The International Cessna 170 Association

I have a very original 1955 170 B. My Amp gauge bounces back and forth and is very hard to get an accurate reading. My IA said I might need to replace it. I really don't want to replace it since it is the original gauge. Is there someone that rebuilds these antique gauges ?

Thanks,

Ralph

rmallicoat wrote:I have a very original 1955 170 B. My Amp gauge bounces back and forth and is very hard to get an accurate reading. My IA said I might need to replace it. I really don't want to replace it since it is the original gauge. Is there someone that rebuilds these antique gauges ?

Thanks,

Ralph

Ralph,

You can find the 30-0-30 and 60-0-60 tulip gauge Stewart Warners on eBay somewhat frequently. Have you load tested the system to ensure that your gauge is not the issue? Do you have an alternator or generator still?

You could try someone like Rudy’s or Keystone, but I’m not sure I’ve heard of folks overhauling ammeters.

I have not load tested it yet. I have a generator.

Have you checked the gauge without the engine running? It should show a steady discharge equal to whatever loads you have turned on (landing light is a good one to use). Tap or otherwise jostle the instrument and see if it shows the wavering you mentioned. If it stays steady, you probably have a generator and/or regulator issue.

As Miles hinted, your bouncing amp meter is likely not your amp meter but something else causing the load to fluctuate wildly and you poor meter is trying to keep up.

Chances are good it is the voltage regulator or your master switch. The master switch is a double pole switch. Half of it switches power to the battery solenoid, the other half connects the field wire on your generator.

As the contacts in the switch wear or get dirty, rather than a clean low resistance contact, there is higher resistance and heat. The heat cause the contact to move changing resistance. This is the bouncing you are seeing if the switch is dirty or bad. Often times cycling the master switch several times cleans the contacts a bit and the bouncing disappears but will likely be back.

Of course dirty contacts in the voltage regulator does the same thing.

Did you know that a ammeter with corroded/loose connections can make you believe you have a bad battery? The battery will refuse to keep/take a charge and when you replace it with a new battery.... the problem is still there. That's because ALL generated current must pass through the ammeter before it gets anywhere work can be done (such as charging, lighting, etc)

I suggest you do as Miles wrote, and also to clean the contacts behind your ammeter and see if the wild thing calms down. Exactly HOW MUCH is it fluctuating? (Small, regular movements can be your strobe system.)

I totally forgot about the strobe. No wonder, I never turn mine on cause of the wild bouncing of the amp meter it causes.

A loose connection anywhere in the system can cause the bouncing. If you have original fuse holders, specifically those under the piano keys but probably not exclusively these fuse holders, the wire connections are all solder connections. These can crack over time.

One extremely nice morning as I commuted to work in my 170 I was thinking what a nice day, when all of a sudden my inside right ankle started stinging. First thought, it's a nice day but early in the year for bees but somehow I was being stung. I reached down to my ankle smashing it into my leg to kill it when I found it wasn't a bee at all. It was a glob of molten solder that had dripped from under the panel and was burning it's way through my sock and ankle flesh and now my hand. Master switch was quickly turned off and once I figured I didn't have a fire I continued the few miles left to work.

On inspection it was found the solder joint must have cracked and heated up so hot it melted the solder. Repair involved cleaning the old solder, dirt and corrosion off best I could and resoldering the joint. It is not an easy procedure and requires a large soldering iron to do correctly. After the repair the joint was cold while in service as i checked it for many flights after and my amp meter was then rock steady.

Bruce Fenstermacher wrote:I... burning it's way through my sock and ankle flesh and now my hand. ... once I figured I didn't have a fire I continued the few miles left to work.....

New Headline Pictured in my mind: EMS-Emergency-Pilot Flies Self to EMS Heli-Pad!

Rebuild your existing Master Switch and then turn your attention to the AMP guage.

For a certainty there are C170s flying w an intermittent connectivity problem in the Master Switch.

Original problem starts with new factory Switch that had one riveted Lug that would work loose if disturbed at a future time.

This is a rare event. Avionics Technicians Sometimes refer to the mass of wiring under the panel as a "Rats Nest."
With the engine NOT running I observed a Tech connect a meter to something and reach his hand under the panel and Grab the Rats Nest, Shake it and the needle on the Meter would wiggle. Avionics were OFF!.

This shaking of the Rats Nest could loosen the lug on the Master Switch 40+ years after new installation.

Took me 20 years to come to realization.

Ask George.

Thanks for all of the replies and suggestions ! Looks like I have some trouble shooting to do ! I never thought about the Master Switch might be the problem !


Ralph