Charging Systems
From Bondline
Contents |
The Basics
The battery stores power chemically, the alternator converts the engine's rotating motion into electrical energy, the Voltage Regulator (also known as the "Alternator Control Unit", or ACU) controls the alternator's output to keep the battery charged, but not over charged (which results in boiling out its acid).
For a more detailed explanation of charging system operation and theory, a quick Google search usually does the trick. Rather than rehash the theory here, Rowand.net has a fairly good explanation of how generators and alternators work as well as the regulator.
Troubleshooting
Mechanical Difficulty:
The usual scenario ... Your battery started discharging during your last flight. You shed loads by turning off unneeded equipment until you arrived at the airport and landed normally. Congrats on handling the in flight portion properly. Now how do you get it repaired?
You need to find out exactly which part failed. The usual FBO response is to replace the alternator AND the voltage regulator. They don't have to do much troubleshooting and they sell lots of expensive parts. A Field Rep for Kelly Aerospace, one of the largest OH shops for alternators, came to an FAA Safety meeting and told the attendees that 8 out of 10 alternators returned to them as cores for OH worked when tested before disassembly. He stated that A&P mechanics in the field were NOT doing their job properly, and that better troubleshooting techniques were needed, rather than replacing so many functional alternators.
If you have a Volt-Ohm meter, you can save lots of time and money. Inexpensive voltmeters are widely available from such vendors as Harbor Freight and Radio Shack, often for as little as $10.
To help you know what wire goes where, the maintenance manual has detailed schematics. Gary Vogt has re-drawn the 76-77 and the 78-79 Tiger/Cheetah schematics in a little less technical detail.
All the troubleshooting described below is done with the engine OFF and the keys in your pocket.
Open the cowl and locate the voltage regulator. On Grumman American singles its on the passenger side just inboard of the motor mount attach point, directly above the battery. On AA-5x you'll see a connector on the bottom with several wires. Look closely and you'll see that they are marked with letters on the regulator. On AA-1x there are only 2 wires and they are crimped to the airframe wiring harness without any quick disconnects.
Visually check all of the wires and their crimped fittings. Pull gently on the crimped fittings, none of them should be loose. After 30+ years in service all these wires warrant close examination. Generally inspect the wires for large cracks in the insulation, or, where appropriate, the shielding. Look for breaks in the shielding pigtails.
Turn on the Master switch (both sides if split switch).
AA-5x series 12V
Locate the "S" terminal on the AA-5x regulator. Using your VOM, measure the voltage between the S terminal on the regulator and a good ground such as the battery's negative terminal. The VOM should read close to 12.6 volts on a fully charged battery. In this example, however, your battery is not fully charged after the in-flight failure of the charging system, so a measurement equal to 10 volts or greater is acceptable. If an acceptable voltage is present, then your Master Switch, Master Relay, Alt Field Circuit Breaker and the associated wiring are all working well enough to charge the battery from the alternator. Continue to the next step.
Locate the "F" terminal and measure the voltage just as you did at the "S" terminal. As before, if the measured voltage is within a couple of volts of the nominal battery voltage, then the regulator is correctly trying to "excite" the alternator field so that the alternator can produce an output. Continue to the next step.
Reset the VOM to measure resistance instead of voltage. Remove the "F" wire from the regulator and measure between the wire and ground. You should measure less than 10 ohms of resistance, and usually about 5 ohms. If you have significantly higher resistance (say 20 ohms or more) then there are problems with the alternator field or brushes, or possibly the wiring going to the alternator is in bad condition. A reading of less than 5 ohms, would indicate a short in the Field wire or inside the alternator. Otherwise, since you have the appropriate resistance between the "F" wire and ground, you are through checking the easy stuff, and are ready to pull the nose bowl.
AA-1x series
Locate the "I" terminal wire on the AA-1x regulator. Using your VOM, measure the voltage between the I wire on the regulator and a good ground such as the battery's negative terminal. The VOM should read close to 12.6 volts on a fully charged battery. In this example, however, your battery is not fully charged after the in-flight failure of the charging system, so a measurement equal to 10 volts or greater is acceptable. If an acceptable voltage is present, then your Master Switch, Master Relay, Alt Field Circuit Breaker and the associated wiring are all working well enough to charge the battery from the alternator. Continue to the next step.
Locate the "F" wire and measure the voltage just as you did at the "I" wire. As before, if the measured voltage is within a couple of volts of the nominal battery voltage, then the regulator is correctly trying to "excite" the alternator field so that the alternator can produce an output. Continue to the next step.
Reset the VOM to measure resistance instead of voltage. Disconnect the "F" wire from the regulator and measure between the wire and ground. You should measure less than 10 ohms of resistance, and usually about 5 ohms. If you have significantly higher resistance (say 20 ohms or more) then there are problems with the alternator field or brushes, or possibly the wiring going to the alternator is in bad condition. A reading of less than 5 ohms, would indicate a short in the Field wire or inside the alternator. Otherwise, since you have the appropriate resistance between the "F" wire and ground, you are through checking the easy stuff, and are ready to pull the nose bowl.
Turn OFF the Master at this time
We are through with the powered portion of the testing, so turn off the Master switches.
Right about now you are wishing that you had installed the split nose bowl STC, because without it you have to remove the propeller to gain access to the alternator.
Mechanical Difficulty: 
Up to this point, we were just CHECKING electrical measurements without any actual disassembly of the plane. If you do not have the split nose bowl STC, then this is the point at which you will require A&P supervision to complete the testing, removal and replacement of the alternator, prop, mounting bracket, etc. FAR 43 Appendix A paragraph (c)(19) permits the owner/pilot to remove and install cowlings without A&P supervision, if the prop does not have to be removed, after demonstrating the ability to do it correctly.
On all 2 place and on Travelers before 75, you can check the wiring by removing the screws on the right-hand lower cowling and letting the cowl side tip out from the firewall and nose bowl just enough to enable you to reach under the front cylinder and touch the wires on the back of the alternator. On 75 Travelers with the "Cheetah nose" and on all Cheetahs and Tigers, you have to remove the nose bowl to continue troubleshooting. That's OK, because you have already narrowed the source of the problem to that area anyway.
With the nose bowl off, or at least the lower nose off if you have the split nose, now remove the alternator belt from the pulley. Our testing so far has not ruled out a worn, loose, or missing alternator belt, so make certain that the belt is in good condition and properly tensioned.
The required amount of tension on this belt is often underestimated. The amount of power required to turn the alternator increases as the SQUARE of its output (charging) current! Lycoming's service information was updated a couple years back, and they now suggest setting belt tension by checking to see if the belt will slip before reaching a specified torque. You can check this by putting a socket on the alternator drive pulley nut and setting a torque wrench to 540 in-lb (45 ft-lb), and then holding the prop and trying to turn the alternator (CCW when viewing it from the front) with the torque wrench. The wrench should indicate 540 in-lb BEFORE the pulley slips. The older method mentioned in the Maintenance Manual, no longer favored by Lycoming, was to press in the center of the belt span between the pulleys and measure the deflection. New belts were to have less deflection than those that were worn. Refer to the maintenance manual if you don't have a torque wrench handy.
If the belt itself is in good condition and is tensioned correctly, the next step is to remove the alternator for inspection. First release the tension on the belt. To do this, cut the safety wire on the bolt holding the alternator to the bracket, then loosen the bolt enough so that you are able to push the alternator up, thus relieving the belt tension.
In the next step, you remove the alternator, which is quite heavy. If you have a portable service table or stand, now would be a good time to position it under the alternator. Now remove the big bolt at the bottom of the alternator (the one that holds it to the engine mounting bracket). While supporting the alternator so the wiring is not stressed, remove the bolt that was safety wired. The alternator should slide off the engine mounting bracket and be attached only by the wiring on the back.
Carefully turn the alternator around, and lift the rubber boot that hides the connection of the big wire to the back of the alternator. This is a VERY common failure point on the aging fleet. The wire breaks off the ring lug that is bolted down, and the alternator output is prevented from flowing back to the battery by the broken connection. Look also at the small wire that is bolted to the same connection. This is the "A" or sense wire which tells the Voltage Regulator what the alternator is doing. The sense wire is also a common failure point, though not as often as the larger output wire. Look at the other small wire which attaches to a different post. This is the "F" or field wire. If the resistance checks were normal earlier, then this wire should be fine; nevertheless, after 30+ years in service all these wires warrant close examination. Inspect the wires for cracks in the insulation, or, where appropriate, the shielding. Make sure the shield grounds at both ends of the F and A wires are in good condition and still connected, and check the sheilding pigtails behind the alternator, they can rub on the baffling and get worn through.
If there are no obvious failures in the wiring, you need to mark them and remove them so that the alternator can be tested at a shop. Pep-Boys and AutoZone will check your alternator for free. If you have an AA-5x series, then tell them its from a 71 Ford Pickup with AC. That is the actual original application for the Ford DOFF-10300J alternator on your Traveler/Cheetah/Tiger. AA-1x use a Prestolite brand alternator, and they should have no trouble figuring out the connections.
After the alternator is spun up and bench tested, all but the rarest and intermittent failures will have been diagnosed, and you will be repairing JUST the wiring, or replacing the alternator or the voltage regulator, and NOT spending money on needless repairs.
Prior to installing the alternator, carefully inspect the U shaped mounting bracket for cracks. They are prone to crack across the middle of the U, often into one of the bolt holes. Replacement brackets are available from any Lycoming distributor or from FletchAir.
Replacement Parts
Alternator and Regulator
If the alternator or regulator have failed, you may want to consider a modern replacement than have the old stuff overhauled. Honestly, for about the same price as an overhaul, you can have a new alternator. Plane Power makes a PMA'd alternator that is about 1 lb lighter, and they claim that it makes power at lower RPM. They also make a PMA'd modern solid state voltage regular. Zeftronics is also a popular solid state regulator replacement. If either the alternator or regulator has failed, have the other tested. In some cases, a failure in one of these causes problems in the other.
Plane Power Voltage Regulator Installation
Mechanical Difficulty:
Gil Alexander says: "This is almost a drop in installation - the second difficulty wrench is for the awkward access to the nuts on the aft side of the firewall. It is a change of an airframe component with a PMA part, so a log book entry and sign-off by an A&P is required.
"The Plane Power generic installation instructions overlook the fact that the Grumman firewall may not be flat in the area under the regulator. This is not a big problem, but it does require the use of a few washers under each mounting bolt to act as spacers. Longer mounting bolts are also needed, and I switched to AN4 bolts for this installation. The firewall is scraped down to bare metal at the mounting bolts, and star type lock washers are used to ensure a good ground connection. The electrical leads just have their old blade type connectors cut off and ring terminals crimped on. As shown in the instructions, one lead is not used in our Grumman installations. As with all of the crimp connections on your plane, make sure that high quality terminals are used (AMP PIDG is one good example), this is definitely one area to stay away from the cheap parts from the auto store.
"This new solid-state regulator gives two major benefits over the old one - a built-in, reliable over-voltage protection circuit, and the ability to add an 'Alternator Failed' warning light on the early Grummans. This is not yet connected, but is simply a wire from the negative side of the indicator to the 'Lamp' terminal, and a jumper from the "Enable" terminal to the 'Aux' terminal."
Plane Plastics Battery Box
Original style replacement battery boxes are no longer available. Fletchair sells the Plane Plastics PMA replacement part but several have had questions on how to attach the drain hose to the larger nub on the bottom of that box. You should avoid using metal fittings to attach the hose since battery acid doesn't react well with metals and can corrode and clog the tube.
Bob Steward's Approach
Bob Steward came up with the following method for attaching the hose to the bottom of the box (presented here with his permission).
- Drill a hole in the bottom of the nub sufficient for the drain hose (available from Lowes, Home Depot, Aquarium store) to fit loosely.
- Dip the end of the hose in ABS Cement (blue can @ Lowes in the plumbing department) a couple times to coat it well.
- Paint the ID of the nub with the same cement and let the MEK solvent soften the box.
- Dip tube again and insert in box (from the bottom up) so the tube is almost even with or a little below the bottom of the box.
- Paint more cement around it, such that the nub is filled with the ABS cement so that acid won't pool around the plastic tube and create a standpipe.
- Secure the drain hose such that the box can be left sitting upright for several hours (overnight if the temps are low) as the solvent flashes off.
When the cement has hardened, the pipe will be glued in place, the excess space from the pipe to the nub will be filled with the same material (ABS plastic) that the box was formed from, and a little drill/countersink work will open the tube and smooth the transition from the bottom of the box into the drain.
Mike Boyd's Adaptation of Bob's Method
Given how bad the vinyl tube on my original battery box looked, I wanted the ability to easily replace it on the new box. I was able to find a poly barbed nipple that was small enough to fit in the nub at my local Ace Hardware store (my local Home Depot and Lowes did not stock them). So, here is another option for those wondering how to setup the replacement battery box. I do not know if the hose on my Cheetah is the typical size so adjust the sizes as needed to fit your situation.
What you'll need:
- Plane Plastics PMA'd battery box.
- 1 foot of 3/8" id (1/2" od) vinyl drain tube.
- 3/8" poly or nylon barbed fitting with 1/8" NPT (pipe thread) end.
- ABS plastic cement (not PVC or others, must be a combination of MEK and ABS resin).
- Drill bit sized to allow fitting to be press-fit into box OR 11/32 drill bit and 1/8 NPT tap.
- Hose clamp
The procedure is similar to Bob's approach, but instead of gluing the hose into the nub, you glue the fitting into the nub. For those that like step-by-step and pictures, here's the process:
- Drill hole in bottom of the nub.
- Thread the hole with the tap, should give about 2~3 threads. (optional)
- Screw or press (depending on method chosen) the fitting into the hole in the nub on the box. The fitting should now stay attached to the box on it's own.
- Set the box above some newspaper so it stands up and the drain points down (I found a couple pieces of 2x4 worked well).
- Take the brush in the cap of the ABS glue bottle, let it stop dripping, then place it into the nub on the inside of the box so it applies a coat of glue on the bottom of the nub.
- Let the glue dry for a few minutes and then remove any of the plastic that dripped down into the nipple. Should be able to easily pull the dried plastic out from the bottom. Also remove any ridges that may prevent drainage into the nub.
- Repeat the last two steps until it builds up enough plastic so any water (or acid) will not pool up in the nub but will flow down thru the fitting. The glue should do this naturally once you have applied enough.
- Push one end of the tubing over the fitting and secure with the clamp.
- Place the battery box in the plane and thread the tube thru the drain hole in the cowl.
- Cut the tube as needed so the tube extends past the cowl like the previous one (should be around a couple inches).
You'll need to cut out the slots for the battery cables (I found a Dremmel tool worked OK for this) since the box comes basically unprepared. That's it. You should now have a battery box that will function as intended.
Why you probably need new parts
This picture shows the internal workings of the original Voltage Regulators that were shipped with the 70's Grummans. Lots of mechanical parts with calibrated spring tensions and several electrical contacts to get worn and/or corroded. If you have one of these, you are awaiting a failure (according to Les Staples at AYA2008) - It's definitely time to upgrade to a newer technology all solid-state regulator.
Alternator Brackets
The alternator bracket is a weak part on our Grummans and should be checked every time it is visible with the nose bowl off. This bracket was still in place, but was just held on by the washer under the rear bolt.
Note that a current new Lycoming part has a different number - 07A21443 - and is much thicker - 1/4 inch instead of 3/16 inch.
