The International Cessna 170 Association

The ammeter under the right control yoke appears to "stick" in position during engine start in our 1953 170B. Tapping the glass front face often free's the needle, and it then indicates correctly (I think!) for a few minutes. After that my voltmeter is my primary source for alternator operation and battery health as I rarely look at the ammeter after run-up.
Has anyone had this problem? And is there a source of ammeters anywhere? Is it possible to take the instrument apart and maybe 'lube' the needle?
Any suggestion or insight is appreciated.
Larry

Most instrument shops can probably fix that for you. It’s a fairly common automotive type gauge. I’m glad you pay attention to it. Many pilots don’t give it much thought but the fact is…during start it should show a discharge…then after the engine starts and gains sufficient rpm it should show a strong charge … diminishing slowly through the taxi-out and run-up. It should be nearly back to “zero” (battery fully re-charged) before takeoff, especially IFR or at Night.

Most of us, immediately after start, look for oil-pressure within 30 secs …per the checklist. We should also look for a “charge” on the ammeter and, if available, also on a voltmeter.

GAHorn wrote:Most instrument shops can probably fix that for you. It’s a fairly common automotive type gauge. I’m glad you pay attention to it. Many pilots don’t give it much thought but the fact is…during start it should show a discharge…then after the engine starts and gains sufficient rpm it should show a strong charge … diminishing slowly through the taxi-out and run-up. It should be nearly back to “zero” (battery fully re-charged) before takeoff, especially IFR or at Night.

Most of us, immediately after start, look for oil-pressure within 30 secs …per the checklist. We should also look for a “charge” on the ammeter and, if available, also on a voltmeter.

Come to think of it…. I may have one I took out of our 172 you can have in-trade….(we have a 50A alternator so we upgraded the original 30A Ammeter to a 60A gauge)…. I’ll go look for it.

Larry,
You mentioned that you have an alternator. How many amp is it? If over 30 amp you may want to replace the ammeter with one that would match your alternator output.

That is a Stewart Warner ammeter. If you need a new replacement, you can get it online at Summit Racing or Speedway Motors for about $30-35, either the 30-0-30 or the 60-0-60 version.

George, Gary and Gene,
Thank you all for your information and suggestions.
I will buy a new one, as I've owned the plane for over 2 years now and have always felt uneasy about the fact the current ammeter only goes to 30 amps with a Cessna 60 amp alternator. My memory of high school electronics does not include what happens to an ammeter if twice it's rated current is sent through it - and I don't want to find out!
Hope it gets in before SSM.
Larry

Larry E wrote:George, Gary and Gene,
Thank you all for your information and suggestions.
I will buy a new one, as I've owned the plane for over 2 years now and have always felt uneasy about the fact the current ammeter only goes to 30 amps with a Cessna 60 amp alternator. My memory of high school electronics does not include what happens to an ammeter if twice it's rated current is sent through it - and I don't want to find out!
Hope it gets in before SSM.
Larry

Please note the difference here between a load meter and an ammeter like in our 170’s that use a shunt and measure across it. Most shunts are external from the gauge and The power does not flow through the gauge itself. The shunt itself also needs to be rated correctly for the system and gauge indication.

jklaerner wrote:

Larry E wrote:George, Gary and Gene,
Thank you all for your information and suggestions.
I will buy a new one, as I've owned the plane for over 2 years now and have always felt uneasy about the fact the current ammeter only goes to 30 amps with a Cessna 60 amp alternator. My memory of high school electronics does not include what happens to an ammeter if twice it's rated current is sent through it - and I don't want to find out!
Hope it gets in before SSM.
Larry

Please note the difference here between a load meter and an ammeter like in our 170’s that use a shunt and measure across it. Most shunts are external from the gauge and The power does not flow through the gauge itself. The shunt itself also needs to be rated correctly for the system and gauge indication.

I think you will find that the original ammeters in C170's have internal shunts, and do not have an external shunt.

Yep..internally shunted. (and is preferred, IMO)