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Can you find my missing 9 volts?

Rosie

Rosie

Well Known Member
My Whelen strobe pack quit working last week, so I checked voltage at the pack; 12.5v (battery voltage). Sent the pack to Whelen and it bench tested OK.

Further testing last night finally revealed that I had 12.5v at the switch output, and ~3.5v at the pack (unloaded, not connected to a borrowed pack). A bypass wire from the buss to the pack was used; strobes worked fine, all good.

Voltage (3.5v) was tested with voltmeter 'red' on the positive strobe wire (at the pack), 'black' grounded to the aircraft (Note: ground wire from the panel buzzed out OK).

I'm yet to buzz the positive wire from the panel switch to the pack, so in the mean time....where is my missing 9v? :confused: Rosie
 
Switches do go bad. It is easy when they fail in an always open or closed condition. Sometimes, they fail with burned internal contacts which causes a high internal resistance and a consequent voltage drop.

Do you have inline splices or terminal strips in the strobe wire power feed? These are additional areas to look.
 
Connectivity

Bad switch? Are you sure you were on the switch output (with the switch in the on position) when you measured the 12.5v at the switch?

My guess is either a bad switch or a bad ground.

Bevan
 
I'll second what Noel said. A Whelen power supply with three strobes pulls at least 7 amps in my airplane. That's a lot of current going through the switch. It will eventually wear out. Mine did, and the way I found it was I just happened to touch the switch and it was VERY hot. After I landed, I noticed my strobes were out. Changed the switch, all OK.

Another reason to upgrade to LEDs!
 
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Pat Hatch said:
I'll second what Noel said. A Whelen power supply with three strobes pulls at least 7 amps in my airplane. That's a lot of current going through the switch. It will eventually wear out. Mine did, and the way I found it was I just happened to tough the switch and it was VERY hot. After I landed, I noticed my strobes were out. Changed the switch, all OK.

Another reason to upgrade to LEDs!
Click to expand...

Common occurrence, especially with Carling switches. Strobe power packs are hard on switches and terminals.
 
Rosie said:
<snip>Further testing last night finally revealed that I had 12.5v at the switch output, and ~3.5v at the pack (unloaded, not connected to a borrowed pack). <snip>
Click to expand...

Let me expand on the switch test (I'm testing at the correct switch)....I disconnected the strobe wire at the switch and measured the voltage at the switch output: 12.5v

I reconnected the wire and measured again at the:
**switch output: 12.5v
**Other end of the wire (no splices, not connected to strobe pack): ~3.5 v

I was tired, it was last and didn't get a chance to buzz the wire from the switch to the pack...will do that tonight, Rosie
 
Do you have a pullable breaker in the mix? If so, try resetting it 30 or 40 times. That may help.
 
No load?

A no current load drop of 9v is hard to do. Put a 1K load resistor across that meter and see if it's not really 0v
 
Rosie,
I don't think buzzing the wire will help.
It sounds like you have continuity that will buzz the test but not enough conductor to carry the large current. Somewhere in the circuit is getting hot.
 
I'll chime in on voting for the switch. If you disconnect the load and check the switch voltage its not the same as checking the switch when it has 7A passing through it.
 
If I'm reading this right and you used a jumper wire from the power bus to the strobe and they worked, you're very close to finding the problem.

Leave the end connected to the strobe alone. With the other end start working your way down the line. I.e. switch input, switch output, etc. At some point your going to reach a point where your strobe do not work. At this point you just crossed the problem point.
 
Rosie,
Your on the right track. Voltage check across a switch with no load is not particularly useful. Report back. Im betting on :cool:the switch.
 
I think I might find something to load the circuit at the strobe, maybe an incandescent landing light? (or several) Something under the range of the circuit and fuse, then check voltage and look for the hot area, switch or somewhere along the circuit. Or, check voltage across devices.

It could be upstream of the switch too, all the way back to the buss.

Good luck, finding the solution will be rewarding and relieving at the same time ;).

Funny story, I was a fresh service writer/mechanic at a dealership, with no service manager on the scene (hard times), I would write tickets on a short and found one mechanic very quick at finding and repairing the issues. I wanted to know how he did it. Reluctantly, he showed me. He would take a heavy wire direct to the battery hot side and touch the wire, it got hot, quick. Then he would run his hand down the harness and find where it got cool, then he had located the area of the short. I coined this technique as "follow the smoking wire" Crude but very very effective, and with skill, no damage. Brilliant.
 
Switch

Like Vern said...and strobes are known switch killers.
And like Kahuna said, you won't see the voltage drop unless the circuit is loaded, or the switch is totally failed. Using a little Bosch cube relay may be a good option if it dies again later or you want to prevent it from happening again.
Tim
 
Bad switch ... and more...

Paul,

I had the same issue on my -7 and Whelen strobe pack at about 125 hours. Output of the switch was ~12.6V and ~3V at the pack. Voltage was measured at that point of connection to local airframe.

The switch had failed AND about 2 inches of wire had "burned" resulting in a very high resistance path.

The root cause of the failed switch and wire was faulty termination at the switch (bad crimp).

I ran a new section of wire from the switch to the Whelen, properly terminated both, and installed a new switch.

All good after 250 hours.
 
bjdecker said:
Paul,

I had the same issue on my -7 and Whelen strobe pack at about 125 hours. Output of the switch was ~12.6V and ~3V at the pack. Voltage was measured at that point of connection to local airframe.

The switch had failed AND about 2 inches of wire had "burned" resulting in a very high resistance path.

The root cause of the failed switch and wire was faulty termination at the switch (bad crimp).

I ran a new section of wire from the switch to the Whelen, properly terminated both, and installed a new switch.

All good after 250 hours.
Click to expand...


The root cause was likely the switch, not the crimp. See http://www.vansairforce.com/community/showthread.php?t=11538&highlight=Carling+switch.

I spent a lot of time and money chasing this down. Here is what a bad switch looks like.... It takes the terminal with it. BTW,this is a strobe switch.




IMG_0935_2.JPG


IMG_0956_2.JPG
 
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Taking voltage readings at various points isn't a good way to troubleshoot.

Because...V=I*R.

If I (current) is very low, the few microamps of current passing thru the voltmeter with a high resistance R, voltage at the switch will read close to the battery voltage.

The real test is to disconnect the switch altogether and measure the resistance with it on. If its more than a few milliohms, it needs to be replaced.
 
I had the exact same problem, my strobe stopped working, low voltage at the strobe input. Re-crimping the wire terminal leads fixed the problem. Problem occurred at 1300 hrs.

I'm the 3rd owner, the main point is don't use automotive parts store crimp connectors, they seem to fail. I've had to replace a lot of them.
 
Kahuna said:
Rosie,
Your on the right track. Voltage check across a switch with no load is not particularly useful. Report back. Im betting on :cool:the switch.
Click to expand...

I'm listening to all the SMEs (Subject Matter Experts) - thank you for all the replies; a new 10 amp switch should arrive today from Aircraft Spruce....with Spruce just over the hill, I get next day delivery for the price of first class mail! By the way, the wire runs are 1-piece (no splices) from the panel to the pack, Tefzel 16 gauge.

I will report back :D Rosie
 
I've burned up 3 or 4 strobe switches. My latest fix is to leave that switch on all the time. Working so far.
 
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rocketbob said:
Taking voltage readings at various points isn't a good way to troubleshoot.

Because...V=I*R.

If I (current) is very low, the few microamps of current passing thru the voltmeter with a high resistance R, voltage at the switch will read close to the battery voltage.

The real test is to disconnect the switch altogether and measure the resistance with it on. If its more than a few milliohms, it needs to be replaced.
Click to expand...

Speaking of Ohm's Law, strobe power supplies are, like most switching power supplies, negative resistance devices.

What I mean is that as the input voltage falls, the input current rises in order to provide a constant output power. Any problems with the input power feed will lead to a thermal runaway... A bad crimp, a bad switch or breaker. Keep your eye on this at every Annual.
 
Voltage loss

You might find a partially broken conductor or if there is a termination(butt splice, terminal strip, etc) between the switch and the load a loose or damaged connection will actually become a heater. If a switch is going bad, you should derate the replacement.
 
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