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Old 09-02-2012, 08:28 PM   #1
PlasticCarsRock
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1995 SL2
Default Alternator: voltage intermittently low

My '95 SL2 has an upgraded CS-130 alternator--same as stock except for a rewound stator, better diodes, and a finned rear housing (http://alternatorparts.com/7130_7140.htm). It's rated for 140 amps, but mine tested at 159 amps at 2000rpm (engine, not alternator). The diodes (rectifier), brushes, and stator have 30k miles. The regulator is from a parts store rebuilt alternator (unknown mileage).

Normally, it starts charging at around 14.3-14.5 volts, and drops off to 13.5-13.7V (varying slightly), once it warms up. This is the same behavior as previous alternators. I monitor it with an AEM digital voltmeter, accuracy confirmed by a calibrated Fluke DMM.

Today, I started out on a 4 hour trip. I'm not sure if it was like this from the start, or if it started a few minutes later, but the voltage was hovering right about 12.9-13.2 volts (on the highway). Suspecting a bad diode, I turned off the a/c (to remove the cooling fan draw). It made no difference. A half hour later, I had to turn on my headlights (storm), and the voltage seemed to drop a bit: (12.6-13.1V). When I turned off the lights a few minutes later, it seemed to increase back to 12.9-13.2 volts. (There wasn't much of a difference--I may have imagined it.)

I stopped for gas after 3 hours, and upon restarting it, everything was fine: 13.5-13.7 volts (with no drop from turning on the a/c, lights, etc). The voltage was normal for the rest of the trip, and several short subsequent trips (several restarts).

The battery is good, and started out fully charged. The warning light never came on. This was a return trip--voltage was fine on yesterday's 4 hour trip (a/c on the whole way).

I'm inclined to think it may have been some sort of glitch with the regulator, but it seems odd for a solid state device to screw up like that, and go back to normal later... I'm going to double check the charging wiring, but I'm fairly certain there are no problems (there was no corrosion at the end of last summer, when the starter was replaced). I will, of course, double check the battery terminals and the two-wire connector at the alternator, but I don't think they could cause this...

Any ideas? Can I trust the alternator?

I have a spare regulator, but due to the difficulty of rebuilding this type of alternator, I'm not inclined to replace it unless it acts up again.

Some more info: Last winter, the alternator took a bath in some ATF (leaking rebuilt PS pump), which resulted in contaminating the slip rings and an intermittent warning light (and voltage drop), but I don't see how this could have done any long term damage. It has been fine since then. Last month, the alternator had to work hard to charge a completely dead battery, but other than boiling over the battery (full charge in less than 20 minutes, confirmed with a hydrometer), it didn't seem to cause any problems. The battery was replaced, although it seemed fine (5 years old, so the reserve capacity was dropping).

Thanks for any suggestions!

...
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Old 09-02-2012, 08:57 PM   #2
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1998 SC2
Default Re: Alternator: voltage intermittently low

I would clean the connector to the regulator just because. While the alternator is spinning I would spray off the slip rings keeping in mind that every electronics cleaner on the market is a mixture of alcohol and petroleum, it will burn.

Those regulators can fail as they have to sense output to control it and if that section of the circuit gets sick then the alternator output is effected. The only other point of failure is the actual application of the excitation to the rotor as it will go to max and that is as far as it will increase. As long as the actual output potential is greater than the battery terminal potential under load the light stays off. There is no monitoring of alternator current flow. The design assumption is that if alternator terminal potential exceeds battery potential then there is a charging current, this is not always the case in real life. If you can actually see the slip rings you can tell by looking if they are the problem. Based on the ATF bath that would be my first guess.

Personal experience is that this is a excitation/slip ring failure.

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Old 09-02-2012, 11:13 PM   #3
PlasticCarsRock
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1995 SL2
Default Re: Alternator: voltage intermittently low

The odd thing is that it was so consistent. When I had the problem with the p/s leak, it would work fine until it dropped out completely. Once the fresh fluid was cleared from the slip rings, it came back full power again. That is, it either worked fine or didn't work at all. It has now been roughly 20-25k miles since that incident, so I'm sure any remaining residue on the slip rings has been long worn away.

In this case, the voltage was maintained just as precisely as normal, but at a lower value. If there was a problem with the slip rings or brushes, I would expect it to either drop out completely, or to be inconsistent. It seems like the regulator was simply regulating to a lower voltage... which doesn't make sense. It remained at the lower voltage for roughly three hours (mostly highway). As soon as I restarted it, everything was back to normal. I realize that the jolt of starting the engine could have "fixed" a loose connection, but I find it hard to believe that 3 hours of driving (including a few miles of very rough road) wouldn't do the same thing (or make it worse).

I'm familiar with how the warning light works, and I am not surprised that it did not come on in this case (the alternator was still charging, after-all).

I have been having a bit of an issue with the belt squealing when accessory load is high (a/c+lights on immediately after a cold start) but it's only noticeable if you really pay attention (not loud), and the voltage is always spot-on, regardless. I have a new OE tensioner, and the belt is in good shape, but I believe it's contaminated with something... been meaning to clean it and the pulleys. I don't see how this could be a belt issue (it wasn't making any noise, and as I've already said, it seemed too consistent for a mechanical failure like that).

I'll clean all of the connections, tomorrow, as well as the belt. I don't think I can get anything near the slip rings, but I'll give it a shot if it seems possible. Hopefully it was a one time thing, or if there is actually something wrong, it will fail soon. I have a spare alternator, and I'll be sure to keep that in the trunk with some hand tools for a little while. Luckily, my daily commute is short enough to make it without an alternator, but I do drive ~500 miles many weekends, often at night (no parts stores open).

The regulator senses voltage from the ignition (the second, non-warning-light, wire), right? (Not from the alternator output...?)

...
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Last edited by PlasticCarsRock; 09-02-2012 at 11:22 PM..

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Old 09-03-2012, 12:10 AM   #4
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1998 SC2
Default Re: Alternator: voltage intermittently low

Regulator has 2 sense points, internal and external, a failure of the external should shift to internal. It is possible that it is a belt issue if you have any squealing at all. They usually slip quietly. If you are not using an OEM tensioner or the OEM tensioner is 10yo or more it may very well be the belt. Worn belts do slip. If they squeak then they are slipping even when not squealing. If the copper oxide layer is heavy enough it will shut down the excitation current in the rotor. The brushes will suck up oil and slowly fail over time. Unfortunately all of the commutator and brush fixes require access to the components while it is running and even on a test bench this is far from easy.

With the belt info that is what I would fix first. Clean all pulleys carefully to remove any rubber dust and/or oil before replacing belt.

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