355 Size of 355 alternator pulley

I have three consecutive failing 355 alternator and I am running out of idea, except for one.

Can someone please measure the diameter of the pulley on the 355 alternator and tell me what that is? If you have it on a bench or can have access to it, or even just post a picture of the front face of the alternator.

 

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Outermost diameter of the pulley face is 66.77 mm. Diameter where the belt rides is 59.87 mm. Image Unavailable, Please Login Image Unavailable, Please Login

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Sorry, these photos always end up sideways when I post from my phone.

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Thank you that is what I needed. The pulley on the alternator I have is too small therefore spinning too fast at any RPM. The voltage generated would burn up the regulator I think.

 

Curious, how much smaller is your pulley compared to the OEM one?

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It's 56mm instead of 67mm so the alternator would spin 18% faster.

The alternator was putting out 14.3V after installation and is now not charging the battery at all. Its voltage while idling is 11.5V. Curiously, the Battery light is never on. Is the Battery light supposed to turn on when the alternator dies? It also does not come on when you first turn on the key like the rest of the other lights. The light bulb is good, not burnt out.

 

87

Thus the engine would have to be running at greater than about 7400 RPM before the alt would be spinning faster than with the OEM pulley with the engine at redline. Maybe the problem is elsewhere?

 

It looks like your voltage regulator has failed. Try replacing it before going further. Is the alternator you currently have an aftermarket or "rebuilt" version?

I agree with "johnk..." that the cause of problem is unlikely to be the somewhat smaller pulley that you have. And too high rpm of the alternator would primarily affect its bearings.

 

The first alternator was bought on ebay from a shoddy rebuilder. It failed.
The second alternator was bought from a crook on ebay. It also failed.
The third alternator was a straight exchange from a local outfit I did years of business with. Very competent. We compared the pulleys and I elected to swap the pulley to a smaller one because that is what I brought in. It failed in 30 miles, so there is got to be a reason for a larger pulley.

Any idea why the Battery warning light on the dash did not come on?

 

Even with ignition on (engine not running)?

I suppose a broken alternator wire (yellow/green) or even a blown bulb would cause this. If your battery warning light isn't coming on, what were your initial signs of failure? Is this why you were asking me about your cigarette lighter voltage?

 

The sign of failure is the battery becomes discharged. I can charge it up with a trickle charger to 13V. When I go to restart the car and it runs, the battery voltage drops below 12V. The alternator belt is on and the two connectors are attached but there is no "charging" action coming out of the alternator. The bulb is good, I took out the instrument panel and measured the bulb. The Battery warning did not come on, and is not on with the key in the ignition turned to position 2.

I asked about the cigarette lighter because there was a battery voltage monitor attached to that port.

 

The battery warning light is lit-up by the current flowing through it to the Voltage Regulator and then through the exciter coil to the ground. If the Voltage Regulator fails, and if it fails in a way that cuts the flow of current coming from the bulb, the light will not come on at all. Another possibility is loss of contact at the brushes or open circuit in the exciter coil but these would probably cause the light to come on if the regulator is good.

 

Have you checked the voltages going to the alternator?

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I get around 12 volts (live battery) on the sense line, and around 12 volts on the other two with the ignition on.

This would at least prove the car side of things is working.

(EDIT: different online sources show different wiring hookups for the harness connector on Denso alternators).

 

No I have not done any voltage measurement on the round connector pins. So, I should turn the battery on, and measure 12V (Battery voltage) at the S pin of the round connector? And should see 12V with the other 2 pins with the igntion switch turned on?

 

So, new theory. Let's say one of the three wires to the round connector is broken somewhere (sounds like your problem Ian), the alternator would work for a short while and then its voltage regulator is fried.

 

Correct.

I'm hoping my alternator is not fried. It may just be that the broken wire was causing the improper operation of the alternator. I've completed the temporary repair and will start the car today sometime.

 

Correction to my diagram. The alternator harness 3-pin plug graphic should say sockets (female), not pins.

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The harness side has sockets, the alternator has pins.

 

The only thing that the alternator warning light reliably detects is a good alternator with the broken drive belt. It often comes on with an electrically bad alternator, but not always.

 

Actually, the voltage should be less than 12V on the other two pins because the ignition will be powering computers, relays, fans, lights and things.

I got some good indications on my car after I started it. 13.8volts at the cigarette lighter with the engine running. I'll give it a proper checkout when the weather improves.

 

So without the alternator installed, battery connected, and the key is OFF, the center pin of the round green connector has 12.8V (battery voltage) and the other two pins are 0V. With the key turned to on, all three pins are measured 12.5V. So, at least the car side of things seem to be working.

The alternator is tested by the rebuilder today and it confirms that the regulator is burnt out. The question is ... why so soon after a fresh rebuild.

 

Probably a good thing, considering the problems I'm having with the car side. A new alternator sounds cheaper.

Does your car have additional earth straps? I noticed what looked like some on my car.

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Yes, an additional ground strap was installed by me.

 

Possibly a poor quality regulator. Get one from a reliable source, like this company: https://store.alternatorparts.com/denos-regulator.aspx

 

Not three in a row ... The last alternator was putting out 14.3V at idle, and the one before that 14.5v at idle. That seems too high to me. How does the regulator regulate the voltage? What function does the voltage at the sense pin have? How is the rotor current a function of the load?