Cherokee 140 and the dancing ammeter

[Fredd Baber]

Good afternoon,

I just completed a VERY long cross country in my Cherokee 140 and had some serious issues with my charging system. On Leg 1, the ammeter suddenly went to zero. After months of it dancing back and forth, I thought the spring had finally given up and the gauge had died. After about an hour and all was looking good, my number one nav/comm went black, followed quickly by my transponder and then the T&B. Before my nav/comm 2 went out, I declared, turn back to an airport I'd just passed and landed without incident. Since this was a Sunday, no maintenance.
The next morning, the A&P arrived and looked it over, tested and recharged the battery, tested the alternator, master contactor and the battery master contactor. A little cleaning and checking the voltage regulator seemed to reveal the issue was that the VR was set to 11.7 volts, far too low. He adjusted it and it seemed to solve the problem. Leg 2, I took off to resume my trip and 45 minutes later the ammeter went straight to zero. No dancing, just straight to zero. I flew it with as low a load as I could (one radio and the transponder) and stopped at the end of leg 2. The mechanic there tested it and said it appeared to be the alternator. He jumped it, I started Leg 3, kept the load low again but this time the needle stayed dead center in the gauge for nearly 2.5 hours. Sunset was coming so I found a place to land and stay the night. The next morning, the plane started like normal (I'm assuming that since the needle was dead center it was charging as per usual) but this final leg 4 the needle swung from side to side (O-60) for the entire flight.

Does anyone have a suggestion as to where to start? VR or alternator? Let the debate begin. Thanks all!

 

[Bob Noel]

Caveat: I'm not an A&P, nor did I stay at a Holiday Inn Express last night

a bad master switch was the root cause of a dancing ammeter in my Cheroke 140.

Put in a new master switch and the ammeter went back to being nice and stable.

however, there are plenty of other possible causes for a dancing ammeter

 

[Bell206]

Does anyone have a suggestion as to where to start? VR or alternator? Let the debate begin. Thanks all!

There are a number of existing PoA threads on this topic if you search. Regardless, I would recommend that your entire charging system to include all the wires and grounds be checked prior to throwing more parts at it. It sounds more like a loose connection or bad ground.

 

[Dan Thomas]

First thing I always did: pull the field wire off the regulator and put an ohmmeter between that wire and ground. You should see 3 to 5 ohms. Turn the prop back and forth a bit (with the sparkplugs disconnected!) and watch the ohmmeter. It should remain fairly solid at 3- 5 ohms. I bet you'll find it really jumpy and nowhere near the range. Maybe just no reading at all. It's worn alternator brushes, 99% of the time, from a lack of 500-hour inspections.

 

[Gary Ward]

I just completed a VERY long cross country in my Cherokee 140 and had some serious issues with my charging system. On Leg 1, the ammeter suddenly went to zero. After months of it dancing back and forth, I thought the spring had finally given up and the gauge had died. After about an hour and all was looking good, my number one nav/comm went black, followed quickly by my transponder and then the T&B. Before my nav/comm 2 went out, I declared, turn back to an airport I'd just passed and landed without incident. Since this was a Sunday, no maintenance.
The next morning, the A&P arrived and looked it over, tested and recharged the battery, tested the alternator, master contactor and the battery master contactor. A little cleaning and checking the voltage regulator seemed to reveal the issue was that the VR was set to 11.7 volts, far too low. He adjusted it and it seemed to solve the problem. Leg 2, I took off to resume my trip and 45 minutes later the ammeter went straight to zero. No dancing, just straight to zero. I flew it with as low a load as I could (one radio and the transponder) and stopped at the end of leg 2. The mechanic there tested it and said it appeared to be the alternator. He jumped it, I started Leg 3, kept the load low again but this time the needle stayed dead center in the gauge for nearly 2.5 hours. Sunset was coming so I found a place to land and stay the night. The next morning, the plane started like normal (I'm assuming that since the needle was dead center it was charging as per usual) but this final leg 4 the needle swung from side to side (O-60) for the entire flight.

Does anyone have a suggestion as to where to start? VR or alternator? Let the debate begin. Thanks all!

I think this is a good read...http://www.radioaviation.com/cherokee_docs/Electrical_System.pdf

 

[Chilito]

Caveat: I'm not an A&P, nor did I stay at a Holiday Inn Express last night

a bad master switch was the root cause of a dancing ammeter in my Cheroke 140.

Put in a new master switch and the ammeter went back to being nice and stable.

however, there are plenty of other possible causes for a dancing ammeter

This happened to me. Ammeter showed a frequent intermittent charge/discharge until one day it just stopped charging at all. Turned out to be one side of the push/pull master switch which powers the S terminal on the regulator had failed after 53 years of service. Switch replaced with a new toggle (they don’t make the push/pull anymore) and it’s charging normally again. Worth a try.

 

[Gary Ward]

First thing I always did: pull the field wire off the regulator and put an ohmmeter between that wire and ground. You should see 3 to 5 ohms. Turn the prop back and forth a bit (with the sparkplugs disconnected!) and watch the ohmmeter. It should remain fairly solid at 3- 5 ohms. I bet you'll find it really jumpy and nowhere near the range. Maybe just no reading at all. It's worn alternator brushes, 99% of the time, from a lack of 500-hour inspections.

Where is the regulator located on Cherokee 140? TIA

 

[Fredd Baber]

Where is the regulator located on Cherokee 140? TIA

Unfortunately in the least accessible part on the plane…under the pilots side panel, against the firewall. You really need to remove the front seats to access it and work on it.

 

[Gary Ward]

Unfortunately in the least accessible part on the plane…under the pilots side panel, against the firewall. You really need to remove the front seats to access it and work on it.

Thank you,

My buddies amp meter has been dancing around for a year or more. Apparently his mechanic is not concerned about it. I fly his plane sometimes and I am concerned about it. I am leaning towards the master switch as the problem. Can you jumper that switch to prove the switch is the problem. I would like to check the brushes in the alternator also if I can get to the regulator as per Dan Thomas suggestion.

 

[Dan Thomas]

You don't have to get at the regulator if it's in a bad spot. Just go to the other end of the field wire, at the alternator, and disconnect it from the stud, and put the ohmmeter on the stud and ground.

If an ammeter has been bouncing for a year and many hours it isn't likely the brushes. They wouldn't normally last that long. Old airplanes are full of oxidized switches and breakers and crimp terminals, and finding the problem gets to be a pain. I made up a simple diagnostic tool: a piece of plywood a couple of inches wide and a foot long with four screws run partway into it. To each screw was soldered a long wire. One of those wires gets connected to the field terminal on the regulator or alternator (F terminal). Another goes to the alternator output. A third goes to the regulator's sense wire (A terminal) and the last one goes to ground. The wires are long enough to reach the pilot's seat, and I'd sit there, run the engine, and take voltage readings at those screws, between the ground terminal and the others. It helped to isolate the problem. If nothing showed up, I'd move the wires to some different spots like the alternator switch or S terminal on the regulator, the bus, and so on, and run it again.

But the person doing the testing MUST know how the system works. Without that, it's just guessing and throwing parts at the problem.

It might even be the regulator's voltage limiter contacts. They're arcing all the time, and they get old and rough and can start sticking together, making the voltage rise further before the coil pulls them open. That makes the voltage jump around, which of course makes the current flow erratic, and the ammeter sees that. The run test, checking the field voltage, will find that, but you need an analog voltmeter. The digitals don't respond fast enough. And a small analog works best, with its small, light needle.

 

[mondtster]

My buddies amp meter has been dancing around for a year or more. Apparently his mechanic is not concerned about it.

If I were your mechanic I likely wouldn't be concerned either. The "amp meter" you are referring to is a load meter. If there are strobes or any other sort of intermittent load in the system it is totally normal for the load meter to "dance".

 

[Gary Ward]

If I were your mechanic I likely wouldn't be concerned either. The "amp meter" you are referring to is a load meter. If there are strobes or any other sort of intermittent load in the system it is totally normal for the load meter to "dance".

Thank you both for the tech.

He flew 68 hours in the past year, annual to annual. It does jump around more when he has the beacon light on, he has no strobes. It is always moving some with only the panel load on it.

We have watched the voltage while the load meter is moving around and it stays pretty steady at 13.8 volts while it is running. But not rock solid, maybe 13.5-13.9? volts. But I was using digital DVM. I do have small analog volt meter I can use on it. Earlier this year is maybe when it started, all I have done is used a DVM on a terminal to insure it is charging. We have flown it together quite a bit since then but don't want to wait until other more pressing problem turns up. His mechanic is in Cleveland and is a personal child hood friend of his for 40 years now. He has been a AP/IA for that long. I helped them both do his annual this year but we didn't address the load meter but he has talked to him on the phone about it several times.

Maybe nothing is wrong? MY 172 load meter is pretty steady with only a very slight movement when the older original beacon flashes. Maybe they are not the same? Is the "amp meter" in my 172 the same setup as his? He probably has a older regulator and I have electronic regulator and a fresh alternator. His alternator is older also.

Sorry for hijack Fredd hopefully it helps you also.

 

[Dan Thomas]

Maybe nothing is wrong? MY 172 load meter is pretty steady with only a very slight movement when the older original beacon flashes. Maybe they are not the same?

Cessna's incandescent beacon system has a big power resistor in its circuit. The beacon's power supply shunts that resistor in and out of the circuit. It either flip-flops between the bulb and resistor, or they're in series and it shorts across the resistor to cut in it and out of the circuit, a method that would minimize the thermal swings of the bulb. I never did analyze that system to see what it did, but at any rate, it means that the current flow remains more constant than if they just switched the bulb on and off.

 

[geneseib]

Does anyone have a suggestion as to where to start? VR or alternator? Let the debate begin. Thanks all!

After chasing an intermittent charging problem in my 180 for over a year, it turned out to be a high resistance connection on the back of a breaker. Finally confirmed it by finding the connection to be hot after a flight.

 

[mondtster]

Is the "amp meter" in my 172 the same setup as his? He probably has a older regulator and I have electronic regulator and a fresh alternator. His alternator is older also.

The 172s I have been around all had a more traditional Amp meter that shows a charge and discharge indication rather than a load meter. The devices are really showing two different things. The traditional amp meters would likely not show much of an indication either side of zero unless something was really wrong.

If the load meter in your friend’s plane only recently started flickering (and there have been no electrical additions that would cause it) that may indicate a problem. I’d consider investigating further but there may be nothing wrong too. It’s hard to say without being the mechanic looking at the airplane.

 

[Gary Ward]

The 172s I have been around all had a more traditional Amp meter that shows a charge and discharge indication rather than a load meter. The devices are really showing two different things. The traditional amp meters would likely not show much of an indication either side of zero unless something was really wrong.

If the load meter in your friend’s plane only recently started flickering (and there have been no electrical additions that would cause it) that may indicate a problem. I’d consider investigating further but there may be nothing wrong too. It’s hard to say without being the mechanic looking at the airplane.

Thank you for your expertise, that make sense to me. If I find anything out about the Cherokee I will post it.

Dan Thomas I do have/remember that large resister in the rear under the inspection cover at the base of the vertical stabilizer. Thanks for your help.

 

[Fredd Baber]

You don't have to get at the regulator if it's in a bad spot. Just go to the other end of the field wire, at the alternator, and disconnect it from the stud, and put the ohmmeter on the stud and ground.

If an ammeter has been bouncing for a year and many hours it isn't likely the brushes. They wouldn't normally last that long. Old airplanes are full of oxidized switches and breakers and crimp terminals, and finding the problem gets to be a pain. I made up a simple diagnostic tool: a piece of plywood a couple of inches wide and a foot long with four screws run partway into it. To each screw was soldered a long wire. One of those wires gets connected to the field terminal on the regulator or alternator (F terminal). Another goes to the alternator output. A third goes to the regulator's sense wire (A terminal) and the last one goes to ground. The wires are long enough to reach the pilot's seat, and I'd sit there, run the engine, and take voltage readings at those screws, between the ground terminal and the others. It helped to isolate the problem. If nothing showed up, I'd move the wires to some different spots like the alternator switch or S terminal on the regulator, the bus, and so on, and run it again.

But the person doing the testing MUST know how the system works. Without that, it's just guessing and throwing parts at the problem.

It might even be the regulator's voltage limiter contacts. They're arcing all the time, and they get old and rough and can start sticking together, making the voltage rise further before the coil pulls them open. That makes the voltage jump around, which of course makes the current flow erratic, and the ammeter sees that. The run test, checking the field voltage, will find that, but you need an analog voltmeter. The digitals don't respond fast enough. And a small analog works best, with its small, light needle.

You don't have to get at the regulator if it's in a bad spot. Just go to the other end of the field wire, at the alternator, and disconnect it from the stud, and put the ohmmeter on the stud and ground.

If an ammeter has been bouncing for a year and many hours it isn't likely the brushes. They wouldn't normally last that long. Old airplanes are full of oxidized switches and breakers and crimp terminals, and finding the problem gets to be a pain. I made up a simple diagnostic tool: a piece of plywood a couple of inches wide and a foot long with four screws run partway into it. To each screw was soldered a long wire. One of those wires gets connected to the field terminal on the regulator or alternator (F terminal). Another goes to the alternator output. A third goes to the regulator's sense wire (A terminal) and the last one goes to ground. The wires are long enough to reach the pilot's seat, and I'd sit there, run the engine, and take voltage readings at those screws, between the ground terminal and the others. It helped to isolate the problem. If nothing showed up, I'd move the wires to some different spots like the alternator switch or S terminal on the regulator, the bus, and so on, and run it again.

But the person doing the testing MUST know how the system works. Without that, it's just guessing and throwing parts at the problem.

It might even be the regulator's voltage limiter contacts. They're arcing all the time, and they get old and rough and can start sticking together, making the voltage rise further before the coil pulls them open. That makes the voltage jump around, which of course makes the current flow erratic, and the ammeter sees that. The run test, checking the field voltage, will find that, but you need an analog voltmeter. The digitals don't respond fast enough. And a small analog works best, with its small, light needle.

Dan Thomas…can I drill holes in a board and use butt end connectors to connect the wires to the screws rather than solder? Just make sure the connector is metal to metal or will the board cause a problem? I like idea of being able to sit in the seat with the needle dancing and being able to see if that is going back to the battery like that. Also, the VR appears brand new.

 

[Dan Thomas]

Dan Thomas…can I drill holes in a board and use butt end connectors to connect the wires to the screws rather than solder? Just make sure the connector is metal to metal or will the board cause a problem? I like idea of being able to sit in the seat with the needle dancing and being able to see if that is going back to the battery like that. Also, the VR appears brand new.

Soldering the wires to the screws is better. Butt connectors don't do well trying to grip something hard and solid like a screw. If the plywood was thick enough you could clamp the wire between washers on the screw.

 

[Fredd Baber]

Soldering the wires to the screws is better. Butt connectors don't do well trying to grip something hard and solid like a screw. If the plywood was thick enough you could clamp the wire between washers on the screw.

That’s what I was thinking…use a 1x4 with 4 holes drilled in it, put the bolt/screw through the hole, tighten with a nut, washer, butt connector, washer, then tighten with another nut to make it secure. Then it’s a matter of connecting the alligator clamp to the opposite end. Use different colored wires and mark as you instructed and test away. Question is, my VR appears sealed. No screws or anything to attach the alligator clamp to…just three wires (one of which is true ground). How do I connect to the A lead as you mentioned?

 

[Dan Thomas]

That’s what I was thinking…use a 1x4 with 4 holes drilled in it, put the bolt/screw through the hole, tighten with a nut, washer, butt connector, washer, then tighten with another nut to make it secure. Then it’s a matter of connecting the alligator clamp to the opposite end. Use different colored wires and mark as you instructed and test away. Question is, my VR appears sealed. No screws or anything to attach the alligator clamp to…just three wires (one of which is true ground). How do I connect to the A lead as you mentioned?

Are there push-on connectors on the VR wires? Sometimes you have to make short adapter wires to fit between the airplane's wires and the VR, with a tap for the meter. If you can't get at the A wire at the regulator, you can use the bus or the alternator's output terminal; that A wire is just sensing system voltage anyway, as well as providing the current for the field.

Alligator clips don't work well on the alternator. Vibration shakes them off. You need to clamp the wire under the nut/washer on the terminal.


If you get it all hooked up, you should see a field voltage that varies as you turn stuff on and off. Turning landing lights on is a good load, and the field voltage should rise some. Load it up, see what the field voltage is, and run the RPM up and down; it should drop with increased RPM and rise as you reduce it. It will always be somewhat less than system voltage, sometimes only three or four volts if there's not much on and the alternator is just topping up the battery. Checking the bus or alternator output (A) you should see a pretty steady reading no matter what, unless you have a whole lot of stuff on and you bring it down to a low idle. Might see some drop there.

 

[TrueCourse]

There are lots of ideas and several very experienced A&Ps giving good suggestions for the OP. The last 140 I flew that had a similar issue that turned out to be loose or frayed ground wire to the landing light.

The load meter vs ammeter phraseology is one of those things that always comes up in these discussions. Two different devices with two different ways of working or purposes. Ammeter has the zero in the middle, load meter has the zero all the way to the left. I guess both can dance!

 

[RonP]

The ammeter in my Cherokee 140 was swinging like a pendulum from 0 to 60 constantly. A&P cleaned the terminals and replaced the master switch. Was good for the next few short flights and then the pendulum returned. The voltage regulator was solid state and the A&P replaced it with another solid state regulator and it had been about 109 hours and never had the problem again. I opened up the ok’d solid state regulator and it had the smell of burned components although nothing was melted or charred.