alternator light on
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alternator light on
Hey y’all, new here. I have a ‘91 IDI 7.3L with an ambulance (fire truck) package sitting on a clean superduty (pre 450) frame. I’ve always had a problem maintaining a charge on the batteries if left sitting more than a week or two but day to day it is fine. I always thought that the drain was something to do with the decommissioned ambulance package and a possible ground. I have not been able to track it down. Cigarette lighter and radio removed as possible sources but no change.
I now have new batteries and a new alternator, LeeceNeville type, that came with three different possible wiring diagrams that suggest copying the original installation but the problem is that I have that darn long term battery drain. I had four wires from the original hookup. When I hooked up two wires, I got a very nice 14+ volts both at the alternator and battery, and a functioning tach (without using the any of the extra wires as suggested). The problem is that the dash alternator light stays on and I have no idea what the remaining two wires are for and I am suspicious of that battery drain, would love to get rid of that drain and I must get rid of the dash light. One of the drawings suggests one of the four wires as an “F” wire, whatever that is, I’ve no idea, and the other as a possible tach wire but I already have a functioning tach with two wires only. It does allow for a two wire installation with tach if the truck was wired that way so that part seems good. Just need to get rid of the dash alternator light.
As compared to each of the drawings, those extra wires in all combinations do NOT turn off the dashboard alternator light. I requested new docs from the manufacturer and will go back again. I am suspecting a failed fuseable link or a relay to the light, just guessing though and would not know where to find them anyway and I have no other ideas or where to start. To make it more complicated, I replaced the O-rings on the injector return lines and found another orphaned, pretty clean, wire beneath the fuel filter that could/might reach the adjacent alternator with no other plug-ups for it to be found but clearly it goes to something!!! This being a decommissioned firetruck, it is otherwise in great shape, I bought it with 26K miles (now 55K) and it has always been garaged, very clean, starts crisply, runs well, no rust, KN Air. Thanks-J
I now have new batteries and a new alternator, LeeceNeville type, that came with three different possible wiring diagrams that suggest copying the original installation but the problem is that I have that darn long term battery drain. I had four wires from the original hookup. When I hooked up two wires, I got a very nice 14+ volts both at the alternator and battery, and a functioning tach (without using the any of the extra wires as suggested). The problem is that the dash alternator light stays on and I have no idea what the remaining two wires are for and I am suspicious of that battery drain, would love to get rid of that drain and I must get rid of the dash light. One of the drawings suggests one of the four wires as an “F” wire, whatever that is, I’ve no idea, and the other as a possible tach wire but I already have a functioning tach with two wires only. It does allow for a two wire installation with tach if the truck was wired that way so that part seems good. Just need to get rid of the dash alternator light.
As compared to each of the drawings, those extra wires in all combinations do NOT turn off the dashboard alternator light. I requested new docs from the manufacturer and will go back again. I am suspecting a failed fuseable link or a relay to the light, just guessing though and would not know where to find them anyway and I have no other ideas or where to start. To make it more complicated, I replaced the O-rings on the injector return lines and found another orphaned, pretty clean, wire beneath the fuel filter that could/might reach the adjacent alternator with no other plug-ups for it to be found but clearly it goes to something!!! This being a decommissioned firetruck, it is otherwise in great shape, I bought it with 26K miles (now 55K) and it has always been garaged, very clean, starts crisply, runs well, no rust, KN Air. Thanks-J
Fleet Mechanic
Joined: Jan 2011
Posts: 1,709
Likes: 4
From: 8600 ft in Colo
Disconnect the positive cable on both batteries, and connect a test light between the positive terminal on one battery and the cable end. With the power drain, the light should light dimly. Then you can start pulling fuses, and hopefully find the circuit that has the drain.
Sorry, I can't help with the alt. wiring.
Good luck!
Sorry, I can't help with the alt. wiring.
Good luck!
More Turbo
Joined: Nov 2010
Posts: 521
Likes: 6
From: Cincinnati, OH
The dash alt light should be your exciter wire. When ignition off the wire has no volts, but when you turn on the ignition it puts +12v on the wire, through the light and then onto the alternator. Once the alternator is charging, this exciter is what causes the voltage regulator to energize and start charging. If your alternator is not charging, then current flows through the light and it is lit up. If your alternator is charging, then it balances the +12v input and keeps current form flowing through the wire and the light does not light up.
An alternator can self-excite when the rpm gets high enough. That is the principle behind 1-wire alternators. It may be your alternator was self exciting when you drive so it does keep the batteries charged up.
The drain is a different problem, do the disconnect fuses and see which one the test light goes out and that is the circuit you have the drain. Your alternaotr could also have internal leak and is allowing discharge through it.
An alternator can self-excite when the rpm gets high enough. That is the principle behind 1-wire alternators. It may be your alternator was self exciting when you drive so it does keep the batteries charged up.
The drain is a different problem, do the disconnect fuses and see which one the test light goes out and that is the circuit you have the drain. Your alternaotr could also have internal leak and is allowing discharge through it.
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