Alternator Voltage Question
#1
Intermediate
Thread Starter
Alternator Voltage Question
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.
#2
Moderator
I would remove the alternator and have it bench tested at a shop. Voltage is just one indicator.
Salim
Salim
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Lexmus (05-23-17)
#4
Moderator
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
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
Last edited by salimshah; 05-22-17 at 09:14 AM.
#5
Intermediate
Thread Starter
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.
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.
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Roop10 (02-04-22)
#6
Moderator
Things are not simple in this case. The battery itself acts like a big clamp on voltage.
Salim
Salim
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#8
Intermediate
Thread Starter
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.
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Roop10 (02-04-22)
#9
Moderator
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.
Salim
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Roop10 (02-04-22)
#10
Intermediate
Thread Starter
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.
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.
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Roop10 (02-04-22)
#12
Moderator
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
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Roop10 (02-04-22)
#13
Intermediate
Thread Starter
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.
#14
Intermediate
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.
. 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.
Last edited by Drcoffee; 05-31-17 at 04:55 AM.
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Roop10 (02-04-22)
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