Sorry if this has been discussed before....I just added an HEI system and decided to add an LED idiot light to the alternator wiring while I was at it. The idiot light lights up when I start the engine and stays on until I rev the engine. I know the alternator is designed to start generating power once revved (this is how guys with 1 wire alternators do it). If I don't rev the engine, it will fire up but then run off the battery. Question is, is the issue with my wiring or with the alternator? With key on, engine not running, the idiot light is on but there is no voltage at the exciter wire where it plugs into the alternator. That makes no sense to me since they are on the same circuit. Is it possible I have the LED wired backwards?? Thanks in advance for the help. I read this forum all the time, figured it's time to join and make it official.
I should probably mention that the idiot light/exciter circuit is getting power from the same wire that powers the HEI distributor. It is definitely 12v switched power.
Sounds like the alternator is not getting reference voltage when the ignition switch is in the "RUN" position. LED is Light Emitting Diode. Current flows one way and not the other way - thus the alternator is not getting the reference voltage. Replace the LED with a small indicator light (conventional filament bulb). If that does not solve the problem, something else is going on.
Makes sense. The reason I initially added the diode was because having the HEI distributor connected to the exciter wire was causing me to not be able to shut the engine off due to the direct alt-HEI flow. I'll try a regular bulb and see if that allows sufficient voltage to the exciter. Any chance it's a bad alternator component, or do we all agree the issue is the diode?
Here are a couple of diagrams I saved over the years. I have used the one in the second photo a number of times and it works good. I'd say your LED bulb doesn't have near the resistance that a regular light bulb would. The 1N5408 diode is a Massey Ferguson tractor number. They aren't hard to find though. You can replace that with the gen light though.
I'll give the standard bulb a try. I am definitely getting resistance because the engine shuts off when I turn the key to off (didn't before the LED). It seems the LED doesn't allow enough voltage to pass to the alt. I have 12v going into the LED, almost zero coming out.
Purpose of the 'ALT/GEN' light is two fold. One is to excite the generator immediately(its a resistor), thus begin charging after startup. Two is to warn the operator/driver that there is a fault in the electrical system if/when a no charge(alt/gen) or no storage(batt) condition occurs. ALT/GEN light works due to difference of (electrical)potential. The light is not grounded. This form of lighting the bulb works like a see-saw. If there is higher voltage on either side of the lamp the lamp will illuminate indicating there is a problem in the charging system. When the voltage is equal on either side then there is no difference of potential, power will not flow through the lamp, and the lamp will stay off. This is why when the rpms drop excessively the alter/gen light will begin to glow as the power across the lamp is now uneven, allowing power to flow through the lamp and illuminate it. It is also a good idea to run a resistor in parallel with the lamp, if the lamp burns out the exciting function of the alternator will still work without having to rev the engine. A diode should not be used in this part of the circuit. Light emitting or not. When the ignition switch is turned to off, power should be removed from distributor(to kill engine) and from the excite circuit of the alt to turn off the self exciting circuit. Although both will derive power from the battery they should not be allowed to derive power from each other or you no longer have a STOP circuit(switch) in your system and the engine will continue to run. If you are using a 10/12SI alternator the wiring in the diagrams above are mostly correct, there are a few things that need to be addressed. T1 in the diagram is correctly wired, although a resistor should be run in parallel across the charge light to keep functionality of the system working if the lamp fails. T2 should not be run directly to the output of the alternator. It should be run to the main power distribution block or main power splice. T2 is the 'sense' wire, it monitors system voltage which will change from load requirements of the system. Running it directly to the battery or alternators Batt. terminal is kind of pointless as that will rarely be affected by system output.
The LED setup will work OK if you put a resistor across the two connections. a 70 ohm resistor will mimic the loading of the 2w bulb that would normally be in circuit; the LED still lights OK with the small amount of current that goes through it. I have had this setup working for the last 12 years on my '50 Chevy truck which has 10 SI alternator fitted.
Ok, back to the drawing board....I replaced the LED with the incandescent bulb that was recommended and the alternator still requires a rev to start generating power and turn the light out. Should I look into replacing components in the alternator itself?
If you have to rev to get it to charge that is the regulator in the alternator. You need a different regulator, that is a feature of a one wire alternator regulator.
Make sure that with the key on that both terminals on the alternator plug are hot...use a test-light or a multimeter. The multimeter will give you more info as to the voltage in the plug...then turn the key off and recheck...the #1 terminal (you will see the numbers embossed into the aluminum housing) should turn off with the key...the other #2 should be hot all the time. Confirm that and report back. Sent from my iPad using Tapatalk
If you are truly using a one wire regulator then you don’t need an exciter wire...relocate the power to the ign. Module to the accessory side of the ign. Switch and use what the factory calls for for a resister. You can tell a self excited regulator by the web between both terminals when the alternator plug is pulled out. If no web it’s not a SE regulator. Sent from my iPad using Tapatalk
It's a stock Jeep cj7 alternator. This issue only came up after changing to an HEI ignition. Before that, the exciter wire did it's job and no rev was required. I'll check voltage at the terminals and report back. Thanks so far for the help
Ok, interesting turn of events. The main red wire on the alternator harness/plug has power both on and off. The exciter wire had zero volts off, and only 0.8 volts on. I then checked the voltage at each of the wires coming from the idiot light. When the light is illuminated, I get 12+ volts coming in but only 0.8 volts coming out the other side of the light. This is the same thing I saw with the LED. It makes sense since the engine is not running (hence the light illuminated like it should) but turning the engine on, none of the conditions above change. Same 0.8volts at exciter terminal and light bulb "output" wire.
Key on, each terminal of the alternator plug. This makes sense since only 0.8 volts is coming from the light at the other end of the exciter wire.
bump. Any recommendations? If I put 12 volts to the exciter terminal on the alternator with the key in the on position, will that circuit be grounded? Should I be reading 12 volts or close to 0? More importantly, do I need to replace my alternator? I have it wired exactly like the first photograph/diagram in this thread.