Problem with Alt-out light/annunciator

[mcgeer25]

Initialy rear Alt annunciator on--when both Alt's on line
Found rear Alt operating below spec , ripple current O.K.-made hair line ajustment VR. The rear voltage now at 27.7. Front voltage now at 28.1 Now both alt in voltage spec. Front Alt light is on. Any help as to why front alt light is on?

[Pete Somers]

You have got to the stage where it is time to throw the book away and start again!!

Start the rear eng and run it at 1700 rpm and check the bus voltage, adjust to as near 27.5 as you can. Load it with about 15 - 20 amps for 10 mins, then take it down to a near cruise load and recheck the voltage should be about 27.5 if not adjust.

Do the same on the front.

Run both engines at 1700 and check the bus voltage is ok.

Due to the long run for the alternator cable for the rear eng you might have to readjust the rear VR to balance the system. Make sure all voltage checks are done at 1700 rpm.

Cheers
Pete

[mcgeer25]

Pete, thanks so much for the info. the run up info was new to the mechanic. What we are doing is replacing the front VR(Cessna $160). the old potting compound would not allow for adjustment and the VR looked like it had been there for a long time. Will keep you posted.
Dick

[Guy Paris]

Pete, Curious as to why the 27.5 volts. Guy

[Pete Somers]

I have replied to Guy direct, however here is a copy for your info.

Sorry Guy been out all day.

The Skymaster alternator system is a odd, as the electrical items are
mounted in the front engine bay including the Voltage Regulators,
which leaves a long run from the VR and the busbar for the rear alternator.
Also the alternators on the IO 360 turn slow. This causes problems with the
resistance of the long run of the cables difference from front to rear
engine and the small size wire they used for the BAT and FLD wiring to the
rear.

If you set the voltage to 28.5 using the 1200 RPM they say in the manual at
max RPM you will find that the voltage is quite high.

If you compromise on this and set the volts to 27.5 at 1700 RPM you will
find the volts ok at max rpm and the system more stable.

Some of the older avionics will not like to run at anything above 28.5
volts.

The older Skymasters with the single voltage regulator are more prone to
unstable balance and voltage variation due to age and only having one
regulator controlling two alternators.

Hope this makes sense, I will post it on the web site so others can comment.
on this.

Cheers
Pete

[Guy Paris]

Thanks Pete, appreciate the quick response and will try this. guy

[mcgeer25]

Thanks Pete and Guy, I will share info. with mechanic
Dick