( 1968 L68 427 )
I got down my to-do list to investigate why my amp meter hasn’t worked since I brought my car home 3 years ago.
I found my horn relay fried years back and the two fuseable links leading directly off of it were fried as well.
The relay was fried because someone had the steering column apart and put the spring that goes directly above the signal cancelling cam under the horn button where it rattled around and made constant contact. No horns were blaring to let them know their mistake because one was missing and the other broken.
Ok so I repaired that putting the spring where it belongs, installed a new relay, new horns and the proper fusable links.
Ok the horn relay has a bus-bar providing a positive + connection to three posts and how the amp meter is wired in confuses me.
Volt meters are as simple as can be, I have antique tractors that have amp meters but the power from the generator or alternator run thru the amp meter to the top post on the starters. This seems to have both posts on the meter connected to the same source. I don’t understand how this works… but the schematic has it like this so I’m doing my best to make it right.
On the wiring schematic the left side post goes thru an orange fusible link to a black & white wire to one post on the amp meter. After repairing the orange fusible link I checked continuity to that post connections (both ends disconnected) and it was good but the amp meter still didn’t work.
Continued….


The center post has a red wire that connects to a junction with another orange fusible link that connects to a black wire running to the other post on the amp meter. I’m not sure where to find that link but it appears to be in the engine bay. That junction also splits off with a wire running to the starter sharing a post with the positive cable from the battery. Both my wiring schematic and the AIM manual show just these 2 wires connected to the battery post… but I have 3 wires. The third being a 10 gauge black wire going too ???
Theres also supposed to be another fusible link down close to where it connects to the starter .
Mine has a bad solder connection apparently omitting the fusible link.
At this point I’m figuring to track down and replace the fusible link that goes to the black wire / second post on the amp meter and repair that then replace the fusable link in the brown wire down at the starter. As for the extra heavy black lead on that starter post I’m at a loss.
Does anyone have an idea what that is for and or why someone might have added it ?

 

Ok, I had a look at where the fusable link may be and its right before the bulk head connection, right under the vacuum tank buried under the drivers side fender.
I could try to unplug the harness at the bulkhead but my gut is telling me it might create more problems than it solves.
Gave it some thought and this circuit has three connections , the horn relay, the starter, and the second terminal on the amp gauge.
I’m thinking the best course of action would be to disconnect all three connections and test continuity between them.
If theres continuity between the horn relay and the starter connections but not between either of those to the the amp gauge I’ll run a jumper to see if I can get the gauge to respond to something like a discharge”—“ by turning on the lights with the key on engine off. If that works I could run a separate wire with an appropriate fuseable link to the amp gauge until I feel ambitious enough to take on the bulkhead connection.
Anyone see fault in this as a solution or temporary one ?

 

Amp gauge is a "shunted" galvanometer circuit. What that means is that it is NOT directly measuring AMPS flowing in the system--it is measuring the small amount of voltage drop across a [calibrated] length of heavy gauge wire in the main power line. This millivolt drop over several inches of wire is proportional to the current flowing thru it, so the "ammeter" just has a scale that reads in amps. Cheap, easy to do and works quite well.
Someone seems to have rewired the ammeter in your car and sent one of the wires to the starter. It won't work that way..or, at least, it won't read accurately, if it reads at all. Both leads from the ammeter go to the bulkhead connector and on the firewall side, blend into the main wiring harness where they attach at two different points to the SAME power wire. These points are about a foot apart; so the wire between where they are connected becomes the "shunt" for the galvanometer/ammeter.
Not sure how to advise you about correcting what you have now. You may just want to install a later vintage C3 voltmeter. However, the lettering on that will be WHITE instead of the pale GREEN as in your gauges.

 

If one of the meters series fusible links was toast I would suspect so is the meter...as a high current thru the meter circuit could have happened. As far as the extra black wire perhaps when whatever event blew out the cars wiring the PO added a replacement wire. Try taking an ohm meter and see if either one (brown or black) goes to the horn relay barrier strip.

 

Another idea for the extra black wire is that it is hooked up to the battery as is the other one...maybe in an attempt to resolve a heat soak problem. See if there are two black wires on the positive battery post connector.

 

If you see the wiring schematic the second lead from the horn relay to the amp meter goes to the orange fusable link to the black wire to the gauge, the red wire runs down to a another fuseable link and the connects to the starter post that the positive battery cable connects to. That is factory but what isn’t on my car is that theres another wire, a 10gauge black wire connected to that post . No idea yet what thats connected to and thats definitely not factory.

 

Didn’t mention it but I made that assumption also and replaced the gauge.
I’m going to disconnect the wires, the battery and take an ohm meter to it today. I hate bubba wiring… unless its me At least then I know what I did.

 

Battery post connection is as it should be. It did have “house”wiring to the battery leading to auxiliary driving lamps up front . The headlamps weren’t working because all the fusable links at the horn relay were fried cutting power to the lights. The PO was working out of his wheelhouse trying to repair wiring and yes, I had a good bit of house wiring in the car.

 

I am not familiar with the 68, but when I bought my 76 the fusible link to the starter had evidently fried so bubba just cut it and put a wire nut on it😜. I replaced the fusible link, still no worry. Removed the gauge, disassembled, found a segment of the fine wire broken. Soldered it back together, now gauge works as it should

 

I had house wiring wire nuts in my car when I got it.
I suppose I could’ve repaired the gauge but I bought a gauge kit off eBay from one of the big Corvette vendors. It was cheaper buying all 4 than two separately so I just replaced the gauge. If I get stumped on this theres a part of me thats tempted to install the volt meter made to replace the amp meter.
Just have to run a positive and negative lead to it and done. I like volt meters better as well as it shows the state of the battery and alternator output.

 

I’m zeroing in on it but what I’m finding is not good but I’ll revise what I said on the wiring. The fusable link was omitted in the wiring by the starter but its the black wire thats spiced into a 10ga red wire thats proper. The smaller 18ga yellow into red with the yellow aftermarket crimped ring terminal thats in question now.
Having removed the electrical tape revealed really terribly soldered connections with no sharing wrap or crimps. These I will redo properly.
The jury is still out on my throwing in the towel and replacing the amp meter with a volt meter.

 

Perhaps this diagram will help. Posted to a old thread on this topic.

 

Good Morning Lt. Mike! Top of the morning to you, sir!

Just beware that your 1968 has unique (1 year) coloring on the gauges so if you mix and match them you will potentially have different colors on the gauges. On my 1968 roadster like yours I have a upgraded alternator and this requires a wiring modification. I ran a #6 marine cable from the alternator output to the battery post on the starter motor and this allows the extra current being generated a direct path to the battery for charging purposes.

My 1968 C3 could have been buggered with as well but on my car there are two fusible links that come from the starter area and go to my Horn relay which then supplies power to the fuse box and the rest of the car. A few years ago I had a electric fan controller self destruct and when it did, it also took out the two fusible links. I bought a set of fusible links at NAPA and soldered them in which made everything work again. They are also available on Amazon. Just be absolutely sure to replace them with the same sizes of links. When the links were burnt open the dashboard and fuse box were all without power, it wasn't until I replaced the fusible links that the fuse box came back to life. I also put a good piece of heat shrink over them to protect the wires.

The ammeter in my 1968 has a current shunt mounted to the back of the meter. The current shunt is a bit different than the circuits described above. The wire that supplies power to the fuse box goes through the shunt and it calculates the power being used based on the current flowing through the shunt. My 1968 C3 is # 2595 which is an October 1967 Corvette assembly.

I understand the desire for a Volt meter as I too wish I had one in my system. Fortunately my Holley EFI system monitors the battery voltage and displays it on the main screens. The Holley EFI software will tell you the voltage and alert you if it goes too low or too high. I would likely keep the ammeter for originality as long as it is in the right "color" for the 1968 models. I have tiny DC Voltage displays made for use on motorcycles in tight places. They are about 1" in diameter and show three spaces, this allows you to see the voltage on the fly. I also have a cigarette lighter Volt meter that just plugs in and displays the voltage clearly.

Having spent many years testing batteries for Westinghouse I learned that using the voltage to determine capacity is not always very accurate. There is value to knowing if the car is charging or discharging with the ammeter just for the comfort of knowing all is okay.

 

Finally...someone who understands that value of the ammeter. It provides MUCH more info on the charging system...if you know how to read it.

 

The shunt resistance presented to the amp meter may be different now with all the wiring, fusible link and connection resoldering changes. Might make your meter read way off....in my case after fixing all this stuff, the meter barely moves either way.

 

You make a good point and it echos what I’ve read in other posts on this topic. Theres a fuseable link buried under the vacuum tank where it goes thru the firewall. Is it good ? No clue . You’ve seen the photos of the soldering work done by the previous owner(s) and there will be my work that will be considerably safer and certainly better looking but yes I’m sure it will differ from what the factory had no matter how hard I try to replicate it. I’ve read that even altering wire length will affect the amp meter to some degree. The best way to correct what I’m dealing with is to replace the harness which I’m not ready to do right now.
For this reason I’ve ordered a volt meter in the style of the ‘68 amp meter. Hook up is simple, a solid ground connection and a line tapping into the ignition circuit and there just happens to be an available pin in the fuse block marked “ign” . I won’t burn any bridges in respect to being able to return it to having an amp gauge but I feel now that trying to make the harness I have now work properly… well I’ll just be chasing my own tail. (Insert raising the white flag emoji here)

(the flute block has been cleaned up considerably since this pic was taken )

 

Reading this its interesting. The line running from the battery post on the starter is a 12ga red wire to a 14ga brown fusable link to a 10 gauge red wire.
14 being the thinnest gauge is definitely the weakest link.

 

Both types of galvanometers circuits as typically used as either voltmeters (with a high series impedance) or ammeters (low impedance shunted) can have some benefits for determining what is going on in the electrical system in our cars. The voltmeter is helpful to see that the generator is charging the battery when the car is running and can also give a relative idea of the state of charge of the battery when the car is not running. The ammeter can show if the battery is taking current and energy is being stored in the battery for later use. Also can show that if the generator is not functioning that the negative current draw is eventually going to drain the battery.

Also here are the fusible links I have found so far on my car built June of 1968.

Main Starter B+, 14 Gauge, Brown. Located by the starter in the harness wiring assembly and connects to the large B+ terminal on the starter. Connected to a 10 gauge wire to the horn relay and then to the rest of the car.

Horn Relay B+ terminal, 16 Gauge Black. Comes off the horn relay and feeds the ignition switch, head light switch, hot battery fuses at the fuse block under the dash, and other misc. locations. Located at the horn relay using a lug to the hot B+ terminal. Connected to an 18 gauge wire.

Horn Relay B+ Terminal, 20 Gauge Orange. Comes off the horn relay and is in series with the Battery amp meter and item 4 fusible link. Located at the horn relay using a lug to the hot B+ terminal. Connected to a 16 gauge wire.

Firewall Harness, 20 Gauge Orange. The other fusible link in series with the Battery amp meter and item 3. Located near the wiper motor relay slightly to the driver’s side on the engine side main firewall harness. Connected to a 16 gauge wire.

Voltage Regulator, 20 Gauge Orange. Connected to the external voltage regulator using a lug to terminal 3. 1968 C3 year only due to later year alternators with the internal voltage regulator. Fed from the B+ 10 gauge wire.

 

I will use this info thank you.
You say your car built June of ‘68 and you’ve found 5 fusible links. Mine is an early build, November of ‘67.
There were a number of changes made after the first few months of production.
I wonder if they made changes in wiring as well ?