What might show up if we had a slipping alternator belt?

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NPMR

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Strange goings on. Tacho fails then starts. Charging light goes off or on.

Yanmar 3YM20 with newish 125Ahr alternator (Big fat belt, not regular V belt). Measured output 14+ volts. And smart charger system.

Wondered how to get to the bottom of things, soonish .. launch date imminent.

Take alternator to someone who could test it?

Other ideas?
 
In my experience a slipping alternator belt has two obvious tell tail signs.
1) the tacho drop and under reads
2) It sounds like an angry cat has it's tail trapped in the belt.
 
Loose connections ?

Charge light is usually based on the exciter lead .... so maybe the lead has bad connection at back of alternator ?

Tacho - that usually taps its power of the alternator as well - some in fact use the alternator output as its gauge ... so loose connection again ?

If you have a good steady 14v ... and batterys are well charged ..... got to be the connections for the low power stuff ...
 
Check belt tension

Dont understand what you mean by "Charging light goes off or on. ". It should come on when ignition is switched on but go off when the engine is running ( but it may need a little burst of revs)

Check alternator connections esp the L&R

Check connections around ignition switch.

Check fuse in instrument panel unless the other warning lights work correctly

Check brushes , but they should be OK if the alternator is "newish" .

If all else fails take to auto-electrician for testing.
 
Refueler said:
Loose connections ?

Charge light is usually based on the exciter lead .... so maybe the lead has bad connection at back of alternator ?

Tacho - that usually taps its power of the alternator as well - some in fact use the alternator output as its gauge ... so loose connection again ?

If you have a good steady 14v ... and batterys are well charged ..... got to be the connections for the low power stuff ...
Click to expand...

There are two excitation circuits, R & L . One (R) directly from the ign switch. The other (L) via a fuse and the warning light ( See the wiring diagram in the owners manual)

Tacho takes its signal from the "Y" terminal on the alternator
 
Refueler said:
Loose connections ?

Charge light is usually based on the exciter lead .... so maybe the lead has bad connection at back of alternator ?

Tacho - that usually taps its power of the alternator as well - some in fact use the alternator output as its gauge ... so loose connection again ?

If you have a good steady 14v ... and batterys are well charged ..... got to be the connections for the low power stuff ...
Click to expand...


Also check for loose connections around the ignition switch .
 
VicS said:
There are two excitation circuits, R & L . One (R) directly from the ign switch. The other (L) via a fuse and the warning light ( See the wiring diagram in the owners manual)

Tacho takes its signal from the "Y" terminal on the alternator
Click to expand...

Point is he reckons hes getting 14v ..... so its most likely he has loose or poor connection on the low power side. Not unlike cars in the old days ... anyone old enough can remember disconnecting a terminal ... bit of emery cloth to clean it all up .. reconnect ...
 
Not sure if applicable to this case, but another sign of slipping belt besides noise and dust, may be the failure of the alternator to provide the maximum available output (wrt battery size, discharge, etc): I have a 90A alternator and 4-500Ah battery bank, when they are discharged at say 30-35% the alternator gives an initial 50-60A; whenever I measure a much lower value say 20-30A (within normal voltagerange) it's always a matter of tightening the belt and there it goes again at a much higher output.
 
JBJag27 said:
Dust from the belt as well.
Click to expand...

The above is a good sign.

Black rubber dust in the immediate area of the pulleys and alternator is often the first sign that the belt requires adjusting/changing.

A high output alternator of the field coil type at full output has very high internal magnetism - in a previous life I measured a Jaguar cars BHP output with everything electrical turned on and then immediately off. The BHP reading, from a new Suntester rolling road dynomometer was 4 BHP less with ALL electrical systems full on.

The effect of trying to rotate an alternator with full magnetism in the field coil will be to put high loads on the drive system. Aggravated by the fact that alternators have a high rotational speed requirement and thus tend to use small diameter pulleys, increasing the tendency for belt slippage.

Modern permanent magnet type alternators do not suffer this inherent problem.

Smart regulators can boost the internal magnetism of field coil types thus causing a further chance of belt slip.

We had a complete power failure of the nav instruments caused by low voltage four years ago on our delivery trip - the battery voltage dropped too low for them to work.

Inspection showed a loose drive belt on the alternator. It had been checked before leaving and was tight.

14 hours later it was hanging off and the front of the engine covered in horrid black dust.

A stripped thread on the adjuster proved to be the culprit! Now fixed.

If you decide to take your alternator for testing, take the regulator as well - both work together.
 
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