tii74

Confused about Voltage Regulator testing

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So when you bridge the voltage regulator per the manual and the L warning light never glows, the manual says the voltage regulator is defective. But when I run the engine with a bridged regulator, I still only get 12.5V. No charging. 

 

Now I’m confused. Which is bad??

 

with the regulator installed, the battery does not charge (12.5v) and the L light is glows (relatively dim). I checked all the grounds. 

 

Any help appreciated. 

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3 hours ago, Tommy said:

I think your alternator brushes are worn out.

 

It's a brand new alternator.  But as we know, that doesn't mean it's not bad.

 

When I unplug the voltage regulator, the light on the dash goes out (without engine running).

 

Any help appreciated.

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So I think it's helpful to understand how the dash light works, as I don't think it's actually a very good indicator of the health of the regulator.  One side of the light bulb is connected to +12V from the ignition whenever the key is on, and the other side is connect to the output of the alternator (D+).  So when the alternator is NOT functioning, D+ is at 0 volts, and thus current flows through the bulb from the ignition to the alternator and it lights up (hence normally on before you start the car).  Now when you start the car, the alternator comes 'on' and starts producing say 14.5V.  Now the D+ side of the bulb comes up to this 14.5V, and the other side is still at +12V from the ignition/battery, and the 2.5V difference isn't enough to keep the bulb lit, so it goes out.  So just bridging the regulator I don't think will help you troubleshoot all failure mods of the alternator and regulator.  I would instead suggest you unplug the regulator and start the car, and check what the voltage of the D+ is at idle.  Then maybe rev the engine up to about 2000 RPM and make sure the voltage starts to climb.  Don't rev higher than this since it's unregulated; don't want it to get too high!  If nothing, then the alternator is at fault.  If you get good climbing voltage then plug the regulator back in and repeat measuring D+ voltage at idle and 2000 RPM.  Should be around 14V and NOT change much with RPM if the regulator is good.

Two things: do NOT measure voltage at the B+ terminal with big red wire going to the battery; the battery voltage will buffer and skew the reading, use the D+ terminal (blue wire) instead.

Also, since it sounds like it's a new alternator, make SURE that the alternator housing is properly grounded to the engine block and/or chassis!  If it's not, then it certainly won't work properly!!!

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Thanks. 

With the unplugged VR, I have 12.5V at the D+ blue wire with the engine off and ignition on. The voltage does not go up with the engine running. Even up to 2000 rpm. 12.3V

 

Based on that, it seems that my new alternator is dead. 

 

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I had a similar problem. New alternator and new internal voltage regulator, but no charge from the alternator when running and no glowing dash lamp either. Took it back and the shop, tested it and conformed it was good, the problem was in my car.

Checked and no lamp on in the dash so I traced this to a break in the blue wire at the alternator plug. Fixed the connection and the lamp came on, and went off when the motor started and all was well. The lamp illuminates when ignition is on, goes out when motor starts, regulator permits transfer of power to the battery from the alternator.

I am not certain of this but I think the blue wire connection to the dash lamp activates the voltage regulator. I think you would have the same problem if the lamp bulb was defective.

Does anyone know if this is correct?



Sent from my STH100-1 using Tapatalk

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10 hours ago, Guy Cocquyt said:

I am not certain of this but I think the blue wire connection to the dash lamp activates the voltage regulator. I think you would have the same problem if the lamp bulb was defective.

Does anyone know if this is correct?

 

Technically not 100% but in practice yes. The lamp circuit feeds magnetizing current to the rotor of the alternator. When it starts charging the regulator controls magnetizing to keep the voltage at the right level. When charging the magnetizing current comes straight from the alt not through the lamp anymore but in the beginning the lamp is needed.   

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The charge light in the instrument panel will not light when you perform the " bridge the voltage regulator" test, because it is not in the circuit. However, your alternator should show an output of >+12.6V. Make sure your alternator is well grounded- this is a common reason why alternators have no or limited output.You said you checked that- how? You said with the regulator in the circuit, the L light is glows relatively dim. Obviously, the bulb in your instrument panel is not bad. Presuming all of your connections are good, this dim light is a classic symptom of a bad rectifier (diodes) in the alternator. You might want to read this: https://www.bmw2002faq.com/articles.html/technical-articles/electrical-and-ignition/the-02-charging-system-r175/

 

From what you have said, it does sound like your new alternator is bad.

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P.S.: check for +12V on D+ at the alternator (the blue wire). It could be that you have excessive resistance in that circuit, which would allow the bulb to glow dimly, but not charge up the alternator.

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8 hours ago, Chris_B said:

P.S.: check for +12V on D+ at the alternator (the blue wire). It could be that you have excessive resistance in that circuit, which would allow the bulb to glow dimly, but not charge up the alternator.

Thanks.

I do have 12.3V at the D+.

I guess it's time to take the alternator out.

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Problem solved.

 

Thank you for the advice.  I redid all of the connections and checked to make sure the plugs were tight.  Turned the key and ... magic, the L light is now bright.  When I started the engine, the light went out and I am getting high 13s voltage.

 

Thankfully, I don't have to remove the PIA alternator.

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