1968 Ammeter Circuit

[franklinair]

I'd like to have a functional ammeter. Our cars are notorious for this gage not to work, or minimally at best.
Looking at wiring diagrams (2 different ones), it appears as though both wires from the ammeter end up at the + Battery post of the starter solenoid (wire # 654 & 655). This doesn't make sense to me. How would the ammeter read a current flow if both wires are terminated at the same point (electrically)?
To compound my confusion, the diagram indicates wire #654 splices into wires #3 & #37 & #38A. 38A goes to the starter solenoid +Battery post, the same as wire #655.
What am I missing?

Neil:undecided

 

[Ruppstang]

Neil PM me your phone #.I think I can help you but too much to type. Marty

 

[robert campbell]

Steve "rvrtrash" where are you???

Rob

 

[rvrtrash]

I was going to jump in but thought Marty had it covered. Here goes for everyone else. The leads from the ammeter connect to the main power wire several feet apart. All wire has a resistance over distance which causes voltage drop and the Ford ammeter uses the difference in the two points to calculate which direction current is flowing and how much. Needless to say, it doesn't work worth a crap because the distance (therefore resistance) is minimal and the gauges aren't very precise. The only fix I could suggest trying, if you don't want to go to an aftermarket gauge, is to unwrap your harness, find the ammeter lead furthest from the battery, cut it and reconnect it further from the battery. This will increase the resistance between the two points and cause your gauge to react more. Go too far and you'll peg or smoke the gauge. To little and it will only fluctuate a little more than before. I can tell you it's very difficult to get a good solder connection on the main wire because of it's size. It acts as a very efficient heat sink. The real answer is that cars with alternators should have voltmeters instead of ammeters anyway (unsolicited personal opinion).

Steve

 

[CougarCJ]

I agree with Steve on the volt meter.

 

[Ruppstang]

The first thing I try is cleaning the terminals that attach to the solenoid till they are shiny. It will not read correctly with corrosion on them. Works on most cars. Did that work Neil?

 

[franklinair]

Haven't done it yet (today we had to T/S & repair the alternator system an an airplane). I'm going to follow Marty's advice, and clean all connections - and see how it goes.
But I must admit I'm skepticle of it working. It just doesn't make sense to me that the 2 wires from the ammeter, terminated at the same electrical point (wires #654 & 655) on the solenoid would sense current flow in either direction + or -. It seems more logical to me that the wires - being connected at the Battery + are really sensing voltage and deflecting the ammeter needle according to the voltage. And Ford designated this indication as current, when in reality it's sensing voltage.
Steve's theory of wire length/resistance, while used in more sophistcated circuits in modern electronics doesn't seem to fit with 40 year old technology in automotive circuits. The automotive grade 18 gage wiring is kinda generic, not calibrated resistive type wire.
I'll get to the bottom of this beguiling mystery, and it may very well turn out to be clean connections, but I'm just as certain that this circuit does not sense current flow. More than likely- voltage.

Neil:undecided

 

[rvrtrash]

Well, I tried. :surrender

Steve

 

[tomcwarren]

Check out the latest (May) issue of Mustang Monthly. They have an article on troubleshooting Mustang instrument panels, and discuss the ammeter & it's function. Seems most of them burn out pretty easily. They also point out that because they're unfused, they can be a fire risk.

Voltmeter FTW! :wink:

Tom

 

[robert campbell]

Boy do I like my voltmeter.......

 

[Mustanglvr]

Well, I tried. :surrender

Steve

That was a good try Steve! :thumb: :grin:

 

[franklinair]

I believe I have figured a way to convert the ammeter to a voltmeter, using the original ammeter & movement. The original gauge will function as a voltmeter and the pointer 's center position will indicate 12 volts. Less than 12 volts deflects the pointer to the negative (discharge) side, +12 volts will deflect the pointer to the positive (charge) side.

There is a company that will replace the ammeter movement, but requires 4 to 6 weeks to modify your gauge cluster + $150. Bummer.

Does anyone have an old/spare cluster that I can borrow to fine tune the modification? I have a customer that owns/operates an electronic repair facility to help me finalize the circuitry. I guesstimate it would cost about $20 (including a fuse to protect the circuit- which the factory ammeter circuit does not have). It would be self contained @ the back of the cluster, and would not disturb the original wiring harness.

Neil

 

[Ruppstang]

Neil I have lots of that stuff I will call you. Marty

 

[di81977]

Check out the latest (May) issue of Mustang Monthly. They have an article on troubleshooting Mustang instrument panels, and discuss the ammeter & it's function. Seems most of them burn out pretty easily. They also point out that because they're unfused, they can be a fire risk.

Voltmeter FTW! :wink:

Tom

Just read on my corvette forum a few days ago about a guy that took his 65 Corvette out for a drive and after about 5 miles smelled an electrical burning smell. He quickly pulled over and disconnected his battery. Turns out his amp guage was faulty and the wires were melting.

There were a couple of key learnings for me with his story:

As mentioned by Tom, the wires going to the ammeter are hot and can be a fire risk. Might want to think about adding a fusible link. Corvette changed their design and added a fuse in 1967.

Second, think about adding a quick disconnect on your battery. Much easier unscrewing or lifting a lever when there is an emergency than looking for a 1/2inch wrench. Could save your car.

david