pulsating instrument lights...

[atsiii]

I have a 1969 T337D. At night I get a high frequency pulsating of the instrument panel lights. I recently installed two new voltage regulators but it did not help. The pulsing of the lights occurs with either voltage regulator selected, and either or both alternators selected... full load or minimum load. Can anyone offer any insights?

Thanks, Allen

[skymstr02]

Try turning off your flashing beacon, and see if the pulsating goes away. This happened when I had a C-150 (same system). The power supply should have a dummy load resistor on the opposite end from the flash head, or two flash heads installed.

Dave
O-2A
68-11055

[Bob Cook]

Allen
if you replaced the regulators make sure they are the correct ones. there are two different regulator voltages depending on mfgr of alternator.

suggest replacing the diodes and especially capacitors. they are 450 - 500 mfd electrolytics. Also consider replacing the smaller diodes located same place. It very well may be the electrolytics.

1. is the current and voltage matched for each engine?
/
2 if you reduce load does the flickering go away?

3 what is the current being drawn at night?

you might want to consider an EI volt/ammeter with shunts. this makes it much easier to monitor what is going on with the voltage regulators.

make sure all you connections are tight. any resistance will cause problems.

there has been lots written on this subject on the previous board.


fyi

bob

[stackj]

Atsiii----

I just resolved a problem with the same symptoms you described with my 1967 C337B. I am going to paste text from a note I sent to a friend of mine below describing the issue.

If you want a little easier to read schematic (in my opinion, for what it's worth) go to the technical data page of this web site. Look under "Mitchell Instrument Cluster Replacement in 1967 C337". There is a link under that heading titled "Electrical System Schematic (PCX format). It is a redrawn diagram of the 38 amp Ford alternator system used in the earlier Skymasters. It applies to serial numbers 337-0526 through 337-0755.

Even if the schematic does not apply to your bird, the principle of the problem might.

Here is the description of what I did from the other note:

I spent a little time with our ’37 yesterday and a couple of days before that. I replaced several switches on the pilot’s sub-panel. Jenny had problems with the left (front) fuel boost pump being intermittent. I traced the problem down to the switch, and ordered replacements for both left and right. I was going to replace all the switches in the alternator field circuit while I was there but couldn’t locate all of them. I replaced the Run/Stby and Regulator Select switches, but it turned out the ‘GEN’ switch (Which turns the alternators on and off) was the bad one. I didn’t have a new one of those, but I was lucky. The switch has a split rocker and is actually two single pole double throw switches built into one. It only used the single throw function, so I was able to invert the switch, swap the alternator field wires to the opposite end and everything worked out fine. It eliminated electrical charging system oscillations I have had since buying the airplane. I’ll still try to get a replacement switch, but probably don’t really need to.

While I had the switch out of the panel, I measured the resistance of the contacts which had been used in the field circuit and found the resistance to fluctuate between 0.3 and 0.6 ohms. The unused contacts measured 0 ohms. I also disassembled and inspected the switch. The contacts which had been used were extrebely burned and pitted (lots of soot on them). The unused ones looked brand new. (Don't disassemble the switch unless you know what you are doing. They are not very complicated inside but can be very hard to impossible to reassemble if you spring the outer tabs too far apart.)

Hope this helps you out.

Jim

[Mark Hislop]

Allen:

I have had this problem, and from what I've heard, it is not uncommon. There are at least two "generic" causes. One is that the regulators are swapping the load back and forth, and the other is resistance in the field circuits which causes the regulators to "hunt"

I had both problems. My regulators were boucing back and forth, and not sharing the load equally. Many people maintain that keeping the electrical system in good shape (as detailed by Bob Cook) will keep the system working properly, and probably they are right. The other thing you can do is get a Form 337 from CPA that will allow you to replace the existing voltage regulators with regulators from 400series Cessnas that are truly "paralleled". I have done this on mine (as well as replacing the diodes and capacitors) and my alternators have shared the load flawlessly for the last 4 years.

If you turn off one alternator (thus there is no question of the regulators bouncing back and forth between each other) and the lights still flicker, your problem is probably one of some extra resistance in the field circuit. This causes the regulator to constantly hunt, and the voltage changes are what causes the lights to flicker. Check any connections in the master or field circuitry, and check the master and alternator switches. You may have to put new connectors on the wire ends. The least little bit of resistance can cause this problem. I had to change out my master switch, even though a quality digital ohm meter showed it was good.

Mark