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I have a 69 SS that did not originally come with the tach gauge cluster. Any car should be the same. I will be putting one in this spring. Currently, I have an aftermarket gauge package with the amp, oil, and temp gauges. Currently, the amp guage is in series with a red wire that comes from the battery/alternator area and the other end goes to the battery terminal on the horn relay (right side screw). I know this is not correct because if my amp blows, I lose my hot or switch power. I have seen diagrams that show where to connect an amp gauge and have also been told by an "expert" that all I have to do is connect the amp gauge from the junction block on the radiator support to the horn relay. Basically, just run a new wire in parallel with the existing, red wire. Is this correct? Second, when I open my door or run the car whether at idle or driving, the amp shows just slightly negative and if I turn the lights on, it goes more negative. Is this correct or should it go positive at some point? If I had a higher output alternator, would it go positive? I thought that it should measure output current because it is connected in series with the alternator before any load. From the end of the wire connected to the horn relay, I have continuity to the back wire on the alternator and the wire going to the junction block to the battery. All wires were disconnected during the continuity test. Any help would be appreciated. Thanks.
 

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To start with I’ll recommend you don’t use an AMP gauge. Starting in 72 the power feed routing was changed on Chevelles. The change helped elevate the hot starting problem. Prior to 72 the power feed was routed from the battery to the J-block, to the horn relay then to the bulkhead. 72 and later the power was picked up at the starter then to the horn relay, mounted on the firewall, and on to the bulkhead. By removing about 6 - 8 feet of wire, less wire in the start circuit = less overall resistance, the voltage to the start solenoid during starting is increased.

The reason I pointed this out is because tan aftermarket amp gauge is wired in series with the power feed from the J-block to the horn relay. To do so requires adding wire, 10 to 12 feet, to the power feed/start circuit. Lower voltage to the start solenoid. This is not the only concern. The wiring to the amp gauge can also short to ground inside the car, a potential fire hazard.

You should consider installing a volt gauge, it is easier to install, is safer and doesn’t add to the hot starting problem.

OK if you want to use the AMP gauge you will remove the wire from the J-block, the one that goes to the horn relay. Using #8 wire, connect to the J-block rout to the AMP gauge. Connect a second #8 wire from the AMP gauge back to the j-block area and connect it to the wire you previously removed. After connecting the AMP gauge turn on the headlights. The gauge should read discharge, if not switch the wires on the back of the gauge.

The AMP gauge measures the current flowing to or from the battery. It doesn’t measure the current flowing from the alternator. With the engine running and a good alternator the current is flowing from the alternator to the horn relay then to the fuse block/ignition switch and to whatever you have on. Some of the current will flow into the battery to recharge it. Typically this current flow doesn’t last long. With a good battery and a good alternator the AMP gauge, most of the time, will not indicate charge or discharge.
 
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