The Cessna 120-140 Association Forum

I feel foolish even posting this, but I want to make sure I do something right moving forward.
I bought my 140 last spring, got signed off and started flying it regular. The ammeter gauge started showing a discharge. I put brushes in the generator and it showed a charge. I discussed this in a previous post and thought I was on to something.
Then cold weather set in and my battery wouldn’t stay charged. I would charge it with a trickle charger and it would start the plane and show a charge on the gauge. But that wouldn’t last long. So I put a new battery in, Concorde RG-35A to a Concorde RG-35XC. All is good, ammeter showing charge. Then 2 months later battery dead. While investigating why a brand new battery went dead I discovered that I had just traded the batteries out and attached the cables as they had been on the first battery. Yep, both were hooked up backwards. The cables come into the battery box, frame grounded cable on the right and solenoid cable on the left. They look like they have been that way since 1947. The battery is just the opposite of the cables.
Question: why does the starter work if it is hooked up to the negative side of the battery? I understand the generator wouldn’t charge the battery if it is hooked up backwards. Is there anything else that might have been damaged because of this?
I am assuming the first thing I should do is install new cables and make sure the ground cable is connected to the negative side of the battery.
Any other thoughts will be welcome.
Thanks, Brent

I use the RG-25 in my 140 and the RG-35 in my Cherokee. The terminals go towards the tail, and that puts the ground on the right and the positive on the left. That is the way all the 140's, I have seen, are. I have never used an XC.

Thanks Randy.
I did turn the battery around, but it looked like the terminals were right up against the back of the box. It looks like I could put a spacer in it and move it forward about 3/8”. Maybe that would give me room.

There should be cutouts on the back of the box for slotted rubber O-rings that the cables go thru.

8209 wrote: ↑Wed Jan 23, 2019 9:29 pmQuestion: why does the starter work if it is hooked up to the negative side of the battery?

The starter is a "series wound" motor. If you reverse the polarity it reverses through both the armature and the field windings. Two wrongs, in this case, make a right.

I understand the generator wouldn’t charge the battery if it is hooked up backwards.

Actually, often in this case the generator will get polarized backwards and will be perfectly happy charging the battery. (Since your battery isn't getting charged I'll venture a guess this did not happen.)

Is there anything else that might have been damaged because of this?

Radios?

First thing to do is get the battery wired correctly. Negative to the airframe and positive to the master relay. Turn on something big, like the landing light and confirm the ammeter shows a discharge. Shut the landing light off and start the engine. Hope for a charge indication at about 1500 RPM. If you don't get it, you probably need to flash the field. To do this, with the engine off and master on, use a piece of heavy wire (12-14 gauge) to jump from the battery terminal on the regulator to the armature terminal, for about 2 seconds. Repeat 3 times. There will be sparks, but you have to power through!

Thank You!
This info should get me back on track.

As Randy said, get the big grommets that protect the wires from the edges of the battery box. Generic AN grommets (AN931-??) are fine. You can work them over the ends of the wires, or cut them and put them over that way. Also, get a MS25171-4S boot for the positive connection.

My head hurts thinking about it, but, I think, if the battery is in backwards and the ammeter is showing a charge (+) it really means discharge, as it is measuring electron flow and doesn't care which way the battery is in. Since the battery is backwards, the gauge would read on the (-) side for a charge. Did you ever see a "discharge" on the meter? If not, I'd guess it's never been charging.

Hopefully, getting the battery in right will fix everything.

John,
My original problem was the gauge showing a discharge with the old generator brushes and old battery. Once I changed the brushes it showed positive charge, but the old battery that I suspected was bad (really just hooked up backward) would not fully charge so I changed it. New battery also hooked up backwards showed a charge but ultimately after a couple of months went dead.
I have charged the new battery up fully with a charger. I will follow your directions on hooking it back up and checking for discharge and charge while running and possible re flashing the field on the regulator and report back this weekend.
I will get it sooner or later, it’s not rocket science right?
Thanks for all your help.

8209 wrote: ↑Fri Jan 25, 2019 1:16 pmit’s not rocket science right?

Nope! '40's automotive technology.