ALT restart batteries

[DrDave]

Learjetter:

I've read through your symptom list. It appears that you have a couple of things in play here. I think we need to establish some baseline items here. I would like to make certain that the front alternator drive coupling is happy. Stick your finger in the fan of the front alternator and see if it turns past the point of medium resistance. We are ruling out that the two rubber pieces aren't mangled.

I'm concerned about the low voltage at 1000 rpm, even considering that it is being read by less than reliable means. Along that line I am curious about the front alternator that will "drop offline." By drop offline do you mean blow the field breaker? I would like to rule out a diode failure in the rectifier bridge. This is a simple test. With the front engine running and the front alternator online with a medium load read the millivolt AC output. Put your meter on the AC scale and select the lowest voltage setting. I'd like to see a number <50mv. You can do your measurement with your test-leads on the battery preferably.

If we pass those two tests we can dig in further to the fun stuff. I have the schematic for the G model. There is only one Alternator system drawing. Let me know what you find from the above two tests.

We will fix this.

Dave

[mshac]

For "one man show" alternator readings, may I suggest you run test leads to the battery from the multimeter that are long enough for you to pass them through the pilot port window, so you can start the engine and take your readings safely from the pilot's seat? You can test the voltage and the AC ripple that DrDave wants to know. Be sure your battery is fully charged before you perform the ripple test (measure AC voltage in a DC circuit). I would perform the tests with both alternators operating, and with each one individually.

[Learjetter]

Originally Posted by DrDave

Learjetter:

I've read through your symptom list. It appears that you have a couple of things in play here. I think we need to establish some baseline items here. I would like to make certain that the front alternator drive coupling is happy. Stick your finger in the fan of the front alternator and see if it turns past the point of medium resistance. We are ruling out that the two rubber pieces aren't mangled.

I'm concerned about the low voltage at 1000 rpm, even considering that it is being read by less than reliable means. Along that line I am curious about the front alternator that will "drop offline." By drop offline do you mean blow the field breaker? I would like to rule out a diode failure in the rectifier bridge. This is a simple test. With the front engine running and the front alternator online with a medium load read the millivolt AC output. Put your meter on the AC scale and select the lowest voltage setting. I'd like to see a number <50mv. You can do your measurement with your test-leads on the battery preferably.

If we pass those two tests we can dig in further to the fun stuff. I have the schematic for the G model. There is only one Alternator system drawing. Let me know what you find from the above two tests.

We will fix this.

Dave

DrDave,
I’ve replaced the old and unhealthy-looking diodes, relays, capacitors and resistors in the front firewall circuit board. Replaced warning annunciator assembly. Replaced some very shady and badly corroded contractors with Sky-tec (battery, starters, ground power). Alternators turned by hand meet resistance in less than 1/8 turn.

When running front engine only, was able to adjust Original Voltage regulator to 27.9v. Rear DGR5-1 VR appears to run at 26.5v. Together: 26.5v. Select either offline after both online, you get 26.5v. New battery voltage is 25.4, and DIS light comes on at 25.9v. All annunciations appear normal. Ammeter still intermittently waves slightly and rythymically Even when beacon off. Have not touched or adjusted overvolt/undervolt circuitry. There is no battery pack in the aircraft so I’ve labeled the ALT RESTART switch as INOP.

I’ll try to remember the AC volt test next session. Thanks for your thoughts.

[DrDave]

Good evening:

You can't like any of that. It sounds like the front engine system is the closest to normal. I would adjust the front regulator to 28.4V. Once it's set there let's see how it handles the load. We also want to see a steady amp meter. If the amp meter is waving at you there is a problem in the field circuit. Let me know what you get and we will diagnose.

One step at a time will win the battle.

Report your findings.

[patrolpilot]

Interesting and timely topic. I recently experienced a significant electrical event after takeoff; I thought I had documented it here but didn't and will add that topic today.

I wanted to mention that as the troubles were corrected, the amp meter continued to have a "hard, full-range cycle. On the ramp, I ran through the Electrical System Ops Check outlined in the POH, and at the end of the successful test, I realized that my amp meter indication had returned to the regular, light, small bumping on the meter.

My battery pack did not pass the Alternator Restart Test, so the batteries are being replaced now while the airplane is in maintenance. I found and purchased the SK337-46A Battery Pack Kit recently. Given what happened to my airplane, my mechanic might replace the existing pack with the kit.

As pilots, we are supposed to perform both the Electrical System Ops Test and Alternator Restart Test at 25-hour intervals. I remember this from attending Cessna Skymaster school in January '78. While in the airplane, the paragraph format in the Manual is tough to follow. I listed them out in checklist form in ForeFlight and also kept it as a document within the app (attached).

From experience, when you push on either electrical test button, push with a clockwise movement on the button. The button is thread onto the shaft of the switch, that shaft is free to rotate. It is impossible to get them too tight, but I have found them missing and on the airplane floor. I haven't experienced the loss since I started adding the rotation with the push.

[DrDave]

Does the regular, small, bumping of the amp meter correspond to any known electrical load?

Dave

[Learjetter]

Originally Posted by DrDave

Good evening:

You can't like any of that. It sounds like the front engine system is the closest to normal. I would adjust the front regulator to 28.4V. Once it's set there let's see how it handles the load. We also want to see a steady amp meter. If the amp meter is waving at you there is a problem in the field circuit. Let me know what you get and we will diagnose.

One step at a time will win the battle.

Report your findings.

Rear alt field circuit to case= 15 ohms. Book value 10-12 ohms. Good diode test on real alternator.. Haven’t check front yet. Forgot about AC volt test, again.

Ran engines separately and adjusted both VR to 28.0v
Running both engines together showed 27.9-28.1v fairly steady, even with a load of full lights. Annunciators appeared to operate normally....but...rear alt out light would blink occasionally. Looking closely at warning board, we determined terminals 1&2 and 6&7 were wired backwards, with terminal 2&6 to cathodes instead of 1&7. Fixed that, re-ran front engine and it shows 30v. Gave up and came home to some 12 year old Glenfiddich and read old Tom Carr posts in the CPA forum.

I think I need to start over, but none of the troubleshooting guides in the service manual match these symptoms.

[Kim Geyer]

don't forget to look at the trip out sensor.
had 2 of our 337s doing stupid electrical stuff and found the sensor had corroded pins.
re pinned the connector and everything good. The other one I replaced the sensor. Its located on the copilots side of the pedestal. little rectangle box about the size of a pack of smokes.

[Learjetter]

Turns out, the wiring diagrams in the Service manual depict the printed circuit board in two places. In one drawing, it shows the circuit one way, and in another, it shows it differently.

When I ignore the drawing on page 4-3, and wired the board as in 4_2_1, magically, the system works. Annunciator lights come on when the switch is off. Adjusted each regulator to 28.0v to give 27.7v (book value with load on). And it all worked. Test flight soon to confirm.

Still have a jittery ammeter, but at this point, I'm willing to live with it until annual when we bring this airplane into the 21st century and install plane power R1224s and an EI Voltmeter/ammeter and pile all the ancient diodes and resistors and capacitors in the middle of the ramp and set fire to them.

That is, if I can convince the local FSDO to play ball and sign off my 337 form.