[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