I decided to check the output voltage on my RX300 alternator due to a recent alternator failure in one of my other cars and was surprised to see it was only 13.7V. I measured it directly at the alternator. My other three vehicles measure 14.2 to 14.4V. Is this indicative of an alternator that is ready to fail? Should I get it rebuilt as a preventive measure? This is the original alternator.

 

I would remove the alternator and have it bench tested at a shop. Voltage is just one indicator.

Salim

 

If it ain't broke, don't fix it. IMO 13.7V sounds totally fine too.

 

Just an insight ... The alternator produces AC voltage in different phases ... [i forgot if it is 4/5/6 phases]. Then each phase is run through a diode [rectifier bridge for all phases] to produce DC voltage. If any diode fails [lets stick with open] then that phase does not produce any voltage. With successive fails the voltage keeps dropping and most important of all current capability just starts going down [as each phase does not produce].

This is why I do recommend having the alternator bench tested for
1. voltage,
2, current (load)
3. Leakage

note: the diode open fails generally happen when the alternator has to charge up dead battery.
I do not have any statistics as to when, but mine failed at 225K miles and by that time i also needed to refresh my brushes and slip rings.

Salim

 

Thanks for the comments. I decided to get more specific data to share.

When I rev the engine under minimal load I have 14.05 V at the alternator. Under full load (lights and max A/C at idle rpm it is down at 13.6-13.7. I am also losing about 0.15V in the Positive wiring from the alternator to the battery. No voltage drop in the negative wiring.

So the first thing I am going to try and fix is the positive wiring voltage drop.

In the mean time, I will do some watchful waiting and see if the alternator voltage degrades further.

 

Things are not simple in this case. The battery itself acts like a big clamp on voltage.

Salim

 

Here is a thread I refer back to and really helps make sense of the electrical system

http://my.is/forums/f221/diy-chargin...attery-426000/

 

After reading the link Dr Coffee posted I borrowed a Fluke multimeter and measured the AC voltage at the alternator as 0.52V. This indicates I have a bad diode. For comparison, I measured 0.05V on another car. I plan to remove the alternator and get it tested and rebuilt.

 

Rectifiers and brushes will run you close to $90 [OEM]. If your slip rings have been gouged beyond re-use that would be additional purchase and the labor is going to be about an hour. You can get a refurbished with life time warranty for not much more assuming you dont have to pay for repair labor. If bearings are shot then $$$ wise referbs would be lot more appealing.

Salim

 

Salim,

That is a good point. I would buy a new alternator from Lexus, but I can't find any available. I think the second best option is a BBB industries new N13844 alternator. It is about $245 from Amazon. Their remans are under the Vision-OE label and are $168 at Amazon. My daughter's car that had the bad alternator was fixed in her college town and the shop put in a Vision-OE alternator. So far so good but it has been less than a month so far. The BBB industries alternators all have 5 star ratings at Amazon, but there are maybe 25 total reviews across all models. They are made in the USA and have a 3yr/36000 mi warranty.

 

Not trying to be a jerk, but other than the voltage reading slightly low (which could be perfectly normal on this car), why are you messing with it?

 

Based on presented evidence, it seems one phase is not being rectified, producing higher AC voltage and less DC voltage. I tossed my rectifier (can't confirm 3 or 4 phase output) but if we go with the assumption that the alternator is producing 3 phase AC, then 1/3 of the output is dead. With his current demand (night driving) there may be a trickle of current left to recharge.

I stand behind my suggestion that have the alternator tested in jig and then decide to keep, fix or replace. I chose to fix mine for less than $100 ... new rectifier bridge and new brushes. I did have to smoothen the slip rings.

Salim

 

I get what you are saying, if it ain't broke, don't fix it. It is an old car, and when another less old car of mine had an alternator failure, I thought I should check this one. Now that I know there is potentially a problem, I think it is prudent to try and mitigate it since I don't want anyone to get stranded on the road for something that I am aware of.

If someone else has an RX300 and can show that the original alternator runs below 14V and has >0.5V AC as a standard operation condition, then I will drop it.

 

I would say, buy a new alternator and be done with it. If you do a search on google (not on CL) you will find a common occurrence of dimming headlights. This happens as the oem alternators are faulty (not all but too many) and do not produce a sufficient steady flow of power. In one of my project threads I mentioned this issue because my headlights would be fine, then fade out for a moment and then return. It stresses your battery because during the fade period, the battery is running the vehicle. If you kill your battery, add another $150+ to the cost of the alternator issue.

. When it comes to electronics, I believe in buying only new, not rebuilt devices. I have had great luck with TYC alternators and have one on my rx300, 4runner and son in laws camry. Years later and no issues. If your alternator isnt putting out a solid 14.3v replace it. Dont F *** with rebuilding these things when a new unit is only $110. BTW check out rockauto.com. You will always save a ton of cash there.
I just checked rockauto and they dont have the TYC. I would check amazon for one. Avoid chinese crud.

 

Diagnostics