No RPM drop on Right Mag Run-up

Webb · #1 12-06-2008, 04:01 PM

I have slick mags

When I do the mag check on run-up, the left mag drops as it should but the right mag does not.

Looking back over the wiring, I noticed the plans called out for a jumper on the right lead (to ground if memory serves correctly) on the starter switch. After looking at the ACS starter switch diagram on line, it states the jumper is for impulse coupling or a breaker mag.

My gut is telling me that this problem is due to the jumper AND I also have a safety issue going on here and a hot mag even if the switch is off.

Will removing the jumper on the starter switch fix this? This seems the most logical reason.

Radomir · #2 12-06-2008, 04:10 PM

Be careful.. you don't want a non-IC mag to be live when cranking.. removing that jumper may just do that...

if both of your mags are impulse coupled.. then you don't need that jumper..

I doubt that removing that jumper will help you.. but you should first determine if you have a live mag or not before messing with the switch..

Mel · #3 12-06-2008, 04:17 PM

The jumper only grounds the right mag during the start cycle, not while running. Yes, you have a problem somewhere.
Check for an open "P" lead or a bad switch. I have seen new switches that were bad.
Remember when checking for grounding of a "P" lead, the mag MUST be disconnected. Otherwise if the points in the mag are closed, it will show a ground even if the "P" lead is open.

Webb · #4 12-06-2008, 04:44 PM

Quote:

Originally Posted by Mel

The jumper only grounds the right mag during the start cycle, not while running. Yes, you have a problem somewhere.
Check for an open "P" lead or a bad switch. I have seen new switches that were bad.
Remember when checking for grounding of a "P" lead, the mag MUST be disconnected. Otherwise if the points in the mag are closed, it will show a ground even if the "P" lead is open.

Mel - Let me see if this is correct. I disconnect the P lead on the right mag. Place a ohm meter between it and ground. Turn the switch to both and it should show open. Turn the switch to right and it should show closed (grounded). If open, then go to switch and check the lugs and see if I get the same. If the switch is bad, then I get the same. If there is break in the P lead, then the switch shows a connection to ground. If open still, then I have a bad switch and order a new 20 dollar part. If the switch shows closed to ground, go after the wire.

Kahuna · #5 12-06-2008, 04:45 PM

what happens when you turn the ingition key off when running?

Webb · #6 12-06-2008, 04:46 PM

Quote:

Originally Posted by Kahuna

what happens when you turn the ingition key off when running?

Haven't tried it. If the p lead is open, it would keep running - correct?

Mel · #7 12-06-2008, 05:00 PM

Quote:

Originally Posted by Webb

Mel - Let me see if this is correct. I disconnect the P lead on the right mag. Place a ohm meter between it and ground. Turn the switch to both and it should show open. Turn the switch to right and it should show closed (grounded).

When you turn the switch to "R" the right mag should be open because you should be running on the right mag. The left "P" lead should show closed.

Mel · #8 12-06-2008, 05:02 PM

Quote:

Originally Posted by Webb

Haven't tried it. If the p lead is open, it would keep running - correct?

That is correct!

Webb · #9 12-06-2008, 05:53 PM

Quote:

Originally Posted by Mel

When you turn the switch to "R" the right mag should be open because you should be running on the right mag. The left "P" lead should show closed.

It sounds like it's my left lead that is the guilty culprit. However, I will do an off switch check before anything. It may be possible that it was one of those days when the rpm's didn't drop and everything is working as it's supposed to.

However, if if keeps on running on the switch off, then I'll roll up the sleeves and go after the left lead.

As usual, the brain trust here is invaluable. Thanks guys.

Geico266 · #10 12-06-2008, 06:01 PM

Quote:

Originally Posted by Webb

If the p lead is open, it would keep running - correct?

Webb, Be VERY carefull! Remember, if the P-lead is broken (open) you can start the engine now by turning the prop. One "click" on that mag with fuel in the cylinder and "BANG" you could have a running engine! I know you know this, but don't let anyone touch the plane until you check it out. I would hate for anyone to get hurt.

An old mechanic (I think it was Mel

) I told me to do a mag check PRIOR to shutting down the engine. One of the "P" leads could have broken inflight and you won't know it until the next start up. What is the first thing we do after we shut the engine down and get out? ...turn the prop.