Alternator (Ammeter) Problem (Piper)

[Rick Potts]

I replaced the alternator in my Warrior (PA 28-151) this week and the
prior problem (the alternator dropping offline periodically, up until
the time it dropped offline and refused to come back on 200 miles from
home) has been replaced by a new problem.

The good news is the new alternator came online when I started the
Warrior's engine this morning and, according to the ammeter, started
producing electricity.

The bad news is the ammeter needle then started to wiggle back and
forth across the dial like a windshield wiper, from "0" on the left to
about "30" on the right, about two times per second. Varying the
engine speed didn't make a difference in how it behaved.

I measured the battery voltage (at the cigarette outlet) and it
remained steady at 14 to 15 volts. It didn't fluctuate at all, even
though the ammeter needle was dancing all around.

The Piper Service Manual's troubleshooting chart says that "excessive
ammeter fluctuation" can be caused by a defective voltage regulator or
excessive resistance in the field circuit. I hesitate to blame the
voltage regulator when the voltage stays so steady, so is "excessive
resistance" the likely culprit?

For "excessive resistance" the manual recommends: "Check all
connections and wire terminals in field circuit for deterioration such
as loose binding posts, broken wire strands at terminals, etc..
Tighten all connections and replace faulty terminals."

I'm no expert at reading electrical diagrams, but it appears the
"field circuit" would include the voltage regulator, the overvoltage
relay and the alternator switch. The output side would include the
ammeter and a large (6 gauge?) wire that runs to the battery
contactor.

The voltage regulator and the overvoltage relay are mounted up under
the instrument panel, so getting to them won't be easy. The alternator
switch, on the other hand, is quite easy to get to so I think I'll
start there.

Question: How is the split master switch mounted? Does the plastic
cover piece come off the instrument panel with the switch mounting
from the front? Or do I have to remove it from the rear somehow? I'd
like to get in there to clean and tighten whatever connections are
there.

Or is the voltage regulator still a potential culprit?

Thanks for any ideas!

------------------------------------
Rick Potts Phoenix, Arizona
N32334 PA 28-151 Warrior
------------------------------------

[CITABRICKR]

Hi RIck,
Since day one with my Cherokee my load meter has done the same thing. During
this last annual I had my original alternator rebuilt due to radio noise and
was sure I would now for once see a steady needle, didn't happen. For kicks, I
also replaced the master switch, which is a popular "fix" in the Piper circles,
no dice. Being frustrated and not too much of a chore I ran a new shielded
field wire thinking maybe that could be a culprit, no dice. Then, as the
service manual states, I jumped the overvoltage relay and the 5A field CD one
at a time and watched the needle trying to isolate one of those units, guess
what ...no dice. So, the only thing I haven't touched so far is my regulator,
but I have never had a charging problem, and the wiggling needle doesn't bother
me enough to spend the $$$.
The master switch is most easily accessed if your center radio
stack isn't full, or if you can take the bottom most radio out (and it has an
open bottom tray) take the plasic overlay off and you will see 4 short screws,
they hold the whole switch panel in, remove them and then remove the two screws
that hold the master switch in to that, then remove the wires from it (labeling
them, of course!!!) It's a little tight in there so good luck, you may have to
work from under the panel. The voltage regulator and overvoltage relay are on
the left cabin wall under the panel almost where the airspeed/ turn coordinator
live, not hard to get to but you'll be on your back, easiest if you remove the
seats.
By the way, for you anomaly buffs out ther, my radio noise was cured
by installing a new BATTERY. Seems the old one had a partially shorted cell
(starts were never great so I just assumed I had one of "those" Cherokee's) and
the alternator was always trying to feed it and leaking to ground. I'm whisper
silent now and I could taxi on the starter alone if need be. Joy. Hope this
helped somehow for you, take care, John Tremper

[Rick Potts]

citab...@aol.com (CITABRICKR) wrote:

>Since day one with my Cherokee my load meter has done the same thing. During
>this last annual I had my original alternator rebuilt due to radio noise and
>was sure I would now for once see a steady needle, didn't happen. For kicks, I
>also replaced the master switch, which is a popular "fix" in the Piper circles,
>no dice. Being frustrated and not too much of a chore I ran a new shielded
>field wire thinking maybe that could be a culprit, no dice. Then, as the
>service manual states, I jumped the overvoltage relay and the 5A field CD one
>at a time and watched the needle trying to isolate one of those units, guess
>what ...no dice. So, the only thing I haven't touched so far is my regulator,
>but I have never had a charging problem, and the wiggling needle doesn't bother
>me enough to spend the $$$.

A friend and I went out to the airport this morning to try to
diagnose the Warrior's electrical problems based upon everyone's
advice. Here's what we found:

1. We started the engine and watched the ammeter do the "windshield
wiper" dance across the face of the instrument. Not good.

2. Using a small jumper wire, we jumpered across the overvoltage
relay. The ammeter still danced and jerked from 0 to 30. Not good.

3. It was hard to get up behind the master switch, but we finally
managed to jumper across the alternator side of the master switch
using a small jumper wire with alligator clips. The ammeter still
danced and jerked from 0 to 30. Not good. If it isn't the overvoltage
relay or the alternator switch, what could it be?

4. We were stumped. On a whim, we tried jumpering across the
alternator side of the master with a larger diameter wire. Eureka! The
ammeter settled down and was rock steady. (We speculate that the small
diameter wire had too much resistance for the load.)

5. We repeated the test several times, and each time using the jumper
instead of the alternator side of the master switch resulted in the
ammeter immediately settling down and remaining rock steady.
Conclusion: The alternator side of the master switch is the problem.

6. We peeled back the plastic overlay from the front of the panel, and
saturated the switch with contact cleaner, then repeated the process
as best we could from the back. Taking that switch out is going
to be a major project, and we thought a little (okay, a lot!) of
contact cleaner might do the trick. The first fluid flowing out of the
switch looked murky and dirty, so I suspect it had 25 years of junk in
it.

7. Started the engine up and without using any jumper cables, the
ammeter was steady and smooth BUT snapping on the fuel pump would
cause the ammeter to drop offline. Using trial and error, we
discovered that gently easing the electric fuel pump switch into the
ON position wouldn't cause any problems, but snapping it on the way I
usually do it (stabbing it with my forefinger) would cause the
alternator to drop offline 90% of the time.

8. Crawling back under the instrument panel (he's a lot more agile
than I am and by now he had the routine down pat), my friend again
jumpered the overvoltage relay. We started the engine and repeated the
test several times. Each time, so long as the overvoltage relay was
jumpered, the ammeter would stay on line no matter how sharply I
snapped the fuel pump switch into the ON position. Conversely, without
the jumper, the alternator would drop offline nearly every time I
turned the fuel pump on. We believe that (a) the fuel pump switch may
be arcing and throwing just enough noise into the circuit to trip the
overvoltage relay or (b) the overvoltage relay is just a little too
sensitive to whatever electrical noise the fuel pump switch generates
when it is snapped on, and that causes the overvoltage relay to
trip. I understand that overvoltage relays are sensitive to line
noise, and we think there may be something about the fuel pump switch
that is causing sufficient noise to trip the overvoltage relay.

9. The problem now is we don't know which to fix. I suspect the fuel
pump switch is to blame, since snapping the other rocker switches
(landing light, beacon, pitot heat) on and off have absolutely no
effect on the overvoltage relay. Ironically, the overvoltage relay
would be quite easy to change out (it is relatively easy to get to,
using Stan O's advice to remove the pilot seat and lay a pad over the
reinforcing stiffener), but it is a $90 part and I hate to change it
if I don't have to. On the other hand, the $40 fuel pump switch is
going to be a real problem to get out, so it may cost the same to
fix once labor is counted into the equation (and if we have to hire
some someone to change the fuel pump switch, I would have them replace
the master switch at the same time to avoid having to pay for the same
labor again later.)

So what do you think? Is it logical to assume that the fuel pump
switch is now the likely culprit?

[CITABRICKR]

I doubt it is actually the fuel pump switch itself causing the alternator to
drop off. Try jumping the fuel pump circuit breaker first and doing the same
test. Also, before further playing with the master switch, try jumping the 5A
field circuit breaker and see what results you get. I can't believe that your
contact cleaner didn't melt the plastic master switch!! I never even bothered
cleaning the old one, but put a new one in with absolutely no results.

[MikeM]

In article <B+GpOYWCcjmUr6...@4ax.com>,
Rick Potts <rpo...@home.com> wrote:
. . .

>So what do you think? Is it logical to assume that the fuel pump switch
>is now the likely culprit?

Rick,

There have several "swishing ammeters" sagas here on '.owning over
the years. Most of the fixes seem to have been tightening/cleaning
ground connections, and replacing the master switch. IMHO all of these
cases are related, and they have to do with added spurious resistance
between the main bus and the voltage regulator.

The Voltage Regulator (VR) in a Piper is a three terminal device. It
"senses" the voltage between its Input terminal, and its Ground
terminal, and if that voltage is less than 14.2V, it internally connects
its Input terminal to its Field terminal, applying the full bus voltage
to the alternator field.

The Alt field has a resistance of about 6 ohms, so the maximum field
current at 14.2V is about 2.4A. If the VR input voltage is less than
14.2V, there is 2.4A flowing from the main bus, through the alternator
half of the Master switch, through the OverVoltageProtection Module
(OVP), to the Input terminal of the regulator. If the bus voltage is
greater than 14.2V, then the current is only a few mA.

Lets assume that for whatever reason, an 0.1 ohm additional spurious
resistance develops between the Main bus, and the Input terminal on the
regulator. Further, assume that the bus voltage is less than 14.2V. The
regulator sees the low voltage, and switches its Input terminal to its
Field terminal, causing 2.4A to flow through the series 0.1 ohm
resistor. The 0.24V drop across the resistor causes the regulator to
"see" a voltage 0.24V lower than the actual bus voltage. It happly keeps
the field current switched on to the alternator (its doing its job).

The alternator has full field current applied, so it happily cranks out
max output (~60A registered as a pulse on the ammeter), causing the bus
voltage to rise. The battery sees the rising voltage, and draws charging
current. You can think of the battery as a huge capacitor; it wont allow
its voltage to change suddenly, so it takes a 1/4 to 1/2 second for the
bus voltage to increase.

How far does the bus voltage have to go before the VR is satisfied, and
switches off the field? With the additional 0.1 ohm resistance between
the bus and the VR Input terminal, the bus voltage has to go to (14.2 +
0.24)=14.44V. The bus/battery voltage finally makes it to 14.44V, and
the regulator switches off the field current. The alternator output
current goes to zero, and the battery is now carrying the entire system
load (the ammeter reads zero). The battery is able to keep the bus
voltage above 14.2V for just a short while (1/4 second), at which point
the VR senses it, turns on the field back on , and the whole cycle
repeats. The period of the cycling is usually between 0.25 and 1 sec.

In engineering terms, the additional 0.1 ohm resistance between the bus
and the VR has introduced positive feedback into the control loop,
causing it to become unstable (oscillate). Lets carry this analysis a
little further, and assume that the additional resistance is
more like 0.5 ohms. The drop across 0.5 ohms at 2.4A is 1.2V, so now the
bus/battery voltage has to get all the way up to 14.2+1.2=15.4V to cause
the VR to cycle off.

Wired between the Master switch, and the Input terminal on the VR is the
OVP module. It contains a relay, with normally closed contacts, which
disconnects its input from its output whenever the voltage between its
input terminal and its own ground terminal is greater than 15.5 +-0.2V.
With the aforementioned 0.5 ohm additional resistance, the bus voltage
can transiently shoot up high enough to trip the OVP, latching its
internal relay in the open position, cutting off the Voltage regulator,
and causing you to have to recyle the master switch to re-establish
the field.

Note that both of these problems result from the same root cause: too
much resistance in the path between the main bus and the input to the
VR. If you only get the pulsing ammeter, or both the pulsing ammeter and
an occasional tripping of the OVP module is only a matter of degree;
with a little bit of additional resistance, you get the one, and with
more resistance you get both.

Now finally to your question about the tripping of the OVP due to the
fuel pump: It is likely that the root cause (additional resistance
through the master switch contacts, etc) is already biasing the OVP
dangerously close to its trip point, due to the cycling. The switching
transient caused by turning on the fuel pump is enough to send the OVP
over the edge. Likely there is nothing wrong with either the pump
switch, or the pump itself.

Your fix will be to find and eliminate the resistance in the path from
the main bus to the Input terminal of the VR. Dont overlook the contact
resistance of the OVP relay and the Master Switch. The OVP should be
checked to see at what voltage it trips out. If its adjustable, set
it to about 15.5V. Check the VR. It should turn on the field below
14.2V. The with the battery removed from the aircraft, and the master
on, the resistance from the main bus to the Input terminal of the VR
should read "dead short". Anything more than 0.1 ohm must be eliminated.

Any resistance between the case(ground) on the VR (or the OVP) and the
firewall must also be elimated by cleaning the connections, and
retightening the hardware. The connection from Battery minus to
airframe, and from main bus to MAster Relay to Battery plus must also be
clean and tight. Finally, the alternator case must have a low resistance
path to the firewall ground (not just the engine crankcase).

--
MikeM, Skylane '1MM

Sent via Deja.com http://www.deja.com/
Before you buy.

[Jay Honeck]

Wow!
--
Jay Honeck
Iowa City, IA
Warrior N33431

[MikeM]

In article <8ohao7$a1a$1...@sword.avalon.net>,
"Jay Honeck" <hon...@avalon.net> wrote:
> Wow!

Are you impressed that someone on here knows Ohm's Law, and
feedback?

[Bob N]

Remove the cover on the over voltage relay assembly and burnish the
contacts on the relay.

[Jay Honeck]

I'm impressed with the depth, breadth, and variety of knowledge displayed on
this newsgroup, and in your posts in particular.

As a stupid ol' businessman, all I know about electronics is what I read --
and I appreciate your answers.
--
Jay

[jack...@gmail.com]

Had a puzzling week with a 1978 Warrior doing the same thing with Load meter, finally located a chafed Alternator output wire to Primer line which now all adds up.

[Equbal Kalani]

I bet you its gonna turn out to be the voltage regulator. Had similar
experience. Mostly needs just a little adjusment. Good luck.

[rtwilli...@gmail.com]

I'm working on a Piper Cherokee that's ammeter is showing a mild draw and then a drop. It maintains a measurement of 10amps for maybe 10seconds then it drops back off to zero. The pilot removed the alternator took it to get tested then brought it back. The pilot also had the voltage regulator changed out. Anyone had a similar problem? I can answer more questions this was just a brief description.

[Charlie Gibbs]

Speaking as a Cessna driver, I see this routinely (although my meter kicks
up to 20 or 30 amps when I first fire up). The alternator is replenishing
the charge that was drawn from the battery by the starter; the meter settles
back down to zero after a minute or so when the battery is topped up.

However, in a Cessna the ammeter measures how much juice is going into
or out of the battery (it goes positive or negative), whereas in a
Cherokee the meter indicates how much the alternator is putting out,
regardless of what's happening to the battery. It can never give a
negative indication. Upon starting, the meter would show the charge
the alternator is putting back into the battery, in addition to whatever
other electrical loads are on the bus. After the battery is topped up,
the meter should drop to what was required to supply whatever electrical
devices are turned on. If you haven't yet turned on lights, radios,
etc. this would be close to zero. (Sorry, I forgot to closely watch
the ammeter on startup last time I flew a Cherokee.)

Have you tried putting a load on the system? Once the engine is running
and the meter has dropped to zero, try turning on the landing light.
You should see a good indication on the meter then. If not, there's
a problem somewhere. Make sure the landing light isn't burned out,
though. :-) (I recently detected a burned-out landing light in broad
daylight by this lack of response on the meter.)

--
/~\ cgi...@kltpzyxm.invalid (Charlie Gibbs)
\ / I'm really at ac.dekanfrus if you read it the right way.
X Top-posted messages will probably be ignored. See RFC1855.
/ \ Fight low-contrast text in web pages! http://contrastrebellion.com

[billqu...@gmail.com]

Hi,
I have encountered a problem with my PA28 Archer II where the Alt Light comes on and off and showing that it is charging between 13 to 14.8 volts, I have replaced the voltage regulator with a brand new unit it is still doing the same thing, also I tried a test where you by pass the over voltage regulator still the same signs, I have also changed the alternator with a serviceable unit and it also had the same symptoms, any one out there ever had this issue?
any insight would be appreciated

Cheers
Bill Q

[gregr...@gmail.com]

I'm having the exact same problem with my Tomahawk PA-38-112. Replaced the battery, voltage regulator, cleaned all the ground connections, no go..My alternator voltage is reading high (15.5 volts) with a pulsing ammeter..looking for help also..


Thanks Greg