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Interesting alternator voltage problem

FireMedic_2009

Well Known Member
I upload most of my flt date to Savvy Analysis so I have a history. My alternator was putting out 13.7 to 13.8V at the end of January. I noticed at the beginning of Feb the voltage dropped to about 12.9V - 13V. My battery was weak and was getting hard to turn over the engine. So I replaced the battery in mid to late Feb. I have a backup alternator (B&C) which bumps the voltage up to 14.1V.

I thought maybe the voltage regulator was going bad. My alternator is 12180N (3-wire 35A). So I replaced the voltage regulator and took it to get tested. The place where I took the alternator to get tested was a place that fixes and builds alternator (I didn't take it to the auto parts store). The guy told me the alternator was putting out 14.2V.

So I'm thinking maybe there is a grounding problem at the battery or engine. Both alternators are bolted onto the engine so it's definitely grounded. The engine must be grounded to the firewall, if not then the backup alternator would have a voltage problem also which it doesn't. The battery, Odyssey PC680, is still strong and easily turns over the engine

I guess the only question is, did the place that tested the alternator put a load on it??? I guess I'll call them tomorrow and ask. If they did put a load on it, does anyone know what the problem could be?

Thanks,
Daren
 
Possibly a faulty connection from the alternator to the battery, corrosion at the alternator connector or in the fuse connector.
 
Alternator

The connection at the B-lead is good and no corrosion. I don't have a contactor or fuse between the B-lead and the main bus, just a 35A breaker on the main bus that the B-lead connects to. Is it possible the 35A breaker is going bad and has some resistance causing the voltage drop?
 
Possibly a faulty connection from the alternator to the battery, corrosion at the alternator connector or in the fuse connector.
 
Yes it is and that size of breaker seems a bit small., which can cause contact problems. What is the rating of the alternator?
 
No corrosion at Alternator terminal

The alternator is 35A. There is no corrosion at the alternator terminal and there is no fuse connector as stated above, only a 35A pop breaker. The lug on the cable at the alternator is solid. I will be checking the connection at the 35A pop breaker to both the bus bar and the alternator cable. If that checks out ok I will be swapping out the 35A breaker with the 35A breaker from the backup alternator. Since the 35A breaker has never popped I doubt my panel is drawing more than 35A. My nav lights are incandescent and I have pitot heat. When I flip them On the voltage drops but breaker doesn't pop. 13 months ago I had the breaker pop and it was due to the rectifier going out. Since the alternator was old I decided to buy a new alternator which was the same one, 12180N 3-wire
 
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If you have not already done so check to make sure the lugs are properly crimped on the cables. I have seen bad crimps cause problems similar to yours.

Good luck,

John
 
The alternator is 35A. There is no corrosion at the alternator terminal and there is no fuse connector as stated above, only a 35A pop breaker. The lug on the cable at the alternator is solid. I will be checking the connection at the 35A pop breaker to both the bus bar and the alternator cable. If that checks out ok I will be swapping out the 35A breaker with the 35A breaker from the backup alternator. Since the 35A breaker has never popped I doubt my panel is drawing more than 35A. My nav lights are incandescent and I have pitot heat. When I flip them On the voltage drops but breaker doesn't pop. 13 months ago I had the breaker pop and it was due to the rectifier going out. Since the alternator was old I decided to buy a new alternator which was the same one, 12180N 3-wire

Still need to check the ground connections from the Alt to the block and from the block to the airframe.

Larry
 
For testing purposes, remove the 35A CB. What happens to the output voltage?

Also, check the condition of the FIELD supply wire terminations and voltage.

And FYI - If the breaker on the B-lead "pops" when the alternator is turning, and energized/enabled, it will fry the rectifiers...
 
Update

So I swapped the B-leads to each others 35A pop breaker to see if the breaker was the problem. Primary alt was the same and the backup alt outputting the same so breaker were ruled out.

So I checked the connector and found it was loose and with a little bit of twisting it can off. I re-crimped it and crimped the other side as well. Voltage is now 13.7V at engine run-up. I also took the voltage at the cigarette lighter while engine is off to compare the voltage the showing on the EFIS. Cig lighter was 12.34V, EFIS was 12.2V.

So there is still about a 0.4-0.5V difference from the alternator and what the EFIS is reading. Could be wire size (I don't the size of the B-lead). I'm not able to test the alternator output at the terminal because I don't have long wires for a voltage meter. Someone mentioned a weak field wire so I don't know what the alternator is actually putting out.

I'll check the engine to firewall ground. Thanks for mentioning the breaker popping will cause the rectifiers to fry. The last alternator popped the breaker. The rectifier was fried but did the breaker pop because of the rectifier fried first? I guess it could be that too but who knows

Question. Someone mentioned grounding alternator to the engine. Isn't the case of alternator already grounded to the engine with the alternator being bolted onto the engine??? Also mentioned the field wire being weak. Will a low voltage of the field wire cause a lower voltage output? Doesn't the field wire turn the alternator On/Off so even if the voltage is a little low but enough to turn the alternator On, is it On 100%?

Thanks
 
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My GRT EIS and EFIS have diodes in each power input so you can have multiple power inputs.

They appear to forward drop .5 to .6 volts as my actual bus from alternator is 14.1 and EFIS voltmeter shows 13.6.

Is this possibly the differences you are seeing? Is there a diode the alternator output passes through on the way to the voltmeter?
 
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Question. Someone mentioned grounding alternator to the engine. Isn't the case of alternator already grounded to the engine with the alternator being bolted onto the engine???
Thanks

That was me. Yes, the alt case to bracket interface usually creates a good ground to the block. However, just as with the ground strap from block to firewall, corrossion can occur and it will increase the resistance. Use Ohm meter. One lead to alt case and the other to engine case. Should read 0 or .1 ohms. Same engine block and firewall.
 
Also mentioned the field wire being weak. Will a low voltage of the field wire cause a lower voltage output? Doesn't the field wire turn the alternator On/Off so even if the voltage is a little low but enough to turn the alternator On, is it On 100%?

Thanks

Yes, you are correct for that (12180N) alternator it is not a "field" wire providing power (volts X amps) but just a voltage sense for turn-on by the regulator. The regulator picks up volts (and Amps) directly (internally) from the B output.

Most automotive are/were for flooded lead acid batteries and proper voltage at the battery terminal would be 14+0.2 volts. So, 14.2 is well within expectation.

Grounding - -would have to be followed alternator-case-battery/airframe. There has to be a substantial battery-case ground wire for the starter.

If the diode is the reason for the EFIS reading offset, then you are good.
 
My GRT EIS and EFIS have diodes in each power input so you can have multiple power inputs.

They appear to forward drop .5 to .6 volts as my actual bus from alternator is 14.1 and EFIS voltmeter shows 13.6.

Is this possibly the differences you are seeing? Is there a diode the alternator output passes through on the way to the voltmeter?

EFIS yes. EIS is not multiple power input unless GRT has changed it since I built. My voltage display does not lead one to believe there is a diode drop before the voltage sensing in the EIS.
 
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