68 Turbo with electrical problem

[cbxmike]

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

[hharney]

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?

[cbxmike]

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.
__________________
Herb R Harney
1968 337C

I'm in the front of the aircraft to prevent it from committing suicide

[hharney]

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

[cbxmike]

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
__________________
Herb R Harney
1968 337C

I'm in the front of the aircraft to prevent it from committing suicide

[B2C2]

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.