RTH - Overcharging Causes 18.5V

[TheForger]

I’m at work so here’s a short synopsis of what’s going on:

• On my morning commute everything was fine until (randomly) my battery, ABS, VSC TRAC, VSC OFF, airbag, and brake lights all turned on
• I looked at both my ScanGauge II as well as my voltmeter in my console and both read between 18.3-18.8V while on the gas
• Radio shuts off
• A/C remains unaffected
• Headlights turn on and do not allow me to turn them off unless I shut the truck down; when I turn the stalk to the OFF position my high beams come on
• When I come to a stop at a traffic light voltage returns to normal around 14.1V and the battery light turns off but all others remain on
• When I shut the truck off and turn the key to ACC I’m reading 12.7V
• No issues starting
• After shutting down and restarting, the dash lights turned off and I’m reading around 14V. The dash lights then turned on when I got on the gas in park when voltage reached 17.5V
• Engine itself is running perfectly with no weird side effects or RPM changes

Could an overcharging issue be indicative of a bad alternator? Should I run out to the parking lot and disconnect the negative terminal for a few hours and see if that helps anything?

 

[cjmoon]

Sounds like a bad voltage regulator computers have some protection and their turning them self off I’d guess to protect from over voltage. I’d unplug the alt if you must drive to avoid frying anything and hope battery power can get you home.

High voltage like that will boil a battery and cause harm outgassing with bad results

 

[TheForger]

Sounds like a bad voltage regulator computers have some protection and their turning them self off I’d guess to protect from over voltage. I’d unplug the alt if you must drive to avoid frying anything and hope battery power can get you home.

High voltage like that will boil a battery and cause harm outgassing with bad results

Good thinking and the likely culprit, although not what I wanted to hear. Since the voltage regulator is internal it sounds like I’m due for a new alternator. I did disconnect the negative terminal and will let it sit for a few hours as a long shot. I will have to drive it home but it’s just a 15min drive and once I make it I’ll drive another vehicle in the meantime until I can get this into a shop. I wonder if this is an issue that can be caught with an alternator test? None of the alternator wires have any corrosion as I just cleaned them 3wks ago.

 

[dogdaysindurham]

@TheForger I believe in the 2000 100 series, one can get an Toyota OEM voltage regulator p/n: 27700-50030 but it's almost as expensive as just getting a remanufactured Denso alternator. In 02/03 the alternator was changed for a higher current unit (130 amp or so?) and the voltage regulator is no longer offered. Only the bearing and brushes are offered.

 

[TheForger]

@TheForger I believe in the 2000 100 series, one can get an Toyota OEM voltage regulator p/n: 27700-50030 but it's almost as expensive as just getting a remanufactured Denso alternator. In 02/03 the alternator was changed for a higher current unit (130 amp or so?) and the voltage regulator is no longer offered. Only the bearing and brushes are offered.

Yeah I've been doing just a little bit of research on my lunch break and I found the pn for the voltage regulator, but IMHO if the alternator has to be pulled out might as well just replace it with a new 100A one. I'm thinking I'll keep the old one and fix it myself some time in the future so I can keep a spare in the truck.

Does anyone know of a way to definitively test whether this is the culprit before I dive into things? Maybe testing resistance values between certain points? By "dive into things" I'm referring to a mechanic because I simply don't have the time to do it myself.

 

[SHIfTHEAD]

The only cause of over voltage is a bad regulator. There are diodes in the regulator that shunts over voltage to ground, and these diodes go bad after a time. Usually they fail Closed, shunting to ground, but they can fail Open, as in your case, causing the over voltage instead of the typical under voltage scenario. Over voltage is much worse for the vehicle, BTW, so don't run it unless you have no choice.

 

[dogdaysindurham]

@TheForger on pg 13, if I recall correctly there is the flowchart for testing the voltage regulator (referred to in the pdf as IC regulator) http://www.autoshop101.com/forms/h8.pdf

 

[nissanh]

Did you notice any loose battery terminal or 12V main wire on the alternator? A loose wire can damage the diodes.

 

[white_lx]

Also check the Alt.S (7.5A) fuse. It provides voltage regulation feedback to the alternator. It is mounted in the little box on the positive battery post.

 

[nissanh]

Check the sense fuse located on the positive bettery terminal

 

[TheForger]

Did you notice any loose battery terminal or 12V main wire on the alternator? A loose wire can damage the diodes.

I did not notice any at all. I replaced the power wire coming from the + terminal to the alternator a few years ago but I recently cleaned all the terminals, including disconnecting the wire harness to the alternator and spraying with electronics cleaner.

Also check the Alt.S (7.5A) fuse. It provides voltage regulation feedback to the alternator. It is mounted in the little box on the positive battery post.

Check the sense fuse located on the positive bettery terminal

I checked both of those and they all look good. Didn’t get a pic of the 7.5 fuse but it’s also good

 

[TheForger]

UPDATE: After letting the truck sit in the parking lot with the negative terminal disconnected for ~3hrs it seems everything has returned to normal. Starts great, idles great, I’m reading 14.1V both while sitting idle as well as under load, and it doesn’t fluctuate more than 0.5V when coming to a stop at a light. Radio, headlights, etc work as they should. This almost makes me feel worse now that it’s hiding itself....I’ve always hated hide and seek On the flip side I made it home with no issues so I will continue to monitor things and keep updating as I go. No long road trips for this rig though, I can’t risk getting stuck out in the middle of nowhere!

 

[nissanh]

Alternators on a toyota can fail at two places, brushes and diodes. I carry an extra diode pack with me as I have replaced the brush kit.

 

[dogdaysindurham]

@TheForger it still could be the voltage regulator and that once the alternator gets hot this will happen again. You might also have a faulty diode pack P/N: 27357-20060. Usually 1 of the 3 diode packs will fail and mask a total alternator failure as the voltage will still be ok, but the alternator can no longer put out the full amount of current.

I think the alternator for your year is like: 27060-50260-84.

The 130 amp alternator for the later year 100 should fit but don't quote me on that (the plug is a different shape IRC, but I remember a thread way back where the person bought the proper alternator plug from Toyota and crimp it in, it came with the crimped ends?). The Denso P/N for the higher current alternator is: 210-0607

 

[white_lx]

Just noticed that your battery indicator is also on. But if the charging is OK, it should not be on. And it should also be off in the case of overcharging.

It could be that the 3-pin connector unseated from the alternator or desintegrated. This results in the sense (feedback) signal disappearing, hence the high voltage output.

The sense signal and battery indicator light signal are on the same connector to the alternator.

 

[BenCC]

I see the scangauge mount warped. Did you use wet sanding when you finished the surface? The printing material, PLA, deforms when exposed to excessive water. I have been using new a material, ABS. I can send you one for free. PM me your address.