Lagging Tach

[Alec H]

I use the engine to charge batteries. I've noticed an increasing lag between raising the RPMs and seeing the change on the tachometer. This lag time can be as long as ten minutes. I know what the engine sounds like at 1600 rpm; but the tach will read 1000. Eventually I see it creep up to the right reading. Why would this happen. The engine is a Yanmar 3JH5E. The control panel is the square rather than the rectangular one. Thanks.

[transmitterdan]

What kind of regulator do you have on the alternator? The tach is controlled by the alternator or in some cases by the external regulator.

[captain465]

If the tach is driven by the alternator, check the belt tension. It should be tight enough to allow only a 1/4" deflection in the longest section of belt between pulleys.

[Jim Cate]

Yep, sounds like a slipping alternator belt. Might be time to buy and carry a replacement belt (if you don't already have one) because sometimes slipping under load is a precursor to abject failure. Not an expert, but I suspect that when it looses tension after long hours of service it is a symptom of failing fibres in the belt, and it escalates kinda rapidly: fewer load carying fibres mean greater loads on the remaining ones, so they in turn fail... cascading events that lead to rapid total failure.

Of course, in your case it could be from other causes, like slipping tension adjustment or oil on the belt and pulleys, but IMO the lagging tach is very likely due to belt slippage from SOME source, rather than some fault in the alternator or regulator.

Jim

[Alec H]

This would seem to be an electronic problem as opposed to mechanical, ie. a slipping belt. The belt has less than 100 hours use, I installed it a few months ago, but I will check tension. Since the low RPM reading jumps to accurate reading after 10-20 minutes running, it seems more likely caused by the signal coming from the regulator rather than the belt suddenly starting to stop slipping.

Agree?

[billknny]

Originally Posted by Alec H

This would seem to be an electronic problem as opposed to mechanical, ie. a slipping belt. The belt has less than 100 hours use, I installed it a few months ago, but I will check tension. Since the low RPM reading jumps to accurate reading after 10-20 minutes running, it seems more likely caused by the signal coming from the regulator rather than the belt suddenly starting to stop slipping.

Agree?

Don't agree.

Regulators ramp back on the field current at the END of the charge when the batteries are totally full. They do not take 10 minutes to ramp up (some do take 60 seconds or so to allow the belt to "seat" before they apply full power to the field.) A problem with the regulator will almost always show as no tachometer reading at all, or a very intermittent one, not one that is just low.

You might have a loose connection somewhere, or a regulator that has an unusual internal malfunction, but a slipping belt that grabs after it gets hot (and softer!) would be the money bet for the symptoms you describe.

[Alec H]

Thanks responders. It was a loose belt.