Ammeter: Stop me if you've heard this one before. - Vintage Mustang Forums

 
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post #1 of 12 (permalink) Old 03-30-2017, 04:23 AM Thread Starter
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Ammeter: Stop me if you've heard this one before.

My just purchased '66 has an ammeter that goes wild; Pegged at cruise and slightly bouncing below center at idle. It has an additional meter added under the hood added by the fellow from whom the PO purchased. It appears to mimic the behavior of the in-dash unit. The lights don't seem to dim/brighten and when I picked up the car after it had been sitting for a month, it started right up. Do I have a problem or is the unit especially sensitive due to incorrect wiring or a missing resistor somewhere in the system?
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post #2 of 12 (permalink) Old 03-30-2017, 07:30 AM
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Put a volt meter on the charging circuit and verify what's really going on. Maybe a flakey voltage regulator or a grounding problem between the engine and body?
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post #3 of 12 (permalink) Old 03-30-2017, 08:37 AM
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Sounds like it is working which is very unusual for a '66 ammeter. I have a '65 GT which includes an ammeter, but of a different design. At idle, the needle rests at or around the middle of the gauge, depending on the state of charge of the battery and the power draw at idle (i.e., lights or fan on, etc). Again, depending on the state of charge, while driving my ammeter moves very far to the right which indicates that the system is charging (it is measuring current, not voltage). However, due to the way the voltage regulator is designed, when the system is fully charged, the needle will usually start to bounce which simply indicates that the current is switching on and off rapidly as a result of reaching a particular design voltage.

If you don't like the behavior, you can replace the ammeter with a voltmeter which will give you a much more stable needle and slightly different information regarding the state of the charging system.

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post #4 of 12 (permalink) Old 03-30-2017, 09:24 AM Thread Starter
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I'm calling it an ammeter, but am I correct, technically? Also, where would the engine to chassis ground typically be located? I'm assuming that one may be added just about anywhere if there is doubt about the connection.

If testing with a digital multimeter, should I simply put the leads between the two battery posts?
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post #5 of 12 (permalink) Old 03-30-2017, 09:39 AM
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I don't think you have a problem. Your amp meter is working, you lucky stiff.

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post #6 of 12 (permalink) Old 03-30-2017, 09:40 AM
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Typically, you would find a ground wire/strap from the back of the passenger side head to the firewall. Many (like me), run a second cable from the location where the negative battery cable attaches to the engine block, to a point on the body/chassis.


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post #7 of 12 (permalink) Old 03-30-2017, 09:43 AM
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There must be at least 1000 posts about non-working Ford ammeters for every 1 post about one that actually works.
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post #8 of 12 (permalink) Old 03-30-2017, 09:58 AM Thread Starter
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Well, I know that I've never seen one this active. On the car I recently sold, if I did something like forget to turn off the lights, when I started the engine the meter would move toward charge. It was obvious, but not pendulum like, swinging wildly, back and forth.
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post #9 of 12 (permalink) Old 03-30-2017, 01:00 PM
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Quote:
Originally Posted by rose62 View Post
Also, where would the engine to chassis ground typically be located?
On V8 cars, it was attached to the rear of the RH cylinder head. On I6 cars, the rear of the head. Both attach to the same spot on the firewall.

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post #10 of 12 (permalink) Old 03-30-2017, 03:20 PM
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It may have been changed to a voltmeter.
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post #11 of 12 (permalink) Old 03-30-2017, 04:02 PM
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It wasn't a very good design...obviously! I relied on more current going through a large wire and less going through a smaller wire known as a shunt. Anything could disturb it such as poor contact on the large wire or the smaller wire, poor grounding (firewall ground strap), etc. Sounds like you have too much current going through the shunt which is usually an indication that the main path through the larger wire is compromised. Lots of times, too much current goes through the small wire when the large wire gets compromised and the ammeter blows out. Your ammeter days may be numbered unless you figure out the problem.

When I rewired my car, I tested how much current it took to make the ammeter to peg full scale to determine how many watts it took and then got an assortment of power resistors and put one at a time in series with the ammeter until I got a good but not too much deflection of the needle. I also put a fuse in series with the ammeter so if the main wire got disconnected (worse case) the ammeter would be protected. Now the ammeter circuit is current limited and it's been working for 13 years. I forget how many amps it took to peg and how many ohms I used for the power resistor, but i can try to find my notes if someone wants to know.

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post #12 of 12 (permalink) Old 03-31-2017, 06:11 AM Thread Starter
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Quote:
Originally Posted by blkfrd View Post
It wasn't a very good design...obviously! I relied on more current going through a large wire and less going through a smaller wire known as a shunt. Anything could disturb it such as poor contact on the large wire or the smaller wire, poor grounding (firewall ground strap), etc. Sounds like you have too much current going through the shunt which is usually an indication that the main path through the larger wire is compromised. Lots of times, too much current goes through the small wire when the large wire gets compromised and the ammeter blows out. Your ammeter days may be numbered unless you figure out the problem.

When I rewired my car, I tested how much current it took to make the ammeter to peg full scale to determine how many watts it took and then got an assortment of power resistors and put one at a time in series with the ammeter until I got a good but not too much deflection of the needle. I also put a fuse in series with the ammeter so if the main wire got disconnected (worse case) the ammeter would be protected. Now the ammeter circuit is current limited and it's been working for 13 years. I forget how many amps it took to peg and how many ohms I used for the power resistor, but i can try to find my notes if someone wants to know.
Might be useful.
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