The International Cessna^® 170 Association

I have an alternator conversion with an electronic controller (regulator). On one flight the system was working normally on the outbound leg. After stopping for lunch, the engine started just fine but the system would not charge. I switched everything (breakers, switches, etc) off and on a few times which made no difference. So I flew back to Fredericksburg with everything shut off except the digital gauges.

The local shop ran a diagnostic on the system and determined the alternator was not working. All we did was open the cowl and disconnect the wires on the controller for the tests and then reattached them, plus check all wiring and connections to make sure everything was secure. When I started the engine to taxi back to my hangar to remove the alternator, the charging system was working even though we had done nothing to fix it.

The controller had been replaced last year so all the connections were clean and corrosion free at least to the eye. The entire wiring harness has been replaced on the charging system so there is no old original wire left in the charging system and the fuses have all been replaced with Klixon breakers. The alternator and controller were new at engine overhaul a few years back and had never caused any problems. My battery is a Concorde RG-25XC AGM that was replaced in 2009 but showing no signs of being weak, it holds 12.5v between flights.

Now, the engine starts fine and the system charges like it should on the ground taxiing for departure, 14.3v and the charge spikes after start and then tapers off as the battery is topped off. Here's the weird part I can't explain. While flying, the system will still show 14.3v but the charge rate shows anywhere from +2amps to -7 or -8amps. Turning on the incandescent landing lights just makes the discharge more, maybe -17 to-20amps depending on what else is turned on. The voltage stays at 14.3v. The amp meter shows +current into and -out of the battery, not alternator output.

I'm no electrical expert so I am stumped and the several A&Ps I have have talked to haven't come up with anything except the alternator is "weak". Any thoughts?

Probably has something to do with that cobbed up mess of extra relays you have mounted on the left upper firewall
Did you check the connections at the shunt? Seems I recall those EI instrumentation having an option of internal or external shunt. I can almost recall doing some work on your plane in that area. Maybe my fault

So my question would be what is pulling -7 or -8 amps and is the load a steady load. If a steady load then it looks like the charge is intermittent and when charging the alternator is covering the load +2. You would think the voltage would vary a bit though.

So here is the first thing I'd have to answer. The shop disconnected the alternator from the regulator. They tested it and determined it didn't work. How could it magically work then when reconnected? Did the shop actually test it correctly? If they tested it again with the same procedure would the test show the alternator is working now as it seems to be? And if so why couldn't they make it work for the first test? If you trust trust the first test then your alternator has to be intermittent.

Richard the first thing I would do is run a jumper wire from the engine to the negative on the battery and the aircraft to establish a new good ground. Before you do anything. Most of your electrical problems are caused by grounding issues.

Bruce Fenstermacher wrote:So my question would be what is pulling -7 or -8 amps and is the load a steady load. If a steady load then it looks like the charge is intermittent and when charging the alternator is covering the load +2. You would think the voltage would vary a bit though.

So here is the first thing I'd have to answer. The shop disconnected the alternator from the regulator. They tested it and determined it didn't work. How could it magically work then when reconnected? Did the shop actually test it correctly? If they tested it again with the same procedure would the test show the alternator is working now as it seems to be? And if so why couldn't they make it work for the first test? If you trust trust the first test then your alternator has to be intermittent.

Bruce-
With everything running and the alternator turned off, the battery was supplying -15 amps, so I suspect the -7amps is the makeup the battery is supplying with the alternator running. The other thing is that the load as observed on the amp meter did fluctuate considerably from +2 or 3amps to -10 or 12amps. Current was never steady but voltage was.

How could it magically work is the $64,000 question! The shop used the mfgr troubleshooting guide which is the standard test he would have done anyway, this guy understands charging systems. I'm thinking testing it again would be prudent. What makes an alternator go intermittent?

canav8 wrote:Richard the first thing I would do is run a jumper wire from the engine to the negative on the battery and the aircraft to establish a new good ground. Before you do anything. Most of your electrical problems are caused by grounding issues.

Thanks Doug-
Bad ground definitely could be the problem. However, when the plane was restored, we ran dedicated grounds from the battery to the firewall and to the engine. That said, I will go back and check all the terminations and then add another ground and see if it makes any difference.

wingnut wrote:Probably has something to do with that cobbed up mess of extra relays you have mounted on the left upper firewall
Did you check the connections at the shunt? Seems I recall those EI instrumentation having an option of internal or external shunt. I can almost recall doing some work on your plane in that area. Maybe my fault

Yeah, what is all that stuff anyway? I never did get it while you were trying to explain it to me. Glad that stuff wasn't on the Private Pilot written, I would still be trying to pass it.

I'll check all the connections there as well and see if it helps, thanks Del.

hilltop170 wrote:What makes an alternator go intermittent?

Bad diode(s)

High resistance in the master switch or alternator output breaker would cause this kind of problem, but it depends on the converted electrical system design. Do you have a schematic?

What kind of alternator and controller do you have? Does the controller have a separate sense wire or does it sense the bus voltage through the power wire? Where exactly does the voltmeter sense the bus voltage, and where is its ground?

DaveF wrote:High resistance in the master switch or alternator output breaker would cause this kind of problem, but it depends on the converted electrical system design. Do you have a schematic?

What kind of alternator and controller do you have? Does the controller have a separate sense wire or does it sense the bus voltage through the power wire? Where exactly does the voltmeter sense the bus voltage, and where is its ground?

Probably all good questions but I have no idea! Its some STC and I wish I did know but I'm not an electrical kind of guy. I would have to check the records.

Richard, is it an Electronics International volt/amp digital? Does it have a toggle to switch from showing volts and amps?
Edit to clarify; is it a dual purpose gauge?

As brushes wear, the brush-holder springs elongate and apply less pressure upon the brushes to hold them against the slip-rings. Varying RPM may cause the brushes to "float"....so that at idle, the alternator may charge...but at higher RPMs it may not.

Having said that..the act of disconnecting wires and re-connecting them may correct a previously undetected poor connection.

While displaying 14.3 volts....switch ON the landing lights and see if the generated voltage drops over the next few minutes....or not. A drop would indicate a weak system, while a steady voltage would indicate things are fine.

An excellent troubleshooting guide is published by Zeftronics website over in Longview, and that may be your best resource in Texas.

wingnut wrote:Richard, is it an Electronics International volt/amp digital? Does it have a toggle to switch from showing volts and amps?
Edit to clarify; is it a dual purpose gauge?

Yes, it is an EI volt/amp gauge with the toggle switch as you describe. I can either look at volts or amps.

gahorn wrote:As brushes wear, the brush-holder springs elongate and apply less pressure upon the brushes to hold them against the slip-rings. Varying RPM may cause the brushes to "float"....so that at idle, the alternator may charge...but at higher RPMs it may not.

Having said that..the act of disconnecting wires and re-connecting them may correct a previously undetected poor connection.

While displaying 14.3 volts....switch ON the landing lights and see if the generated voltage drops over the next few minutes....or not. A drop would indicate a weak system, while a steady voltage would indicate things are fine.

An excellent troubleshooting guide is published by Zeftronics website over in Longview, and that may be your best resource in Texas.

Thanks George-
Floating brushes sounds like a possibility because it charges steady while on the ground and varies in the air. I'll check the voltage in the air with the landing lights on next time I fly it.

Richard, I had a similar situation with my RV8 alternator. Turned out to be worn bearings causing too much movement of the armature.
Rob

Thanks Rob, another good lead to follow.