Starting issue, or two?

nobubbles63 · #1 07-25-2020, 03:29 AM

I have had a curious issue develop over the last three flights. The aircraft is a 400hr RV7A with a Lycoming O-360. Two Slick 4300 mags. Normally starts first turn of the key after 3/4 blades.
Suddenly, this last week...starter turns over at a good speed on the key, but no attempt to catch light until the key is released. Once key is released back to "both" she comes alive. This happens when cold or hot and is only on the last few flights.
AND...when she does burst into life, get the RPM up and then turn on the alternator...we have a deafening alternator whine and the ammeter shows charging of between 7-10A which lasts for a few seconds before falling back to the normal +/- 1A and the whine disappears.
I am racking my brain to see how these issues are related and what has suddenly changed. The 1yr old battery is good showing +12V before cranking, the starter turns over healthily..??
Any ideas or suggestions please..?
Thanks
Derek
G-RVEM

Walt · #2 07-25-2020, 05:30 AM

Sounds like the left impulse coupled mag is failing. When you release the switch it lets the right mag come to life.
Or I suppose something could have happened to the switch and it's not ungrounding the left mag in the start position.

nobubbles63 · #3 07-25-2020, 06:59 AM

Thanks Walt, that sounds right along with what I was considering. Could easily pull the switch first and check that all is okay in there before pulling the mag.
I'm thinking the alternator issue may be coincidental but not related.
Derek

PilotjohnS · #4 07-25-2020, 07:58 AM

The high alt current could be because you are using more battery during start. Maybe the alt belt is slipping when trying to provide the high current. You might want to check the alternator belt.

RV7A Flyer · #5 07-26-2020, 02:18 PM

Quote:

Originally Posted by nobubbles63

I am racking my brain to see how these issues are related and what has suddenly changed.

My money is on "they're not". Mags don't have anything to do with the alternator (or battery, for that matter). That's the point.

Sounds like a) bad p-lead on the left, maybe? AND b) alternator failure of some kind, perhaps?

NewbRVator · #6 07-26-2020, 02:47 PM

400 hr. You likely need to tension the alternator but you might want to make sure you have adequate clearance with the current belt to do that without rubbing. If not switch to a shorter belt.

Bad mag or bad switch.

nobubbles63 · #7 07-27-2020, 02:42 AM

Thanks for the responses. I have dissembled the ACS switch to check for issues..all clear. Wiring all good. Still trying to source an ACS service kit in UK with no luck!
I agree the two issues are not related but thought I would ask.

Related question...I have seen various schools of thought on the shielding of the p-leads. I believe Bob Nuckolls suggests that in addition to being connected to the mag (done) that the shield is attached to the GND at the switch. The switch GND is NOT then connected to the aircraft.
Mine is not, the shields are not attached at the switch, and GND is to airframe as per ACS switch instructions.
I'm going to change it and hoping this is the "better" arrangement..any advice to the contrary?

nobubbles63 · #8 08-03-2020, 03:27 AM

Got to the bottom of this....
Both mags in for service - left mag was perfect?? Right mag coil was failing??
Turns out the original builder had the mags wired wrong way round at the switch, so the left mag/impulse coupling was not doing the starting at all...may explain some of the reluctant starts!
Alternator issue not related...just trying to keep up with the starting effort I think but will be coming off for service anyway.
Just wanted to thank all for the suggestions.
Derek