Charging system diode

#1

In order to energize my alternator I have the wire running to the power relay under the hood. When I went to get my annual sticker they wanted to check my killswitch and it failed. I installed the biggest diode I had at the track and it was 760ohms forward. I started the car and the alternator didn’t get enough power to start charging. After I got my sticker I twisted the bare wires on each side of the diode together and forgot about it. How a 20ga wire can power the car baffles me but it’s true.

Due to my primitive EE knowledge I can’t decipher the information on digi-key to buy a diode that will work. I don’t see anything about a forward resistance rating and am not sure how low it needs to be to get power to the alternator.

I would prefer a part # to electrical terminology and theory because I have plenty of homework already.

#2

[quote=“turbo329is” post=65781]In order to energize my alternator I have the wire running to the power relay under the hood. When I went to get my annual sticker they wanted to check my killswitch and it failed. I installed the biggest diode I had at the track and it was 760ohms forward. I started the car and the alternator didn’t get enough power to start charging. After I got my sticker I twisted the bare wires on each side of the diode together and forgot about it. How a 20ga wire can power the car baffles me but it’s true.

Due to my primitive EE knowledge I can’t decipher the information on digi-key to buy a diode that will work. I don’t see anything about a forward resistance rating and am not sure how low it needs to be to get power to the alternator.

I would prefer a part # to electrical terminology and theory because I have plenty of homework already.[/quote]This diode you put into the system…You did this to replace some kind of OEM functionality or was it intended for some other obscure purpose?

#3

The alternator needs exciter power when the ignition is on. Normally this is provided via the alternator/battery bulb in the cluster. A diode isn’t needed, any source of switched power applied to the alternator exciter wiring will suffice.

The failure to pass the “kill switch test” says that the kill switch isn’t properly wired or that you don’t have the correct kill switch. In addition to severing the main power connection from the battery a separate pair of contacts must interrupt power to the DME or the coil. With those conditions satisfied the engine will stop when the kill switch is activated.

#4

Once the alternator begins charging the energizing wire becomes positive and sends power back to the system. This is enough to keep it going for long enough to fail tech before it burns up at least. I started the engine, cut the blue wire and then hit the kill switch and it worked.

I get why we aren’t understanding each other now. I thought that the instrument cluster had a diode in it that prevented this and that everyone with a stock instrument cluster just had to cut the two wires going back to the battery. I just checked the charge system diagram and

[attachment=1837]chargelight.PNG[/attachment]

As you can see even thought it’s sideways that the 1.5mm blue wire is positive which I am running to switched power via a relay and feeding a positive splice under the dash. I like my way better so I just need to measure the resistance across the bulb and resistor and put enough diodes in parallel to get that value.

#5

Turbo,
I’m not quite following your description of how you now have it wired, could you make a sketch and post or PM? Some voltage regulators are designed with the voltage drop across the resistor/lamp used to set a specific input voltage range at the D+ input and also to limit the current in the field coil windings. I would recommend you put something in this line to perform the same function, a power resistor works nicely. Diodes ideally don’t have any resistance in the forward direction and the resistance they do have is extremely non-linear. That’s why you won’t find a forward resistance listed on the Digikey data sheets. What you’ll find instead is a forward voltage drop at a specific current. Paralleling diodes won’t help you, if you need more forward drop you need to wire them in series. But again I’d use something to limit the current. The field windings in the alternator are effectively a dead short when the alternator isn’t turning. Something somewhere should be getting very warm indeed.

#6

Ah, but none of that is necessary if the kill switch is properly implemented. Done correctly the kill switch will interrupt primary power AND power to the DME or coil. If you take away DME or coil power the engine will stop regardless of what the alternator is doing. It sounds to me like your kill switch is only disconnecting the battery. In which case the alternator will continue to run the engine electronics if it is producing output. Rather than screwing around with the exciter circuit, fix the kill switch problem.

#7

Find the old threads on wiring the kill switch.

#8

You still want exciter current to the alternator before the car gets started. Many alternators build up enough residual magnetism in the rotor to be self exciting, especially after 25 years, but you don’t want to count on it. As others have mentioned you also want some kind of load for the alternator to dump charge into when you abruptly interrupt the main battery load. I haven’t researched all the relevant threads to wiring up a kill switch but one easy way to do it (if you have the switch placed as close to the battery terminals as you can get it for maximum short protection) is to wire the NC (main) terminals of a 4 terminal or 6 terminal switch to the battery 3/0 gauge lead and wire the smaller 12 gauge lead to one NO terminal with a resistive load on the opposing terminal. This takes care of everything at once.

#9

Thanks.

I think once I figure out the resistance through the cluster I can figure something out. I have a longacre 4 pole NC switch so I’m not going to worry about a shunt and hope to never use my kill switch again while the motor is running. If I do I probably have bigger issues anyway.

#10

[quote=“jlevie” post=65790]The alternator needs exciter power when the ignition is on. Normally this is provided via the alternator/battery bulb in the cluster. A diode isn’t needed, any source of switched power applied to the alternator exciter wiring will suffice.

The failure to pass the “kill switch test” says that the kill switch isn’t properly wired or that you don’t have the correct kill switch. In addition to severing the main power connection from the battery a separate pair of contacts must interrupt power to the DME or the coil. With those conditions satisfied the engine will stop when the kill switch is activated.[/quote]

This is probably the easiest way ^

If the coil can’t spark, car won’t run.