Time to replace alternator?

[Teddyhherrera]

Mid flight I had my ammeter go negative with the red voltage light on my 75 F. I landed without incident, checked everything and proceeded home. Mid flight, it happened again and I landed again just before landing without incident and once on the ground, I cycled the master and it appeared to have resolved the problem. 

After troubleshooting connections, I e settled on needing a new alternator. F1/f2 resistance test yielded 5.2 ohms apparently normal range is 3-4. 

Am I correct in thinking I need the new alternator and if so, which one is recommended for best bang for my buck? I'd like to go to an a&p confidently as I find myself without a reliable one at the moment.

[MikeOH]

I think it's premature to blame the alternator.

I would have an A&P carefully follow the Plane Power troubleshooting guide: https://planepower.aero/wp-content/uploads/2018/02/Troubleshooting-Single-Engine-Externally-Regulated-Alternators.pdf

 

[N201MKTurbo]

Read my other post on alternators. Remove and check the brush holder and brushes. They are relatively cheap and easy to access.

[1967 427]

Did you measure the resistance in your test leads?  They can easily add 1 Ohm to your reading, taking the resultant measurement down to 4.2 Ohms, and keep in mind the last time your meter was calibrated.  But what do I know, I was am ME, not an EE.

[A64Pilot]

Being as it’s intermittent I’d be more likely to suspect the voltage regulator than the alternator.

If memory serves and being older it’s not what it used to be but the voltage regulator is powered from the backside of the master switch, it uses spade terminals and they are prone to being loose, cleaning and a slight squeezing of the female spade (wire side) is all it takes if that’s the problem.

Also check the field wire on the alternator for condition etc, but they usually break and of course that’s not intermittent.

[Teddyhherrera]

14 hours ago, 1967 427 said:

Did you measure the resistance in your test leads?  They can easily add 1 Ohm to your reading, taking the resultant measurement down to 4.2 Ohms, and keep in mind the last time your meter was calibrated.  But what do I know, I was am ME, not an EE.

Yes, we did this. Variation is at about 0.1

[Teddyhherrera]

22 hours ago, MikeOH said:

I think it's premature to blame the alternator.

I would have an A&P carefully follow the Plane Power troubleshooting guide: https://planepower.aero/wp-content/uploads/2018/02/Troubleshooting-Single-Engine-Externally-Regulated-Alternators.pdf

 

I'm going to do this tomorrow!!

[Teddyhherrera]

15 hours ago, N201MKTurbo said:

Read my other post on alternators. Remove and check the brush holder and brushes. They are relatively cheap and easy to access.

75 f. What's the easiest way to remove it without pulling the prop?

[Teddyhherrera]

13 hours ago, A64Pilot said:

Being as it’s intermittent I’d be more likely to suspect the voltage regulator than the alternator.

If memory serves and being older it’s not what it used to be but the voltage regulator is powered from the backside of the master switch, it uses spade terminals and they are prone to being loose, cleaning and a slight squeezing of the female spade (wire side) is all it takes if that’s the problem.

Also check the field wire on the alternator for condition etc, but they usually break and of course that’s not intermittent.

I had to replace it 2 years ago. Would the VR have gone bad that quickly? My last VR failure the panel just shut off

[N201MKTurbo]

7 hours ago, Teddyhherrera said:

75 f. What's the easiest way to remove it without pulling the prop?

The brush holder connects the field wires with the brushes and supports the brushes. You can easily remove it to inspect it. Just remove the field wires, then remove the two bolts outside of the field wires and pull it out. Inspect the connections on the back side of the field wire studs. If they are loose, you have found your intermittent connection. To replace the brush holder, push the brushes down and stick a wire through the little hole to hold the brushes down, slide it back into the alternator, put the bolts back in and pull the wire out which lets the brushes contact the slip rings. This can all be done easily without removing the alternator

[Teddyhherrera]

8 hours ago, N201MKTurbo said:

The brush holder connects the field wires with the brushes and supports the brushes. You can easily remove it to inspect it. Just remove the field wires, then remove the two bolts outside of the field wires and pull it out. Inspect the connections on the back side of the field wire studs. If they are loose, you have found your intermittent connection. To replace the brush holder, push the brushes down and stick a wire through the little hole to hold the brushes down, slide it back into the alternator, put the bolts back in and pull the wire out which lets the brushes contact the slip rings. This can all be done easily without removing the alternator

nice! thank you!

[A64Pilot]

17 hours ago, Teddyhherrera said:

I had to replace it 2 years ago. Would the VR have gone bad that quickly? My last VR failure the panel just shut off

Shouldn’t, they usually last decades it seems

When you fix it please report back, I think it helps the next person who has your problem

Edited April 21 by A64Pilot

[Teddyhherrera]

Here at the airport. Cleaned field terminals ohms between f1/f2 improved. However, after leaving the master on for a while after a run up and taxi, voltage regulator is warm/hot to the touch. disconnected and was getting 12.49 to the VR and 10.7 coming out the other side. 

 

[N201MKTurbo]

4 hours ago, Teddyhherrera said:

Here at the airport. Cleaned field terminals ohms between f1/f2 improved. However, after leaving the master on for a while after a run up and taxi, voltage regulator is warm/hot to the touch. disconnected and was getting 12.49 to the VR and 10.7 coming out the other side. 

 

That sounds reasonable. If you had the field wires off, did you pull the brush holder out it look it over? You were 90% of the way there.

[Teddyhherrera]

25 minutes ago, N201MKTurbo said:

That sounds reasonable. If you had the field wires off, did you pull the brush holder out it look it over? You were 90% of the way there.

i tried to wiggle it out but it wasnt cooperating. i may try tomorrow when i have more patience.  what do you think about the voltage regulator heating up?

[A64Pilot]

I believe the VR heating is somewhat normal, it I believe functions by reducing voltage, before the solid state days that was often done by wasting the excess power as heat .Warm is I believe normal, smoking, burn your fingers hot is not.