely fault is in bad wiring connections. However if these are good the next failure point from wear is the brushes that rub on the slip rings. It isbest to remove the alternator for this check. They should have reasonable spring pressure onto the slip rings and the rings should be smooth and clean.
Bad brushes are usually indicated byt he igntn (gen) (alt) light not lighting when the engine is stopped but power is on. good luck olewill

 

[billmacfarlane]

I had a TWC on my last boat which performed faultlessly for 11 years. However a pal of mine also had one and his batteries went flat as the TWC wouldn't switch in. It turned out that the ignition lamp though still lighting had gone high resistance and this was enough to prevent the TWC from switching in. Strange but true.

 

[lydiamight]

Thanks for everyone's help. Went down to the boat today and managed to get the alternator off. The two main connections were fine, but there was a thin wire hanging loose from the wiring loom and two spade connectors on the back of the alternator--neither with a wire on them. Crimped a new spade terminal on and then guessed--the two spade terminals were marked R & L--so I put the loose red wire onto the terminal marked R. Put it all back togerther again and BINGO, I now have batteries that show they are charging when the engine is running. Damn hot today struggling in the confines of a small engine compartment, must have lost some pints of body fluid. The satisfaction of getting it working made that cold beer taste even better!