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Alternator Drawing Current #4728529
04/16/18 08:18 AM
04/16/18 08:18 AM
Joined: Jan 2014
Posts: 447
Australia
Spetz Offline OP
Spetz  Offline OP
Joined: Jan 2014
Posts: 447
Australia
Hi everyone,

I noticed my car had a parasitic current draw when off.

After circuit testing it turned out that removing the hot side from the alternator (the thick red wire) got rid of the current drain.

The alternator however still produces voltage etc.

The ECU is showing up an error for the FR terminal however from my testing the voltage at the FR terminal is fine.

Could the current draw (which might be faulty diode?) cause the error code?

Engine and ECU are from a Mitsubishi FTO 6A12 MIVEC.


2003 Mitsubishi Diamante 6G74 3.5L. 5 Speed auto. 155,000 miles. Shell Helix Ultra 5W40 at 6k OCI
Re: Alternator Drawing Current [Re: Spetz] #4728533
04/16/18 08:20 AM
04/16/18 08:20 AM
Joined: Mar 2009
Posts: 2,637
Cedarbrae, Ontario
xxch4osxx Offline
xxch4osxx  Offline
Joined: Mar 2009
Posts: 2,637
Cedarbrae, Ontario
Bad diode in the alternator maybe?


2015 RAM SXT Crew Cab 5.7 with 6 speed tranny.

2008 Mazda 3 GS Sport Hatchback 5sp MT (Girlfriend's car)

Re: Alternator Drawing Current [Re: Spetz] #4728574
04/16/18 09:08 AM
04/16/18 09:08 AM
Joined: Jan 2014
Posts: 447
Australia
Spetz Offline OP
Spetz  Offline OP
Joined: Jan 2014
Posts: 447
Australia
Yes that is what I have read - But could a bad diode cause an FR terminal error?
As the FR voltage seemed fine when testing it


2003 Mitsubishi Diamante 6G74 3.5L. 5 Speed auto. 155,000 miles. Shell Helix Ultra 5W40 at 6k OCI
Re: Alternator Drawing Current [Re: Spetz] #4728585
04/16/18 09:18 AM
04/16/18 09:18 AM
Joined: Sep 2007
Posts: 989
TX, USA
JMJNet Offline
JMJNet  Offline
Joined: Sep 2007
Posts: 989
TX, USA
I can be wrong but Diode is supposed to limit the electrical current to go one way to the direction it supposed to go.
In this case charging the battery not the other way around.

For alternator, you cannot measure voltage, you need to measure amp-acity (amperage).
That is why we use load tester.
The load tester apply a certain amp (load) to the system and see if it can still hold the voltage after the load.
You may do it this way, though. If so, forgive me for the suggestion.


  • 2017 Toyota Sienna
  • 1995 GMC Suburban 6.5L Turbo Diesel


Re: Alternator Drawing Current [Re: Spetz] #4728607
04/16/18 09:36 AM
04/16/18 09:36 AM
Joined: Apr 2017
Posts: 1,481
Ontario, Canada
mightymousetech Offline
mightymousetech  Offline
Joined: Apr 2017
Posts: 1,481
Ontario, Canada
If you have a bad diode, you will get a parasitic draw, and the alternator will be sending some AC current to the battery. Not good for the life of the electrical system. Use your meter to measure alternating current at the alternator, if you see any, it is toast.


Mighty Mouse Tech
BMW Tech, Former Acura Tech
2010 Civic Si Castrol 0W40, Redline MTL
2013 BMW 135i M Sport Edition Castrol 0W40, Redline MTL
Re: Alternator Drawing Current [Re: Spetz] #4728736
04/16/18 11:21 AM
04/16/18 11:21 AM
Joined: May 2009
Posts: 12,250
N.H, U.S.A.
ARCOgraphite Offline
ARCOgraphite  Offline
Joined: May 2009
Posts: 12,250
N.H, U.S.A.
I had a power seat wire shorting in my old Buick Electra, Only when you sat on the bench and it hit a seat spring! Hard to find that puppy!


2014 Nissan Rogue S CVT, OC#9 41392mi-QSUD10W30 + M110A CHAMP Filter; 2017 Subaru Crosstrek CVT OC#2 ?? mi-SOA/Idemitsu 0w20 + SOA Filter
Re: Alternator Drawing Current [Re: Spetz] #4728864
04/16/18 01:12 PM
04/16/18 01:12 PM
Joined: Jan 2006
Posts: 734
Los Gatos CA
djb Offline
djb  Offline
Joined: Jan 2006
Posts: 734
Los Gatos CA
That's just at the transition point for a generation change in alternator regulator design.

The first generation of electronically regulated alternators were powered through the warning light. The regulator used that small current as it's power source until the alternator started producing power.

The second generation monitored the phase pulses to delay starting up the alternator. It wouldn't try to generate power until a certain RPM was reached.

The third generation changed how the regulator was initially powered. It used the warning light circuit only as a signal that the ignition was switch on, while drawing its operating power from the main battery lead. This might seem a meaningless change, but it allows the regulator to signal other faults using the warning light, without shutting down its own power.

The fourth generation is always powered by the battery. It stays in very low power mode until it detects that the engine has started and has continued to run for a few seconds. These generally have a digital output to the ECU, communicating the status and errors using LIN (sending messages over a serial communication bus) or DFM (digital field monitor, a pulse proportional to how much power the alternator is trying to put out).

A bad diode is the traditional way that alternators have a parasitic draw. The failure is usually caused by reverse polarity during jump starting, and the current drain is often high.

An electronic regulator adds a new failure mode, either continuously leaving its power on (a 1-100 mA) or even providing field current (up to a few amps). The ECU probably already knows what the fault is, but if you can't read the codes, removing the regulator will usually narrow down the fault.

Re: Alternator Drawing Current [Re: Spetz] #4729215
04/16/18 07:19 PM
04/16/18 07:19 PM
Joined: Jan 2014
Posts: 447
Australia
Spetz Offline OP
Spetz  Offline OP
Joined: Jan 2014
Posts: 447
Australia
Would removing the hot side wire from the alternator and starting the car for a few seconds to see if the error code still shows up be a good way to test this?

Or how can I test it?


2003 Mitsubishi Diamante 6G74 3.5L. 5 Speed auto. 155,000 miles. Shell Helix Ultra 5W40 at 6k OCI
Re: Alternator Drawing Current [Re: Spetz] #4730145
04/17/18 04:52 PM
04/17/18 04:52 PM
Joined: Sep 2015
Posts: 1,552
USA
mk378 Offline
mk378  Offline
Joined: Sep 2015
Posts: 1,552
USA
You know there is a problem inside the alternator. The question is whether to poke around inside the alternator or replace the whole unit.

Re: Alternator Drawing Current [Re: Spetz] #4730650
04/18/18 06:19 AM
04/18/18 06:19 AM
Joined: Aug 2005
Posts: 910
dayton oh
kc8adu Offline
kc8adu  Offline
Joined: Aug 2005
Posts: 910
dayton oh
if you can buy the rectifier assy and feel up to the task replace it yourself.
get the brush assy too.
you may be able to get a full rebuild kit.
i would rather do this myself or use a good local rebuilder vs a 3rd world cheap rebuild from a big box.

Re: Alternator Drawing Current [Re: kc8adu] #4730895
04/18/18 10:59 AM
04/18/18 10:59 AM
Joined: May 2012
Posts: 4,484
Baltimore, Maryland, USA
zzyzzx Offline
zzyzzx  Offline
Joined: May 2012
Posts: 4,484
Baltimore, Maryland, USA
Originally Posted By: kc8adu
if you can buy the rectifier assy and feel up to the task replace it yourself.
get the brush assy too.
you may be able to get a full rebuild kit.
i would rather do this myself or use a good local rebuilder vs a 3rd world cheap rebuild from a big box.


I agree with this comment. It's easy enough to do too.


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