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No RPM drop on Right Mag Run-up

Webb

Webb

Well Known Member
Sponsor
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.
 
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..
 
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.
 
This way?

Mel said:
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.
Click to expand...

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.
 
Nope;

Webb said:
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).
Click to expand...

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.
 
Will try it tomorrow

Mel said:
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.
Click to expand...

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.
 
Webb said:
If the p lead is open, it would keep running - correct?
Click to expand...


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;):D;)) 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. :eek:
 
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Thanks Larry

I appreciate the reminder. I know but it's always a good ideal to treat it like it might go off.
 
Webb said:
I appreciate the reminder. I know but it's always a good ideal to treat it like it might go off.
Click to expand...

Story Time: I was working on the -3 with the 0-235. Pulled the carb off for rebuilding, had the exhaust manifolds off, valve covers off. I had been adjusting the priming setup about 1/2 hour before I grabbed the prop and turned it. Click, bang. The prop came around and hit the back side of my hand. :eek: No blood, but my hand was swollen and sore for a month! A good reminder of my dumbness. :cool:

Before this happened I just did not believe / realize an aircraft engine could fire on one cylinder and have enough power to do any damage. I look at props in a whole new lite now. :rolleyes:

Be careful out there! ;)
 
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Though I had it

Pulled the top cowl and with just a little tug on the P lead, the left terminal end ring on the mag broke. Hmmm. That must be it. Loosened up the wire (too much tension), put a new ring on and put it all back together.

Fixed - right - Wrong - Did the Kahuna check after it was all back together and it still ran in the off position. Darn....I knew I should have put the ohm meter on it while I HAD it apart and the cowl off. At least the terminal ring is not the problem and both P leads are secure on the mags.

Before I fly next, I'll be under the panel to check the switch wiring just in case the nut that turns the nut is his worst own enemy.

Fortunately I'm in a T-hanger so I'm not worried about someone turning the prop when I'm not there. However, I will get to the bottom of this soon so I don't have to worry ME turning the prop.
 
Intersting. Let us know what you find out is the problem Web. It's gonna be simple, just a PITA to track down.

I'm just glad you found the broken "P" lead on a mag check instead of my way! :eek:
 
Did you check the ground connections at the mags?

You said you checked the "P" leads. Did you check the ground leads at the mags?
 
Ground leads looked OK

Mel said:
You said you checked the "P" leads. Did you check the ground leads at the mags?
Click to expand...

The ground leads looked fine. Also - aren't they for proper shielding? Since I am getting no noise on the radio, I think the shielding is good.

I suspect trouble at the switch. Either a connection or the switch itself. Since it's a new switch, odds are connections. If the connections look OK, then I'll move on to electrical testing.

More to follow........
 
Webb,
The shield on the mag is the ground point for the P Lead. I'm not sure how you've got the mags wired at your ignition switch, but the ground connection on my mag is connected to the p lead via the ignition switch. When the switch contacts are not made (open) the P lead is ungrounded (hot). Check to make sure that you have continuity from the mag ground wire to the P lead with the mag switch off and selected to the opposite mag. This places the P lead to ground shutting down the mag.
 
50% right

Rockyjs said:
Webb,
The shield on the mag is the ground point for the P Lead. I'm not sure how you've got the mags wired at your ignition switch, but the ground connection on my mag is connected to the p lead via the ignition switch. When the switch contacts are not made (open) the P lead is ungrounded (hot). Check to make sure that you have continuity from the mag ground wire to the P lead with the mag switch off and selected to the opposite mag. This places the P lead to ground shutting down the mag.
Click to expand...

I'm first going to check the terminal ring connections at the switch. Since the wiring was done early on at the panel, it's possible that I could have broken a terminal ring in handling multiple times. There is also terminal block that the P leads break that I did for easier panel install. If the break is there, I'll just run a whole new wire and bypass it. Easy 1st step. I'm hoping that I just broke a ring. Tracing electrical problems is a major pain.

If it is not a physical issue that I can readily see. Then it's meter time.

I know I'm 50% right. Both sides were wired at the same time and done the same. Since 1 side is working and 1 is not, I think I've got a good chance of figuring this one out in a short period of time. On the trace, I'll check the switch first and make sure the left (and right) side are getting switched to ground before pulling leads off the mags. Time to get out the trusty meter and verify the switch, the wiring, etc..

I'm going by the airport before work, getting under there to visually verify connections so I can plan my course of diagnostics should it all appear solid. Since the wiring for the switch was done over a year ago, memory of exact connections are a little fuzzy. I also need to pick up the blueprint so I have a fresh memory.

I've got a friend who is joining me on Saturday morning to help trace the electrical. He is just started the empanage on an 8 and is exceptional mechanically inclined and knows his way around an airplane so I have no doubt that we will lick this problem.

Electrons - Watch out, we coming!!
 
No puffs yet

Geico266 said:
Don't let the white smoke devil out!
Click to expand...

No electrons have escaped.

Went by the hanger before work this morning. As you know, you can't just "look" at something. I pulled the P leads, and jumper. According to the volt/ohm meter, the switch works as designed. Guess I'll order a few feet of shielded wire from ACS and run new lines. Instead of getting frustrated at trying to track it down, I'll just run 2 new lines and completely bypass the terminal block but leave myself an extra foot. It will take me less time to replace both P leads instead of trying to solve it.

I can prepare the mag ends at home and get them ready to take to the airport at home so it should take just about the same amount of time to pull and replace the top cowl as running new P leads.
 
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Webb said:
Instead of getting frustrated at trying to track it down, I'll just run 2 new lines and completely bypass the terminal block but leave myself an extra foot. It will take me less time to replace both P leads instead of trying to solve it.
Click to expand...

Probably a very good choice of actions.

If this doesnt fix it, at least you have eliminated things down to either the switch, or a problem internal to the mag.

Good luck, hope the wire works magic for you.
 
Good idea Webb (replacign the leads) - if I have access, I almost always just run new shielded wires from one end to the other, and then I know that if it doesn't work, it's ME who screwed it up....

Paul
 
Hot mag fixed

Oh yes - the hot mag is fixed. Works perfect.......except I now remember why the terminal block was such a dandy ideal.....That little 30K ohm resister shunt that provided the connection for the tach.

Started up the engine. Hey, I get a drop in rpm but.....no tach. No biggie. Just cut the two leads and install at the starter switch with resistors.

The joys of aircraft ownership.
 
Tach too

Ah yes, the tach is now fixed. Thanks to all that helped me figure this one out.

The terminal block that was on the sub panel was such a great ideal at the time. P leads broke there and provided a nice place to put shunt resistors to the Dynon EMS. The problem was I should have put the terminal block on the forward side of the sub panel, not the aft side. I couldn't get my hand up there to re-do and had to bypass on the fix.

When I ran new P leads with no break directly from the key switch to the mags and bypassed the terminal block, I lost the connection to the EMS.

The fix occured by cutting the Dynon leads off the terminal block and bringing them off the key switch with a 30K resistor in line for the left and right leads into the Dynon EMS.

Here is the lesson learned for others. If you don't get a rpm drop on mag check, you probably have a hot mag. At idle, turn the key off. If the engine still runs, you have a hot mag and this is a big safety issue.

Case closed.
 
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