Scirocco Register

I was checking my Bosch 90amp alternator tonight, put the red multimeter lead on the red cable/bolt and the black on the block, it was kicking out 13.92 volts.
Checked the battery with the engine running, 13.60 volts.

Haynes manual does not give any readings for what I did, anyone any thoughts or experience?

ianaudia4 wrote:with the engine running, 13.60 volts.

You have basically "lost" 0.32 of a volt. Now whilst this may sound like a large amount, it can be happily used (and lost) in the circuitry between the alternator and the battery and everywhere in between. The wire itself will absorb some voltage (called voltage drop) as it will have some resistance inherently built in, be that terminal connections, the wire itself, the chassis of the car between the earth strap on the battery and the alternator.

I wouldn't be overly concerned by this small a drop. If it was 10% or more (so 1.3 V), then yes start to ponder some more as to how you can remove that loss from the system.

as said, voltage drop small enough not to worry about. however, knowing what some of you guys can be like, i'd be guessing you want to find it

measure from the red wire on the alternator and the case of the alternator. this will tel you the voltage being put out by the alt, and therefore the most you'll be able to get.

now move the -ve first to the engine block and then to batter -ve. ideally, you should get the same reading each time. if it dropped when you moved to measuring from the engine block then you need to take the alt off and clean the points where it bolts on, similarly where the mount bolts on to the block. if it drops when you move to measuring from battery -ve then either the main earth points are needing a clean (probably the one on the gearbox) or the earth cable will need replacing.ter or batter

now trace where the big red wire goes. it will either go from the alt to the starter motor and then on to the battery, or there will be 2 separate big red cables coming from the battery, one to the alt and one to the starter.

ok, this time keep one connection on the alt case and mov the +ve to the next point along the big red wire (starter or battery as appropriate) and measure again. if this is lower you will need a new red cable. repeat at the battery if this isn't what you just did.

makey sense?