Cessna 150 (O200) alternator problem

[Pedals2Paddles]

A few questions troubleshooting an electrical problem. Found the battery dead (Gill 25). Removed, recharged, and load tested it. It passed with flying colors. Ok, super. Now why was it dead.

Engine running, 1700RPM, no lights or other loads on, it holds proper voltage and is charging the battery. As soon as I turn on the beacon or the landing lights, it tanks. Voltage drops well below 13 and shows discharge.

Testing the alternator, obviously with everything off, resistance to ground on the alternator field should apparently be 4 ohms from what I've read. I read 34 ohms. Can anyone confirm or deny the implications of this? With the master on, I get the requisite battery voltage on the S and A terminals of the voltage regulator. This is the extent of what I can test that I know of.

So my question is, does this sound like it's the alternator? The 34 ohms sounds like it to me. If so, is it relatively simple to unbolt and replace? Of course it would need to be signed off by an A&P. But the work, I feel we should be capable of doing ourselves. Is it oiled or dry where it bolts in? Gaskets?

Thanks!

 

[weirdjim]

A few questions troubleshooting an electrical problem. Found the battery dead (Gill 25). Removed, recharged, and load tested it. It passed with flying colors. Ok, super. Now why was it dead.

Engine running, 1700RPM, no lights or other loads on, it holds proper voltage and is charging the battery. As soon as I turn on the beacon or the landing lights, it tanks. Voltage drops well below 13 and shows discharge.

Testing the alternator, obviously with everything off, resistance to ground on the alternator field should apparently be 4 ohms from what I've read. I read 34 ohms. Can anyone confirm or deny the implications of this? With the master on, I get the requisite battery voltage on the S and A terminals of the voltage regulator. This is the extent of what I can test that I know of.

So my question is, does this sound like it's the alternator? The 34 ohms sounds like it to me. If so, is it relatively simple to unbolt and replace? Of course it would need to be signed off by an A&P. But the work, I feel we should be capable of doing ourselves. Is it oiled or dry where it bolts in? Gaskets?

Thanks!

It doesn't MATTER what the alternator field reads. If the battery is fully charged but the bus voltage drops to less than 12 volts with a reasonable load (like the nav lights or beacon) then one of two things is possible ... a high-resistance connection between alternator and battery bus OR a defective alternator. If the alternator output is at 13.6 (or higher) but the battery bus is 12, the answer is connection. If the alternator output drops below 12 with load, it is the alternator.

Gaskets (if necessary) will be supplied with alternator. HOWEVER, if your A&P has an automotive electrical repair shop they like, that is a legal repair if the A&P signs it off as such. And one HELL of a lot cheaper.

Jim

 

[Pedals2Paddles]

It doesn't MATTER what the alternator field reads. If the battery is fully charged but the bus voltage drops to less than 12 volts with a reasonable load (like the nav lights or beacon) then one of two things is possible ... a high-resistance connection between alternator and battery bus OR a defective alternator. If the alternator output is at 13.6 (or higher) but the battery bus is 12, the answer is connection. If the alternator output drops below 12 with load, it is the alternator.

Gaskets (if necessary) will be supplied with alternator. HOWEVER, if your A&P has an automotive electrical repair shop they like, that is a legal repair if the A&P signs it off as such. And one HELL of a lot cheaper.

Jim

With the lights on, the voltage is between 12 and 12.5, and the ammeter shows discharge. Bad news. I think anything less than 13 is bad, as opposed to less than 12.

So it is actually legal to have reputable automobile alternator shop fix it? We were discussing that. It would cost a few hundred bucks instead of $1000 for a new one. We kinda laughed and said that makes too much sense so it must not be allowed. Hell they can probably add a few more windings and crank a few more amps out of it while they're at it

 

[Dan Thomas]

It doesn't MATTER what the alternator field reads.

It sure does. At about four ohms we can get three or more amps through the rotor and make a strong field to generate lots of power at any RPM. At 34 ohms we can get maybe a third of an amp through it, and get a really weak field that's useless for any load.

I've often found oil- or grease-contaminated field brushes. They degrade into a resistant scum on the slip rings and reduce the field current regardless of what the regulator throws at it.

Dan

 

[Pedals2Paddles]

Thanks Dan. I found a lot of sites explaining that it should be 3-4 ohms. And saying what it means if it is open or shorted. But nothing about higher than normal. That basically confirms to me the alternator is toast and it isn't just the regulator.

So this leads me back to having an automotive alternator shop rebuild it for us?

 

[weirdjim]

With the lights on, the voltage is between 12 and 12.5, and the ammeter shows discharge. Bad news. I think anything less than 13 is bad, as opposed to less than 12.

So then what the hell does the resistance of the field winding have to do with it? You've got a bad alternator. Take it to an alternator fixit shop that can assess it and fix it properly. If they ask what it is off of, tell them an airboat or field generator. Tell them airplane and they will tell you to take it to an airplane $hop.

So it is actually legal to have reputable automobile alternator shop fix it? We were discussing that. It would cost a few hundred bucks instead of $1000 for a new one. We kinda laughed and said that makes too much sense so it must not be allowed.

What is "allowed" is between you and your A&P. Tell me, from the outside of the case, can you tell that the alternator shop replaced brushes, bearings, diodes, or other internal parts? Does it work and meet specifications (13.6 to 14.4 volts under maximum load).

Hell they can probably add a few more windings and crank a few more amps out of it while they're at it

Now you have crossed the line between fixit and illegal modifications. Don't do it.

.....

Jim

 

[Pedals2Paddles]

It mattered because we're trying to determine if it is likely the alternator or likely the regulator. One costs $100, one costs $1000. If the 34 ohm reading was meaningless or normal, we'd probably start with the regulator. But if that 34 ohms is indicative of the alternator being shot, we'll go right to that. Dan's post tells me the 34 ohms is indeed indicative of an alternator problem, and the symptoms are in fact exactly what we're experiencing. I think we're definitely going to use a local alternator shop to repair it.

Does anyone have experience with removing it and putting it back? How involved and messy is this? Is there a gasket? Is there oil behind it?

 

[Capt. Geoffrey Thorpe]

Does anyone have experience with removing it and putting it back? How involved and messy is this? Is there a gasket? Is there oil behind it?

At this point, you really want to have the manual in front of you. A brief search will turn up what you need.