I've read over most of the previous postings on how to verify if an amp gauge (in a '66) is working or not but seem to have run into a snag.
When my Dad and I reworked just about everything on my '66 Mosport Coupe we didn't get to the instrument cluster. The amp gauge seemed to be a dead player for a lot of years and I wasn't ready to yank the whole cluster till I was ready to clean it all up as well as the steering column, etc. When I recently removed the cluster and column, I already had a replacement amp gauge on hand figuring that the factory gauge was bad. One of the old Forum posts showed how to bench test the gauge with just a small 1.5 or 9v battery. Tried it and the factory gauge moved (both ways when leads were switched). Hmmm, maybe it's something in the wiring after all. I've already pulled the firewall connector and that looks fine and verified that the Black and Black/White wires are good to the firewall connector and, on the black wire, good from the connector to the positive post on the starter solenoid. The Black/White wire which heads up to the horn relay also proved that the wiring is good from the firewall forward.
Now what? By the way, at the present time I've got the cluster out, wiring hanging down and steering column out so I won't be firing up the car in the short term but was hoping to at least get the 'hand over hand' investigation out of the way while it's all exposed.
Lastly, for the 3500 miles that I've put on this since we went through most of the car, the charging system (stock 37A, 10DN alternator) has been working fine. It's just the amp gauge that sits dead still regardless of electrical activity. Thanks

 

It sounds like you already did this, but if not, you can use a test light or voltmeter to verify that you have power at both female terminals of the ammeter connector. You should read battery voltage on both of them.

 

Unfortunately, I didn't check that before stripping down the dash, probably because I had (incorrectly) assumed that the gauge was defective and didn't suspect anything else. With all the wires hanging down loosely and unconnected, I don't think I might try verifying voltage through the two wires (black and black/white) will I get things back together.

Other than the wiring that I've checked so far, is there anything else I should be looking at? Since the horns worked and charging system worked I didn't think the voltage regulator (new as of 3500 miles ago) or horn relay (not new) would be suspect but a while back I did learn that components that I wouldn't normally suspect could cause a problem (like the dimmer switch being responsible for normal driving light low beam problems) could be the culprit.

Also, is there a way to bench test a horn relay?

 

Doesn't it need to be running to check this? One side should be battery, the other alternator.

Vet65te - If you don't have a wiring diagram, get one. DrRebuild has a great one in color:
http://docrebuild.com/oosoez.html

Will help a lot following wires.

 

Yes, I've been following the wiring diagram from the '66 Service Manual.
That's how I was able to figure out the two wires at the amp gauge were Black and Black w/White Stripe and following that diagram led me to the solenoid post for the Black wire and Horn Relay for the Black w/White Stripe. Having checked (and verified continuity) of those runs along with the firewall connector I'm looking for additional info on what next to look for.

 

No need to check the horn relay - it doesn't "do" anything for the battery gauge except provide a connection point; both the screw terminals are on the same common buss, and the relay has nothing to do with that buss. The relay portion only makes the horns work.

 

Then it seems that if I've found the wiring to be intact I'm down to a bad connection either at the gauge or at the firewall connector. The gauge twin connector and the firewall connect both looked good but I'll pop them loose and clean them out with with a wire brush and some wet/dry paper and hope for the best when I finally get it all back together. Thanks to all for the assist.

 

If your are not comfortable with hooking up the battery to test the connections (can't say as I blame you), you can do a test with your ohmeter. If you have a digital meter it will probably have an option for an audible 'beep' to tell you when you have continuity.

Get a long length of wire and connect it to the terminal at the horn relay. Connect the other end to your ohmeter. Connect the other ohmeter probe to the black/white ammeter terminal and see if you have a complete circuit. Then do the same test for the black wire at the solenoid.

 

Actually, I did get adventurous and first checked battery voltage at both the black and black/white wires with the firewall connector disconnected and then after reconnecting it (and taping anything dangling from the open cluster area) checked for voltage at the two wires on the amp gauge connector. Result: I've got voltage...both times. So, it looks like the wiring and connectors are doing the job. I'll have to wait to get it all back together before finding out if the 'difference' in amp flow shows on the gauge. One thing I did notice was that the thin bayonet connectors on the male plug at the engine side of the firewall, while not corroded or crappy looking, were a bit flattened and I did coerce them more open than they were with a small flat blade screwdriver.
Did this help the connection, I hope so, but I'll still probably have to wait till I get it all buttoned back up.

Now, when I finally get to that point, if I find the amp gauge needle still sitting dead still (same battery voltage showing on both posts), then where do I check to see why the alternator voltage isn't registering? Not sure if I phrased that correctly but I think you get the idea.

 

Looks like you have done everything you can regarding the wiring. I will go out on a limb and say that the ammeter is faulty. I don't know if that test you used with the 9 volt battery is valid or not. I have a 700 page book on electrical systems and the only thing it doesn't show, is a way to test this gauge.

 

If you have voltage at the connector and the gauge is good the only thing left is the connection at the gauge itself. It has to work then.
Good luck

 

If you have it that far apart (cluster out) why not replace the harness? The cluster is the worst part of a harness replace, sounds like you are there already. Lectric Limited has a complete harness for about $900. They say the harness was designed to last 10 years, not to mention how many hands have butchered the wiring in 40+ years. I did mine last year, had several electrical issues that were fixed by this.

http://www.lectriclimited.com/mainpage.htm

The Service Manual diagrams work but there is nothing like a color diagram.

 

That little test of the amp gauge using either a 1.5 or 9 volt battery wasn't meant to verify accuracy but was more intended to show that the amp gauge will indicate the 'different' readings between the two posts. In my case proior to pulling everything apart, the gauge just stood still with no movement at all.
At first I suspected (and maybe wrongly) that the gauge was defective but after trying the simple test with the 1.5 volt battery (found on a previous posting on this forum) the needle moved to indicate the difference. I also tried the test on the new gauge fresh out of the box.
Result was the same movement.

Regarding replacing the harness, while the underdash harness is somewhat reasonable at around $400 by itself, it's typically found to be in good shape even considering the age of the cars since it's protected inside and away from the underhood heat and road dust and dirt. My harness appears to be in good condition and other than the one set of 3wires that had obviously been cut and respliced poorly to the wiper switch,the underdash harness is in serviceable shape. There is no brittle insulation to be seen or any other crappy repairs. I've verified that there is continuity from the engine compartment in to the two leads on the amp gauge.
Once the cluster and column are back in place I'm hoping I'll see the normal amp gauge reaction as the alternator does it's thing. Are my fingers crossed...you betcha.

 

With all the testing you have done , plus your purchase of a new gauge, I am betting it will work fine.

 

And I'm hoping you win that bet because while I did buy that new gauge, finding the old one reacting to the bench test with the 1.5 volt battery just like the new one made me decide to put the 'old' one back into place.

I might have mentioned before that I'm not the most gifted person in the world when it comes to electrical theory but I am good at following instructions and testing recommendations. Of course, with my luck that also means I have crossed fingers on both hands.
The phrase from my past that comes to mind is...
'firing for effect'.