I replaced my burned out ammeter. I have 12 volts to both terminals, but it goes negative and stays there if the key is on or the door is opened tuning on the courtesy lights. Horn relay seems to be fine. Buzzer works as well as the horn. confused.

 

Does it go positive with the engine running? Going neg not charging is normal.

 

stays hard to the negative running or not. Only at zero when off with doors closed.

 

The ammeter gauge is fed by dedicated wires that run in parallel with the main power supply wire between the battery terminal on the starter and the junction terminal on the horn relay. Nothing else can be connected to or fed from the ammeter wires. My guess would be that somebody needed a 12V supply and found one of the alternator wires. I would look for a jumper wire connected to the ammeter wires. It is likely going to be on the wire that comes from the starter terminal since the gauge is reading negative.

Explanation
The wires to and from the ammeter are thin (numerically high) gauge wires that have a much higher resistance than the main wire so they only carry a portion of the current between the main junction (horn relay) and the battery terminal on the starter. (note the ammeter connection is technically not on the starter, but is made inside the wiring harness) . This is referred to as a "shunt". The gauge is calibrated to read the small current through it and estimate the current through the main wire. Anything that influences the current through the gauge would affect the gauges reading. Since the gauge reads negative, current is going backwards through the gauge. This means the draw is on the battery side of the gauge. Since it goes full negative, the current draw is beyond the capacity of the gauge and is likely the reason the first gauge failed.

Both wires to the ammeter should read 12V since one is fed from the junction on the horn relay and the other is fed, in essence, from the battery. If you have a really good volt meter, you should be able to read a very small voltage between the two.

If you are thinking about replacing the wire, remember that the wire must have the same resistance as the original. This means the wire must be the same length and gauge as the original wire.

 

Rethought this. The draw is likely to be the wire that goes from the ammeter to the horn relay.



Then this changes too. Since the gauge reads negative, current is flowing from the battery to the car. This would indicate the draw is between the gauge and the horn relay.

 

2mnyvets said "If you are thinking about replacing the wire, remember that the wire must have the same resistance as the original. This means the wire must be the same length and gauge as the original wire. "
I went thru this several weeks ago. I had replaced the wire from the horn relay with the wrong gauge and the ammeter tended to go negative, except when the alternator was in strong charge. When I put the correct gauge back in, it worked like it was intended to.

 

Re-tightened wire screws at horn relay and it now works! Thanks for all the help!
Bill

 

I was going to say check your horn relay connections because I had a similar problem you had just not all the way to the negative peg after I did my headlight relay mod. I used the horn relay as power for my lights as this is just like hooking it up to the back of the alternator or + post of the battery. I found my connection a little loose. Once I tightened that up everything worked normally again.