Charging system troubleshooting

[rupertjl]

I used the 170 electrical manual and the Cessna 100 series manual to troubleshoot a no charge situation on my A model. Twice now, I've replaced the generator due to stopping at the point when I run the field terminal from the generator to ground and running the engine with no charge showing on the volt meter. First time was with the generator that was installed on the engine when I bought it in 03. The second time and most recent time was with a generator I had laying around that was thought to be good. It ran for about 20 hours and then I had the same condition. So i broke down and got an O/H unit and installed it last night. I went through the same troubleshooting techniques again and have the same condition of no charge. I have flashed the field after each install through the voltage regulator (Arm terminal to Bat terminal on VR). I checked the voltage at the field terminal on the generator last night with the master switch on and I have no voltage at the field terminal on the generator.

Should i have battery voltage at the field terminal on the generator with the master on? And if so, i'm guessing I should start at the battery and check my wiring into the cockpit and back out to the VR.
Fuses have been confirmed to be good and I cannot see any visible problems with the wiring that is within view on the engine side and underneath the instrument panel. I have not gotten up high behind the ignition switch yet to look at wiring.

Any guidance would be helpful. My mech and I are trying to troubleshoot effectively without just throwing money at a new VR.

Thanks!

[rupertjl]

i reviewed my troubleshooting, I have not yet grounded the field terminal of the generator to ground and ran the engine with the new generator installed. I guess I need to do that first. And my assumption that battery voltage at the field terminal with the master on is incorrect, I should only see voltage there if the engine is running correct?

[n3833v]

After installing a rebuilt generator, you need to polarize the fields by touch shorting the battery to armature. To see if the generator will charge, ground the field terminal on the generator while running and it should charge full. The regulator regulates this field system as to how much charge is needed in the battery.

John

[rupertjl]

John, you're suggesting the battery positive terminal to the Arm terminal on the generator instead of doing that through the VR?

[Bruce Fenstermacher]

Confirm all is grounded well. Engine to airframe to battery to VR base. Bad grounds or often missing straps are the culprit.

[n3833v]

You can do it at the regulator if it is hot with the master.

John

[GAHorn]

Rupert, if you have that 170 Electrical service manual you have misread it apparently.

Checking for battery voltage at the field terminal of the gen is not a valid test for several reasons I won't go into here.
Think about this: The gen field terminal...is connected to the reg field terminal...by a SECOND circuit inside the cockpit master switch when that switch is ON. Test for continuity from gen field to reg field with the master sw. on. A simple way to do this is 1) check that the field term of the regulator is grounded with the sw on. 2) check that the resistance from the DISCONNECTED gen field wire to the reg field terminal is zero. Then reconnect that terminal if OK.
REmember, the master sw has TWO circuits inside it: One grounds the battery solenoid (to put the battery online. The SECOND connects the gen field term to the reg field term. A bad master switch circuit will prevent a good gen and good reg from working.

To polarize the system, turn the batt master ON, and momentarily connect the reg ARM terminal to the reg BAT terminal. That's all.

If you connect the gen field term to ground, a properly polarized gen will charge, because you've bypassed the regulator in this test.

The elect. service manual has illustrations in the back showing how to make these tests. (Fig 2 will complete the circuit, bypassing the mstr sw circuit, to confirm whether or not that is the problem.

Any number of things can cause a good reg and good gen to not produce current. Defective wiring, bad fuse holder, failed fuse (don't trust just looking at it...use a volt/ohm meter), regulator whose base is not properly grounded to firewall, firewall not well-grounded to battery AND engine...

ONe of our older members has just spent over $1,000 with his local mechanic (who now advises him to throw it all away and spend another $1400 on an alternator conversion)... when in fact the NEW solid state regulator same mechanic sold him for $300 (and which advertises in catalogs for $150) ... was burned up in an improper polarizing activity. HIs mechanic condemned the new generator.
When he removed the gen as I advised him, and used the "motor" test on it... turns out to be a perfectly good generator. (this test proves that a gen is really a motor. Ground the field term, clamp a good battery gnd to the case, put your foot on the gen which is really gonna JUMP when you connect the POS bat terminal to the gen ARM terminal ...and the generator becomes a MOTOR. Don't let it run but for a moment tho', or you will overspeed the generator.)

(Our friend has now replaced that expensive/smoked solid state regulator with a $50 automotive type and things are charging just fine.)

What amperage gen do you have?

[rupertjl]

Thanks guys,
Update: I flashed the field from the battery positive terminal to the generator arm terminal and then grounded the field terminal on the generator and ran it up. At about 1500 rpm the volt meter finally jumped and showed a charge that was changeable by Throttle movement. No then hooked the field terminal back up to the VR and ran up and the volt meter showed a regulated charged the whole way through static full RPM.

So why wasn't flashing the field through the VR as the troubleshooting guide and the 100 series manual suggests work? Is it more likely I have an intermittent ground issues on the VR?

I have started with a known good battery. I intend to trace wiring and possibly replace the 20 year old VR (Zeftronics).

[rupertjl]

To answer your generator question George, I have the 35 amp generator and the corresponding 1350 Zeftronics VR

[GAHorn]

If your gen field terminal was loose, corroded, or simply not well-connected...and if removing it for the test, then reconnecting it restored it.... then that might explain it. (I always use "star" washers in electrical connections.)

Our Member who just spent over a $1K called me yesterday to report that by following the troubleshooting in the Elect Svcs Manaul he was able to discover his gen was fine. He replaced the $300 solid state regulator he'd been sold (apparently there was no smoke left in it) with a $31 NAPA automotive original style and things started working just fine.

However he is suspicious of his battery solenoid, as he discovered during testing that it sometimes failed to put power onto the buss. He disassembled it (original box style) cleaned the contacts and all is working fine.

[gfeher]

He disassembled it (original box style) cleaned the contacts and all is working fine.

I love that original box style solenoid for that reason. When I did my first annual on my plane a year and a half ago, I needed to install new battery cables. They certainly appeared to be the originals (and no log entry showing they had been changed), as both had many broken strands. Since the solenoid appeared to be original as well (original box style, grungy, and no log entry showing it was a replacement), I bought a Skytec replacement thinking that I should replace the solenoid as well. But after removing it, cleaning years of accumulated crud off the cover, and opening it up, I was shocked to see that it looked brand new inside. I showed it around to the other guys in the shop and they were amazed as well. So it went back on for what I hope to be many more years of service. I'm a believer in that little box.

The Skytec replacement was put to good use as well. About 2 weeks later, a friend getting his private ticket who owns an early C-150 was about to take his check ride. The day before the check ride, his master solenoid fails. He and everyone else in the shop were in a panic, as there was no time to order a new one before the check ride. I suspected that the Skytec replacement I had bought earlier was the same model for his plane. Turns out it was, and after a quick trip home to get it, we had him back in business in an hour. He passed his check ride, and he still owes me a beer for the solenoid. Come to think of it, though, I still owe him a few beers for passing his check ride.

[DaveF]

My airplane still has the original master contactor. It's classic old style machinery, held together with screws, easily disassembled for repair.