My answer above is a bit lame so I did a few drawings which I hope help. If I have managed to confuse anyone beyond all reason, please feel free to email me and I will see if I can confuse you more.

 

What gets me this is an original wired truck with nothing added or removed.
It's been in the family since new and the gauge always moved 1/2 of a nnedle width left right maximum.
Question; where is the shunt located on a 68 CS?
I'm thinking check all connections for looseness or resistance at all connections from shunt to gauge.
How high of a DC voltage can be applied thru the gauge to get a full sweep from zero or center position to full discharge then reverse polarity to do the same for a full sweep zero to full charge?
I just want to check gauge function for a full needle sweep.

.....=o&o>......

 

Sorry, Carl, I do not have a wiring diagram for these trucks. All the 1972 shop manual says is to turn the lights on with the engine off, and if the meter does not show a discharge (after you check the connections on the rear of the meter) to replace the meter.

The 85 manual I have is a little better - it shows a Orange fuseable link from the batt terminal of the starter relay, that somehow changes to a black/orange wire that goes to one side of the ammeter. Where the fuseable link and the black/orange wire join, there is also a smaller yellow/white dot wire that goes to the "A+" terminal of the voltage regulator.

The other side of the ammeter goes to the batt term of the alternator, with a smaller wire going to the ignition switch. The shunt wire, if used, goes from the same terminal that feeds the ignition switch to the battery side of the amp meter.

Why don't you disconnect the ammeter and replace it with a multimeter? You can start on a high scale and drop to lower scales until you know what sort of currents you are dealing with.

Once you know that you can use a 12 volt battery and some resistors to build a test circuit for the stock gauge. You can even run your multimeter (set for amps) in series with the stock ammeter to see what current reads on the stock meter.

I wish my 72 was running, it has the same lazy gauge as yours. But I am still doing the engine swap so it will be a while before I get back to the instrament cluster.

 

Thanks ***** B, this is on a friends truck, on my 68 it moves a total of 1/8" or 1/16" plus / minus and been that way for years. Will be pulling the cluster forward to change lamps then a good time to do meter testing.

.....=o&o>.....

 

I know this post is a little old, but I might have something to add. I remember from a long time back a Ford service man telling me they could make a lazy ammeter more responsive. He said they would take fine wire and wind it around the coil in the ammeter to strengthen the magnetic field. You would have to solder it to the existing teminals and make sure you had the windings going in the same direction as the original windings. I don't remember if he said they added the coils on top (parallel) or if they disconnected one end and added the windings to (series) the original windings. I'm thinking it would be more effective to put them in series.