1968 %&#@$ Ammeter/Circuit

[franklinair]

I feel like I'm flogging a dead horse:
The ammeter reads continuity with an Ohm meter, barely deflecting the needle. (about the needle width)
The 2 wire circuit, one lead to the + post on the solenoid, one lead going to the Alternator reads 1.25 Ohms.
With power on (Battery only) I turned on the headlights expecting to see the Ammeter needle deflect negative. Nada, barely discernable.
I hate having to resign myself to having a non functional Ammeter.

Neil

 

[stangfan]

My understanding is that the ammeters are so old that they rarely register much of a change in current. The one in my GT/CS seems to pop up above the vertical centre line when the engine is running, but not by much.

 

[whodat]

I had mine converted to a volt gauge. When I put everything back together it didn't work. I still haven't removed the cluster to see what the problem is because it's such a pain in the butt to do.

 

[nikzack]

Have you checked continuity of each individual wire to make sure the wire itself without power is good?

 

[franklinair]

I did a continuity check on both wires (power off/Battery was disconnected). I checked this circuit from the Red & Yellow wires at the Instrument Cluster connector and read 1.25 Ohms after having cleaned all connections. In essence, you're reading a loop due to the fact that BOTH wires (Red & Yellow) terminate on the same terminal on the Starter Solenoid due to a spliced wire in the engine compartment harness.
The wiring diagram shows the Red wire going from the Ammeter to the +(Battery) terminal on the Starter Solenoid. The Yellow wire goes from the Ammeter to a Splice in the engine compartment which branches to the Alternator AND the very same terminal on the Starter Solenoid as the Red wire, effectively making them the Same point electrically.
I'm sure Ford's theory was just to look at the direction of current flow between the Battery & Alternator, but in order for it to work you'd have to have a VERY sensitive coil in the Ammeter- which these cars obviously don't have.

Neil

 

[CougarCJ]

My electrical guys here at work both say that to properly test these ammeters, you would have to have a battery and something to act as a draw, with the ammeter between them. This will only test the negative side of the ammeter though. They think that the little coil creates a magnetic field that moves the needle. Please let me know if this is incorrect?

 

[franklinair]

The problem as I see it, is such a minute current differential and an insensitive meter movement, the system as designed is doomed to mediocrity at best. I did order a new ammeter from CJ Pony Parts, but I'm not holding any high expectations. I do have an idea as far as modifying the circuit, and I'll keep everyone posted.

Neil

 

[franklinair]

Did some further checking (while I wait for the new ammeter).
The two wires (Red#655 & Yellow#654) that go from the ammeter to the + side of the starter solenoid read different resistance. The Red wire read 1.5 Ohms from end to end, and the Yellow wire read .25 Ohms. They both appear to be 18 gage, equal length. These readings were from the Instr cluster connector to the Solenoid. The only thing I can think of for the resistance differential is the fact that the Yellow wire has a factory splice that branches off to the Alternator (Green/White#3).
I have the Mustang wiring diagram, and made a simplified drawing (for my simple mind).
Steve seems to understand this circuit, but I don't. To me, if the 2wires are connected together at the solenoid (same electrical potential) I cannot grasp how the Ammeter would sense a + or - current flow direction. Unless the Ammeter functions as the old style 'induction' type that senses direction of current flow, but they had a 'loop' that sensed the current flow direction, rather than the 2 posts on the Mustang Ammeter.

Neil

 

[rvrtrash]

Neil, both wires should have the same resistance for the ammeter to even have a chance to work, and the lower it is, the better. Wires, and the connections, build up corrosion after time that increases resistance. All wiring has resistance in it. The longer the length and the smaller the wire diameter, the greater the resistance. The Ford ammeter circuit uses that wire resistance between the two connection points to cause a small amount of parallel current to flow though the ammeter, in whichever direction the current is flowing (charge or discharge). The closer the ammeter circuit resistance is to the resistance of the main harness wire between the two points, the better it will work. Unfortunately I think it was a case of one engineer not talking to another, because even when new, the circuit didn't work very well, and only got worse with time. At one point I toyed with the idea of moving the wire spliced in to the main harness a few feet further down the harness to increase the resistance between the points, but finally decided to just get a voltmeter from RCC instead. Ammeters are a holdover from the days when cars had generators, because generators produce varying levels of amps, speaking simply. Alternators should have voltmeters, because alternators produce varying levels of voltage. Hope this helps some.

Steve

 

[franklinair]

After much frustration, poor engineering on Ford's ammeter circuit design, and Steve's input I have decided to convert the electrical indication to a Voltmeter configuration. I've communicated with Rob McMullen of RCC Innovations and ordered a Voltmeter for my CS. Simple wiring mod, and it will be done. When I get it completed,I'll be glad to share the info. (I'm also going to install his Minitach in place of the clock.)

Neil

 

[rvrtrash]

Neil, here's the link to the write up I did and posted in the restoration section, in case it helps you at all.

http://www.californiaspecial.com/forums/showthread.php?t=10184

Steve