Headlights dead, ammeter on the frits

[DattMtn]

My '74 Cherokee recently acquired an electrical issue in the headlights circuit. I have replaced the headlight switch as I thought it was the issue to no avail. The headlights and side markers are dimly flashing when the headlight switch is pulled. When the headlights are turned on the ammeter is bouncing all over the place too. All the other lights (brake and turn signals) still work. I unplugged the dimmer switch on the floor and the ammeter still goes wild when the headlight switch is engaged. Any idea where to start looking to troubleshoot this issue? There is no visible corrosion on any of the grounds and I have started to clean them up as precaution but nothing seems to be fixing this issue.

https://youtu.be/YoFOR9vrbDk

[letank]

time to bypass the ammeter, disconnect battery, and reaching behind the dash -if you can- connect both ammeter wires to the same post for a quick test. Various mods are possible

check this

http://www.madelectrical.com/electricaltech/amp-gauges2.shtml

[DattMtn]

Nailed the issue

I found the ground wire from the ammeter to the dash ground was burnt out. It looked like the original approx. 16awg was melted and then the dash was grounding through the headlight circuit. The burnt out headlight switch was the symptom not the cause! I have replaced it with a 12awg fresh wire and soldered on the pin connector for the gauge clip. It was an easy fix once I found the culprit.

I did however break my bezel on the gauge cluster, I was lazy and instead of taking off dropping the column I forced it too much. Not too happy about that one but oh well.

[nograin]

Glad you found the real cause and fixed it.

Be aware that neither wire on an ammeter is a ground. Both are always hot. One side connects to the battery, and the other to distribution point and then onto the alternator.

With the engine off, all power comes through the ammeter to the distribution.
With the engine running, power comes from the alternator (as long as its output voltage is higher than the battery's). If the battery is low, the ammeter will show charging.

[tgreese]

Glad you found the real cause and fixed it.

Be aware that neither wire on an ammeter is a ground. Both are always hot. One side connects to the battery, and the other to distribution point and then onto the alternator.

With the engine off, all power comes through the ammeter to the distribution.
With the engine running, power comes from the alternator (as long as its output voltage is higher than the battery's). If the battery is low, the ammeter will show charging.

This. All vehicle current goes through the ammeter. That loop is 10 ga wire. Look here http://oljeep.com/edge_74_tsm.html trace through wire 12 and wire 55, both 10 ga.

Best solution IMO - Keep the distribution point, remove the ammeter, and run the alternator charge wire directly to the battery. Install a voltmeter.

[nograin]

I beg to differ on part of that. Once the battery is charged, no power should be flowing through the ammeter. It's got the potential (14.5 Volts), but no flow (0 amps) since there is no demand.
When additional equipment is added to the battery post, its another story.

[tgreese]

I beg to differ on part of that. Once the battery is charged, no power should be flowing through the ammeter. It's got the potential (14.5 Volts), but no flow (0 amps) since there is no demand.
When additional equipment is added to the battery post, its another story.

Yeah, that's true. The alternator will self-regulate so that it supplies everything needed for the car - then there's no current demand on the battery and no current through the ammeter. Equal potential on both ammeter posts.

Once the demand from starting is made up, the battery should not discharge again until there is more demand on the alternator than the engine speed can support. That's a fairly uncommon situation, ie when you are running a winch or welder, or when there is something wrong with the alternator or regulator.

[mattparliament]

I found the ground wire from the ammeter to the dash ground was burnt out. It looked like the original approx. 16awg was melted and then the dash was grounding through the headlight circuit. The burnt out headlight switch was the symptom not the cause! I have replaced it with a 12awg fresh wire and soldered on the pin connector for the gauge clip. It was an easy fix once I found the culprit.

I did however break my bezel on the gauge cluster, I was lazy and instead of taking off dropping the column I forced it too much. Not too happy about that one but oh well.

can you clarify what actual wire you fixed then if there isn't a ground from the ammeter?