Alternator low charge at idle

[helly]

Hello!

I just got a mx5 (Miata) NA (1990) and I noticed that the voltage at idle is quite low, 12 – 12,5.

Today I took out the alternator and checked the bushings etc, everything looks ok, except for the small bearing, it’s not 100%.

I put everything back together and installed the alternator in the car.
When I start it (cold, 1500 rpm) it charges about 14v. I then turn on the lights, turn signal, fan, etc. It still charges good, 13.6 – 14 v. Then when the engine gets hot, and the rpm drops to about 800. It then only “charges”/ stops charging 12.5v (battery voltage, and then drops). If I turn off the lights etc, the voltage is about 13.

What’s wrong? The belt should be tight.
A new alternator in Norway would set me back 420 USD + VAT 😮

Any ideas?

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Topic

[college man]

The voltage regulator may be going out. does the alternator
spin free with the bearing?

http://www.ericthecarguy.com/faq/solving-automotive-electrical-problems?start=2#SolvingAutomotiveElectricalProblems

[helly]

I can spin the bearing, no problem. It’s just a little noisy.
I think I’ll pay someone to check the alternator. Even at 1000rpm it charges 13-14 volts, untill I turn on the lights / a fan.
At 2000 RPM I can have everything on and it still charges over 13v. So it “works” for now, but I’ll need to fix it / buy a new.

[Josh G]

I was thinking voltage regulator too (which can be replaced separately). Here’s a link on testing it

http://www.freeasestudyguides.com/alternator-full-field-test.html

It’s not the best link, but it’ll give you an idea of what I’m talking about then from there maybe you can find a youtube video showing it!

[EricTheCarGuy]

You may not have a problem. It IS charging, just not as much as you’d like to see and that’s probably due to the idle rpm, which in your case I think 800 is normal. Have you checked the belt tension? It could be as simple as that or it could be that there isn’t a problem since it charges fine at higher RPM’s. Sometimes it’s best to leave things alone rather than go looking for a problem.

[Julian]

Any chance the car is running underdrive pulleys? 👿

[Tom]

Nice call on the underdrive pulleys. Might not be the answer, but way to think a bit outside of the box!!!

[helly]

I’m back!

Couldn’t login to the forum for a couple of months.

The alternator was bad, I tried to change the bushes, but still not enough volts. It just got worse.
A long drive with the fans on and the battery slowly drained.

Bought a new one and everything is good 🙂

Thanks for the input!

[Paul]

Just curious, cant you have the alternator remanufactured in norway? Its pretty common here in slovakia, i paid 90 euros for the reman and montage+demontage…but sure everything is a lot more expensive in norway…

[helly]

You can, but they charge about 35 Euro just to check the state of the alternator.
I got bushings from Ireland (ebay) for 7 Euro, but they didn’t help.

I paid 160 Euro for a new one from Germany (IL Motorsport).

[college man]

[quote=”helly” post=88850]I’m back!

Couldn’t login to the forum for a couple of months.

The alternator was bad, I tried to change the bushes, but still not enough volts. It just got worse.
A long drive with the fans on and the battery slowly drained.

Bought a new one and everything is good 🙂

Thanks for the input![/quote]

Glad you worked it out.

[EricTheCarGuy]

Glad you found the problem. Thanks for the update.

[Author]

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