Funny acting alternator..

[jeeper]

Working on a Honda Odyssey for a neighbor. The van has a rebuilt title from a wreck.
They told me they quit driving it because it shut off on them, after the lights were all flickering and such. I assumed alternator went bad, drained battery, car died.

Van has sat for a year or so now.

Battery was toast, but I was able to get it functional again. Can sit for many days, not drain. Stays topped off.

Alternator was giving me 11ish volts when I first checked. No brainer it's bad. I went to change it, but when pulling into the garage all the 'check charging system' lights turned off. I was getting 14.3 volts.

I've put about 25 miles on it now. I keep my scan gauge plugged in, and carefully watch the readout for volts. The voltage fluctuates a ton. about 70% high 13+ volts, and about 30% low 12 or less volts. The check charging system lights on the dash flicker on and off often.
Random things will change the low volts back to 14v, like turning on the headlights, or restarting the van.

I have never seen an alternator fluctuate like that. My experience says they are on or off.
Good battery connections. I haven't checked other places.

Is this normal behavior for a failing alternator?

 

[Hickey]

Probably the voltage regulator, but maybe a terrible ground cable.

 

[1969honda]

I'm with @Hickey on bad voltage regulator. What year is the vehicle? Some newer vehicles I believe use the ECM to control the alternator output as well. It might be having problems from there depending on how hard it was hit?

 

[DaveB]

I learned something new last week when my son's alternator failed. He was getting the charging failure light on and off but couldn't figure out why. Then one morning on the freeway up by Layton the alternator pulley came off. Luckily he made it off the freeway. What I learned is that many of the newer cars have an alternator pulley clutch. The clutch engages when the motor is under a load but disengages when the motor is not loaded. My son's alternator clutch pulley failed and unthreaded itself off of the alternator which then caused the serpentine belt to come off.

 

[Mouse]

Diode in the rectifier bridge can act weird when starting to fail as well. Definitely check grounds. A ground directly to the alternator case is a good idea

 

[Unimog]

I would check the grounds first. This is exactly what a bad battery ground or engine to frame ground would look like.

 

[lhracing]

“rebuilt title from a wreck.” If it was a front end crash I would be very suspicious of a poorly repaired wire harness. So many of these don’t wiring repairs done properly.

 

[UNSTUCK]

I learned something new last week when my son's alternator failed. He was getting the charging failure light on and off but couldn't figure out why. Then one morning on the freeway up by Layton the alternator pulley came off. Luckily he made it off the freeway. What I learned is that many of the newer cars have an alternator pulley clutch. The clutch engages when the motor is under a load but disengages when the motor is not loaded. My son's alternator clutch pulley failed and unthreaded itself off of the alternator which then caused the serpentine belt to come off.

I believe you have the alternator mistaken for the A/C compressor. 😂



Just kidding.



I think.

 

[DaveB]

I believe you have the alternator mistaken for the A/C compressor. 😂



Just kidding.



I think.

Funny thing that new AC compressor is charging the battery just fine.