The Ford Barn

old31 · 07-28-2025, 10:54 AM

My ammeter just died.

What tests would you perform to verify that the ammeter is the culprit?

Also, what other tests do you recommend if the ammeter is okay?

nkaminar · 07-28-2025, 11:25 AM

Use your mulimeter to measure the resistance at the two terminals. If it is infinite then it is toast. The resistance should be very low.

Also test with a your car battery with a light bulb in series with the ammeter. The light bulb will restrict the current so that you do not accidentally blow out the ammeter.

Marshall V. Daut · 07-28-2025, 12:05 PM

Turn the headlights on without the engine running. The ammeter needle should dip into the discharge (negative) side of the face. Pull the instrument panel forward and switch the two wires on the back of the ammeter around. Turn the lights back on. The ammeter needle should go into the charge (positive) side of the face. If the needle responds as described in both tests, the ammeter is o.k. The problem lies elsewhere, such as in the wiring inside the terminal box or a burned-out generator/defective cutout.
Marshall

Marshall V. Daut · 07-28-2025, 12:15 PM

By the way, while the instrument panel is pulled forward, wiggle the ammeter terminal posts to check for looseness. If there is movement, tighten the lock small lock nut between the ammeter and the wire end. If there is still movement, the ammeter is probably broken inside. Also check for signs of burn splotches on the back of the ammeter, indicating a loose wire or loose terminal post.
Marshall

Bob Bidonde · 07-29-2025, 11:42 AM

Here is a temporary roadside fix for a failed ammeter.

Joe K · 07-29-2025, 01:35 PM

Sponsored Links (Register now to hide all advertisements)

Replacement ammeters vary in their quality.

The common replacement is 30-0-30 which are not original in markings but seem popular in production because of the 12V conversion crowd.

Finding a 20-0-20 is only slightly more difficult and one can resort to an Ebay "original" - although the bezel is usually tarnished as a result of nearly 100 years in the environment. The bezel ring can be removed and re-plated with one of the "foam brush" replaters commonly used for small area nickel. (www.caswellplating.com)

Repops can be found in 20-0-20, usually seen with an aluminum bezel ring.

One downside I've seen with the repop is they can be slightly "undersized" and bending the tabs out won't hold them in the panel hole. A solution to this I've found in backing up the meter in the hole with a 1-1/2" rubber o-ring such that the tabs bear on the thin o-ring rather than the panel back - it's a way to compensate for something which SHOULD have been made better.

Joe K

ronn · 07-29-2025, 03:03 PM

make sure the back posts arent touching the tank............

stevemclark · 07-29-2025, 05:12 PM

If you can't find an o-ring and get a reproduction and it is loose ,put a little RTV silicone dot on the back side of it after it is mounted (less messy)in the opening and spread it to the rear of the dash panel . Make sure it is straight.Let it cure before mounting dash panel in place.

old31 · 08-03-2025, 05:04 PM

Follow up:

All the tests that were mentioned to do on the above posts on the ammeter proved that it was ok. It was a bad wire to the cutout.

nkaminar · 08-03-2025, 05:17 PM

Ahhh, The old bad wire thing. It is always a bad wire or bad connection somewhere.

Glad you got it fixed.

07-28-2025, 10:54 AM	#1
old31 Senior Member Join Date: Jan 2016 Posts: 2,460	31 ammeter died My ammeter just died. What tests would you perform to verify that the ammeter is the culprit? Also, what other tests do you recommend if the ammeter is okay?

07-28-2025, 11:25 AM	#2
nkaminar Senior Member Join Date: Jan 2012 Location: Western North Carolina Posts: 5,863	Re: 31 ammeter died Use your mulimeter to measure the resistance at the two terminals. If it is infinite then it is toast. The resistance should be very low. Also test with a your car battery with a light bulb in series with the ammeter. The light bulb will restrict the current so that you do not accidentally blow out the ammeter. __________________ A is for apple, green as the sky. Step on the gas, for tomorrow I die. Forget the brakes, they really don't work. The clutch always sticks, and starts with a jerk. My car grows red hair, and flies through the air. Driving's a blast, a blast from the past. Last edited by nkaminar; 07-28-2025 at 04:15 PM.

07-28-2025, 12:05 PM	#3
Marshall V. Daut Senior Member Join Date: Apr 2011 Location: Davenport, Iowa Posts: 2,627	Re: 31 ammeter died Turn the headlights on without the engine running. The ammeter needle should dip into the discharge (negative) side of the face. Pull the instrument panel forward and switch the two wires on the back of the ammeter around. Turn the lights back on. The ammeter needle should go into the charge (positive) side of the face. If the needle responds as described in both tests, the ammeter is o.k. The problem lies elsewhere, such as in the wiring inside the terminal box or a burned-out generator/defective cutout. Marshall

07-28-2025, 12:15 PM	#4
Marshall V. Daut Senior Member Join Date: Apr 2011 Location: Davenport, Iowa Posts: 2,627	Re: 31 ammeter died By the way, while the instrument panel is pulled forward, wiggle the ammeter terminal posts to check for looseness. If there is movement, tighten the lock small lock nut between the ammeter and the wire end. If there is still movement, the ammeter is probably broken inside. Also check for signs of burn splotches on the back of the ammeter, indicating a loose wire or loose terminal post. Marshall

07-29-2025, 01:35 PM	#6
Joe K Senior Member Join Date: Jul 2010 Location: Cow Hampshire Posts: 4,610	Re: 31 ammeter died Sponsored Links (Register now to hide all advertisements) Replacement ammeters vary in their quality. The common replacement is 30-0-30 which are not original in markings but seem popular in production because of the 12V conversion crowd. Finding a 20-0-20 is only slightly more difficult and one can resort to an Ebay "original" - although the bezel is usually tarnished as a result of nearly 100 years in the environment. The bezel ring can be removed and re-plated with one of the "foam brush" replaters commonly used for small area nickel. (www.caswellplating.com) Repops can be found in 20-0-20, usually seen with an aluminum bezel ring. One downside I've seen with the repop is they can be slightly "undersized" and bending the tabs out won't hold them in the panel hole. A solution to this I've found in backing up the meter in the hole with a 1-1/2" rubber o-ring such that the tabs bear on the thin o-ring rather than the panel back - it's a way to compensate for something which SHOULD have been made better. Joe K __________________ Shudda kept the horse.

07-29-2025, 03:03 PM	#7
ronn Senior Member Join Date: Feb 2011 Location: NNNNNNNNJJJJJJJJJJ Posts: 7,648	Re: 31 ammeter died make sure the back posts arent touching the tank............

07-29-2025, 05:12 PM	#8
stevemclark Senior Member Join Date: Oct 2019 Location: Central Shenandoah Valley,Waynesboro,Va. Posts: 144	Re: 31 ammeter died If you can't find an o-ring and get a reproduction and it is loose ,put a little RTV silicone dot on the back side of it after it is mounted (less messy)in the opening and spread it to the rear of the dash panel . Make sure it is straight.Let it cure before mounting dash panel in place. __________________ It's not the destination ,it's the journey .

08-03-2025, 05:04 PM	#9
old31 Senior Member Join Date: Jan 2016 Posts: 2,460	Re: 31 ammeter died Follow up: All the tests that were mentioned to do on the above posts on the ammeter proved that it was ok. It was a bad wire to the cutout.

08-03-2025, 05:17 PM	#10
nkaminar Senior Member Join Date: Jan 2012 Location: Western North Carolina Posts: 5,863	Re: 31 ammeter died Ahhh, The old bad wire thing. It is always a bad wire or bad connection somewhere. Glad you got it fixed. __________________ A is for apple, green as the sky. Step on the gas, for tomorrow I die. Forget the brakes, they really don't work. The clutch always sticks, and starts with a jerk. My car grows red hair, and flies through the air. Driving's a blast, a blast from the past.