where to check for bad voltage reg?

:Confused: Trying to find out why my battery was dead last week. Threw a slow charge on it and it has behaved for about a week. Now, I was out last night and noticed that the CD player kept going off and on, I turned off the fog lights and it worked fine. I also noticed that when I stopped and the car was idling, the lights got very dim. This is new for my car (1974 911 Coupe). I have looked at past posts and figured it might be voltage regulator or altenator...any help is appreciated as far as how to check. I have a voltmeter.


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Shakenbake (Chris Baker)
1974 911 Coupe

Hopefully Warren will comment. He has forgotten more about electricity than I will likely ever know.

Voltage is like pressure. Your car needs at least 12, but the alternator and regulator keep more pressure than that in the system. This keeps the battery "full."

You can hook up a voltage regulator pretty much anywhere (anywhere "hot" that is) in the system and measure system voltage. 13.8 volts is very typical while the engine is running. 14 volts, maybe 14.5 is okay.

Yours may be falling too low. Anything under about 13.8 volts is suspect.

The battery alone (wiht the engine off) should measure more than 12.5 volts.

Any service station with a load tester is likely to offer free system testing to motorists. The trouble is they probably will not even know where the alternator is located in your car.

If your system (with the engine on) does not have at least 13.5 volts, I'd suspect a problem with yoru alternator, regulator or elecrtrical connections. Check electrical connections first as they can be cleaned cheaply. Voltage less than 13 is definitedly a problem.

Is this close to accurate, Warren?

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'83 SC

The only suggestion I would add to S'mans excellent advice is to perform the system voltage check with a fully charged battery, otherwise your readings will appear too low even with a good charging system trying to make up for a weak battery.

Ask me how I know...well, it happened to me and I thought my alternator went bad when in fact my battery was bad.

Cheers,

Joe Garcia
86 Carrera
[This message has been edited by stlrj (edited 08-10-2000).]

[This message has been edited by stlrj (edited 08-10-2000).]

Guys,

Super's post is correct, assuming he meant Voltmeter instad of 'regulator' in paragraph 3. Overall, though, he is correct ... especially good, for one not experienced in electronics! The acceptable range for battery/system Voltage, measured at the battery terminals, or fuse block (such as the one in the engine compartment) is 12.6 to 14.5 Volts ... anything outside that range is considered abnormal, and troubleshooting/repair will be required!

There is not always a clear-cut method of determining whether an alternator or Voltage regulator is bad. There are certain conditions/situations where the Voltage regulator is clearly indicated as the offending part, and others where the alternator is definitely indicated. A spare Voltage regulator is a good part to have on hand, both for emergencies, and for troubleshooting, and contrary to many people's beliefs (a myth, actually)... putting a new, known-good Voltage regulator in a system with a defective alternator, does not 'hurt' or 'kill' the Voltage regulator! Sometimes you need to verify that a 'good' Voltage regulator will not correct the electrical system's condition, then you know the alternator needs corrective attention.

Robert Bentley publishes several English-tanslation Bosch publications that have very good information on electrical systems and alternators, such as the 'Automotive Handbook,' 4th Ed., which is available from Pelican and others!

Chris, you did not say whether you have noticed the alternator warning lamp on when you noticed the headlamps dimming. Does the warning light come on when you turn the ignition on? Have you noticed it on recently while the engine was running? The reason I ask is that the warning lamp must be 'good,' and should be checked each and every time you start your car, because it is an interal part of the exitation circuit load, and the regulator will NOT be able to do its' job properly if the warning lamp is burned out, loose, or disconnected!!!

There are three wires connected to the Voltage regulator on the 'console,' or fuse/relay/regulator/CDI panel on the left side of the engine compartment. The brown wire -- 'D-', is ground, and the resistance measured from the regulator terminal to the frame of the alternator, and engine block, and any bare metal on the chassis should be less than 1 Ohm, always!!! A higher resistance than 1 Ohm is an indication of a high resistance/corroded connection somewhere in the wiring harness! The blue wire -- D+, is the exitation output from the alternator, and is connected to the warning lamp, and is used an input signal to the regulator ... it should always be higher than the battery Voltage, or the exitation diodes are bad inside the alternator. The black wire -- DF, is the output control signal from the regulator to the alternator, and should be a fluctuating DC Voltage that may show on the AC Voltage scale of a digital or analog multimeter as a 1-5 Volt signal ... if it is a steady DC Voltage, then the regulator is bad, or the brushes in the alternator are bad, or the field winding in the alternator is either open or shorted. If you want to determine which of those alternatives is most likely, turn the engine off, disconnect the black wire from the regulator, and measure the resistance from the black wire back to the alternator to a good ground connection, such as the brown wire at the regulator. If you can't get a good resistance reading, either the brushes are bad or the field winding is open. If you get a steady, non-zero resistance reading, the field winding and brushes are good, and the regulaor is bad!

Good luck, and good hunting!

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Warren Hall
1973 911S Targa

The nice remarks feel good, Warren and yes, I meant voltmeter. I do wish I understood these matters better as I love puzzles. As do many of my BB mates.

Here's another tack... Tires stores have these load tester things that they will usually hook to your car for free. they can sometimes identify a bad battery or bad charging. Is it difficult to attach these testers to Porsches? Can they isolate voltage regulators problems from internal alternator problems?

Perhaps they can only see unspecific 'bad charging.' the technicians will insist on the battery being charged for the test.

Once it is determined that the chargins system has a problem, then wouldn't it make sense to just remove the alternator, particularly if the v. regulator is internal. It will have to come out anyway.

And Warren has been repeatedly correct about another thing. Starters and alternators can undergo a quality rebuild at any competent elec motor repair shop at reasonable (usually) cost.

Is the '74 v. regulator internal?

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'83 SC

Unless a later, all-in-one, alternator has been retrofitted, the '74 has the separate regulator, mounted over on the console ... not too easy to get at the wiring, either!

Not easy by any means to hook a 911 alternator to a load box, as the 'hot' wire from the alternator goes straight to the starter 'hot' lead ... heavy red wire, about 10 gauge, I believe.

As much trouble as it is to remove the alternator, I thought most folks would like a few details about things to check before pulling the fan belt off!

A few points about common problems:

1. An overcharged battery, boiling the water out ... sulfur smell, along with high (16+ Volts above idle) system Voltage is most likely a bad Voltage regulator.

2. Overcharged, cooked battery with NORMAL, 13.5-14.5 Volt system operating Voltage (when engine is running) is most likely a bad battery with a shorted cell.

3. A system that runs down a fully-charged, or new battery in a few hours, is most likely a bad alternator with multiple shorted rectifier diodes ... it takes TWO, minimum, one in the positive array, and another in the negative array!

4. A system that has normal operating Voltage, but dim lights at night, and a slowly dropping battery charge is most likely a bad alternator with one or more open rectifier diodes or open stator winding ... each bad diode or winding drops 1/3 of the load capacity!

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Warren Hall
1973 911S Targa

Good things to know. It seems to me that, for many motorists, the 'load box' test would be a simple and useful thing. Perhaps the motorist could crawl under and attach the 'hot' lead at the starter for the technician. I also suspect that, with internal regulators (like I imagine my car has) there is not much opportunity to isolate the regulator from the alternator problems, and removal is necessary anyway, if either problem exists.

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'83 SC