Magneto Switch Wiring

[Jon Wilder]

Hi all.

I understand the concept of the ignition switch grounding the non-impulse coupled magneto during the cranking cycle. However, in the electrical diagrams for a 1975 Cessna 172M, it does not show to do this, even though the aircraft only has one impulse magneto.

Was this not a thing to do until sometime after 1975?

 

[bnt83]

Hi all.

I understand the concept of the ignition switch grounding the non-impulse coupled magneto during the cranking cycle. However, in the electrical diagrams for a 1975 Cessna 172M, it does not show to do this, even though the aircraft only has one impulse magneto.

Was this not a thing to do until sometime after 1975?

First question I would ask, what P/N magnetos are installed, not model numbers, the actual part numbers from the data tags on the mags, then you can confirm if the shorting bar should be installed for a non-impulse mag if one is actually installed.

 

[Bell206]

However, in the electrical diagrams for a 1975 Cessna 172M, it does not show to do this, even though the aircraft only has one impulse magneto.

In general, you'll find the grounding of the right mag during start with a rotary type switch is done internally within the start switch, so it wouldn't show on the Cessna diagram.

 

[Magman]

Issue or just curious?

I did not look look at the diagram but find this interesting.

Since the jumper is an optional component of the Switch should it be included in the aircraft wiring schematic?

 

[bnt83]

In general, you'll find the grounding of the right mag during start with a rotary type switch is done internally within the start switch, so it wouldn't show on the Cessna diagram.

You would think so, the 1968-1977 Cessna 177 Service manual shows a strap between two terminals and a flag note explaining it on the drawing. I removed this strap on our 177 when replacing the single impulse mags with dual impulse mags via field approval.

 

[Magman]

My post #4 was similar to Bell’s due to my slooow typing.

bnt. Why did you need a Field Approval? Lycoming authorizes Dual IC on nearly all engines. I am a fan of this; particularly for clubs/ rentals.

 

[Jon Wilder]

Issue or just curious?

I did not look look at the diagram but find this interesting.

Since the jumper is an optional component of the Switch should it be included in the aircraft wiring schematic?

Just curious. Even though it's not impulse coupled and "shouldn't" be able to generate spark, sometimes it "could" generate a very weak spark, which "could" fire the mixture in advance of the IC mag (the impulse coupling retards the timing of the IC mag during cranking), causing possible damage to the starter via starter kickback. Thus grounding it while cranking.

@Bell206 ours has a GRD terminal right next to the R terminal that grounds when the switch is in crank. The R terminal does not do it internally (confirmed). Thus you "should" wire a jumper strap between the two when the right mag is not an IC mag. But the service docs don't show this.

 

[Magman]

On “ N 172s “ that are converted to 180 hp the jumper is often overlooked.
( D- mag to singles).

Some aircraft with dual IC also wind up with the unneeded jumper and are only using half of the available ignition.

A timing light will tell the tale in either case.

 

[Bell206]

ours has a GRD terminal right next to the R terminal that grounds when the switch is in crank. The R terminal does not do it internally (confirmed). Thus you "should" wire a jumper strap between the two when the right mag is not an IC mag. But the service docs don't show this.

While my memory is getting a little gray, I recall any ground jumper or strap was either installed by the switch vendor and the switch ID'd by P/N or the vendor supplied the jumper with the switch. Either way it may not show up on the aircraft diagram. If it was shown with a note as indicated by bnt83, then its just one of those differences in who wrote the diagram specs. But technically, if the part is not a Cessna part then no requirement to be listed or shown on diagram. Maybe check to see if you have the correct switch P/N installed? But I wouldn't change it if your current switch is working properly. More curious.

the 1968-1977 Cessna 177 Service manual shows a strap between two terminals and a flag note explaining it on the drawing.

Did the flag note reference any other document on the strap?

 

[bnt83]

While my memory is getting a little gray, I recall any ground jumper or strap was either installed by the switch vendor and the switch ID'd by P/N or the vendor supplied the jumper with the switch. Either way it may not show up on the aircraft diagram. If it was shown with a note as indicated by bnt83, then its just one of those differences in who wrote the diagram specs. But technically, if the part is not a Cessna part then no requirement to be listed or shown on diagram. Maybe check to see if you have the correct switch P/N installed? But I wouldn't change it if your current switch is working properly. More curious.

Did the flag note reference any other document on the strap?

 

[Jon Wilder]

In looking at ALL 172 wiring diagrams (even the 1996 and on), none of them show to install the shorting bar.