alternator diodes

[old cars]

Alternator diodes finally blew at 150k miles- so replaced with new/rebuilt from supplier.
Three weeks later, same issue - apparently an underlying issue, have no idea what the problem is. Only thing not kosher in the system is direct wiring of one of the electric cooling fans through an aftermarket thermostat .

Any help would be appreciated !

[yoda067]

make sure the battery if fully charged before running new alt or it will blow under the load.

[robyrob]

could be a few different things - a bad fusible link wire, any loose connections, a bad sense wire going to the regulator, or a bad wire going to the computer on the field response circuit in the regulator, and as the last poster mentioned if you don't charge up the battery BEFORE replacing the alternator, you WILL burn up the alternator if you try to charge up a dead battery with it.

what year is it? around 1997 most models switched to a regulator with a field response circuit in the regulator - if you aren't careful replacing the alternator in models around that year with the EXACT same one, it will do some very strange things.

start by checking your connections and fusible link

[old cars]

It's a 95 - The fuse link is good.
Will make sure battery is fully charged- but as it took three weeks to blow this set , doubt that was the cause.

Can't tell about the two wires that plug in , and are apparently tied to the regulator and or the ECM . Any test procedures for those?
Thanks !

[robyrob]

if you are looking at the top of the plugin on the alternator, it should be labeled "S - I (or F) - L - P" (although it is probably upside-down when you are looking at it)

Delco used one regulator to cover many functions in many different applications, so that regulator plugin can have anywhere from 1 to 4 wires going into it, but it only NEEDS 1 to activate it. You said that you have 2 wires going into the plugin, you need to determine which ones they are to check them. There is no information sent from the vehicle computer to this alternator, so we don't need to worry about that.

The "S" terminal is the battery sense wire, it usually a thicker gauge and if it is present it should read fairly close to battery voltage. It is used to detect the state of charge of the battery, so that the alt can respond appropriately. If it is NOT there, this same function would be done through the main battery wire coming off the back of the alternator, which should also read close to the same voltage at the stud on the back of the alternator as it does at the positive terminal on the battery. There will be some voltage drop between the battery and the alternator, if this is too great, it will prevent the system from charging (usually the alt will burn up trying to send voltage to a battery that isn't getting there).

The "I/F" and "L" terminals can work in a number of different ways depending on how the charging system is set up on the vehicle; one can be the Field and the other the Light, or you could have just a Light wire or just an Ignition wire. The bottom line is that at least one of the 2 center wires should be hot with the key on. (the exact voltage isn't too important because most replacement regulators will "turn on" with less than 7 volts)

The "P" terminal is not essential to the charging system, it is basically an Alternating Current tap, which is normally used to feed a tach or other accessories.


It wouldn't be a bad idea to actually get the battery tested as well, one weak or dead cell could also be a potential problem where the alternator would misread the load and overcharge.

[old cars]

Hey Thanks for the great info!

Have been too d### busy to check the site-
the good news is the new replacement alternator is fine. I had them check the first replacment one , something with the stator- not the diodes this time. Just a bad rebuild. Advance auto parts did not give me a hassle either replacing it.
Had them check the new one before I took it home.
Hey I am getting preety quick at changingthese, so if yours goes out, just call me.