Overcharging problem

[Jim Smith]

Hey Guys. On my '69 turbo model my Davtronics display shows my voltage alternating between 28 to 30 V. My original equipment red overvoltage light flashes as it's on the verge of over-voltage. I have replaced both voltage regulators with Zeftronics and the problem persists. Switching from one alternator to the other or one regulator to the other doesn't solve the problem. Thoughts? Thanks Jim

[hharney]

Have you tried to adjust the regulators to a lower voltage output? They are externally adjustable.

[Jim Smith]

Herb, my original factory ones were adjustable, but I don't recall seeing anything where the Zeftronics are adjustable.

[B2C2]

Hi Jim

I have a 169 T337D as well. I recently rebuilt the front alternator, and due to the fact that the field circuit was shorted out also replaced both stock regulators with the Zeftronics variety. I also replaced the battery with a Concord platinum series as the Gill I had was marginal at best capacity wise. Finally I also bought a battery minder and have been keeping my new battery connected to this when not flying. After doing this I have also seen the voltage jumping up and down as you describe, and the OV light flashing on and off. I also have a VI meter so I can see the actual voltage, not just the light and it seems to be similar to what you are seeing of 28-30V. Finally I also see the alternator lights flickering indicating the alternators are offline, but I see current so I know they are not. All this appears to settle down and stops completely after about 10 minutes of flying. I believe the reason for this is that the Zeftronics regulators regulate to 27.5V. This is close enough to the battery voltage for a "just off the charger" battery that with the forward drop of 0.7V across the alternator isolation diodes the system doesn't charge the battery all the time, leading to the alternator offline flicker indication. I think the regulator doesn't like the fact that the battery is so close to the regulation voltage and this causes the voltage to jump around as it seeks to get charging going. After a few minutes of running current off the battery for part of the time, the battery voltage drops enough that current flows steadily into the battery but doesn't increase the battery voltage to the point where this won't happen. This leads to the end of the flickering and OV behavior.

You don't mention if you did anything besides replace the regulators, or if the problem is persistent or just around at start up. Anyway thought Id pass this along as its similar to what you are seeing.

Ben

[Jim Smith]

Hi Ben. I'm sorry you're having a similar problem but good to know I'm not the only one, especially on the same model and year. The problem typically seems to be worse when the battery is at peak charge or the electrical system has a minimal load. To temporarily cure the problem I will take the alternators off-line and drain the battery some and then turn them back on. I use a post it note that says 'alts' that I stick right in front of me to insure I don't forget to turn them back on! I spoke with the original owner of the plane who owned it for over 40 years and said this was never a problem for him. It makes me wonder if the pilot who delivered the plane to me might have pushed the 'alternator restart' button. I've heard pushing this button when not necessary can cause odd electrical system issues. I think it somehow affects the diodes in the alternators. I've also seen other posts where someone had similar issues and found each one of the alternators had one bad diode. This could explain how the current alternates from 28-30V as it passes through cycles in the alts. Also, the only time I have flickering alternator lights is occasionally when the plane at idle, and I understand this is a normal occurrence. Let me know if you find any possible solutions and I will do the same. Thanks, Jim

[hharney]

Jim

You may want to check the resistance on the alternator field wires. I have seen where there is a splice or a bad connector that increases the resistance to the point that the regulator can't compute the load.

[Jim Smith]

Thanks Herb. I have seen other comments regarding a similar problem and it related to resistance on the field wires. Since it's a parallel system, I'm going to assume since it occurs with each alternator individually or both at the same time it must be at some point after where the field wires connect jointly. Thoughts?

[hharney]

It will depend on which alternator is taking the load, assuming it is not paralleling. It could be either one or both. Start by inspecting the field wires for any suspect connections. Compare resistance on each regulator and determine if it is one. If both are the same it could also be in the control board (I think your model has this).

[B2C2]

Hi Jim

I did an experiment yesterday where I took the battery off the charger for two days then started up the plane. All of the symptoms went away, indicating that in my case it is an effect of a fully charged battery being close to the regulator output voltage. This is consistent with it not showing up with my old beat up battery. I don't think this is much to worry about as it doesn't exceed 30V by much and fixes itself after a few minutes of running even for a fresh off the charger battery. I should mention that in addition to the new regulators and rebuilt alternator, I went through the field wiring with a meter and found that under some circumstances I could see high resistance through the alternator switch when I actuated it repeatedly. so I replaced it. Also there is a jumper lead that goes across that switch to give the regulator access to both alternators. Take a look at this as mine was heavily oxidized (black) and so I replaced it with fresh crimps. Check all the crimps associated with the field wiring for good measure. Also check the connections to the lugs attached to the field terminal on the alternator. My front alternator field lead had one strand left from lots of vibration.

Ben

[Jim Smith]

Ben that sounds like a lot of great things to check and will go through each of those items. Also my battery was pretty much shot and have replaced it but not had a chance to fly and test yet. That could very well be the source.