Mag always grounded

DesertNomad

DesertNomad

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DesertNomad
During an inspection on a Cherokee 180, we installed a new ignition so that we can have a single key for door/baggage/ignition. Everything was working fine beforehand. Now it seems the right mag is always grounded. We swapped the old ignition in with the same results. I have certainly seen broken P-leads with a mag that remains hot, but I have never seen one stay permanently cold. It's a known-good mag.

I think the only electrical things that were touched were all spark plugs, a condenser on the alternator, and the ignition switch.

Has anyone seen these symptoms? Obviously there is a hidden ground somewhere that is keeping things cold.

*We = the AP doing the work and me standing there. :D
 
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In order of likelihood, either the new switch is incorrectly wired, the P-lead is sorted to ground somewhere possibly from being disturbed, or the new switch is bad.
 
Dana said:
In order of likelihood, either the new switch is incorrectly wired, the P-lead is sorted to ground somewhere possibly from being disturbed, or the new switch is bad.
Click to expand...

I think we eliminated the switch as it does it with the old one reinstalled. We closed it up late yesterday so hopefully a fresh look this morning reveals something. Just frustrating.
 
Step one, disconnect wire from magneto.
Step two; hook a multimeter to wire disconnected from step one. Turn multimterer to horseshoe mode.
Step three: play with ignition switch. making damn good and sure prop is clear of obstacles incase starter engages by accident.

Is the switch actually switching the circuit? No? disconnect wire from switch and test wire for shorts to ground. Wire shorts to ground while playing with it? Replace wire.

Then test switch with multimeter in horseshoe mode.
 
Just in case it‘s a 200 Bendix; with a little box at the breaker

compartment.

Note there is a capacitor in there with a long strip used to contact

the p-lead. it also grounds the mag with the p- lead removed.

Even a slight looseness of the knurled p- lead nut will allow the

strip to ground and kill the mag.
 
Magman said:
Just in case it‘s a 200 Bendix; with a little box at the breaker

compartment.

Note there is a capacitor in there with a long strip used to contact

the p-lead. it also grounds the mag with the p- lead removed.

Even a slight looseness of the knurled p- lead nut will allow the

strip to ground and kill the mag.
Click to expand...
That's easy to find, too. Take the wire off the switch and use a mag timer (NOT a multimeter) to check the grounding. If the timer implies that the points are closed, move the prop backwards until they open. If the timer still shows a grounding, that reed in the mag is likely not opening. Or the points aren't opening (!). Removing the lead from the mag should show "points open."

That lead is obviously grounding somewhere. Taking it off the switch (to isolate the lead) and the mag show unground it entirely. If it doesn't, start searching. It could be something as dumb as the shielding touching the P-lead wires at the crimp.
 
Seems all you did was replace the ignition switch right? Since nothing else was disturbed the problem is almost 100% going to be there. Either p-lead is shorted to shield or you hooked it up wrong.
 
It would help IF the type mag was known.

Mag timing is normally checked during scheduled inspections.With a 200

there are at least 3 ways to do this:

1. Remove breaker box cover

2. remove p- lead and use an adapter

3. connect light at switch

Any one can introduce problems too.
 
A dead mag is not good.

A hot mag is dangerous.

Recently I met a mag on a Cherokee 140 that could be shut off with the

Switch ; sometimes.

Mag drops were normal which should indicate that the Switch is functioning.

The Switch would kill the mag IF shut- off above 1000 rpm.

At lower engine speeds the engine would continue to run.

P- leads and ground were intact on both engine and Switch ends.

After much diagnostic time we realized the ground for the p- lead

was the culprit.

The system is grounded via shielding from the Switch to Airframe ground;

not the engine end.

The pic shows the p-lead box on the forward side of the firewall.

The screw shown has the ground terminal attach behind the firewall

by means of a nut. After 60 years the Terminal had some corrosion

that required cleaning up. This is a rather hazardous situation that

results in hot mags when parked but can be resolved with a few contortions.
 

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Your post implies (at least to me) that a run up was done, and when dropping the running mag, the engine completely died. Am I correct in my speculation?
 
That‘s about right.

At low rpm ( 600 ) shutting Switch OFF would have no effect.

At 1100 the same action would would cause the engine to stop firing and

rapid rpm drop. I did not bring it to a complete stop though

Both were done repeatedly. Should have taken a vid.


Particularly nasty is there was normal mag drop on each mag.

Since it has a Starter Button there is not an AD mandating the check.

Essentially the engine could have fired with the key OFF and removed.

Addressing the corrosion at the high resistance p-lead to airframe

ground point and it works fine now. Very odd and still don’t quite

understand it but operation is normal now.
 
Clip4 said:
Just replace the P leads.
Click to expand...
Throwing money and time at the problem is a poor way to fix things. It could be the P-leads. It could also be the switch, the mag, a shorted crimp terminal, even a bad external filter capacitor on the mag, if there is one.

On the switch there is a shorting bar between a couple of the terminals. It's not always required; depends on the installation. It's there to short the right mag to ground when the engine is cranking so that it cannot cause a kickback. The impulse is on the left mag, so it is used for starting. Some engines have two impulse mags and that shoring bar is not required.

Now, if someone put the airframe ground wire on the wrong ground terminal as well as the right one, that mag would be dead all the time.

1695753530018.png

#5 is the airframe ground. The dedicated starting ground is the one next to #1, which is the right mag terminal, and you can see the shorting bar between it and #1.
 
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I agree with Dan.

It would help if we knew type mags.
 
Dan Thomas said:
Throwing money and time at the problem is a poor way to fix things. It could be the P-leads. It could also be the switch, the mag, a shorted crimp terminal, even a bad external filter capacitor on the mag, if there is one.

On the switch there is a shorting bar between a couple of the terminals. It's not always required; depends on the installation. It's there to short the right mag to ground when the engine is cranking so that it cannot cause a kickback. The impulse is on the left mag, so it is used for starting. Some engines have two impulse mags and that shoring bar is not required.

Now, if someone put the airframe ground wire on the wrong ground terminal as well as the right one, that mag would be dead all the time.

View attachment 120897

#5 is the airframe ground. The dedicated starting ground is the one next to #1, which is the right mag terminal, and you can see the shorting bar between it and #1.
Click to expand...
New P leads are not expensive and one of the most over looked items on aircraft.
 
Clip4 said:
New P leads are not expensive and one of the most over looked items on aircraft.
Click to expand...
I know that, but if the new ones don't fix the problem, they were not at fault. Furthermore, intalling them can be a first-class pain. Time ($) consuming. This in on a Cherokee, getting under the panel to cut the wire ties and install new leads and connect them is another pain altogether. Firewall sealing as well.
 
Wheels said:
I’ve heard that this solution works best and fastest if you start by replacing the plane with a new one.
Click to expand...
Perhaps start with a new mechanic first, then a new plane if that doesn't work.
 
Dan

Would the grounding strap fit if improper install?

On some mags I would disconnect p-lead to make Right Hot and use a long

wire to cockpit to ground for start.

OP gone quiet?
 
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