Computer and Alternator?

[Transam388]

As of late when I am at a stop sometimes my tach and speedometer will start jumping around. The car is running fine but this seems to have gotten worse lately. I do have an underdrive pulley so my alt is not putting out as much and am wondering if this is causing this.? The computer not getting enough juice or what? Once I move from the stop all goes back to normal but once moving and the revs are down it will still move a little but much less. Have been thinking about going to a FAST 140 amp alt instead of the OEM 102 amp. The car does have custom tuning if that has anyting to do with this. Just want to get some opinions and possible ideas of what might be causing the guages to jump around. Thanks!!

[12secSS]

Do you have the overdrive alternator pulley? If you do not, I would strongly suggest you get one, even with your proposed 140amp alt. It could either be the alt, battery or starter causing your issues. Is it hard to start? If not, where is the alt needle at (charging volts), does it dip down? Before I added the OD Alt pulley, my volts would drop to under 10 volts at idle, after I added it, the volts stay above 13V even at night.

[Transam388]

Well I do not have any issue starting the car and do not have the over drive alt pulley. The odd thing is I have had the underdrive for a while and this had happened before but went away quickly. Up untill recently it has just gotten worse and happening frequently. The lights don't seem to dim or anything and the charge although slightly lower from the UD still stays above the 10 range. Like you said, I'll just take the cheap route for now and see if the OD alternator pulley takes care of this. Thanks for the post!

[tblu92]

If you have a cammed car you autimatically set the ilde up faster because it won't idle down there at 550. I would reccomend setting the idle up to around 750 -775 with an automatic and that would cure the low output of the alt. With a M6 you can set it up even higher to around 8-900.Putting a pulley on the Alt to speed it up kinda defeats the purpose of having an underdrive pulley in the 1st place.I have a 98 with a 221/221 cam and it idles at 750. I also have an ASP 25% underdrive pulley too and I have no issues with the alt. At Idle it still register 13.8 volts.