Alternator is putting out 16.3 volts

I have a 2004 Toyota Matrix XR. I’ve been having a lot of issues with the battery light. Currently with a new alternator (not rebuilt) it seems to be putting out 16.3 volts. My dash looks really bright and my headlights look really bright. I checked the voltage using the cigarette outlet to my volt meter.

My questions are:

Is there any way something other than the alternator be causing me to be getting 16.3 volts? I personally don’t think so that it would have to be the regulator in the alternator. before I loose me cool at the auto store I hope you can give me some information on this.

This is the second replacement for the Alternator. Is it really possible that I may have a two newly purchased alternators that are bad?

Could something in my car be causing the alternators to be bad.

For more information on my issue from the start look up “Battery Light On, Battery and Alternator good, no problems starting” from my original post of this issue.

For the 1.8L 2ZZ GE engine, it appears the voltage regulator function is part of the alternator. I’d guess 16.3 volts is too high of output voltage, indicating something’s amiss if you did that measurement at idle. Make sure your volt meter is accurate, faulty volt meter’s are not uncommon, and suggest to measure right at the battery rather from inside the car. From what I can see the alternator output voltage to the battery should always be less than 15 volts with engine rpm under 2000 rpm. Another test you could try, there should be an A connector and a B connector on the alternator. The A connector should have 3 wires. The voltage measuring from A-3 to chassis ground it appears should be 13.2-14.0 volts with engine running at 5000 rpm, fully warmed at 230 deg F. From A-1 to ground, 0-4 volts with ignition switch ON, but engine not running. Besides the alternator itself, which includes the voltage regulator, the only other gadget I’m seeing that would be involved with the charging circuit is something called the “combination meter” which is associated with the dashboard charging light.

Suggest to locate an auto parts store that has an alternator test fixture. A auto-electrical repair shop that rebuilds alternators , starter motors, etc would likely have this too. Note that on some newer engine designs the alternator voltage regulator function is done as part of the engine computer software, so good idea to figure out if that’s the case on yours.

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I pulled the alternator and took it to Advance Auto and the alternator tested fine and passed on their testing machine.

I’m not sure of the method to test the wires going to the alternator how to touch the contacts through the harness? I’ll see if I can test them and figure out if any are reading bad.

Could something being shorted cause the voltage to go up? This is what confuses me. Also, if I had something shorted shouldn’t that drain the battery? I haven’t had any issues of low battery, starting the car or the lights being dim. Everything always worked fine except I have noticed the headlights and dash being very bright. The only thing I noticed being unusual is when I had the air conditioning on and the fan on high was the fan seemed to change speeds every 10 to 15 seconds. It did this one time after I put the newly purchased refurbished alternator in. I haven’t noticed this since I put the next alternator in, the new, not refurbished, alternator. But it’s possible I just haven’t tried this. I was afraid to use the car with it showing 16.3 volts.

I wonder if there is a disconnection between the alterantor and the battery. There is usually a heavy fuse, fusible link, etc between the two.

You’re checking the voltage at the lighter which comes directly from the battery and you need to check it at the battery positive terminal.

When a load is not applied (as in a break between alt and batt) then the voltage will peak.

Right now I have the battery disconnected and the alternator out. I’m going to check the resistance levels from the positive battery clamp to the positive alternator connection and see if there’s any resistance. I’ll also check the negative battery clamp to the engine to see if there’s any resistance.

Let me know if there are any other suggestions to do before putting everything back together?

Is it normal for an alternator to put out 16.3 volts if it thinks the battery is low?

Check all fuses and the fusible link.


I don’t think so. It might happen briefly with a newer car designs that use the computer to perform the voltage regulator function. But over 16 volts seems unlikely with the design you have, voltage regulator built into the alternator. I expect you have something wrong with one or more of the four wires that connect to the alternator.


" Four wires connect the alternator to the rest of the charging system.
• B is the alternator output wire that supplies current to the battery.
• IG is the ignition input that turns on the alternator/regulator assembly.
• S is used by the regulator to monitor charging voltage at the battery.
• L is the wire the regulator uses to ground the charge warning lamp."

Note: Alternators usually come equipped with a way to bypass the regulator in order to measure the output voltage with full field applied. This “full field test” is for test purposes only, but if your alternator was set up in that mode it might put out 16 volts. See page 4 in the link above.

While the alternator was out I checked the continuity and resistance of the positive battery connector to the positive connector on the alternator and I checked the negative battery connector to the engine. Everything tested good however it probably needs testing with some current and amperage. I also checked all the fuses and none were blown.

The Denso alternator just came in yesterday and I picked it up and put it in. My voltages are reading normal now around 14.3 volts however my battery light still isn’t turning off. Not having the high voltage I felt comfortable driving it that I wasn’t going to blow out the computer and electronic circuits. I went to Advance and had them test it with their tester. Everything showed perfect and it matched what my meter was showing from the cigarette outlet.

When my car is at idle and with a load the volts drop into the low 13 volt range which the guy at Advance thought was too low. With the engine revving with a load it put out around 14.0 and 14.3 without a load. I think everything looked good and it wasn’t overcharging like the old alternator.

I feel like the everything is working fine and I want to drive the car but I’m afraid to with the light on. If the light had never came on I feel like I would still be driving the car without any issues. I never had any signs of low battery or problems with starting the car.

I want to check the three pins in the connector for the alternator but I’m now sure how to do it. How do I get to the pins if it’s connected to the alternator? Do I check it unplugged from the alternator? Can you have it unplugged without causing damage to alternator? Any assistance would be appreciated.

Forget continuity tests . . . you could have a multi-strand wiring harness with all but one strand cut clean off, but that one strand will pass the continuity test with flying colors, with maybe 0.2 ohms end to end, but it won’t properly carry the load it needs to

You need to perform voltage drop tests, if you want to know the true condition of the wiring


It appears the problem is you were supplied w/a bunch of junky alternators. You aren’t the only one with this sort of problem. I’ve been the victim of multiple junky starter motors supplied from auto parts stores. Additionally you were told that prior alternator bench-tested “ok” when it obviously wasn’t. Given all that, suggest to take your auto parts business elsewhere if possible. Or insist on a Toyota or Denso branded part to begin with next time.

You’ll have to get that dash warning light resolved, b/c that’s important to know when it actually isn’t working. If you have 14volts output and the warning light is on, there’s still something wrong. Start by probing the “L” connection I suppose. It’s possible you have a wire harness problem, or the Denso alternator is also junk, or the prior alternator’s high voltage has damaged the warning light circuitry. A proper alternator bench test fixture should show the problem if it is the alternator itself. But who knows whether that parts store uses a proper alternator test fixture. Or if the staff knows how to interpret the test results.

For a valid measurement it has to be measured with the plug connected to the alternator. I’ve never had to do that particular measurement, but the common methods I’ve used in that situation: (1) determine if there is a way to “back-probe” the connector, i.e. access the connection from behind the connector; or (2) partially disconnect the connector, leaving it still fully connected electricity-wise, but exposing some copper; or (3) use a sharp pin to poke through the wire insulation and make contact with the copper inside. Apply electrical tape or a dab of rtv to the hole afterward. Sometimes none of these methods work and I’ll have to make up a test-adapter.

I’m curious as to why you won’t take this car to a competent mechanic for a diagnosis rather than going back to parts stores who are not intended to do that?


17 volts, higher voltage again. Ok, a quick update. I decided to just drive the car around the block. I have my meter connected through the cigarette outlet and I start the car. To my surprise, with the newly put in Denso Alternator, the meter goes up to 17 volts. It only took a few seconds to go up that high and I could see the lights on the gauges and radio getting brighter as it happened. This is the same alternator that a day earlier when it was freshly put in seemed to work fine. So what happened sitting overnight that made this change? I no longer think it’s the alternator (was hoping). There’s something electrical going on.

And if that is the case then you need to find a shop to correct this . At this rate the parts store may start locking the door when they see you

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I had suggested 4 or 5 days ago that the circuit between the alternator and battery be checked to see if a fusible link or main fuse had given up.

I guess it can be assumed that you have yet to do this.

It’s quite common to overthink things and assume the worst while ignoring the simple stuff.

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The alternator is FAULTY… Your voltage reading at 16.3 was too high…anything more than that is also too high.

Either ask them for a replacement unit… or make your unit fail and get a new one that way under their warranty… Whichever comes first. Tell them you do not care about their in store test results… 16.3 is simply out of range, and that is the end of the discussion as far as you are concerned.

You need a new alternator…or at least a re-replacement unit from where ever you purchased this one.

Thanks Hoda_Blackbird, that’s kind of what I’ve been thinking to replace the alternator but after this being the third one I’m second guessing myself.

So I’m having some new things happening. On occasion the alternator seems to work and I’ve had the battery light turn off for brief moments. I found that if I turn on my lights and run the inside heating/cooling fans for several minutes before starting the engine that sometimes the alternator will run at normal voltages. Yesterday I started the car and it was around 14.3 for a few seconds and the battery light was off. Then it started jumping all over the place and started an upward trend. The dome light in my car was flickering and when the voltage started to get high the battery light turned on. Once it got up to 16 + volts I turned my car off.

I think turning on the lights and the fan before starting then it working more normally is an indication of bad cables or resistance. I’m still confused with the voltage reading getting that high even if there is a cable issue. My gut still tells me bad alternator/voltage regulator.

As said, I would suspect the battery circuit. I would start by removing the battery cables, clean both ends of each cable, and reinstall. Even if you aren’t in the “rust belt” states, they can build corrosion under the connections (not just at the battery). I’d also check the frame to engine and alternator to engine connections. You need a non-resistance full circuit for it to function properly. I’d also check the battery isn’t failing.

You could also try testing battery voltage. Press your V meter to the battery post (not the cable ends) and see if the voltage is lower. If there is little resistance in the wiring circuit, you’d should see the battery receiving 16V… which points to a bad alternator… and eventual fail for the battery.

It is possible you are getting defective parts. I received 2 defective alternators from the my local parts house, before I went to Napa. If this continues, you are likely to blow expensive electronics.

I haven’t read carefully, but I see nothing about a new battery, I’m thinking broken post internally.

I also checked a couple places for a battery for this car, they both only showed AGM type batteries.

The first thing I purchased after the battery light was on is a new battery and a serpentine belt. After that I tried replacing the alternator.

Last night using a wire brush I cleaned both battery cables (including where the small positive wire screws on the positive connector), the positive connector that goes to the alternator and the ground wire that’s under the battery plate. This did not fix the issue the battery light is still on. The voltage’s on this check were all good. That doesn’t really mean anything because it’s hit or miss if the voltage is going to shoot up to 16 or 17 volts. Although with good voltage readings I’m not sure why the battery light is on.

I also checked the voltages on the three wire connector to the alternator per Advance Auto support. I put the car in the on position with the engine not running, disconnected the 3 wire connector and checked the voltages to ground. I don’t have all the info right now since I don’t have the paper I wrote on with me but one wire was the same as the battery, 14.5 volts, one was slightly less 14.1 and the last one was about 9 volts. I have to call back the Advance Auto support, I think it’s Carquest, and continue with their troubleshooting.

So the white wire going to the 3 wire alternator plug is the sense wire and is supposed to be a wire going to the battery (i assume with a fuse in it). This wire was showing around 9 volts yesterday and when I checked it today it showed less than 1 volt.

Does anyone know how the sense wire is wired? Sounds like it’s breaking somewhere and is losing its connection. Is this simply a wire that goes to the fuse block than to the battery? Theoretically could I just put a fused line to the positive battery to the white wire going to the alternator and fix the issue?