Battery light came on

[towpro]

Sitting at red light, ac off, blower on 2, rear defog turned on. Both wipers on, head lights and fog lights turned on. Battery light came on. Turned off defog switch and withen a couple seconds battery light went off.
Battery is 6 months old.
Is this light triggered be a low voltage event?
(like load was more than alt put out at idle and voltage dropped

Is this an indicator my altinator is going bad?

 

[ltd02]

That's pretty much what mine did when the alternator was failing, but check voltage at battery with engine off and again with engine idling. IIRC, over 12.5 when off and over 13.5 when idling (I think idling voltage depends on temp).

 

[Billwill]

Yeah check the voltages as stated above.

Also check the battery terminals are clean and tight, the negative lead from battery to chassis is clean and tight and the strap grounding the engine to chassis is clean and tight.

Probably the alternator...you can remove the brushes and check if they are OK.
Most shops cannot test these alternators properly as the voltage is regulated externally.

Is this problem on a CRD?
The CRDs have a one-way clutch/coupler on the alternator....these are prone to failure...normally you will see red rust on the front of this coupler when the coupler is starting to fail. You can replace the coupler by itself but special tools are required to remove/replace it. Early couplers were dry but there are grease-packed couplers available which last longer.....made by Litens!

If you remove the alternator the coupler should rotate in one direction only...if it easily spins in both directions it has definitely failed.

 

[towpro]

does the alternate have a history of taking out the BCM? I would hate to take out a $1K BCM by driving it waiting to install a $120 alt.

I know my buddy's derango has his alternator take out the BCM. Later they recalled the alternator, and paid him back for the replacement but refused to pay for the BCM.

 

[Billwill]

The BCM has absolutely nothing to do with the alternator.....the PCM or ECM for CRD adjusts the voltage output from the alternator. I have never heard of a bad alternator taking out a PCM or BCM.

Have you done the checks listed above?

 

[JasonJ]

Battery is 6 months old, or was INSTALLED 6 months ago?

When I worked retail auto parts, we pulled batteries for sale from the rack when they were 1 year from date of manufacture. We topped them off with a quick charge and sold them at a discount after that.

A battery sitting on the shelf at the parts store, warehouse, or dealer shelf could be a year old by the time someone buys it... and not all places check and top off voltage as needed. Just sitting there, the battery will lose 0.10v per month. 3-4 months of that and it'll technically be 50% charged or less. Put in a vehicle, yeah it'll probably still start it and everything will look normal.. but low voltage on a battery in a situation like that will over-task the alternator and could (usually does) lead to alt failure.

 

[towpro]

The BCM has absolutely nothing to do with the alternator.....the PCM or ECM for CRD adjusts the voltage output from the alternator. I have never heard of a bad alternator taking out a PCM or BCM.

Have you done the checks listed above?

Thanks, I just called it the wrong name.
also Its not a CRD, its a gas engine

 

[towpro]

Test results. Battery sitting voltage was like 12.7 (been sitting for a week).
Headlights fog lights, ac, defog switch turned on for load.
Cold start 13.8-14.2v. as motor heated up and idle dropped I got to the point I was seeing 12.5v. alt is not carrying load at idle. As soon as I turn off defog switch (load) voltage goes back to high 13's. At anytime during test if I raise rpm voltage jumps back to high 13's.
Guess alt is going bad.