Rough idle when something electric on... Lights, radio

[SweetCars]

I got a fuel injected mexi beetle. I rebuilt the engine allll new, every sensor is new. I put on a really nice rebuilt Bosch 55amp alternator. I mean nice cause they did i reallyyyy cleand and nice job look nothing less then new. Everything was working perfect for the first 10 000km.

Now when i turn sonething as the light on, idles wrong and i even hear the belt scream a little bit. Seems cant stabilize the idle. Rough idle.
I think its the alternator the problem. How cani be sure?? I am almost ordering a brand new bosch.

Thanks for your time and help.

[bluebus86]

you can test the voltage easy on the alternator, when running at speed (ie 2000 rpm or so) measure the voltage across the battery, it should be 13.5-14 volts about. if it is only 12 volts, then the alternator is not providing enough volts to charge the system normally.

also to before I forget, to test the alternator, turn on the headlights while engine off, then start engine and rev it a bit, if the alternator is working, usually the lights will get a little brighter, than when engine was off, best seen at night. This is kind of the "I don't have a volt meter handy" test. It is a good test too.

Also the alternator could be ok, but you have a short some place drawing down the voltage, and making the alternator work harder (hence the belt slips and you hear it) could be when you turn on an accessory there is a huge current draw, because of a short that drops the voltage.

now of course the voltage drop wont likely be seen across the battery, unless if is a huge short that soon melts wires, you likely have a minor short some place in the circuit that powers the offending accessory. The drop however we know IS effecting the engine's operation, likely either the power to the spark coil (ignition system, including electronic ignition module if applicable) and / or the power to the ecu (injection brain or computer).

I would thus check voltages on the input to the ignition and injection systems both with and with out the accessories turned on. See which circuit is cutting out (voltage drop ) and then also try to isolate which accessory is the culprit, if it is the headlights, then try removing one headlight fuse at a time, to see if the problem only occurs with right or left light. keep dividing up the current path on the circuit that causes the poor running, and you will be able to pinpoint the short.

I don't have any injection model beattle wire diagram to help you. Hopefully you have access to one, that will help you a lot.

I know the old D-jet injection system (circa 1070's VW) doesn't like to run at less than about 9 volts, and can act a little funky even at 11 volts.

PS make sure the belt tension is correct on the alternator, I am sure you probably have, but I just want to throw it out there just in case. also check the simple things, like the ground straps, and battery terminals for dirt, rust or loose bolts. a bad ground here can really cause the charging system to over work, trying to push power thru high resistance.


Also don't get fooled into getting a new alternator with out first testing your battery, some fail early, if a cell shorts for instance you will constantly be about 2 volts shy of 12 v, that caused my D-jet car to run not horrible, but a little funky, once the new battery went in, the car ran smoother, felt like more power.

Anyway them there is my thoughts on where to look maybe.

Good Luck
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