AOG-in my hangar!

[Plummit]

Originally Posted by gasman

This #2 cable was grounded first to the firewall brace on the engine side before it went to the panel... RIGHT?

Nope. If the battery was grounded to the airframe first I wouldn't have had a problem -and- I don't believe the shunt would have worked. Actually, I'm not convinced the shunt was being used for measurement purposes anyway. I found the wires were not secured tightly to the shunt!

Originally Posted by gasman

If the shunt was mounted to the cabin side of the firewall, was the cable also connected (grounded) to the firewall? If it is, then the shunt is not the starting problem unless you don't have a ground cable from the firewall or battery - to the motor.

I'll get back over to the hangar this morning and check it out. I've never considered that I had a "starting" problem just because the engine would come up on compression and the starter wasn't powerful enough to drive the crank through it. The engine has 9/1 pistons, and it's difficult to pull the prop through by hand let alone the little starter.

I will confirm the ground path today. So far I've just been looking for the cause of the electrical failure. Even with this poor excuse for an electrical system, it's worked for 10 years and 1000 hours....

-Marc

[GalinHdz]

Originally Posted by Plummit

It just didn't feel like a dead battery to me, but I had kept my old battery and it was on an approved maintainer so I swapped it out. Same thing, nothing. Got my VOM out and checked the volts, 13V+. Then I put the lead on the positive terminal and the ground on the airframe. Read 13V+ but when I hit the master the voltage dropped to zero.

It is not uncommon for a bad battery to show normal voltage if no load is applied. But when a large load (like a starter) is applied, the voltage can go down to zero.

Originally Posted by PaulvS

I wonder how is it possible for the VOM to read 13 V+ between the battery positive terminal and the ground on the airframe, when the battery ground connection is broken due to the failed shunt?

A VOM only needs microamps to work so he might be getting this ground to the airframe via some other wire (not just the big wire) which would be just enough for the VOM to show voltage. But not being able to see this particular installation it is hard to say.

PLUMMIT: Put your airplane battery in your car and see if the engine turns on. If the car engine turns on, then your battery is good and you have a bad starter. If the car engine doesn't turn on, then your battery is bad.

[Plummit]

Originally Posted by GalinHdz

It is not uncommon for a bad battery to show normal voltage if no load is applied. But when a large load (like a starter) is applied, the voltage can go down to zero.

Yup, I'm aware of this but as I stated, the problem was the poor wiring path the builder chose. I will fix it and get it right. I'm not going to replace the fried shunt. I'm going to correct the ground wiring and then I can diagnose any problems I *may* have with the starter or battery. I suspect (hope) that once the wiring is correct everything (the starter and battery) will work even better.

I'm sure glad I didn't wire this; I'd be too embarrassed to post about it!

-Marc

[OKAV8r]

An additional item to consider is high resistance in the master contactor.

[Brantel]

Was it a shunt or possibly one of these current limiting fuses? They can look very similar when installed.

[climberrn]

Originally Posted by Brantel

Was it a shunt or possibly one of these current limiting fuses? They can look very similar when installed.

True, but on a ground line? Doesn't make sense.

[Brantel]

Originally Posted by climberrn

True, but on a ground line? Doesn't make sense.

Lots about this that doesn't make sense. I recommend a full trace of all the main power wiring in this bird.

[Plummit]

Originally Posted by Brantel

Was it a shunt or possibly one of these current limiting fuses? They can look very similar when installed.

Shunt. And I agree, it doesn't make sense.

-Marc

[Plummit]

So today I joined the shunt lugs together, tested the electrical system and everything worked. It was raining so I didn't check the Skytec 149NL starter. I'll try it tomorrow weather permitting. Fortunately I have another brand new starter if the old one is bad.

-Marc

[Tim Lewis]

Originally Posted by Plummit

... Got my VOM out and checked the volts, 13V+. Then I put the lead on the positive terminal and the ground on the airframe. Read 13V+ but when I hit the master the voltage dropped to zero.

Hmmm. good voltage on the battery terminals but nothing when using the airframe. I've got and open ground.

Started tracing the #2 welding cable all the way to the panel and then I saw it: a shunt mounted to the cabin side of the firewall that let the smoke out.

I had never seen a shunt on a negative cable but hey, at least it happened at home!

-Marc

Are you saying that you initally measured the voltage from the positive terminal of the battery to aircraft structure (ground) on the fuselage, and that voltage dropped from 13V to zero when the master was switched on?

That would seem to indicate either a dead short is being placed across the battery, or else the VOM reference point (the airframe itself) had jumped up to 13V!

There should be a short, direct ground connection between the battery and the airframe back at the battery. Is there not? That connection establishes aircraft structure as ground.