Alternator Field Circuit Breaker Tripping.

[Paul462]

Any ideas on how to fix an inappropriately tripping alt field CB would be appreciated in my 1968 T337C, dual alternator installation. During the last hundred hours or so, the alt field CB has taken to tripping occasionally upon landing (after power reduction during the flare), and once during approach (23 in/2400 RPM). In all cases the pulse & strobe lights were on; in one case the pitot heat was on (on approach), in one case they were all on.

We disconnected the HI/LOW volt warning system (its on the same CB); the problem persisted.

The problem occurs with either regulator.

Yesterday I tried load-testing on landing, as follows:

Yesterday I flew N2576S to Flagler, turning off the Front alt/batty switch prior to landing Flagler (then progressively adding load), and turning off the Rear alt/batty switch landing KEVB (New Smyrna Beach), adding load there too. The results:

REAR ALTERNATOR ONLY: Approaching Flagler, Front alt/batty switch OFF, 22 in/2400 RPM: avionics ON, beacon & strobes ON, pulse lights ON – 27.2 volts.
Pitot heat ON: 28v.
Landing & taxi lights on: 27.4v.
20 in/2400 RPM: same
15 in/2400 RPM: 27.5v.
Upon landing, the EDM 760 alarmed @ 23.8v.
Approx. 2 minutes after landing, while taxiing up to the self-serve pumps, the alt field CB tripped.

It took 27 minutes to be able to re-set the alt field CB.

FRONT ALTERNATOR ONLY: Approaching KEVB, Rear alt/batty switch OFF, 22 in/2400 RPM: avionics ON, beacon & strobes ON, pulse lights ON – 27.2 volts.
Pitot heat ON: 26.5v.
Landing & taxi lights on: 27.2v.
20 in/2400 RPM: 26.8v.
15 in/2400 RPM: 25.9v.
Upon landing, the EDM 760 alarmed @ 23.6v.
Turning off the runway after landing the alt field CB tripped.

At one point around when I added the landing/taxi lights it seemed the FRONT alternator voltage was oscillating up and down within a 1v range.

The two alternators appeared to perform in the same ballpark, although the front one landing at KEVB seemed to be a little weaker than the rear one (judging from the voltage it was able to maintain). Is it normal for the voltage to fall that much on landing (to approx. 23.6v)? I’ve been seeing that EDM 760 alarm for a while now.

Any thoughts, other than weak alternator(s)?

[loy53]

Paul,

I had the same problem with the alt field CB tripping .

No. 1 Check all the gound wires

No. 2 I had a loose wire on one of the CB for the alternators.
also while I was trouble shooting the problem I burned up
both regulators.

To solve my problem No. 1 I tightened the loose wire
2 I changed the CB on both alts. & the field CB
also I had to change both reg.
It has been about 20 months ago no problems yet??
hope this helps.

Howard

ps. my 337 is a 1965 model

[Paul462]

Thanks, Howard!

[CO_Skymaster]

Paul,

When I bought my Skymaster over 4 years ago, I keep having the same problem. The rear alternator had failed and the front alternator circuit breaker kept tripping off line. I solved by problem unscrewing, cleaning (with cleaner and steel wool) all the contacts. There are the 3 contact cylinders on the front firewall on the pilots side. I removed all the wires, nuts, and components and cleaned them. I also cleaned the contact on the voltage regulator and overvoltage circuit. After that, my front alternator never went off line. I had the rear replace and have not had the problem since.

Karl

[Paul462]

Thanks, Karl!

For our future reference, I received the following from Tom Carr (Cessna Pilots Assoc., ret'd.) I forgot to tell him we've already replaced the CB:

At 2011-09-09 13:31:10, Tom Carr said:

Paul

From your very detailed description I would go along with your diagnosis of weak alternators but would recommend doing some more checks before just replacing them. As you may or may not know the 1968 337C and T337C had their own charging system design that is different from the Skymasters before and after that model.

Since both regulators trip the breaker I feel the regulators are trying to do their job, holding the voltage to a set point, 27.5-28.0 volts. As more loads is placed on the alternator or alternators the field current is increased to keep the voltage level to that set point. Typically 3 amps to the field of the alternator will give you max output, about 35 amps. A 10% drop of max output is normal once the alternator comes up to normal operating temperatures. I do not see where you are drawing more than 35 amps on a single alternator so the alternator is not at max load. The #20 gage wire feeding that circuit is protected by the 5 amp breaker. I do not see a problem with that configuration since it works quite well on all the other Cessna models. Normally a bad regulator, internal short or field wire short will burn out the regulator before the 5 amp breaker opens, so again I do not see a problem with the regulators. You could have a weak breaker since once a breaker is tripped several times the bi-metallic strip inside will weaken and cause the tripping action to occur before it should. I am guessing that 5 amp breaker is an original unit, 1968, so replacing it would be my recommendation just as preventative maintenance.

A test I would do would be to check the voltage to the input wire of each regulator, pin 1 without the engines running, just the master/alternator switch on. It should be the same as the battery voltage. If not then check across the master/alternator switch to see if there is internal contact problems causing the voltage drop. Not uncommon for those switches to be a high resistance point causing charging system problems. Make sure all the wire connections on those switches are in good shape as well. Select the standby regulator and check to voltage to that regulator as well.

If the voltage reading on the input to the regulator matches the battery reading then check the field terminal on the alternator. It should be very close to battery voltage, reading a couple of tenths of a volt less would be normal. Those checks will confirm the regulator circuitry is in good shape.

Next I would check the voltage to the diode blocks coming from the master contactor. Specifically you want the bottom side of the diodes where wire K-PB53 front engine and wire KPB70 rear engine attach. Again this test is done without the engines running, only the master has to be on. That diode connection point is terminal one on the circuit board. You should be reading battery voltage on those points. If not, look for corrosion on the wire terminals used on the heavy gage feed wire, KPB54 at the bus bar and master contactor. That KPB54 wire Cessna used was aluminum and is very prone to corrosion at the terminal and wire swage point.

The next problem area is the large feed through diodes themselves. The diodes, Cessna part number 85HF20, are designed to allow voltage/current to flow from the alternator to the bus only when the alternator has output. At low engine RPMs a bad diode will block the flow when it should be allowing the flow. To test the diodes requires the respective engine to be operating. Looking at the diode circuit board pin 1 is to the master contactor bus side of the system and pin 3 is the connection to the alternator output. With the respective engine running at idle RPM measure the voltage on pin 1 and use that as the reference value. Measure the voltage on pin 3 and you should see a considerably lower number; 10 volts would not be unusual. Increase the RPM to 1000 and still monitoring pin 3 you should see the voltage come up to the regulator setting, 27.5-28.0 volts. There is typically a .25-.50 voltage drop across the diode. Any more than that number and I would recommend replacing the diode. It has been my experience replacing just one diode is not worth the effort since the diodes must work together. If I had one bad diode both were replaced. That gave me the best long term results.

Up to this point I have not talked about any alternator issues. You can replace regulators and alternators till your bank account runs dry and still will not have a functional charging system if the control circuitry has faults. Correcting the circuit breaker, switches and wiring problems must be done first.

On the CPA web site is Tech Note #50 Alternator and Voltage Regulator, which details the charging systems used in the Cessna single engine models. While that note does not directly apply to your 1968 T337C Skymaster, the operation and test procedures mentioned in that note are the same for the components used in your charging system. I would pay close attention to the figures that show the bad and good alternator diode wave forms. You must have good diodes in the alternators for your system to work.

Also on the Tech Note list, look at Tech Note 337-T04 Skymaster Alternator Drive Couplings, and verify you have functional couplings. A slipping one piece coupling will mask a charging system problem and let you spend a lot of money needlessly trying to resolve the issue.

Hope this helps.

Tom Carr

CPA Tech Staff-Retired

[Jay Lessen]

Paul Once had this happen on a 67 found wire from circuit breaker panel to contactor by pilots leg loose at contactor. checked three planes all were loose. tightened it and no more problems. jay

[hharney]

Some very detailed and valuable information. My hat is off to Tom for his procedure. Back when Pete Somers was around these answers were normally from him. It is nice to have contacts that can help in these situations.

This issue that you are experiencing appears to have plagued several folks that have posted right away. Might be something to put on the agenda for the next SOAPA meeting.

Please let us know what you find. Good luck

[Paul462]

The first item on Tom's list following "replace CB" turned up a voltage drop at the input wire of the regulators compared to the battery voltage - about .1 - .3 volts. We checked the master switches and aluminum No. 6 KPB54 wire, and found the voltage drop to reside in its approx. 31 in. length. The end attached to the main bus had 2/3 of its strands disconnected and frayed at the connector. The swaged connector was loose, and rotated semi-freely about the remaining strands!

We replaced the KPB54 wire with copper (also No. 6), and a jumper on the board for good measure (also No. 6 Cu wire). We then ran the other two multimeter checks Tom suggested which didn't call for engines to be running, and everything checked out.

We're hoping this fixed the problem. I'll let you know after test flight - apparently we broke a P-lead on the rear R mag while multimeter-testing, which we fixed yesterday, then observed oil in the rear engine, which we'll try to troubleshoot tomorrow. It's always something...

[Paul462]

The broken P lead turned out not to be the problem, but rather it was a broken impulse coupling spring in the mag. Had to take off the starter and some baffling to remove the mag. Mag checked out OK - now let's go flying!

After landing, the regulator now maintained voltage at 27.5 - 28 v even with the pitot heat, landing & taxi lights on, and everything else I could turn on, so long as RPMs were around 1300. Hot dog, it may be fixed!

[hharney]

Way to chase it down Paul. Lot's of good help out there and on this post. Copper cables are a good idea. Blue skies