1969 Cherokee 140 ammeter swings

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RonP

Pre-takeoff checklist
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Started my '69 Cherokee 140 for a flight today. Nothing out of the ordinary for the 20 minute flight to pick up a friend. After starting again the ammeter was swinging like a clock pendulum from 0 to 60 to 0. I cycled the alternator a few times, shut off everything, cycled alternator and still no change. Cycled main power and still no change. Note that with the alternator off the ammeter goes to 0. The rate of the swing was RPM related; low RPM slow rate, high RPM high rate.

I shut everything down and let it sit while I pondered the electrical diagram and my own experience with electrical systems. After about 20 minutes I started it for the short flight back to my home base airport. The ammeter read steady and showed typical current readings when the alternator was loaded. It worked perfectly for the 20 minute flight.

Electronic problems that are intermittent are always hard to troubleshoot compared to hard failures. I "assume" the voltage regulator is the original 1969 mechanical voltage regulator. My armchair diagnosis is the voltage regulator allowed the alternator to go over voltage and the over voltage protection relay kicked in shutting the alternator down. Then the voltage dropped and the alternator output climbed and the over voltage relay kicked in and the cycle continued.

As to what resolved it in the 20 minutes it was shutdown I can only attribute to temperature compensation that is present in the mechanical voltage regulator or a stuck field contact that cooled off and unstuck.

I don't want to wait until a hard failure to occur and want to address it now. Certainly I can relate the symptoms to my mechanic. If he can't diagnose the intermittent problem definitively I would have him replace the voltage regulator with a modern solid state unit and work our way to the alternator.

Anyone seen this problem before and if so what was the solution without shot gunning replacing parts?

Thanks,
 
Might be worn and burned voltage regulator relay contacts. If they stick, the relay won't open easily and the voltage will spike, causing the overcurrent, until the contacts open and the voltage falls and they close and start that all over again.

But that's a semi-wild guess. As far as the regulator having any intentional overcurrent protection in 1969, I don't think so. There would need to be some other component in the charging system to do that, maybe an overvoltage sensor, that has an automatic reset. Cessna's had to be manually reset by turning the alternator off and on again. I can't remember if I saw any overvolt stuff in an old Cherokee.
 
The regulator is likely not seeing the bus voltage accurately. Check for a loose connection at the battery, alternator and regulator.
 
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fixed the same issue on my '69 Cherokee
 
Timbeck2........how much were all the parts shown and installation costs? I like the readout to select between volts and amps. Lots of good info to ensure the charging system is working! Thanks for passing it on.

I have been poking around the internet regarding my issue. One website stated their issue was the alternator switch. Simply spraying contact cleaner in the switch and actuating the switch many, many times cured it. The theory was the switch contact when dirty causes a high resistance path. As current is passed through the contact it creates a hot spot. The contact then thermally bends and the switch acts like the old mechanical turn signal flashers heating up and cooling off a bimetallic element making and breaking contact. May make sense in a way since after a period of time shut down the symptom went away. Could be when I actuate the switch on startup and shut down sometimes the contact is a clean spot, sometimes the high resistance spot. Sounds reasonable on paper but not sure switch contacts act the same way when the contact gets hot as a mechanical turn signal flasher. Would also surprise me if the alternator switch is not sealed where spraying contact cleaner can penetrate to the actual contacts. Seems benign enough to try though.

Another website stated the over voltage relay when it kicks in shuts the alternator field down killing the alternator output. It also latches in the kill position until the master is cycled unlatching the relay. This prevents the alternator from cycling on and off. Given this is how the over voltage relay works my symptoms of the ammeter needle swinging would not be caused by an over voltage condition kicking in and dropping back out by itself.

A loose wire at the battery is easy to check. The regulator and alternator are a little harder to see. Worth a peek though. Not sure how a loose wire fits the symptoms since it worked fine after a shut down period. That still seems to indicate a thermal issue or something getting "unset" due to being idle. However being an electrical engineer and learning early in my career paper theories work perfectly ever time but in the real world weird stuff happens more frequently I can not dismiss any technical theories.
 
Alternator $545
Voltage regulator $195
Volt/Amp gauge $335

Installation would depend upon who is doing it for you and their shop rate. For me, I have an A&P that supervises everything I do and another soon to be A&P that is an electronic wizard who checks/fixes all my work so it cost me nothing but a few breakfasts and/or lunches.


Ryan, I know you are reading this. ;)
 
Following up on this thread with a similar issue. I had an alternator failure in flight a couple weeks ago in my 182. The alternator and voltage regulator were replaced as a result. When I picked it up I noticed some momentary ammeter spikes after startup, but attributed that to the battery still being weak. Flew again last night and noticed that the ammeter surges at low RPM. It will spike and then return to zero (or slightly above) in a rhythmic pattern every 2 seconds or so. I noticed that the cockpit lighting would brighten with the spike. This only seems to happen at low RPMs. As the RPMs increase, the surge slows in frequency and then completely stops above about 1600 RPM. Above that point, everything seems normal. I'm going to talk to the shop that did the replacement, but wanted to ask here for any troubleshooting advice. Thanks.
 
I also trouble shot the same issue recently, and it's easier than I thought. I found an article that helped a lot on the Cessna pilots subscription site, but also used an article from hartzell. Here are basic steps.

1. Check grounds starting from the battery, using a voltmeter to register voltage drop rather than measure resistance: about .2v is maximum drop. You should do this strap by strap especially and include the voltage regulator and the alternator.
2. Turn on master, check that the voltage regulator sees the same voltage as the battery voltage.
3 with Master on, check that the field wire on the alternator is roughly 1-2 v less than the battery voltage.

If all those check out, it's probably your alternator. If not, it could be the grounds or other working,voltage or the regulator.

The alternator should also be magnetized when the field wire is energized by the master switch, so testing that with a hacksaw blade to feel the pull with the master on and off is also a good idea after the above.

Hope that helps.
 
Thanks. I'll give that a shot.

The shop that replaced the alternator two weeks ago told me they thought this was interference from the aeroflasher beacon when they did the work (Unfortunately, they aren't located at my home base; I had to divert into a different field when the previous alternator quit). So I tested that theory this evening. At initial startup, everything was completely normal, even with the beacon on. So I started loading up the electrical system - avionics, nav lights, cockpit lighting, etc. Everything was still fine until I added some bigger draw items. Both flap movement and the pitot heat caused the surging to reoccur. It's been a while since my last EE class in college, but now I'm wondering if the alternator is having problems with the electrical load at low RPM.
 
Now that I see you replaced the alternator, i actually suspect the vr...

But starting with the battery and working your way forward as per the above shouldn't take more than an hour, and you will have a definitive answer.
 
I usually start with a visual inspection into the rear of the alternator case where the diode block is. Then check the connections at the v/r, wiring goes bad and causes issues there too. Ohm out the wires between the v/r and the alternator next. When you get to the time to swap parts--the V/r is cheaper and the solid state ones have solved a lot of issues for me in the past. Finally, the alternator should get a disassembly and good visual inspection every 500hrs anyway, so if it hasn't been done now is your chance. For the belt-driven 12v ones, a good local auto shop can usually test them for you...

An overhauled alternator can run $500, but a new set of brushes and a good cleaning could be less than $20.
 
Thanks for the inputs. The alternator was overhauled two weeks ago (just before install), and the voltage regulator is a brand new Plane Power solid state type installed with the alternator. Possible that I got defective parts, but I'm going to look more closely at the wiring and the install. Appreciate it.
 
You can isolate it better than just checking connections. You need to take some voltage measurements while the thing is doing its funny stuff. I still have a special tool I made a long time ago: a piece of plywood about ten inches long and a couple of inches wide, with four screws in it. To each of the screws is soldered one end of a long wire, and at the other end of the wire there are alligator clips or just bare, twisted wire. The four screws are labelled ground, battery +, field #1 (alt) and field #2 (reg). The ground wire goes to a good ground point on the firewall. The field #1 goes to the F terminal on the regulator. The field #2 goes to the field terminal on the alternator. Battery goes directly to the positive terminal of the battery. You could add a fifth screw and label it Alt Output, and attach its wire to the fat terminal on the alternator.

The wires are taped to the side of the fuselage out of the way. The plywood sits in your lap and you use a voltmeter to take measurements while the alternator is charging and/or acting up. The battery wire should show a bit under 13 volts when the engine is dead, and around 14 volts when the alternator is running. If the voltage changes a lot with RPM, or it drops badly with a heavy load, the alternator might be bad, depending on what the F1/F2 wires tell you. With a steady load, the field voltage should fall when the RPM increases, and rise when it slows. At a steady RPM, the field voltage should rise with added load and fall with load removal. If it does all that, the regulator is OK. If there's a voltage difference between the two field wires, you have a faulty connection or corroded wire somewhere along that field path between the regulator and alternator. Common, that is.

Now, if that battery voltage falls badly with added load, as mentioned earlier, and the voltage at the field rises and falls properly, that alternator probably has a problem, and that, most likely, is contaminated or worn field brushes. A "new" or overhauled alternator is actually more prone to that. Manufacturers and overhaulers, even big, well-known outfits with "Aerospace" in their names, have this quality control problem. They put far too much grease in the rear alternator bearing, and when the alternator is assembled and the shaft enters the bearing, that grease squeezes out and when the rotor spins the grease flies around and ends up on the slip rings and brushes, forming a tough black sludge that forms a big resistance to the field current flow. Low field current means low alternator output. The regulator will raise the field voltage to its limit to try to compensate, but high field voltage means nothing if the current can't get to the field.

If the ammeter is flicking back and forth, you should see battery voltage doing the same. If the battery voltage is steady, I'd suspect a tired ammeter. If the ammeter is flicking along with battery voltage, and the field voltage is steady at both field wires, it's not the regulator. Might be the alternator output filter, normally found on the firewall, or a bad connection somewhere between the alternator and bus. If you have the fifth screw, and there's a voltage differential between the alternator output and the battery, there's a bad connection somewhere along the alternator-bus-battery path, or a bad ground. If the field voltages are jumping along with the ammeter, it might be the regulator, but it might also be the regulator's sense wire from the bus or alternator output stud. Some systems simply tied the S terminal to the A terminal and fired both with the alternator switch. Cessna's ALT switch controls the S terminal and the A goes to the alternator output. Piper often ran the A to the bus. If that A terminal doesn't get a good, solid reference voltage, the ammeter will reflect it. The regulator uses the A voltage to determine what to do with the field voltage. If the A terminal is connected to some breaker on the bus that feeds a fluctuating load like a strobe, any resistance between the breaker and bus is going to make the voltage fluctuate on that A wire and the regulator is going to go nuts trying to correct it, and the ammeter will flicker.
 
Had the master switch replaced today which may solve my problem. I originally posted that twice the ammeter swung like a pendulum after start up and cycling the alternator and master switch did not correct it. Only shutting down for 15 or so minutes resolved it. It only happened at the second startup of the day after either taxiing to the fuel pump or flying 30 minutes for fuel. One more piece of info I left out. The swing after start when it pendulums goes from 0 to 40 and slowly decreases to 0 to 30, 0 to 20 finally swinging between 0 and 10 (the ammeter normally sits around 10 when it is not swinging). It also happened about 5 flights apart from the first time it happened to the second time. Once resolved by the shut down it is good for the rest of the day.

I took the old master switch apart and found the contacts were not shiny but black on the alternator side. The contacts are about 1/8" diameter and carboned pretty bad. With my finger nail I could scrape off the carbon revealing shiny copper. The battery side still had a few shiny copper spots.

I would theorize not having a clean resistive contact would be a little higher resistance which can create a hot spot or voltage drop causing a false voltage reading by the voltage regulator. Why just the second start up and not every time I can't explain. Will not know for sure if this solved the ammeter pendulum problem until it either happens again or never happens again. If it does then next stop is the voltage regulator.
 
Not super helpful, but just want to throw out that the voltage regulators are probably the most frustrating thing for me on the Seminole. I'm not exaggerating when I say that one alternator will indicate 0 probably at least twice a week on most of our Seminoles. It's not uncommon for us to squawk the airplane, maintenance adjust the voltage regulator, and then have the airplane be squawked again that same day--multiple times in one week. I have no clue if your issue is the same, but just figured I'd toss this out there.
 
rt4388 said:
Not super helpful, but just want to throw out that the voltage regulators are probably the most frustrating thing for me on the Seminole. I'm not exaggerating when I say that one alternator will indicate 0 probably at least twice a week on most of our Seminoles. It's not uncommon for us to squawk the airplane, maintenance adjust the voltage regulator, and then have the airplane be squawked again that same day--multiple times in one week. I have no clue if your issue is the same, but just figured I'd toss this out there.
Click to expand...
There's a paralleling wire between the two regulators to force them to split the loads evenly between the alternators. That wire might be loose.
 
RonP said:
Had the master switch replaced today which may solve my problem. I originally posted that twice the ammeter swung like a pendulum after start up and cycling the alternator and master switch did not correct it. Only shutting down for 15 or so minutes resolved it. It only happened at the second startup of the day after either taxiing to the fuel pump or flying 30 minutes for fuel. One more piece of info I left out. The swing after start when it pendulums goes from 0 to 40 and slowly decreases to 0 to 30, 0 to 20 finally swinging between 0 and 10 (the ammeter normally sits around 10 when it is not swinging). It also happened about 5 flights apart from the first time it happened to the second time. Once resolved by the shut down it is good for the rest of the day.

I took the old master switch apart and found the contacts were not shiny but black on the alternator side. The contacts are about 1/8" diameter and carboned pretty bad. With my finger nail I could scrape off the carbon revealing shiny copper. The battery side still had a few shiny copper spots.

I would theorize not having a clean resistive contact would be a little higher resistance which can create a hot spot or voltage drop causing a false voltage reading by the voltage regulator. Why just the second start up and not every time I can't explain. Will not know for sure if this solved the ammeter pendulum problem until it either happens again or never happens again. If it does then next stop is the voltage regulator.
Click to expand...

I never had the pendulum effect but an intermittent problem solved by replacing the master switch as you did.
 
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