Thorp Air Command - T18.net

I managed to scare the sh!t out of myself this morning.

Since I pulled the comm radio and transponder this spring and then reassembled everything I have had a no charge situation (caution light and steady discharge)...I knew or thought I knew it would be easy to diagnose so I hooked up a volt meter this morning and pushed out to start up and see what was up.

As expected battery voltage dropped to 12.5 after the start or so from the 13 it had been before I cranked it over. Knowing I have jostled a bunch of wires while working under the panel I reached under the master switch and wiggled the wires, hoping a loose connection could be identified....well holy crap.... SMOKE and HEAT from the master and the voltage jumped to 18 and the amp meter to 60 amps....

I shut down and turned off the master immediately and got out the fire extinguisher...thanks god it wasn't needed. Looks like a couple of the leads on the back of the master touched (they are not the covered kind (!!!) just bare female plugs (I will fix that).

Do you think I have done any other damage; where do I start?

start by disconnecting your negative on the battery. this way you protect yourself from shorting anything else out. never work on the electrical with the battery hooked up.

if possible, undo the top half of the panel and tilt the panel towards the seats so you can get a better view of the backside. look for aNY bad connections and replace or repair.

Disconnect the battery!!!

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Dean Cuke
Visalia, California
N772DM
KVIS

I would do what Sparky sez ! Good advice. It only takes me a couple of minutes to disconnect the battery...only a milisec to fry expensive black boxes.

RN

If you have a split Cessna-type master switch, it most likely has both battery and ground on wires to it with the master on.

I'd check for additional damage on the master, of course, and the breaker for the alternator, assuming you have one. It should have tripped if you had the ammeter pegged for more than a few seconds.

Beyond that, assuming the radios and lights were off, there probably is no damage. If they were on, I'd operationally check them and let it go at that. The battery does a lot to protect those things.

But, assuming your regulator is on the firewall or internal to the alternator, the alternator won't run away like that from anything normally behind the panel. The easiest way to check it, if you can get a hold of a variable DC voltage supply, is disconnect the negative battery terminal, set for the supply for 12 volts, and hook it up at the battery wires. Turn on the master and alternator switches, and you should see a little less than 12 volts at the alternator field terminal. If you turn the voltage up slowly, the field voltage should go away at 14-14.7 volts. At 15.5-16 volts, the overvoltage relay should trip if you have one.

This is all assuming you have a separate voltage regulator, and an alternator not a generator. Assuming all that, it is probably likely that your original charging problems were due to the regulator stuck "off", then your short broke it loose and it stuck "on". Also possible that the + wire on the split master was disconnected, causing lack of charge - however that doesn't explain the run-away alternator, and you need to find that. I'd be curious as to what you do find.

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Rich Woodcock
N114RW - T18CW

Still trying to find out whats going on with this...

Let me start off by saying I don't understand electrical systems very well so forgive me..

I replaced the master switch, its a split Cessna type. The top wire on the alternator side was burned off, I replaced the connector and improved the protection at all the terminals so there is less likely to be a repeat (heat shrink over the ends of the terminals covering them right to the end).

I removed the Delco Remy alternator and took it to a local auto shop, they tested it and said it was working fine so no apparent damage there. I checked all the wiring and the ground from the output wire shield to the alternator case, it was fine.

I checked what I believe are the field wires (the plug that goes into the alternator case) and found 12.7 volts between either terminal and ground. 0 volts between the two.

I started the T-18 and once again....no charge, my alternator warning light is illuminated and the ammeter shows a discharge when operating the landing lights etc but no charge.

The breaker for the alternator had popped (30 amp breaker) I reset it but it has made no difference.

I believe the Delco-Remy has an internal regulator; I assume its working fine since the shop said output was good.

I had to connect the two wires coming out of the case (looks modified there) for the alternator to make an output (I assume that is like turning the alternator side of the master on).

I don't have access to a variable DC power supply so no go there...any more thoughts, what have I missed?

The fact that the 30 amp breaker tripped would indicate some sort of whodeedo (problem) is going on. There is a reason for that BIG breaker in the system and it is one I would not like to reset in flight should it trip. [:0] I don't have a lot of faith in auto parts stores testing my electrics. I would take the alternator to an electric shop to have it checked. [;)] I would also trace each wire from the panel to the alternator to check for any broken insulation that may be causing the "short." Especially where they exit/enter the firewall. I have seen my share of "non-standard" wires in airplanes. A meter might be handy to "ring out" the wiring and check for continunity.

RB
NX115RX

There is something odd here - you've gone from overcharging to not charging at all.

I presume that you have a wire that goes from your alt master (Cessna switch) directly to the two wires on your alternator (or through about a 5 amp breaker), otherwise you would have a regulator in there. My previous post assumed a separate regulator.

Does the battery hold a charge and start the airplane?

Your 30 amp breaker would most likely be connected to a third wire off your alternator - a fat wire, size 10, at least. That would go directly to the battery after the breaker.

Assuming the above is now correct (and only if that is correct - if not email me off line, and I'll give you a call), here's how I'd tackle it:

1. Assume that you don't know whether the alternator is putting out or not - with the breaker popped, the battery won't charge. Remove the fat wire from the alternator and check for voltage between the fat wire and ground at the engine with the master on. You should have battery voltage there. This proves continuity through the breaker, wire, master, and connection to the battery.

2. Connect a charger there, negative to the alternator cae or engine block, and charge the battery, still with the master on. Charging the battery from the alternator location gives a good indication of any bad connections. If the battery is very low, or bad, you very easily could pop a 30A breaker. When the battery is charged, you'll have over 13.5 volts on the battery, and low current from the charger.

3. You should check the sticker on the alternator. If you got a 60 amp alternator, and a 30 amp breaker, you've got a problem. But, a 60 amp alternator requires #8 wire - about the size of a pencil, after the insulation is removed. The wire should be sized for the alternator. The breaker should be sized for the wire.

4. Remove the belt from the alternator, and with the field wires hot, spin it (either way) by hand while reading voltage from where the fat wire would be connected to the firewall ground. You should see something DC. Not much, 2-3-4 volts perhaps, but something.

5. There should (theoretically) be a wire from the case of the alternator to the engine block, and absolutely a solidly connected braided ground strap from the engine block to the firewall. Make sure they are there. Charge current goes from the alternator to the pos battery, out the neg battery, to ground, through ground back to the alternator case. Any bad connections will cause trouble. Normally not enough current to start the plane, but nonetheless. . .

6. Once your battery is charged, connect everything back up, turn off the master, and let it sit overnight. When you return, your battery voltage should still be approx 12.5 volts.

7. Start the plane with the avionics, alternator, and lights off. Voltage should be about 12 before start, 9 or less while cranking, and return to about 12 volts when running.

8. After the plane is running, turn on the alternator switch. You should see the voltage increase over 1 volt, and the ammeter (if you have one) show no more than half scale, and decrease to about zero within 3 minutes. If the voltage is over 15, shut down, and throw the alternator through at auto-parts store window.

Hope this helps. . .

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Rich Woodcock
N114RW - T18CW

Thanks for the step by step for the electrically impaired I followed the check list and found..

1- I have battery voltage here (12.66 vdc).
2- I connected the charger and yup, it charges the battery (I had it on the 2 amp setting).
3- I have a Delco--Remy 1102 497 61A, other markings are 5A22 12 VNEG. The breaker was definitely a 30 amp but the wire is #8 size???.
4- With the belt removed I was able to spin it by hand and got a whopping 170 millivolts at max spin (ouch).
5- all there, fwiw the plane starts great...about 10 starts so far troubleshooting
6- The battery is a new PC680 Odyssey.
7- 12.6 or so at the start, drops to 9 or so and 12.6 when running.
8- Not done since alternator still US.

I assume that step 4 showed it is a failed unit (despite what the 17 yr old said at Canadian Tire).

My next step is to take it to an alternator/starter shop for repair. I want the same one back since it has been modified with the wires that interrupt the field allowing it to be shut down. Does that sound right?

Merci buckets

Tim

Let us know what the Doc sez at the Electron Hospital. I wouldn't be surprised if the brushes are worn to limits.

RB

Thanks and will-do...

Spinning it by hand doesn't prove much, except you'll get nothing if the field is open.

If the auto shop said it's good - it probably is. Those test unit do check it for output. I'd put it back in try it again.

What's "US".

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Rich Woodcock
N114RW - T18CW

US = Unserviceable

I took it to a big and well know shop that specializes in starters and alternators and even got to stand beside the tech when he tested it. It works, put out 42 amps on the test stand... he ran a series of tests on it and says it healthy.

I am officially baffled.... I will now trace every wire to see whats broken, burned or disconnected.

OK..."Me."

RB

Tim does the breaker trip immediately, or after a certain time period?

lance38dt

Tim,
I can't help you with internal modifications that may have been necessary to allow your alternator to be shut down. I just don't know. I did set up an automotive alternator that had two field connections and an internal regulator, similar to yours, and all that was necessary was to remove voltage to the field to shut it down. So, it doesn't sound right, but, again, I don't know.

But, nothing is broken or disconnected, or you wouldn't have been able to see what you've already seen. High resistance is possible - and you can check that. Remember, if the breaker pops, the battery won't charge, and the 30 amp breaker is small for a 40 amp alternator. If you have #10 wire from the alternator, 30 amps is about as big as you should go. 40A is OK for #8, and 60 for #6 wire. It seems most likely to me that the 30 amp (somewhat abused) breaker is bad.

As to checking for resistance, you can strip or pierce the insulation on the two field wires, and measure voltage to ground with the master and alternator on, and the engine not running - with the wires plugged in. It should be within a volt of battery voltage. To measure resistance of the fat wire, you'll need to hook up the charger again, and measure voltage from the charger connection to the battery. Assuming 2 two-amp charge rate, you should read less than 0.05 volts from the positive charger connection to the positive battery connection, and less than 0.03 across the 30 amp breaker. Likewise, you should read 0.05 or less from the negative charger connection to the negative battery connection. But, most of those high resistance connections will still allow some charge, so it really sounds like the breaker.

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Rich Woodcock
N114RW - T18CW