Charging system not working. Can I swap in a GM SI alternator?

[Sabaka454]

Hi everyone, I am working on my friends 86 Dynast Cuddy with a 3.0L 140 alpha one outdrive and when I got it running I checked the charging system output and 12.18 Volts. I have a new GM SI alternator sitting in my garage but not sure if it's ok to try and swap this one in. Should I just have hime buy a direct fit? Thanks!

 

[wire2]

Re: Charging system not working. Can I swap in a GM SI alternator?

Is your GM alt. a marine version, with very fine mesh on the vents? If not, no. Spark risk.
You say 12.14v running, I'll guess at idle? That's a wee bit low but still ok. It should rise to 14-14.8 at higher rpm's, does it?

 

[Sabaka454]

Re: Charging system not working. Can I swap in a GM SI alternator?

No my Alternator is for a GM truck. I checked it at idle. I figured at idle it should have 13volts but I didn't throttle it up because the boat is in my driveway and the out drive was in a tub. Anyway It had 12.38 once started dropped to 12.18 I turned on the blower and it dropped even less so I just assumed no charging system output.

 

[wire2]

Re: Charging system not working. Can I swap in a GM SI alternator?

I see nothing really unusual about that. If you want to test if the alternator is working, just unplug the field wire momentarily, I think you'll see the volts dip to 9-10 and slowly drop from there.
Keep in mind, auto makers switched from generators to alternators many years ago b/c generators produced virtually no current at idle, (and were high maintenance). Alternators produce *some* current at idle, but not a lot. The specific voltage also depends somewhat on the condition/age of the battery.

 

[Sabaka454]

Re: Charging system not working. Can I swap in a GM SI alternator?

hmmm.. well I'll have to throttle it up and see then I guess. Thanks for your help