Ammeter problem
Team Owner
Joined: Jul 2006
Posts: 22,192
Likes: 4,180
From: Kannapolis NC
It's in the red wire running from the horn relay to the starter solenoid, at the fusible link connection it turns to a brown wire (the link) and then back to red. Look where I added the red arrows on the first page of this diagram:
Corvette_1972_wiring.pdf
Corvette_1972_wiring.pdf
Last edited by 65GGvert; Nov 21, 2015 at 04:39 PM.
Instructor
Joined: Jun 2015
Posts: 212
Likes: 29
From: State College Pennsylvania
I agree with 454. With an open connection on the red feed wire from the starter, the entire current load pass thru the meter. Then the meter becomes part of the charging circuit, not a shunt across it. Since full defection of the meter needs only about 4 amps, the calibration markings on the meter are irrelevant. The meter coils will be melted very quickly.
Team Owner
Joined: Jul 2006
Posts: 22,192
Likes: 4,180
From: Kannapolis NC
Voltage is higher on the horn relay end than the starter end, or vice versa, so current flow is opposite. It's the same principle as normal operation, only full current is flowing through the meter. Doesn't mean the link is the problem, but I'd sure check it before replacing parts.
Last edited by 65GGvert; Nov 22, 2015 at 07:23 AM.
Burning Brakes
Joined: Jul 2015
Posts: 823
Likes: 57
From: Austin TX
Wish I could tell you. I don't even have a wiring diagram for my car yet (just bought an '82 this week). The shunt is probably a section of resistance wire built into the engine compartment harness, but that's just an educated guess. My last C3 Vette was more than 30 years ago, and I don't remember this area of the cars very well. I'm sure someone here will know.
Thread Starter
Advanced
Joined: Feb 2015
Posts: 55
Likes: 0
I still have yet to bring it in to my mechanic to check the shunt, but I took a few more specific readings (see below) and now I'm more confused than ever. For instance, does it make any sense that the reading is higher with more accessories on? 
In Drive while moving +40
In Drive at idle around 0
In Drive w lights on -40
In Reverse while moving +40
In Reverse at idle -40
In Neutral at idle +20
In Park no accessories +40
In Park w lights only +30
In Park w lights + heat +20
Car Off 0
In Drive while moving +40
In Drive at idle around 0
In Drive w lights on -40
In Reverse while moving +40
In Reverse at idle -40
In Neutral at idle +20
In Park no accessories +40
In Park w lights only +30
In Park w lights + heat +20
Car Off 0
Le Mans Master
Joined: Jul 2006
Posts: 8,117
Likes: 104
From: charlotte north carolina
Your ammeter is the charging circuit, not the red wire that it should be. Whenever you have a current draw or surplus (battery charging) the meter deflects. The amount the meter deflects is a function of the alternator RPM (possibly a low alternator output at low RPMs) and the total current the car demands. This why you are showing varying meter readings. At speed the battery charges, but at idle it discharges. There is a fusible link in line with the large red wire that runs from the starter to the horn relay near the starter. The link is burned, there is a bad connection at the starter or horn relay, or the red wire itself is open. If you keep driving the car as is, you'll be installing a new meter very soon. Carefully check the condition of the lugs swagged to the wire. Check the fusible link. You'll find your problem.
Burning Brakes
Joined: Jul 2015
Posts: 823
Likes: 57
From: Austin TX
These meters must have a full scale sensitivity of several amps, even without the shunt. Otherwise, the car wouldn't run. Doesn't the meter circuit even supply current for the starter solenoid? I'm surprised the meter hasn't exploded already.
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Melting Slicks
Joined: Jan 2014
Posts: 2,478
Likes: 254
From: Downers Grove Illinois
Your ammeter is the charging circuit, not the red wire that it should be. Whenever you have a current draw or surplus (battery charging) the meter deflects. The amount the meter deflects is a function of the alternator RPM (possibly a low alternator output at low RPMs) and the total current the car demands. This why you are showing varying meter readings. At speed the battery charges, but at idle it discharges. There is a fusible link in line with the large red wire that runs from the starter to the horn relay near the starter. The link is burned, there is a bad connection at the starter or horn relay, or the red wire itself is open. If you keep driving the car as is, you'll be installing a new meter very soon. Carefully check the condition of the lugs swagged to the wire. Check the fusible link. You'll find your problem.
