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66 coupe dash installation problem

1K views 13 replies 4 participants last post by  sixfifty 
#1 ·
My 66 project is getting close to the finish line and I have encountered my first real problem and it has be scratching my head.
When I pulled the dash apart, I pulled one wire at a time and labelled the location on each wire with tape and a marker. Today when I was hooking up the wires to reinstall the instrument cluster, I got a small spark when I hooked the yellow wire onto the ammeter. When I attached the red wire to the ammeter, the gauge immediately filled with smoke. I unhooked the wires as fast as I could but I'm afraid the damage is done to the gauge.

Anyone got an idea why this occurred? Was there a step I missed on the instrument cluster hook-up, like grounding the housing first or....?

Any help will be much appreciated
 
#2 ·
Sorry no help here but looking to learn. I wil be assembling my 66 coupe shortly and hope to avoid similar issues. You got sparks and smoke with your battery disconnected?
 
#3 ·
I'm with Fooz---I don't see how you would have gotten a spark and smoke unless the battery was connected. The first rule in working on anything electrical in a car is to first disconnect the battery. That said, I don't have a clue what caused the problem, other than a possible short somewhere, as long as you put them on the correct terminals (red on the inside, closest to the speedo). The red wire should go to the front post on the starter solenoid. The yellow goes to the alternator, but also to the same post on the solenoid (not really sure how that works. . .). Neither terminal on the meter should show a short to ground with the red and yellow wires not connected.

But I'm afraid the meter may indeed be toast.
 
#4 ·
The battery was connected, I was checking bulbs and such prior to this happening. Nothing under the hood has been disconnected except the coil wire so I couldn't have mixed up any wires there. I checked the labels on each wire as well as the colors as I hooked up every single wire. I am stumped.
 
#5 ·
I don't see any easy way to explain what you did but you have something connected very incorrectly or your ammeter was defective - it certainly will be defective now that its smoked. You are the only one who was there to do whatever so you will have to figure it out by looking at what remains. There is no procedure to hook up one wire before another one; it makes no difference. Reversing the wires will make no difference either except that the meter needle will move in the wrong direction(s).

Attached is my standard redrawing of the '66 ammeter wiring. Starting with 1966 the ammeter was closer to a low impedance voltmeter than an ammeter. Its called a shunt ammeter since most of the measured current does not go through the meter but through a 'shunt'. There are NO grounds involved except for the light bulbs involved with lighting the instrument cluster. The ammeter is also not connected in any way to the other gauges or their CVR so whatever they do has no affect upon the ammeter.

Both sides of the ammeter are connected to +12V at different ends of the same wire in the alternator wiring. The ammeter is simply measuring the voltage across a 22" length of 12 gauge wire which acts as the 'shunt'. Its always hooked up, it never gets turned off, it works 24/7 with, or without a key any time the battery and/or alternator are hooked up.

There should never have been a spark by connecting either wire. You should have stopped right there since something is way wrong in your wiring or how you marked it. Neither ammeter terminal should have any path to ground through the meter for electricity. It only conducts electricity internally from one terminal to the other. The max voltage across the ammeter should be maybe 1/4 V which will never generate a spark. The normal voltage across the ammeter is a few millivolts; one millivolt = 0.001 Volt.

It sounds like you connected one side of the ammeter to ground. Since its resistance is normally about 1/6 Ohm it will try to draw about 70 Amps when grounded on one side; that is 100x the current required to peg the meter needle. That will instantly destroy the ammeter and it may not have been too kind to the small wires involved either. Probably the meter acted as a fuse and your wires are OK. If so, that should allow you to measure and see what you did wrong to get this to happen.
 

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#9 ·
Both sides of the ammeter are connected to +12V at different ends of the same wire in the alternator wiring. The ammeter is simply measuring the voltage across a 22" length of 12 gauge wire which acts as the 'shunt'. Its always hooked up, it never gets turned off, it works 24/7 with, or without a key any time the battery and/or alternator are hooked up.
Ivy66gt, I have been wracking my brain trying to figure this out and, while rereading this thread, came across this little nugget from your post. If I'm reading this right, either of the wires that hook to the ammeter should read battery voltage when probed with a multi-meter, correct?
If so, then I do have a wiring problem somewhere. The yellow lead for the ammeter shows battery voltage when checked but the red lead does not (although it does not show that it is grounded either).
I really appreciate the help and the patience!
 
#6 ·
Thanks for the usual thorough explanation and drawing, Ivy66GT, although I should have been able to figure the shunt part out myself. I had to replace the ammeter in my '66 years ago, and had to put a resistor in the red line to get it to read properly. I don't recall if that was before or after I had run new 10ga wires to it, although I suspect it was after. Perhaps less resistance than in the stock wires. I think my logic at the time was probably along the lines: it's an ammeter, so it measures current, so the bigger the wire the better. Obviously that was all wrong.

I'm in the process of replacing the stock instrument bezel with a 6-gauge cluster, which has a voltmeter, labeled as such and connected to switched 12v.
 
#7 ·
Ivy66gt, thanks for the diagram and the advice. I had a couple of local mechanics (friends of mine, luckily) come over and check out the wiring, we spent an afternoon going over everything and they both agreed that everything is hooked up right and the wiring matched all the diagrams we found online (your drawing was one of the ones I found while searching the 'net, by the way) and no shorts to ground were found anywhere. Their consensus is that the gauge must have been bad since all the wires check out. I've got a new one on its way and will post the results when I hook it up. If it happens again, I guess I'll wire in a voltmeter with new wiring instead.
 
#8 ·
An internally bad meter is a possibility. But how, and when did it get that way? Had the smoked meter ever been connected to a harness previously? Or was it a meter not known to have ever worked correctly?

There is usually just a single loop of wire inside one of them that passes near the needle which has a small magnet on the pivoting end. The ends of the wire loop should connect only to the terminals on the back side. Is there enough left of the smoked one to look inside?
 
#10 ·
Obviously I'm not Ivy66GT, but yes, both terminals should show voltage. You need to check continuity on the wire not showing any. The way I would do it: set my volt-ohm meter to ohms and run long leads to the dash connector and the end under the hood. All this with the battery disconnected, of course. I'm sure there are other, perhaps easier ways to check.

That said, I don't see any way the lack of voltage would burn the meter out. There has to be (or have been) a short short to ground somewhere.
 
#11 ·
Yes, as Charles indicated, you should measure continuity from either ammeter wire to the alternator output. When operating correctly, both wires will have '12V' on them with one having just a few millivolts more than the other one.

Both the yellow and red ammeter wires are rather small (only 16 gauge) so its possible that the red one now shows no continuity since it burned up from too much current while you were generating smoke inside your ammeter. If so, that would not be the cause of your problem but a result of your problem.

I agree, the cause of your problem is a good mystery.
 
#12 ·
I found the problem with the red wire having no power to it. When the painter had removed the solenoid from the inner fender to paint the engine compartment, he had reattached the red wire to the starter side of the solenoid instead of the battery side. The rest of the wires are right, I moved the red wire to the proper location and now there is power to both the red and the fellow wires.
I tried to test the gauge but, as expected, it is dead. The replacement gauge arrived yesterday but it was damaged in shipment so it will be late next week before I can explore this further.
Thanks again for the info and assistance
 
#13 ·
Ouch. That explains a lot. The starter windings are meant to drawn 100s of amps so they will look much like a short to ground for the ammeter. The only part I don't understand is why you got a small spark when you connected the yellow wire to the ammeter. But after yellow was connected to one side you had +12 inside the ammeter. Then when you connected red you were trying to power the starter through the ammeter. NOT a good thing as you quickly discovered.

Sounds like you have it figured out now if you can only get a working ammeter. I am curiouis to know of the repro ammeters have similar sensitivity to the originals. When you get it working let us know how far the needle moves with headlights, brake lights, etc.
 
#14 ·
Ivy66GT, I didn't think that through about the starter, it sure would cook things quick.
I'm still baffled about the little spark on the yellow wire too but everything is checking out and I can't think of any cause for it.
I'll let you know when the ammeter arrives and post about its operation.
Once again, many thanks!
 
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