Ammeter Dance on PA-28-140

bmooresbmbmooresbm
in General Discussion
Hello all. I purchased a 1976 PA-28-140 earlier this year and just completed my check ride a couple of weeks back. Recently I started noticing the "ammeter dance". Before this everything was ammeter was always stable. I have read various posts about checking the alternator capacitors and diodes, and potention issues with the alternator field circuit and master switch. The interesting thing about my problem is that when I switch off the battery side of the master, the ammeter reads a nice constant value like it should. We I turn the battery switch back on it starts to dance again. Any ideas or suggestions? I briefly looked under the seat at the battery and master battery relay today. Nothing stood out as an obvious problem but I only had about 10 minutes to looks.

Thanks for the help.

Comments

  • bigandybigandy
    This may sound crazy - but I would suggest you replace your master switch.

    I had the same issue with my Cherokee Six several years ago - that is, with the engine running, and the master switch on, the ammeter would oscillate (wobble).

    The root cause was that the alternator field switch (the second half of the master switch) had got corroded over the years and was now introducing a resistance to the alternator field connection. This then offset the voltage being sent to the voltage regulator, which in turn caused an oscillating circuit. What was happening was that the field winding was getting a slightly lower voltage than normal due to the resistive load of the switch, in turn, the voltage regulator was then increasing the output to the field winding, which in turn causes a higher voltage to be produced on the output of the alternator. The voltage regulator then compensates by lowering the voltage to the winding, and then the output changes accordingly. Since resistance of the switch is not zero, this voltage/current change is not directly proportional to the output voltage of the alternator, and you get this nice ammeter dance as the voltage regulator keeps over compensating with it's adjustments.

    To check this - you can remove the master switch, and then get a good multimeter (with good probes) and attempt to measure the closed resistance of the switch - it should be almost 0 ohms if the switch is good, and it would read a few ohms if the switch is corroded. Another check you can do on the ground (not in flight..) would be to bypass the field side of the master switch with a good piece of wire - this would effectively provide a 'no - corrosion' path.

    Alas - this Piper master switch is not as cheap as the Cessna variant - I think I paid about $50 several years ago.
  • n36601n36601
    I'm not real familiar with the -140, but is your "ammeter" actually a loadmeter? A loadmeter will act exactly as you have described, that is showing the varying load as the electrical system demand changes, and no movement when the alternator is off-line. The most common varying load on the system is strobes. That is, strobes that use flashtubes. If you don't have a loadmeter or strobes, then disregard my entire suggestion. If you actually have an ammeter, then I agree with BigAndy.

    VFRs, without TFRs,
    Dennis_ARR
    Warrior-1.jpg
    1978 PA28-161
    KARR – Aurora, Illinois
  • bmooresbmbmooresbm
    It's definitely an ammeter. When I first bought the plane it was functioning fine. It just recently started doing the dance. I'm not sure that it would be the ALT side of the master switch, since I can leave that on and switch off the battery and everything seems fine. The tells me that there isn't anything wrong with the alternator field circuit.
  • n36601n36601
    I'm confused by your description of what is happening and actual switch positions. How fast is the rise and fall in the ammeter? How does the addition of various loads affect the results? There's nothing connected to the external power receptacle, right? Is your serial number higher or lower than 25000? When you switch OFF the BAT switch is the ammeter needle fully to the left, or centered, when the engine is running?. Is this a 1976 140 or a 1967 140? A bit more information and a description of troubleshooting results is needed here.

    VFRs,
    Dennis-ARR
  • bmooresbmbmooresbm
    It's a 1976 s/n 28-7625143. The flickering is really fast, several times per second. I can also hear the oscillation in my headset. Normally, on runup @ 2000 rpm I see about 15-20 amps. Now it averages the same but bounces +/- 10 amps. If I turn of the battery side of the master during the runup it will read a constant 15 amps. Of course, if I turn of the ALT side then I see 0 amps. I was able to check the resistance across the field circuit breaker and master switch (alt) side and only had about 0.1 ohms.

    Thanks for all the suggestions.
  • n36601n36601
    Here's the wiring diagram for your -140
    mini_-140-SN-7525001-7625234.jpg

    Your "ammeter" although actually called an ammeter in the diagram is really a loadmeter. It shows only the load that is on the alternator at any given point in time. Thus, it reads from zero amps up to whatever the system is rated. An ammeter will show current flow to and from the battery. Therefore, a true ammeter will read something like -60 - 0 - +60 amps. Since yours is wired to the alternator, not the battery, it can only read the load on the alternator which is always positive. Further, your wiring indicates an interlock between the BAT side of the MASTER and the ALT side of the MASTER. This interlock will turn OFF the ALT side any time the BAT side is OFF. However, the ALT side can be independently turned ON or OFF, to energize or disable the alternator. If your interlock is not functioning in that manner, someone has messed with it. It should be returned to the original operation. I am curious about the indications on the ANNUNCIATOR panel. Is the ALT lamp illuminating when you turn OFF the ALT side of the MASTER? It should! It should not illuminate when the BAT side is OFF. With an accurate voltmeter, preferably digital, read the voltage at the battery during run-up. It should be very close to 14.1 volts, with the ALT switch ON. With the ALT switch OFF, it should read 12.5 - 13 volts.

    With the exception of the 15 amp reading, on run-up (which seems impossible) with the BAT switch OFF, the problem that you describe sounds exactly like a battery that is beginning to weaken. You don't mention if the plane has been upgraded to strobe lights, but the oscillation/tone your hearing in the headset would be the strobe power supply cycling with each flash of the strobes. This is easily verified by switching OFF the strobes.

    I would try swapping in a known good battery and note the results. For this test you could use any automotive battery, since you're not going to fly with it.

    My final thought is that somehow your alternator is being energized when the ALT switch is OFF. You could test for voltage at the F terminal of the alternator. It should be zero, when the ALT switch is OFF.

    Here's a link to an excellent tutorial on our charging systems. http://www.nflite.com/ChargingSystem.html

    Good luck!

    VFRs, without TFRs,
    Dennis
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