Electrical Problems

[Hyflyer86]

Hi Guys:

I recently had a local shop change the front alternator on my '73 P337 due to a fault light. When they did the test run, still had the light; the gear doors opened; the OAT/Tach CB popped; the warning horn was sounding. I took it back to the garage. I am an A&P but never worked on a 337. I just bought it a month ago.

First the gear doors; scary. I couldn't find a faulty wire or circuit in the front engine compartment or NLG well. I went to the wiring diagram and looked for logic that would allow the power pack to start and the door to open. I disconnected the #4 terminal block wires in the NLG well. That did it. I suspected a shorted diode in the circuit but I could not find the diode. Anybody??

I pulled the dual warning and over voltage sense boxes. Warning box looks good. Over voltage box had shorted diode and burned trace. I replaced/fixed those. I re-installed both but did not power system up yet. I suspect that will fix the alternator light. The warning horn is real and I suspect that a gear switch is tripped.

Back to the electrical schematics to look for the popped CB on the OAT/Tach lines. I know the mechanic removed the mags to get the alternator out. I think that he may have connected the mag tach wire to the alternator instead of the tach points? I have not been able to ascertain that connection at this time. I am open to suggestions where to look for that gremlin.

Bill N86C
Hyflyer86

[Skymaster337B]

Try a new battery. Even if voltage is good it could have weak cells.

[hharney]

sounds like you need a different mechanic

[Hyflyer86]

Yes, I do! This is the first time that I have farmed out any maintenance on any airplane, for this very reason. What is particularly bad is that I don't know what he really did so that I could easily make the corrections. On the other hand, it will give me the opportunity to learn the aircraft very quickly. I would like to fly it though, now and then!

Hyflyer86

[Gord Tessier]

They are an easy fix. There is a micro switch on top of the throttle linkage on both engines. It's attached to a silver roundish cam. Simply undo the little screws and move the switch so it hits the flat spot on the cam at the desired place. Then tighten it down.

[Hyflyer86]

Thanks Gord:

Switches are ok. The warning box is messed up. I am working on it now. Thanks for responding. I appreciate it.

Hyflyer86
Bill

[JamesC]

If the switches are OK then consider thoroughly cleaning up the banana plug type connector in the wiring just before switch. If it is corroded/coated with oxidation then the resistance can be high enough that the gear horn can sound.

[Hyflyer86]

Thanks James:

Good advice. I did look at the connections and they look ok. I pulled the warning box and it has issues that I am fixing at the moment. I still have a popping CB on the "oil dilution" circuit. I have changed the CB to no avail. I can't seem to find the short in that maze of wiring.

Thanks for the help. I really appreciate it.

Bill; Hyflyer86

[DrDave]

On my 67 B model that is the QB1 wire. It comes from a 2 amp breaker. Without the oil dilution system the wire capped off. I believe that breaker also powers some instruments.

Is there anything that doesn't work in the panel?
Do you actually have the oil dilution switch/valve?

This is an easy circuit to trace. The bundle from the circuit breaker panel goes forward and turns around the edge of the lower panel. It joins up with the bundle coming to/from the front engine. The bundle is right behind the lower switch panel. I would start by identifying/tracing the QB1 wire. I don't recall what other wires are hooked to the load side of that breaker. I think it's the engine gauges. It's worth looking close at that area. I've included a pic of the back of my engine inst. panel that I just finished cleaning up the wiring. The spade terminals can get loose and cause problems.

A methodical approach will identify the problem. I'm not excited about just throwing parts at problems like this.

Dave

[Hyflyer86]

Thanks Dave:

On my P model the cb is labeled "oil dil , tach, oat". The tach and oat both work even with the cb popped so I think that only the oil dil is connected. I changed the cb just to eliminate that but no joy. I don't believe that I have oil dil on the plane. I have tried to trace the lone wire from the cb but cant ID it thru he firewall. It has to go to the aux pump and I am looking there now.

I have also jacked the a/c and believe the landing gear switch is faulty. Waiting for switch to arrive. That should fix the gear problem. We are gaining on it. Thanks for the post. Much appreciated.

Hyflyer86
Bill N 86C

[DrDave]

Okay, so someone has gotten creative behind the panel. I would tend to disregard the breaker labeling in this case. The wire is going somewhere. You only have two and a half options for the popping breaker. There is either an excessive load to whatever it's hooked to now or there is an intermittent short. The half option is a bad breaker that trips all by itself. That is a much less common situation. You should also be on the lookout for something that was disconnected and the wire left dangling.

Dave

[Hyflyer86]

Hi Guys:

Well, I have solved my alternator problems. After changing the front alternator, I still had warning lights. I pulled the warning/control box and replaced a diode. I calibrated the control voltage for front and rear alts. I replaced a bad wire on the firewall unit. All good.

After the mechanic changed the alt for me and powered up the system, the gear doors opened, and the warning horn sounds when you turn on the master. I have changed a couple of components in the warning box and the throttle switch on the front engine which was failed. Still horned.

I have jacked the plane, checked all up/down switches, squat switch, landing gear selector switch, and the up and down lock circuits. They are all good. Cannot get the doors closed. Could swing gear by putting power on the solenoid and operating the master but the gear would not lock up or down due to tripping breaker.(p-switch not in circuit with hot wire). I could lock the gear down with the hand pump.

Now, off the jacks. Not getting power to the solenoid through the pressure switch. Tracing the wire from the gear ind cb and lose it in the large bundle behind the panel. No continuity from cb to solenoid, p-sw at powerpack plug or ind lites, including Press to test. My LAN Cable tracer doesn't seem to work either. It loses power just after the connection at the cb. I think I am at wits end due to brain size.

Can anyone offer trouble shooting help? How about the contact info for an expert to come and solve it? Southern Oklahoma.

Bill Hyflyer86

[Red Air Rambo]

Hi Bill,
My 73 P model was having some electrical issues and it turned out to be a bad contactor. We ended up replacing all three due to age. I would check the voltage coming through the battery cable to the contactor too.

Brent

[Hyflyer86]

Thanks for the reply Brent. I do have power at the LG contactor but no power to activate it when commanded. I will check all of the contactors on my 73 -P too. Thanks again.

Bill Hyflyer86

[Red Air Rambo]

Just to clarify I was talking about the three main contactors located on the firewall beneath the pilots side cowl. There is a battery, front, and rear. In my case the battery contactor went south causing all kinds of strangeness.