68 Turbo with electrical problem

[hharney]

Nice post Ben, thanks for contributing

[DrDave]

Greetings:

I am just finishing a wiring upgrade on a 172. Everything was "working fine." I found 426mV of voltage drop in the field circuit. High voltage drops/resistence in the field circuit cause the voltage regulator to be unstable, aka nervous needle. This does not include the 0.8-1.0 volt drop across the voltage regulator. I replaced the 1968 circuit breaker and replaced the regulator with a new Zeftronics unit. The old VR charged at 27.7V. The Zeftronics unit will charge at 28.8V. I replaced all the wiring in the field circuit. This includes the breaker to master switch, master switch to OVR, OVR to VR, VR to alternator. I found high resistance/voltage drop across the master switch. The total system voltage drop after the repairs is 125mV.

I then moved on to the alternator output circuit. I found a couple of questionable connections. I replaced the alternator to alternator breaker wire. The alternator breaker had a higher than expected voltage drop. It was also a 1968 breaker. After the new breaker was installed I chased out the charge wire that goes off the bus to the amp gauge and back to the battery.

Surprise time! The photo below shows the terminal connection at the amp gauge. There were no symptoms of failure evident prior to this discovery. I built a new cable to replace the cooked cable and terminal. The last wire to be replaced runs from the amp gauge to the line side of the starter solenoid. This wire completes the charge circuit back to the battery.

It is important to consider that the field circuit has nine or more connections. The alternator output can have ten connections before it gets back to the battery.

A good diagnostic procedure is to draw the system out in a block diagram. The field excitation/starter/charging system is not magic nor is it complicated. There is a series of connections that have to be clean and tight. You can have component failure. A little understanding of the system and a thorough inspection will reveal most of the common culprits. Having a meter to do voltage measurements and voltage drop tests is essential.

Dave

[cbxmike]

I recently had a mechanic with superior skills (superior to mine) that I know and trust look at my charging problems. He did find corrosion on the main bus bar behind the CB's and cleaned and repaired. Replacing the bus bar is probably the next action. In fact the alt field CB, I think that's what it's called, was not making good contact. We thought we had possibly found the smoking gun so to speak. After cleaning and correcting the bus issues we ran both engines and not getting any charge now from either Alt on either VR. Just battery voltage. Oddly enough, the alternators only have about 300hrs on them. So we checked the voltage through the alt switches. That was good. Then the voltage to both VR,s. That was good (batt volt). So the next was the field voltage at each Alt. That was the same (batt voltage). My question is this: Is it more likely to have two failed VR's or two failed Alt's? Or better stated, is it possible that the VR's are both bad causing no output from the Running alternators even though we get (batt voltage) from each Alt field test? Does getting batt voltage at the alt field yet no output from the alternator while running confirm bad alternators? I know, that was more than one question.

[DrDave]

Okay, it's time to dig deep into the bag of diagnostic tricks. One thing that we want to know is whether the alternator field is actually energized. For this we are going to perform the wrench trick. Take a small (3") steel wrench and balance it on top of the alternator. Have a friend turn the alternator/master switch on. You should see the wrench move. The wrench moves when the field is excited in the alternator creating an electromagnetic field. If the wrench moves it is ready to charge. Twice I worked on charging systems where it turned out the the alternator wasn't turning. In one instance the rotor shaft was broken. The other alternator had sheared the coupling.

In the field circuit there are multiple connections. The field circuit connects bus bar - field breaker - alternator switch - regulator switch - over volt relay - voltage regulator - alternator. There can be a bad connection at any point along the way. The best way to diagnose the circuit from the bus bar to the VR is using a volt meter on millivolts between the bus bar and the battery lead of the voltage regulator. Do this with the entire circuit hooked up and energized. A number around 100mv. is acceptable. Check the voltage drop from the field lead of the VR to the field terminal on the alternator as well.

If all of that checks out you are down to checking the output circuit of the alternator. You want to check voltage drop across the alternator circuit breaker and related wiring from the alternator to the bus.

If all the wiring checks out and the wrench trick makes the wrench move you are down to looking closely to the VR

Your system may differ slightly from mine but the basic idea will be the same.

Report your findings.

[cbxmike]

Thanks for all that. I will keep you posted on what we find. The "wrench check" makes sense. I also may have access to a known, properly working VR. We may swap it into my system to check.

Mike

[cbxmike]

Let me just start by thanking everyone for assisting me in trouble shooting this issue. The recommendations have been spot on and it's been a great learning experience. As a former Air Force avionics tech who worked on F105's and F4 Phantoms many years ago, to becoming an Army Warrant Officer flying and working on all kinds of aircraft both fixed wing and Rotorcraft, to now Captaining large Boeing aircraft for a major Airline, I can honestly say; I am humbled by the simple charging system of my 68' T337. To own an aircraft for as many years and not truly understand the heart and soul of the charging system is somewhat embarrassing. But when it works great and kinda lasts a long time, that's a testament to Cessna and the engineers back then with their slide rules.

Ok, tonight we made some testing progress but are still not getting any charging from either Alt on either VR. Based on the arrangement of my Turbo to include extra hoses for an aftermarket oil screen system, the ability to balance a wrench on top of either alternator was not fesible. We started back at the main bus and the Alt Field CB. The main bus where this CB is mounted was damaged and the screw whole was cracked through on one side. We had noticed this before and cleaned up the bus and used a washer to better support the CB. Today we removed the original CB and bench checked it. It seemed solid until light vibrations or jarring occurred, then some resistance was noted. I purchased a new 5amp CB from a Cessna dealer and it tested rock solid. We also added an extra length of aluminum where the cracked bus was to beafen up the area. Removing and replacing the bus will occur in a couple of months during the annual inspection. We were hanging our hopes on the bad Alt Field CB being the heart of my dual charging issue. After running both engines, we still got no charge. So we started checking line voltages and came up with the following:

--All the switches were cleaned and then checked. No apparent voltage loss through them.
--Charging circuit diodes both checked good. 0.4 ohms in one direction and open the other way. At least these seems like reasonable diode results and they were identical.
--The voltage at either VR input was exactly the same as Batt voltage. No apparent loss of voltage through the switches and harness to the VR. The output at either VR was approx 1.5 volts lower than Batt voltage. Not sure if that is a normal reading.
--We checked the coil or field resistance from each Alt and noted both around 16.5 ohms. But when the prop was moved, the readings would jump anywhere from 0-200 ohms. Either engine. Always settling back to around 16 ohms when movement stopped. This allowed us to believe neither shaft was sheared but curious why the readings fluctuated so under slight rotation. I can only surmise that worn brushes making weak contact on a rotating surface would cause such fluctuations.

I will have a new VR on hand in a few days and after test running with it, we will be stuck pulling Altenators if that doesn't change the results. Unless we are still missing something else. If not, I would take suggestions on sourcing of Altenators.

Thanks,

Mike

[DrDave]

Mike:

Good job of getting into this. Thank you for the excellent operative report. You can substitute a compass for the small wrench. I'm not convinced that you have a double alternator failure, that would be quite rare. Do you have a run/standby switch in your model?

It is normal to have a one volt drop across the VR. Keep at it. You're getting close. At this point in time it would be a good idea to charge the battery. We don't want the alternators to go full tilt when they come back on line. I neglected to mention that I took the Alternator System page of the electrical manual and had it blown up to 20" x 30". This makes the job much easier to trace out all the wiring.

Dave