The International Cessna^® 170 Association

Hi all,

My alternator is intermittently dropping out. The bus voltage drops from 13.7 in flight to 12, then deteriorates further as the battery discharges. If I turn the master switch off and back on, the voltage comes back to 13.7, without fail. Then a few minutes later it will drop back to 12; lather, rinse repeat. The problem has not shown itself on the ground as yet, only in cruise flight.

I have an O-300A with an Aero Electric (Ford) alternator, and a Zeftronics R15V00 controller.

I first saw this problem last spring, and it only happened once every few hours. The alternator had a lot of hours on it (at least 500 that I am responsible for), so I had it rebuilt with new brushes, bearings, seals and coupling parts (per the applicable TCM Service Bulletin). That didn't solve the problem, but it got a lot worse last weekend -- coincident with the cold weather, but I don't know if the cold had anything to do with it.

The master switch looks like the original Cessna equipment. I've tried fiddling with it to see if it would affect the bus voltage, but no, it's either off or on as far as I can tell.

Has anybody ever seen a problem like this, where the alternator cuts out and comes back after you cycle the master switch?

Thanks for any pointers you can give me!

I'd suspect the switch. On way to test it might be to temporarily remove it from the circuit by connecting the leads together and see if the problem goes away and stays away.

N9149A wrote:I'd suspect the switch. On way to test it might be to temporarily remove it from the circuit by connecting the leads together and see if the problem goes away and stays away.

I'd agree if the switch is DPDT and the alternator field circuit goes through it. If it's SPST, and just controls the master relay, then I don't think it would explain the symptom. I'll have a look at it tomorrow. Thanks!

John

The master switch definitely is (or should be) a DPST, with the master contactor on one pole and the field circuit on the other. If the problem occurs in flight again, try pulling and jiggling to see if it has any effect. I was having a problem with my nav light switch, a very similar switch to the master, but only single pole. I had to wiggle and jiggle and hold my mouth just right to get the nav lights on. I dissassembled, cleaned, and reassembled the switch and had no more problems.

Let us know what you find.

Miles

cessna170bdriver wrote:....Let us know what you find.

Miles

Thanks, Miles. You can count on it!

John

A friend who also has a 170 with your same charging system equipment was having almost the exact same problem. When he charged the battery, the alternator worked fine for a while. As it got some use, the alternator would quit but would fire back up after cycling the master. Googled a troubleshooting guide for the zeftronics regulator and found a problem (can't remember exactly what) with the meter. Just over a 100 bucks for a new regulator from Spruce and bingo, problem gone.

LBPilot82 wrote:A friend who also has a 170 with your same charging system equipment was having almost the exact same problem. When he charged the battery, the alternator worked fine for a while. As it got some use, the alternator would quit but would fire back up after cycling the master. Googled a troubleshooting guide for the zeftronics regulator and found a problem (can't remember exactly what) with the meter. Just over a 100 bucks for a new regulator from Spruce and bingo, problem gone.

That's where I think I'm headed, but I wanted to check out any other possibilities before spending the money. Thanks!

John

Troubleshooting alternator systems, courtesy of Zeftronics:

cessna170bdriver wrote:The master switch definitely is (or should be) a DPST, with the master contactor on one pole and the field circuit on the other....

Of course you're right, Miles, and I would have known that if I had looked at George's excellent book before posting (it's at the hangar).

George, thanks for the documents!

I've been trying to contact Zeftronics tech support, and they called me back this afternoon. Their suggestion was to make sure the connections from the alternator's output to the "A" terminal of the controller are clean and solid. If they're not, they may arc, which causes a voltage spike. The controller's overvoltage circuit will pick that up and shut down the alternator to protect the aircraft's equipment. This sounds very promising to me, so I'll work that angle next chance I get to spend some time in the hangar. It's time to go over all my alternator wiring and clean things up!

Thanks for all the help!

John