Low voltage warning

[TuomoMooneyFlying]

When idling on the ground at 1000rpm, my JPI has now on several occasions flashed a low voltage warning, showing 12V or dropping to 11.8V. When I do my run up and at takeoff/cruise power it shows between 13.8 and 14V.

 

So, is that normal or what could be failing? Battery or alternator or? Thanks in advance!

[Ragsf15e]

2 hours ago, TuomoMooneyFlying said:

 

When idling on the ground at 1000rpm, my JPI has now on several occasions flashed a low voltage warning, showing 12V or dropping to 11.8V. When I do my run up and at takeoff/cruise power it shows between 13.8 and 14V.

 

So, is that normal or what could be failing? Battery or alternator or? Thanks in advance!

 

I also have an F and mine shows about 13.8 in idle, 14v at normal power above that.  Since you have an f, I assume alternator and not generator?  What draw do you have on?  Lots of old (non led) lights, pitot heat, landing light, etc might cause this…

We need @PT20J…

[TuomoMooneyFlying]

Only the rotating beacon, but I had switched the fuel boost pump off a while earlier after startup, could that kill the voltage so severely?

[Ragsf15e]

6 minutes ago, TuomoMooneyFlying said:

Only the rotating beacon, but I had switched the fuel boost pump off a while earlier after startup, could that kill the voltage so severely?

Doubtful.  It would likely take significant draw at idle.  How old is the battery?  Has it been capacity checked?

[EricJ]

Do you have a generator or alternator?   It is most likely the alternator or the regulator if you have an alternator.   Something is just weak and not getting up to capacity at low rpm.

[TuomoMooneyFlying]

Doubtful.  It would likely take significant draw at idle.  How old is the battery?  Has it been capacity checked?

Battery is from previous owner, at least two maybe three years old. Gonna need to check the logs.

Hmm.. if it’s not “normal” then gonna get my A&P looking into it. Weird thing is that at cruise it’s making good power, so just idle is causing issues.

Thanks for all the responses!

[PT20J]

The first thing I would do is get out a DVM and measure the bus voltage with the master and alternator switches on (engine off) and compare it to the JPI to make sure that it's not an instrumentation error. If that's good, I would measure the voltage at the field terminal of the alternator and make sure it is near the bus voltage. Since the alternator is not putting out any voltage (the engine is off) the voltage regulator should be commanding maximum output from the alternator and it does this by applying max voltage available to the field. If that works, I'd suspect the alternator. The first thing to check would be the brushes if the alternator has a lot of time on it.

Skip

[Ragsf15e]

49 minutes ago, PT20J said:

The first thing I would do is get out a DVM and measure the bus voltage with the master and alternator switches on (engine off) and compare it to the JPI to make sure that it's not an instrumentation error. If that's good, I would measure the voltage at the field terminal of the alternator and make sure it is near the bus voltage. Since the alternator is not putting out any voltage (the engine is off) the voltage regulator should be commanding maximum output from the alternator and it does this by applying max voltage available to the field. If that works, I'd suspect the alternator. The first thing to check would be the brushes if the alternator has a lot of time on it.

Skip

Do what Skip said for sure.  Theres a Zeftronics Troubleshooting guide around here or on their website too.  Skip, I thought the ts guide said to expect ~0.5 to 1v less than bus voltage on the field wire with just the master on?

@TuomoMooneyFlying one thing to know, many mechanics are not that great at troubleshooting electrical problems. The tests @PT20J outlined, are very easy to do but most will skip them and start replacing parts. I would definitely recommend educating yourself and maybe even helping or doing a few tests to help narrow down the problem. It can get kind of expensive quickly without fixing anything. The system is not that hard though and there are people on here who can easily help you get it right.

[PT20J]

The field voltage may be a bit less than bus voltage, but it should be fairly close. This is just a coarse test - you’re looking for gross errors like no voltage or a significantly low voltage. 

[Guest]

Start simple, is the belt slipping?

[Shadrach]

Nine replies and we still don’t know if we’re talking about an alternator or a generator.

If it’s a generator it’s not going to show max voltage at 1000 RPM. Depending on the voltage regulator, it will likely still show a positive charge with everything on, but just barely. Below a thousand RPM, The system is likely discharging. 

[MikeOH]

5 hours ago, M20Doc said:

Start simple, is the belt slipping?

@M20Doc

How do you determine that?

[Guest]

48 minutes ago, MikeOH said:

@M20Doc

How do you determine that?

Reach into the cowl with an appropriate sized wrench and see if the pulley will turn easily.

[MikeOH]

25 minutes ago, M20Doc said:

Reach into the cowl with an appropriate sized wrench and see if the pulley will turn easily.

@M20Doc

Should it not slip at all; as in, the engine will turn, or is it a 'feel' thing where some slippage will occur?

[PT20J]

There's a spec for that 

SI 1129D Alternator Belt Tension, Methods of Checking.pdf

[Guest]

1 hour ago, MikeOH said:

@M20Doc

Should it not slip at all; as in, the engine will turn, or is it a 'feel' thing where some slippage will occur?

You shouldn’t be able to turn the engine through a compression stroke!  

I have one of the belt tension meters shown in the SI that Skip posted, but it doesn’t fit in most locations.  You want it snug enough that it won’t slip easily, but not so tight you overload the bearings or break the belt.

[TuomoMooneyFlying]

4 hours ago, Shadrach said:

Nine replies and we still don’t know if we’re talking about an alternator or a generator.

If it’s a generator it’s not going to show max voltage at 1000 RPM. Deending on the voltage regulator, it will likely still show a positive charge with everything on, but just barely. Below a thousand RPM, The system is likely discharging. 

It’s an alternator.

[Ragsf15e]

5 hours ago, Shadrach said:

Nine replies and we still don’t know if we’re talking about an alternator or a generator.

If it’s a generator it’s not going to show max voltage at 1000 RPM. Deending on the voltage regulator, it will likely still show a positive charge with everything on, but just barely. Below a thousand RPM, The system is likely discharging. 

I was kind of assuming alternator since it’s an F.  Were older Fs made with generators originally?

[Shadrach]

46 minutes ago, Ragsf15e said:

I was kind of assuming alternator since it’s an F.  Were older Fs made with generators originally?

Yes. We’re still running a generator with a zeftronics VR. We keep talking about converting it, but we never feel at a loss for capacity.  It will go someday if I embark on my quest for 1100lb useful load.

[Shadrach]

4 hours ago, Ragsf15e said:

I was kind of assuming alternator since it’s an F.  Were older Fs made with generators originally?

There’s a reasonable chance that your bird was delivered with a Delco Remy 50amp generator.