Delcotron 12SI problem

[Bonnewagon]

My alternator is acting weird. It is the 80's vintage large 12SI 78 amp model, that works fine, except for one thing. When I turn the car to "run", the gauge reads 12 volts, or less if it's been sitting awhile. If I turn on a load, like lights, the gauge drops a bit, like it should. But then when I hit "start", and the motor fires up, the voltage gauge just sits at 12 volts, or whatever it was. It will stay there until I get the motor to rev to 2500 rpm, then it comes alive and jumps to 15 volts. It will then read normally all day, even after the motor drops to it's normal 650 rpm idle speed. If I shut the motor down, and restart, it begins all over again. I'm thinking the gauge is OK, but that whatever voltage is needed to "excite" the alternator, it doesn't react until the 2500 rpm is reached. Does this make sense, or am I completely backwards on this?

 

[papastoy83]

Check the small wire in the connector on the back of the alternator. Should be about 10-12 volts.

 

[Bonnewagon]

Roger that. Thanks!

 

[Bonnewagon]

So here's what happened. Cold motor off, red "batt" terminal is 12.31 volts, as is red terminal "A" which is wired to "batt". Brown terminal "B" , field, is .16 volts. With key in "run", "batt" and "A" are still 12.31 volts, "B" is still .16 volts. Start motor, choke on, 1500 rpm, "batt" and "A" are now 11.14 volts and "B" is still .16 volts, motor is running on battery voltage only. Choke off, idle at 650 rpm, same thing. Rev to 2500 rpm, "batt" and "A" jump to 14.49 volts, "B" is 14.69 volts, slightly more than system voltage. Idle down to 650 rpm, same readings.

 

[Longroof79]

Mark,
Happy New Year, my friend.
I believe that the 10 and 12si alternators have a low charge rate at low RPM...or I should say, they don't put out the kind of amperage or volts that the newer type alternators put out at low rpm. That's why I went with the CS130 alternator which puts out 105 amps. The CS144 alternator has a larger body and is rated @ 140 amps.
You may want to consider going with one of later type alternators. They're easy to adapt, too.
I also changed over from a 12si. When I turned the headlights on, heater blower, or wipers, it seemed to put a load on the alternator. The belt would screech like a banshee. I think it was only a 63 amp unit. :?

BTW, was that film clip from the Lil Rascals?

 

[Oldsmoletick]

Nothing to add, just subscribing, my Eagle with the same alternator, does the same thing :lol: .

 

[papastoy83]

Stock gauge package?

 

[Bonnewagon]

Hey there Jack, same to you and yours, yes, Lil' Rascals it is. I believe "Algebra" the mule is towing that bus they cobbled together. Early Hot Rodders! That's the screwy thing- after I hit 2500 rpm, it wakes up and she's a regular powerhouse at any speed. Even at 650 rpm idle I can have everything running and she still puts out 14 volts. My other idea is that the car was originally a 63 amp 10SI on a V6 w/idiot lights. The alt wiring has the brown field wire going to the alt idiot light which is fed by the gauges fuse and has a diode in the line. My current alt is a 78 amp 12SI from an '80 301 Firebird that had AC, heated rear glass, and courtesy lights and is more than adequate. I also switched to stock gauges with which the volt gauge is just wired into the instrument cluster 12 volts and the brown field wire "should" then go to the choke heater relay which I may or may not have. I just hooked the volts gauge up to the existing wiring but had no problems and then I did another change. The volts gauge I was using had a tiny little red alt idiot light at the bottom of the gauge, which probably satisfied the field input. But when I switched to the '78 silver gauges, no idiot light. I can't remember if this problem coincided exactly with that change, but I don't think so. Olsdmoletick, is your alt factory or an upgrade you swapped in and do you have gauges or idiot lights?

 

[jrm81bu]

Mine does pretty much the same thing. After starting it sometimes needs a little rev to energize. However mine on occasion will stop charging at idle if say the fans kick on and I have the headlights and everything on at the same time. But as soon as the rpm comes up it goes right back to charging. It's slightly unnerving but it has never caused an actual problem.

 

[papastoy83]

According to the shop manual, brown wire at back of alternator goes to "b" terminal of carb choke relay under left side of dash.( relay is strapped to a wiring harness) Continues thru the relay coil to choke heater fuse. I would check with an ohmmeter to make sure u have continuity from the choke heater fuse to the back of the alternator(brown wire). If all thats good, then I'd say the voltage regulator is bad in the alternator. This is assuming that u have stock gauge package. If u have idiot light dash, brown wire goes from back of alternator to generator bulb, thru the bulb to gauges fuse.