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  #1  
Unread 11-19-15, 05:32 PM
cbxmike cbxmike is offline
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68 Turbo with electrical problem

I've owned a 68 Turbo for over 13yrs and it has been a reliable and sound aircraft. Recently though, on a trip back from the Bahamas, I noticed a low voltage indication on my digital volt/amp meter. The system could not handle any load other than a radio without the voltage dropping down to batt voltage or below with a heavy load and just slightly above batt voltage with most accessories turned off. I cut the trip short and landed in JAX due to hard IFR up ahead. While in flight I trouble shot some and on either relay 1 or 2 I got the same low voltage problem. While on the ground, at lower rpms, the system discharges. With the alt switches OFF, the discharge isn't as bad but still is in a low voltage condition. Turning the alt switches on causes a higher amp draw in the negative range. At higher rpm I get some charging, but only enough to keep the batt topped off and run one radio. I flew the plane back on a clear day with one radio and transponder running and was able to keep the voltage just above 24v. Turning on even a landing light causes voltage to drop down to around 23v.
I have read several excellent threads on 337 charging issues and will start with my wiring, checking for poor connections and corrosion. I was primarily looking for someone with knowledge of the 68 system to help me on where exactly to start. A lesson on how the system and lights truly work would help as well. I fly larger jets for a living and do have plenty of piston time, but can't completely understand the dual alt, dual regulator relationship.

Thanks, Mike
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  #2  
Unread 11-19-15, 10:13 PM
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hharney hharney is offline
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Hello Mike

I assume you are referring to the Regulators when you indicated the relay 1 & 2?

How is the battery condition? How old? Probably already have addressed this?

Could be regulators, have they ever been replaced?

Do you have a service manual for schematics?

I have seen a lot of posts about poor connections at the bus bar.

Do you have a regular A&P that you work with?
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Unread 11-19-15, 10:46 PM
cbxmike cbxmike is offline
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Herb,

Thanks for responding. Below are some answers to your Q's. Hopefully someone with a similar year as you who has seen symptoms as this can head me in the right direction. Pulling Altenators and throwing big money at this problem is not where I want to start. Checking obvious things like connections is. The bus bar is an excellent place to start.

Hello Mike

I assume you are referring to the Regulators when you indicated the relay 1 & 2?

--Yes, I meant to say Regulators.

How is the battery condition? How old? Probably already have addressed this?

--Battery is less than a year old and is in excellent condition.

Could be regulators, have they ever been replaced?

--Not since I have owned the plane.

Do you have a service manual for schematics?

--I have a service manual but haven't dug into it yet.

I have seen a lot of posts about poor connections at the bus bar.

--That's a great place to start looking for wiring and connection issues.

Do you have a regular A&P that you work with?

--I do but he's slammed right now with work.
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  #4  
Unread 11-20-15, 09:19 AM
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If you are sure about the battery then the next place to start is the charging system. You indicated that the charging system indicated a discharge issue, and you landed and made a second flight with the same condition.

The engines started fine for the second flight? No battery issue after having the discharge indication?

During discharge situation, could you determine if it was both front and rear or did you try to isolate it besides the regulator select? Did you try switching off the alternators one at a time?

Here are some threads that may reveal some insight. Just be careful that the system is the same as yours as Cessna did some changes throughout the years. Your '68 is different than the newer G models

http://www.337skymaster.com/messages...ight=discharge
http://www.337skymaster.com/messages...ight=discharge
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Unread 11-20-15, 11:37 AM
cbxmike cbxmike is offline
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If you are sure about the battery then the next place to start is the charging system. You indicated that the charging system indicated a discharge issue, and you landed and made a second flight with the same condition.

The engines started fine for the second flight? No battery issue after having the discharge indication?

--Yes, Battery was fresh after my initial landing and subsequent VFR flight home only because I shed enough load and only ran one radio. The system would charge around 24.9v but the second I put the taxi or landing lights or any other accessory, the system charging voltage would drop below 24v. After a 2hr flight at 24.9v on one radio, the battery would be topped off again.

--For years prior, I could get 26-27v no problem with either regulator selected. There would be about a .2-.3 difference between the two. Now both regulators are acting the same in a discharged or "weakened" state which from a trouble shooting stand point would kind of eliminate, at least for now, the regulators I would think. What are the chances of them both going out at the same time and acting the same? Now it is as if the system was weakened somehow but still working only at higher rpm's. On the ground just a full discharge. Again, while on the ground if I switched the Alt's off, the amount of discharge would lessen. Turning them on at low RPM caused a higher discharge.

During discharge situation, could you determine if it was both front and rear or did you try to isolate it besides the regulator select? Did you try switching off the alternators one at a time?

--I did and my findings seemed to indicate that both ALT's were acting the same. On the 68 system, do both ALT's get Regulator voltage from one Regulator at a time?

--This is truly strange and may be a diode or wiring issue since I do get charging (a greatly diminished charging voltage only at higher rpm). If the Alt's were shot, would they put out at all? Would worn out brushes cause that or do these Alt's even have the spring style brushes inside like other Alt's?

Here are some threads that may reveal some insight. Just be careful that the system is the same as yours as Cessna did some changes throughout the years. Your '68 is different than the newer G models

http://www.337skymaster.com/messages...ight=discharge
http://www.337skymaster.com/messages...ight=discharge
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  #6  
Unread 11-20-15, 01:06 PM
B2C2 B2C2 is offline
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I think it helps when trouble shooting to think of an alternator as a current controlled voltage source. The output voltage depends on the field excitation current, with a higher current value leading to a higher output voltage. The regulator looks at the output voltage of the alternator and then raises or lowers the field excitation current for the field winding in the alternator to maintain the correct output voltage from the main winding. Since the output voltage is not where it should be, but not zero, and varies with speed, the field windings, field excitation circuits and regulators seem to be working. I have a 1969, On this plane one regulator feeds both alternators. The regulator switch chooses which one. There is a common wire that goes from the buss, through the field circuit breaker, to the regulator selector switch, to the regulator, back to the regulator selector switch, then to the alternator selector switch where it splits to feed the front and back alternators. Since your problem appears to effect both alternators equally, I would suspect a problem of higher than normal resistance in the common leads, meaning check the connection from the buss, through the field circuit breaker, to the regulator selector switch. Also the lead from the regulator selector switch to the alternator switch. High resistance in this common lead will starve the regulator for current, leading to low output voltage from the alternator. The regulator selector switch has two switches, one of which feeds the buss voltage to the regulator, and one of which feeds the regulator output to the alternators. I had a problem with voltage instability that was plaguing me for some time. When I put in a new avionics stack, I had the avionics shop look into it. They did a really smart thing and starting jumpering from the buss bar forward, bypassing components until they saw a stable output result, then replaced all the crimps between the buss and the good result point. I had already had my A&P replace all the switches in an attempt to fix it without a good result. Voltage is now rock solid. If you decide to try this yourself, DO NOT bypass the regulators. This could fry your airplane as the output of the alternators in an uncontrolled state can exceed 60V. Based on my experience if you have an avionics outfit that is good I would take the plane there. The are much better with electrical issues than your typical A&P.
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Unread 11-20-15, 05:27 PM
cbxmike cbxmike is offline
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Thanks B2C2! That's exactly what I was looking for and now have the perfect place to start. Your explanation of the function of the Regulators and their relationship with the Alternators is what I suspected but couldn't put into words. My AI and I have discussed this issue to some degree and our thoughts were the same. We asked ourselves, how does an alternator or better yet both alternators put out some voltage but not be completely failed? And to complicate things when the suggestion is that a regulator must have gone bad yet the symptoms of this problem appear with either regulator selected would point to a commonality between the two regulators.

This may not be the silver bullet to my problem but I'll let you know what we find. Thanks!
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Unread 11-21-15, 11:25 AM
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B2c2

Nice post Ben, thanks for contributing
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  #9  
Unread 11-28-15, 11:21 PM
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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
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  #10  
Unread 05-12-16, 12:02 AM
cbxmike cbxmike is offline
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68 turbo with charging issues

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.
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Unread 05-12-16, 04:40 AM
DrDave DrDave is offline
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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.

Last edited by DrDave : 05-12-16 at 04:53 AM.
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Unread 05-12-16, 06:47 AM
cbxmike cbxmike is offline
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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
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Unread 05-13-16, 12:59 AM
cbxmike cbxmike is offline
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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
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Unread 05-13-16, 01:18 AM
DrDave DrDave is offline
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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

Last edited by DrDave : 05-13-16 at 03:48 AM.
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Unread 05-13-16, 08:24 AM
cbxmike cbxmike is offline
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Dave,

The compass is a great idea. I'll have to explain to my teenagers why the compass app on their smart phones won't work for this.

Yes, I agree to that a dual Alt failure is hard to believe. I typically fly long trips so I guess one could have failed initially and I didn't recognize it. I have the Alt restart button. Correct me if I'm wrong but wasn't that used on older style Alts that did not automatically flash their field when running?

I've heard from another Skymaster Mechanic friend who brought a dozen or so O-2's out of the dessert many years ago that the mating surface for the Altenators can be tricky or uneven due to a poor engine case design in that area and that oil leaks after replacing Altenators can be problematic. Anyone able to confirm this?

Oh yes, I almost forgot to mention that I'm kind of a battery guy and I always put the battery right on a charger after running the engines. I typically keep it on a "maintainer" float charger while sitting in the hanger. It's made for aircraft applications and not something generic. Some believe in these "tenders" and some say no. I've had good luck with them over the years.

Thanks,

Mike
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