[urq] sudden stumbling

Steve Eiche seiche at shadetreesoftware.com
Fri Jun 30 12:11:42 EDT 2006

Where are you taking your reading?  I am guess that it is not right at the alternator.  That would be the best place to start to test the actual components.  My guess is that your has a problem with the main battery cables or ground straps, perhaps at the infamous "splice" where the main battery cable is cut and spliced and has the two crimped wires that feed the fuse block.  Here is a link to picture of that was in my car:

I would also highly recommend checking the condition and connections of the ground straps both at the battery and from the left motor mount bracket to the chassis.  I have seen then in pretty bad shape.

The way to test the system would be to start at the alternator (not the battery) with the engine running (turn on some things like the lights and heater fan for a bit of a load)  with the - lead of the meter on the case of the alternator, and the + lead on the main + (30)connection.  Here you should see 13.8V or more.  Record this voltage.  Then move the - lead to the chassis.  This will test the voltage drop (resistance) in the engine to chassis strap.  Record that value.  This is the value to use as your baseline, as it is the highest voltage that you could possibly see in the car.  Now check that battery voltage and record that (use the chassis as the - reference).  Now you can go anywhere in the system and see where the maximum voltage drops occur.  A drop in volt means resistance, possibly a bad connection or undersized wire. Remember that you can see voltage drops on the + or - connections to any component.  The chassis will always be your - reference, the + reference depends upon where you are in the system. In the case of the dash gauge, you are going through multiple connections, with other loads being connected in between.  While this does give a good reading of what the "system" of electrical consumers is seeing for voltage, it does not tell you what the alternator is actually putting out (voltage reading taken right at the alternator).  Which way is better is the subject of some debate.  Ideally, if the wiring was properly sized and in excellent shape, the readings would be extremely close.  With ur quattros, the wiring was often not properly sized and the connections often corrode, so you will not likely see "ideal" readings.

I wish I could tell you exactly where your issue is, but the above method will let you troubleshoot any part of the system.


Eric wrote:

I noticed another interesting issue today on my way to work.  I have
always had a low voltage condition on my car, but no amount parts
replacements or load reduction mods have helped.  I installed a 2nd volt
meter to monitor the alt directly.  Normally, when its cold the alt
reads 13.5v and the dash voltage reads 12.  In the mid afternoon heat
today, My alt dropped to 12 and my dash to 11.  Now for the interesting
part.  As I sat at the light, my alt voltage would DROP a full volt as I
revved my engine.  Why would it do that?  It is a new and tested
alternator.  I wonder if I have corrosion somewhere that is causing this
low voltage.  The fuse box is brand new.  Any thoughts.  I don't blame
the ignition system for not working very well at 11V!



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