The International Cessna^® 170 Association

OK, so during a run up I experienced a 200-300 RPM mag drop on the left Mag, I was nearing 500 hours on them so I pulled it and found oil all over it. I sent it back to get overhauled. Columbia Aircraft in Bloomsburg PA did the overhaul, said the front seal was bad and there was oil all through the mag. They zero timed it and we put it back on, timed the engine, still showed the same symptoms. I swapped plugs, top to bottom, the mag drop did not follow to the right mag. So I thought ignition harness. Got a new set of those, put them on tonight and same problem. I pulled all the plugs and benched checked them, all ok. I checked the P-leads, they both seem fine, I checked the ignition switch, it looked ok, no broken wires or anything. The airplane shuts off when the switch is turned to off.
What did I miss? Any ideas? My mechanic and I are at a loss now...

v/r,
Jud

A couple things I would do:
Make sure you are actually selecting the mag you think you are selecting. Do a continuity check on the wires from the switch. Remember, when your switch is in "Left", you are really grounding the right mag.
Run it on the bad mag, shut it down and check for cold exhaust stacks. If they are all about the same temp, it's most likely in the mag.
Switch mags to see if the problem travels.

Do you suppose you have the mags timed incorrectly? Did you take then forward through the impulse coupler, then back it up to set the timing? Did you check BOTH mag's timing?

The mechanic and I did as you said George, for both Mags, but at this point I'm ready to check everything again.

Did you see the post just above George's?

I did see your post Ron, and I will attempt to troubleshoot as you say, but thinking out loud, instead of swapping mags, if I took the P-lead off the back of the left mag, and ran the engine, do the same mag check where I ground the right mag (by placing the switch to left) and seeing if I have the same problem, that would eliminate the P-lead back to the switch for faulty wiring. If I get the appropriate mag drop, then it's from the P-lead back, correct? If I have the same problem, it points to the mag or the plugs (since the wiring harness is brand new).

Another thing I thought of was a sticking valve might create the same problem, but I would think I would have the rough drop in both sides when checking the mags correct?

I'm going out tomorrow AM to troubleshoot.

v/r,
Jud

First I would make sure the right mag p-lead is grounded when the switch is in Left, and vice-versa. This is because you suspected the left mag, had it repaired, and there was no change.
Then I would run it on the bad mag, shut it down, and see which, if any, stacks are cold. If all the stacks are about the same temp, I would suspect the problem is in the mag. I know you said it was rebuilt, but you don't know for sure if the internal timing is correct. The rebuilder may have missed the e-gap, or got the gears on the wrong index mark. Or the condenser could be weak. Lots of things in the mag to check.
First, though, make sure the p-leads go to the correct mag.

Are you sure when you had the mag overhauled they put the oil seal in the right way....a common problem....maby you have the same issue.....

Update:
I checked P-leads for continuity with the switch, both mags, both checked good. I unhooked the left mag's P-lead and ran the engine, grounded the right mag and had the appropriate 50 RPM drop. Hooked the P-lead back up to left mag and checked again, got the 300 RPM drop. So I swapped P-leads left, to right. Problem did not follow. I traced wiring back to the switch, all looks ok...so now I suspect the problem is internal to the switch. Going to get a new one of those and see if it cures it.

What an ordeal!

v/r,
Jud

If every thing checked out as you say.....it's the mag....go to what you fixed last ....there is usualy your problem

Brad,
I don't think it's the mag because i took the P-lead off and ran the engine and grounded the right mag and the left mag (the one that was overhauled) gave me a good 50 RPM drop and was steady. I think I didn't really even need to overhaul the mag, since it's the same symptoms are present before and after the overhaul. The new Bendix switch will be here Tuesday, we'll find out then.

I had the same problem flying across Canada enroute to the Galveston Fly-in last year. The problem was one side of the mag switch sooted-up with black powder. We cleaned the contacts and all was good.

rupert

Is it a keyed switch? If you can't wait until tues switch the leads on the switch itself and see what happens.

Guys

A P lead either grounds the mag points or it doesn't. A mag switch only closes the grounding circuit with the P lead or it doesn't. Of course there could be an intermittent condition that could not ordinarily drop the RPM 300 RPM. If the P lead or switch is broken then your mag will not be grounded and you get no drop. If the P lead and switch is good the mag points are grounded and that mag is shut off completely. You are running on the ungrounded mag and the resultant RPM drop is because of different timing and the lose of additional spark and combustion in the cylinder. If both P leads are good but some how the switch was being intermittently grounded on one side when it was suppose to be open then switching off the good side might cause a greater RPM drop but more than likely it would not be consistent and repeatable.

A simple quick test of your P leads would be to run the engine and shut off both mags with the mag switch. The engine should die completely. This doesn't completely trouble shoot the P lead and switch but its a big first step. BTW I do this before every flight as part of my mag check.

These are Bendix mags so you need to have a P lead adapter in order to time them. Do this. Remove both P leads. This will illuminate the P leads and the switch from the problem.

Put the P lead adapter in one mag. Run the engine and with a good jumper wire ground the P lead adapter to the mag case the other mag will not have a P lead or adapter and can not be grounded. Check the RPM drop.

Shut off the engine with the fuel cut off and change the P lead adapter to the other mag and repeat the engine run. If you get the same or close mag drop in each case then the problem is in the P leads or the switch. I'll bet you will find it's a mag.

The circumstance that would have to take place in the P leads and switch where one mag would be grounded and the other mag would pulsed on and off to cause a 300 rpm drop consistently would be 1 in a trillion I'd think.

I'm not saying your wrong, I'm just wish electrical snags were always that simple but often a circuit that requires the designed voltage, current or resistance will behave quite differently when that circuit decides to design itself differently, as in a ground thats not quite ground or an open circuit that has leakage to ground.