My alternator charges briefly over 17v when cold

**Milton477** · 8th October 2010, 09:41 AM

Hello folks,

My 10 month old reconditioned alternator has started playing up. First thing in the morning, at idle the voltage is around 14.5v, 14.6v. As soon as I increase the revs, the voltage shoots up briefly to over 17v, the alternator charge light comes on along with some of the amigos & the radio dies. Just as quickly, the voltage drops back to 14.5 ish for a few seconds & then shoots up again. Turning on the headlights seems to control these spikes & after 5 mins or so, all returns to normal with battery voltages at 14.2. The vehicle runs normally during these spikes.

I am running a standard 120amp alternator & a dual battery system with an ARB Isolator & a Piranah Dual Voltage Display. The main battery was replaced 6 months ago with a Battery Centre recommended 950CCA battery & the second battery is a 120aH AGM battery.

Would I be wrong to assume that the alternator is faulty or is there some other gremlin I should be looking at first?

**biggin** · 8th October 2010, 11:48 AM

If there is nothing obviously wrong, I would start faultfinding by returning the battery setup to standard single battery. At least you can then discount the dual battery system as source of your problems.
If the problem remains, I would start pulling fuses one by one.

**XHPBT1** · 8th October 2010, 03:17 PM

The Alternator actually produces around 120Volts AC there is a regulator on the back that drops the power down to 14.4V DC, there must be something wrong with the regulator !! take it to an AUTO ELEC if you cant get to one PM me and ill try help over the phone for you!

Cheers

**Milton477** · 8th October 2010, 03:28 PM

Originally Posted by XHPBT1

The Alternator actually produces around 120Volts AC there is a regulator on the back that drops the power down to 14.4V DC, there must be something wrong with the regulator !! take it to an AUTO ELEC if you cant get to one PM me and ill try help over the phone for you!

Cheers

Thanks, I suspected as much. Probably a bad solder joint that needs a bit of heat to expand a bit. Just as well it has a 12 month guarantee. I will return it for another tomorrow. Pity they are so hard to change.

**Blknight.aus** · 8th October 2010, 06:40 PM

Originally Posted by XHPBT1

The Alternator actually produces around 120Volts AC there is a regulator on the back that drops the power down to 14.4V DC, there must be something wrong with the regulator !! take it to an AUTO ELEC if you cant get to one PM me and ill try help over the phone for you!

Cheers

what.... no, no they dont, they can but the regulator stops them from doing so. (actually if you change the diodes and the regulator you can get 240vAC from them quite easily.

the regulator controls the excitation winding in the alternator that dictates how many magnetrony thingys turn up at the output windings which directly controls how many voltybars come out of the alterntor windings to be rectified by the diodes prior to being pushed into the battery.

**dutchviking** · 13th October 2010, 07:42 PM

Just installed an AEDO dual battery tray and a National Luna battery isolator with a dual battery monitor. When I started the Td5 Discovery 2 the first time the monitor started to beep and indicated an over voltage situation. It was charging the main battery with a voltage (measured at the poles) of 17.6 volts

My disco has nearly covered 200000 km but this explains why my last battery only lasted for 3 years. Have now installed a brand new alternator which peaks the first 5-10 minutes or so at 14.8 volt, then drops below 14.65 Volt, the trigger voltage for the monitor to set an over voltage condition.

My advice, check your voltage right after starting to prevent swapping light bulbs and batteries regularly

**haydent** · 28th November 2012, 12:52 PM

same problem here as starting post,

high voltage over 17v on higher that idle revs

alternator light comes on in dash as does other 3 amigo style lights.

stop revs and the go out.

im about to try switching the battery with a spare but it sounds like alternator problem.

i hope the regulator can just be replaced as the alternator is only about 2 years old though not genuine lr part so maybe explains the problem...

**Milton477** · 28th November 2012, 02:36 PM

Soon after my original post, the offending alternator died. The current one never gets above 13.7V so it would seem that there are different flavors out there charging to different voltages.

**haydent** · 28th November 2012, 04:50 PM

thanks, i didnt worry about trying a second battery , it had to be the alternator to be putting out such a too high voltage.

so i just finished removing it and will take it into auto elec tommorow.

**haydent** · 30th November 2012, 06:05 PM

well i feel like im going to cry and the missus is going to go tropo at me when she finds out...

i got a new set of brushes and regulator through ultra tune for $160, and just finished swapping them over and putting the alternator back in

and its still doing it !!! ;(

im thinking now there must be something broken in the alternator itself...

also when putting the vaccuum pump back on i broke one of the bolts...

it seems my standard kingcrome torque wrench aint so accurate down in the low torques that these little bolts needed, this is the second time ive run into trouble with trying to do low torque with this wrench, previously stripping the thread on the centrifuge filter cover bolt hole.

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My alternator charges briefly over 17v when cold

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