c170b53 wrote: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.
Sometimes things (even electrical) are that simple and we try to make them more complicated.
Lets assume both mags are good and when grounded with a good ground there is a 50 rpm drop when the engine is running on either mag. How is it a bad ground could cause the engine to run with a 300 rpm drop?
Here is the only way. First one mag would have to be grounded completely so the engine is only running on one mag. Then the mag that is running the engine would have to have an intermittent short oscillating on and off so quickly and consistently shutting of the one remaining mag quickly and smoothly so that the engine runs 300 rpm slower. Remember the engine has run 300 rpm slower on one mag several times. What are the chances of a intermittent short so consistent as I've described being created more than once? I don't even think it could be created once to be honest.
CAUTION - My forum posts may be worth what you paid for them!
Bruce Fenstermacher, Past President, TIC170A
Email: brucefenster at gmail.com
Have you ever witnessed a lamp burning dimly due to a poor ground connection? A magneto, dimly grounded, might produce a spark so ineffective as to keep the engine running, yet not mimic a well-grounded magneto. With the opposite magneto grounded, a poorly isolated magneto might run like a dim lamp.
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention. An originality nut (mostly) for the right reasons.
My last post went away ....so I'll try again...I am with Bruce on this....If the problem didn't travel with the P-lead change It seams the switch isn't an issue..... also it is improbable, but not impossible to get a bad harness off the shelf, I meant earlier to ask if you did a Hi-tension test on the spark plug leads?
gahorn wrote:Have you ever witnessed a lamp burning dimly due to a poor ground connection? A magneto, dimly grounded, might produce a spark so ineffective as to keep the engine running, yet not mimic a well-grounded magneto. With the opposite magneto grounded, a poorly isolated magneto might run like a dim lamp.
WHAT?
So what your saying George is that somehow a resistance between the P-lead and ground is created and that this resistance is enough to still allow the coil to collapse when the points open but with a lower voltage than normal but still enough to spark the plugs. The reduced spark retards the combustion thus slowing the engine down.
Like I said in my early post I'll bet it's a mag problem and nothing to do with the P-leads and switch.
CAUTION - My forum posts may be worth what you paid for them!
Bruce Fenstermacher, Past President, TIC170A
Email: brucefenster at gmail.com
On the idea of doing a hi-tension test......it will also show a crack in the distributor housing....where if you are using the same mag and it wasn't found.....there could be your problem. I usually like to order overhauled-exchange parts for mags and carbs when I have a problem.....simply because the problem that I had should go away.....not to say that another won't crop up.....I just have another avenue to check, and in theory the problems shouldn't be the same.....something that will drive you crazy........as you already KNOW!!!!!!
rupertjl wrote: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
John E. Barrett
aka. Johneb
Sent from my "Cray Super Computer"
What I am saying is, Bruce, that sometimes illogical problems result from improbable defects.
The Hi-Tension leads should be tested. Most mechanics have a hi-tension testor available to them. I have one I'll happily loan out to a member if they'll pay the freight both ways... (but for that kind of money they could purchase one and OWN it.)
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention. An originality nut (mostly) for the right reasons.
gahorn wrote:What I am saying is, Bruce, that sometimes illogical problems result from improbable defects.
OK George I agree with you and that is why you might have noticed I DID NOT bet anyone a St Pauly Girl that I was right. But you example of a light bulb being dimmed by a bad ground is a common problem and a light bulb filament glowing is a much simpler operation that the process of charging and collapsing a coil. My description of the process was meant to point out how improbable if not impossible that scenario was.
I think the text of Jud's that John highlighted points out that Jud already tested this enough by switching the P-leads with the problem not following clearly indicating that the issue is isolated to a single mag or it's related high tension wires and plugs. Yet it appears for what ever reason Jud and his mechanic did not see this and continued to look at the P-leads and switch.
When I read posts like Jud's I am reminded how lucky I am to have several local knowledgeable friends to bounce these problems off of. And between us we probably have enough parts to trouble shoot nearly anything. I know we have several serviceable mags, harnesses and even a mag switch or two laying around. I only wish I could send then through the Internet to members for their trouble shooting purposes. May have saved Jud a $185 switch.
CAUTION - My forum posts may be worth what you paid for them!
Bruce Fenstermacher, Past President, TIC170A
Email: brucefenster at gmail.com
someone explain to me (I'm slow to follow sometimes) of why it would be my mag when I disconnected the mag from the switch (by unscrewing the p-lead) and ran the engine and it gave me a proper RPM drop when I grounded the right mag (meaning the left newly-overhauled mag us running the engine.) If it's the mag, why wouldn't it exhibit the same roughness all the time, whether the mag is connected to the switch or not?
To answer other questions, I did test the new leads with the high tension tester.
Ahh!!!!! The Queens English. I believe that if we look closer at what Jud has written we will realize Jud has isolated the problem to either the P-leads or the Mag Switch. When Jud ran the engine with the P-lead disconnected from the left Mag and shorted out the right mag the engine ran on the left Mag and only lost 50 RPM he than hooked up the left Mag P-lead and grounded out the right Mag and the engine ran and lost 300 RPM, Jud than switched the P-leads and the problem stayed with the left Mag position of the Mag switch leading to his conclusion that the switch must be the culprit. Richard Pulley experienced the same problem and fixed it by cleaning the switch contacts.
Jud it would be interesting to know (with the P-leads disconnected and the Mag switch in the both position) how the ohm meter readings of the two P-lead terminals compare to each other.
johneeb wrote:
rupertjl wrote: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
John E. Barrett
aka. Johneb
Sent from my "Cray Super Computer"
Yes, it seems he has a "dim" mag switch in one position. (It's leaking to ground when the opposite mag is grounded.)
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention. An originality nut (mostly) for the right reasons.
John-Thanks for saying what I've tried to say but failed miserably...I knew what I was trying to say but couldn't get it to come out right on the screen.
Bruce-It's not solved yet but when the switch arrives Tuesday, I'll see if I am indeed that lucky
I appreciate all the help from everyone, like Bruce said, not all of us our so lucky to have expertise at the hangar and I rely a great deal on the folks here to work through a problem like this one. Well worth the yearly membership!
