New Alternator and same issue

[atomicboy]

I noticed last year on my '68 911S that the battery light (alternator light) was glowing slightly at night. It would get brighter and brighter the more accessories I used. Using a DMM the voltage at the battery while the car was running would never go above 13 volts at any RPM and the battery seemed to be getting weak. I just replaced the battery and the alternator with a new Valeo internally regulated model (bypassing the old external VR) and the voltage is now 12.8 idling and 14 at higher RPM's with all the lights and accessories on. The problem is that the battery light still acts the same. It glows dimly at low RPM's and then gets brighter and brighter the more accessories I turn on. Would the parallel resistor (91ohm 5W) mentioned in the tech bulletin solve this problem or is this an issue with the light/diode itself?

Any input would be greatly appreciated.

Thanks, Ron

[brads911sc]

Have you made sure your fan belt has the correct tension in it?

[atomicboy]

Yes. Belt tension is ~1/2" deflection with reasonably light thumb pressure and engine can be turned over by turning the alternator pulley nut. I'm assuming that since I'm getting higher voltages at higher RPM's it's definitely charging. I'm out of ideas at this point.

Thanks for the quick reply,
Ron

[timmy2]

Try the parallel resistor mod. Sounds like the diode isn't burdening the excitation circuit correctly.
Easy to do.

[47silver]

14 volts should be sufficient to stop the light. I think that the resistor is only there to initiate charging. If it a rebuilt alternator I would have it checked.

[timmy2]

Originally Posted by 47silver

14 volts should be sufficient to stop the light. I think that the resistor is only there to initiate charging. If it a rebuilt alternator I would have it checked.

I don't have a schematic for the '68 but I'm assuming the design is the same as my '78.
In my '78 the diode provides a regulated current and a minimum threshold voltage value to the regulator circuit that controls the output to the generator field (excitation)(DF). Due to deign changes with newer alternators the resistive component of the circut is too large to control the output correctly. That is the reason for the parallel resistor. It modifies the total circuit current value by changing the resistance.
This is why the factory mod exists.

I used to build and service 2-500 KW EG sets, I have a fairly decent knowledge of their operation. (Car alternators are just a lot smaller) (Excitation controls output)

If it doesn't make a difference with the resistor installed then I would suspect your ground(s) at the alternator to the engine, or the VR itself.

[47silver]

i think he said he had 14v. if that is the case the light should be out. i only say this as a couple years ago i replaced my alternator, and still had the problem, then i replaced it again and still had the problem. i finally took my old one to a local shop with one of the rebuilt ones and the rebuilt was bad.
i had him just redo the old one and have not had any problems since.

[timmy2]

If it is the VR in the new Alt then it will need repair.
Due to using the newer stlye alt with an internal VR the mod should be done.
Not doing the mod may have caused the vr to fail.

[atomicboy]

I tried the resistor mod but the issue is still there. Battery light comes on from very dim to brighter depending on what I have running and the RPM's. I did a voltage check across the light and here's what I found: 900rpm= +0.8v across the light and it's very very dim. Turn on the lights, fan, wipers and signal and the voltage goes negative (-0.8v)and the light is very very dim. 2500rpm the voltage goes up to +1.6 and the light is brighter. The more accessories I turn on at that point the more positive the voltage goes up to +2.4v with the light getting brighter. When the voltage is positive I have up to ~14vdc at the battery so it seems that any time there is a difference between the battery and the alt. no matter the polarity the light will be on. I am considering installing a diode at the battery light to only allow it to illuminate when the battery is discharging although this would only be treating a symptom and not the issue.
Thanks for all the input.

Ron

[304065]

LOL, you're trying to charge the battery through the light!

The problem isn't in your excitation circuit (the "blue wire.") If that were the case, you wouldn't have 14V.

Here's how the G light works. With the rotor stopped, current flows from the battery into the ignition switch and back to the VR. Since you changed to an internal VR (not recommended by me for a host of reasons, but we can leave those for later) and the voltage on D+/61 (the trio output from the stator) is zero, then the VR sends current to the rotor, magnetizing it, and to ground.

With the rotor turning, a current is induced in the stator which goes to the main diodes and also to the trio, which results in positive voltage on D+/61. Voltage builds up and you get +14V output. VR senses this and then tells the VR to cut off current to the rotor, and the output voltage falls below the 14V "set-point" in the regulator, causing the VR to power the rotor back up and the whole thing repeats itself, keeping the output voltage at the 14V set-point. And so it goes in happy equilibrium.

All right, back to the G light. When current is flowing from the battery to the light to the rotor to ground, the light is on. When the voltage is the same on both sides of the bulb holder, because D+/61 has positive voltage on it, no current flows and the bulb goes out.

YOUR issue is that you have more voltage on the D+/61 side than the battery side! Hence my tongue-in-cheek comment about trying to charge the battery through the bulb.

Think about it-- when you turn on accessories, these are pulling the battery voltage DOWN. With a lower voltage on the battery side than the D+/61 side, the light gets. . . brighter!

All right enough fun with Electronics. How do you fix it?

Your '68 has only one pathway for current to get from the alternator to the battery, unlike in later cars. Current goes from B+ on the alternator through a red/white wire over to a two terminal strip on your electrical console in the left side of the engine compartment. From there it connects to the main harness through about a 10 gauge black wire. This wire runs forward to the left front corner, then down through a hole and over to the right along the crossmember that the shocks mount to, to the starter terminal. From the starter terminal, a large black wire runs forward to the battery.

Somewhere, Ron, along that path there is a high-resistance connection that is preventing the alternator voltage from getting to the battery. It is probably corrosion. So to fix it:

1. Disconnect the battery. Take it out of the car and charge it while you are working. If you haven't tested it lately, do that: if you haven't replaced it for a few years, replace it, that could be your problem right there. What static voltage does it give you when you measure it with engine off?

2. Clean battery to ground terminal at left front of trunk.
3. Clean positive battery clamp. This comes apart with a flat screwdriver.
4. Clean starter lug and 10 gauge wire ring terminal. M8 screw on starter, 13mm ATF wrench. Be prepared to get greasy.
5. Clean 10 gauge cables (red/white and black) on electrical block on console. Be CAREFUL do not break the bakelite fins, the center fin is easily broken! Philips #2.
6. Clean B+ on back of alternator.

That should fix it for you-- be extremely careful, the battery current can kill you or burn down a nice early 911S! Good luck and please let us know how it works out.

[atomicboy]

Thanks for the detailed explanation. I knew that with the alternator running properly it should equalize across the battery light but the change in potential across it had me stumped. I'll check and clean all the connections and then let you know. The battery is new with 13.4v as well as new battery connections. I replaced the starter 3 yrs ago but I'll go over that as well. From your description I should be able to check resistance from the alternator B+ connection back to the + battery clamp and see something significant.

I'll update when I'm done.

Thanks again,

Ron

[304065]

Ron,

One calibration-- typically a new battery will be showing +12.5V or thereabouts when fully charged. When discharged to 25% capacity, the voltage is around +12.25V. It would not make sense to see more than 12.5V with the engine off, if you are then I would check the test equipment.

If on the other hand you are seeing 13.4V with the engine running, that makes more sense.

In any event, if the battery is new you should be OK.

I like to use a piece of emery cloth on all the connections until they are bright metal. You should see a nice brass color under the factory ground points.

Good luck!

[atomicboy]

I've got a few more measurements and I'm starting to suspect the new alternator. Just for reference, I'm using a calibrated Fluke and have double checked some of the readings with a generic DMM and they all match. Battery is reading 12.9vdc and was reading 13.25 just after turning off the engine.
Resistance from B+ terminal on alt to positive battery clamp is 0.4ohms. That seems to rule out a highly resistive connection. I checked voltage from B+ compared to D+/61 (taken directly from alt) with engine running and always get D+ higher. The difference will vary from 0.8 vdc to 2.0 vdc depending on rpm and accessories but D+ is always around 14vdc. That is, D+ will stay at 14vdc but as I turn on accessories B+ will go lower. Sometimes as low as 12 vdc. If the D+/61 terminal is fed from a separate diode trio I suspect one of the main diodes has failed resulting in lower voltage as the load increases.

If anyone has an opinion on this theory please advise.

Thanks,

Ron

[304065]

Ron, a "calibrated Fluke" LOL does that mean it's occasionally perfect? Wish I could afford one of their fine instruments.

All right, 12.9VDC, fair enough, but it must still be holding a surface charge. I bet it drops to 12.6 if you let it sit overnight, but in any event, the battery is not your problem.

Resistance down the line is 0.4 ohms. That's not a bad voltage drop considering the twists and turns it takes, three separate wires with five connections. Agree that's probably not it; and

D+ is higher than B+. Well they both come off the stator. You said this was a new, internally regulated alternator? Highly unlikely you have a main diode failure unless somebody hooked it up backwards. Does the Fluke show frequency? You would definitely see ripple in the output if a main diode were blown, and the voltage would be reduced. I don't think this is the problem- I think that the sag is coming from the accessory side.

OK, your battery clamp-- you say this is new? Is it a 914 part number, the squared one, like the top clamp in this photo?



The accessories drive off the smaller black cable that goes in the clamp. If there is high resistance there, this could be causing the drag. This cable goes to the fuse box, to the unfused side of #3, and then branches out. So you might check there. Somehow you have such a load on the battery that it's pulling down B+ and reversing the current flow through the lamp.

That is a more likely explanation to me than a bad diode out of the box.

[47silver]

i had 3 bad alternators out of the box. 2 were bad electrically and one had bad bearings. I recommend getting an alternator shop to rebuild the original alternator. The original alternators had pressed in diodes that are more robust than the loose diodes used on the rebuilt one. cost is roughly the same if you don't need a new stator or such.
The other problem with rebuilt is that though the numbers match, the fit of the alternator may not match your fan shroud.