Alternator check
 

Hi guys I need a little advise for an electrical issue.. I have a 96 xjr, had new alt in Dec but yesterday battery light come on and off in rev range erratic volt meter gauge,
then volt meter dropped to the 12v Mark from the normal 14v. check at battery 12.20v now and light not on but volt meter reading same as multi meter 12.v ..
Question is what wires to check on back of alt to confirm alt and not car side? before I remove to try and get replacement as they want it checed and they will check before replacment is offerd many thanks Steve

You probably don't want a lecture on punctuation, but it would be much easier to understand what you are saying if you used some!

Anyway, since the alternator has an internal regulator, there is little on the "car side" that can cause your symptoms other than a bad cable or ground. Measure the voltage on the big wire to alternator case ground and make sure it is about the same as the battery voltage you are reading. If it is higher at the alternator, then you have a bad ground strap or bad power cable connection. First suspect would be the bulkhead connections at the firewall.

I had issues with the battery dying not long after I got my xj I could only get about 12.5 volts at the battery took the alternator out and had it bench tested and they said it was ok.

I went round all the charge circuit cleaning connections and such still no good! In the end I found the earth strap connection from the bell housing to the chassis under the car looked ok but was bad (after following some posts on this forum).

I couldn't get the bolt out of it so I fitted another one next to it and hey presto! 14.2 volts at the battery it costs about a fiver for one of these it's well worth changing this before you spend a load of dosh on battery or alternator.

GOOD LUCK I HOPE THIS HELPS YOUR CAT

Sorry on a mobile phone, not the best...thanks for the help

Checked again tonight whilst engine cold.charging normal..
Recent the engine with multi meter on battery's 14.5 v..but when I get the red battery light on dash it's over charging up to 16v...then it dropped to battery voltage and the voltage gauge confirms this....check alt wiring ok check voltage at alt output wire and it's the same as at battery..

If you are overcharging, you have a bum regulator. Since it is inside the alternator, change it out before you burn up your battery, lamps, or modules!

It sounds like your alternator is kicking out good power. Checking the earth straps sounds like wise advice. If there is resistance there then voltages measured else where could be higher or lower than they should be and current could be flowing through the wrong circuits due to poor earth.

The fact you got a new alternator could mean you changed it because of a fault the car still has. You might break this one if you don't fix the fault or you changed the last one when it was not broken.

Hi since last post t he alt doesn't charge at all 12.4 at battery start the car 12.02 running.
checked at the alt charge out just battery voltage..so looks like alt caput.good earth strap good earth on engine block...alt out tonight to get changed...

Just got my alternator tested and it showed 13.89 with no load/13.74 loaded with 'excessive ripple' of 270mV.

Is it time to change the alternator?