Alternator light stays on at warm start until acceleration

[gordon415]

I recently upgraded to a rebuilt Bosch alternator for a 320i and a new internal voltage regulator. Brand new ground wire and new wire connectors on the back, too. I removed the 2002 separate voltage regulator. 

 

Seems to be working fine. On cold starts, no problem. On warm starts, the alternator light in the dash stays illuminated until I give a little gas.  Is this anything to worry about? My previous 2002 alternator never did this. 

[Mike Self]

Could it be that the fan belt is stretching just a bit when warm, allowing some slippage, but contracts when cold to grip properly...or could something slippery be getting onto the belt (leaking water pump, perhaps) and lubricating it a bit?  Just a guess...I've seen stranger things...

 

mike

[SydneyTii]

The alternator also needs about 1 to 1.2k rpm to be “excited” before the light will go out, that’s an official term btw!!

[2002Scoob]

I'd check your belt-tension when warm, but don't over-tighten. 

 

With my car, anything less than ~750-800rpm and the low-voltage/alternator light comes on, so basically just cold-starts and warm-ups.

 

Above 800, or once it's warm and all is good. 

 

 

[xr4tic]

Maybe it has the wrong pulley?  If it's a larger pulley, it will spin slower.

 

If it's new and under warranty, you could try and get a replacement, or just learn to blip the throttle on warm starts.

[Healey3000]

I don't believe there's anything wrong; that's the way these alternators function.  Like SyndeyTii said, they need a  bit of a jump in RPM to self-excite.

[Stevenc22]

Could be the higher output alternator needs more rpm's before it excites...

[gordon415]

Thanks everyone. The need for rpms totally makes sense. I need a good amount of them to get excited myself. Good to know it's not a problem. 

 

I'll double check the pulley and the tension on the fan belt, too. 

[jimk]

The E21 probably has a post start rpm target above the normal idle speed, so i t wouldn't be even noticed there.  I don't notice it with the same alternator and my Haltech EFI because I have a post start idle increase also.  This is the std vacuum dash pot that is on american carbs.