ALT light takes 10 minutes to go out

[Swede]

So.... Anyone have any idea why it takes 10 minutes for my ALT light to go out after I start my Runner up in the morning???

A little history first. I replaced my alternator recently with a HO one from DC Power. I verified with Dave (@ DC Power) that the alternator is working properly (he's been great to deal with btw). So there is something else going on with my charging system.

What happens is this... I get in the rig in the morning, start the thing up and the "ALT" light stays on and there is NO output coming out of the alternator. After 10 minutes (like clockwork) all of a sudden the light goes out and it's charging. Every morning it's exactly the same too. Continuity on the 100 amp fuseable link is good. The 7.5 amp fuse in the cab is good and so is the one under the hood.

Incidentally, I cut the "ALT" wire (yellow) and checked the output voltage and there isn't any on the fuse box side of the lead but there was on the alternator side though.

Seems so odd that once it's warmed up it's fine but it's just in the morning after it's been sitting overnight that I have an issue.

So... any ideas???

 

[fourwd1]

The charged light is controlled by the L terminal of the alt, which is an output of the IC Regulator in the alt. So that points to the alt as the problem. Get a DMM and monitor the battery voltage from start up until the light goes out.

 

[Skidmarcx]

That's weird... Sounds like a regulator issue... DC Power makes a good unit I'm surprised to hear about this

 

[Swede]

I am getting 12.5 volts at start-up when the "alt" light shows and about 15 volts when it kicks on.

 

[Swede]

Just to clarify, when I cut the yellow wire, I wasn't getting any voltage on fuse box side of the wire. I was getting voltage from the alt side though. (I'll go back and edit my first post).

 

[nissanh]

15 V is too high. It can damage other electrical parts. Max is about 14.3 V.

 

[Swede]

15 V is too high. It can damage other electrical parts. Max is about 14.3 V.

I thought that too but after some research, 15v is pretty normal and within specs. It's also only at 15v for a few seconds too and settles down to the high 14's.

 

[Swede]

Anyone else???

 

[TheDurk]

Anyone else???

I'll play. Definitely a head scratcher. The external circuitry is stupid simple. Two grounds, battery, and the lamp/exciter lead. I'd confirm the 12v+ and the two grounds, as well 0-4 volts at the last with the key on and engine off. If those check out, then your issue pretty much has to be internal to the alt.

http://web.archive.org/web/20100203050630/http://www.ncttora.com/fsm/1996/SIL/techinfo.toyota.com/ileaf/ewdtmc2/ewdtmpdf/ewdsourc/2002/024runne/systemci/014cs.pdf

And yeah, above confirms 15.1 Volts at cold start-up is within spec.

The charged light is controlled by the L terminal of the alt, which is an output of the IC Regulator in the alt. So that points to the alt as the problem. Get a DMM and monitor the battery voltage from start up until the light goes out.

This is imprecise. The charge light wire is not an output, it provides excitation voltage to the alt until the alt itself starts charging. If there is no charge light circuit, the alt will never start charging (or only at high rpm). So on the battery side of the light or on the battery wire itself you have battery voltage of 12.5 volts. Once the alt is charging, it is producing ~14V so there is no flow from the battery to the alt through the light. Thus it goes off giving evidence that the alt is charging. You won't see much voltage on the light side yellow wire with engine running but alt light will come on if you ground it. Don't do that to the alt. side yellow, though.

I'd take a very close look at both battery connections and the big alt cable. It's possible something doesn't make good contact until that something warms up. Alt can't charge if it does not have good connection. Look for hidden corrosion in the cables near the terminals. Swap for new if there is ANY bubbling of the insulation. Clean up your battery and alternator grounds and the points where they attach.

 

[Swede]

I know the ground at the battery is good but where is the other one?

 

[TheDurk]

I know the ground at the battery is good but where is the other one?

The second appears to be a case ground to the engine through the mount so check the engine ground point on the left of the engine near the diagnostic plug. This ground is the source of many weird behaviors and certainly could be one that expands into better contact after a few minutes of runtime as the engine heats up. Worth checking.

 

[Swede]

DC Power makes a good unit I'm surprised to hear about this

Like I said, the alternator is working fine. It's something else. I upgraded that ground wire too that grounds the block to #4 awg, maybe I'll double check the contact there.

 

[Swede]

Thanks for the added info "Durk", it is extremely helpful.

I have a brand new (660cca) starter battery, (as opposed to my other deep cycle aux battery which is 6 years old) with brand new #4 awg wire going from the "+" post to the fusible link and then to the alt. #4 awg from engine ground to "-" post on the battery and then #4 awg from "-" post to ground. All the terminations are brand new as well.

I'd like to add that when I was having issues with the OEM alt, I inadvertently grounded out the output post on the old alternator with a small wrench causing a huge spark. Not sure if this caused the problem.

 

[Swede]

And just to emphasize the thought that it's temperature related, I parked the Runner in the garage overnight Friday night (as I was planning on changing out the LBJ's on Saturday) and sure enough when I started it up in the morning, FULL CHARGE and immediately too. (It was about 15 C in the garage and close to freezing outside) I think tonight I will pick up a small space heater and leave it in the rig overnight to see if I can isolate whether the problem is in the cab or not as it will be parked outside overnight.

 

[Swede]

Guess I should update this thread. Turns out Dave from DC Power sent me another alt a few weeks later and this one worked perfectly right away and has been working great ever since. Guess it was a faulty one after all. What a pain. DC Power was good to deal with just the same and I'd still recommend them to anyone who wants a direct bolt in replacement option.