Cherokee 235 Hot Left Mag

[Eric S.]

Hi Folks,

Both my A&P and my self have been tearing our hair out over an issue I have with a Left hot mag on my 64 Cherokee 235.

Basically the issue is that effectively no matter what I can't shut the engine off with the key and it''s all down to the left MAG continuing to fire.

So far the following things have been done:

1. Continuity check has been completed on the entire circuit from switch to P-Lead. (Key on L results in ground continuity to the right MAG Key on R results in continuity to the left MAG.)
2. P-Lead to Left MAG has been replaced (Twice).
3. Ignition switch has been replaced.
4. Manual flip switch has been connected to Air frame and directly to P-Lead on Left MAG. (Effectively bypassing the entire ground circuit, engine keeps firing.)
5. Left MAG was sent off to Aircraft Magneto Service and bench tested. (The Plead contact point inside the MAG has been replaced and MAG was sent back)

Symptoms before MAG was sent off:

• Motor at 2000 RPM Mag check on L shows no drop, and on R shown 75 RPM drop.
• Motor at Idle Key in off position motor continues to idle.

Symptoms after MAG was returned:

• Motor at 2000 RPM Mag check on L shows 50 drop, and on R shown 75 RPM drop.
• Motor at Idle Key in off position motor continues to fire but is very rough.

Now after speaking to the MAG overhaul people they said to try to ground the MAG to itself, which I will try next and to look for the engine ground strap and test that.

Now I'll test the grounding the MAG to itself the next time I can get to my plane but in the meantime I wanted to put this out there to see if anyone has any ideas.

Also does anyone know where the ground strap on an 0540-B4B5 engine is as I can't find the part location in either the PA28 or 0540 parts or service manuals.

Plane Specs:
64 PA28-235
0540-B4B6
Bendix MAGs (Left with impulse coupling)


Sorry about the long post.

 

[magman]

 

[EchoDelta]

 

[Eric S.]

 

[Domenick]

 

[cappy48]