Alt light question (resistor)

si2t3m · 07-12-2002, 10:14 AM

Hi,

I found the tech bulletin regarding the upgrade of an internal regulated alternator in an early car.

Tried to source the resistor 911 641 981 00 (91ohms, 5watts) at the dealer but they told me NLA.

So i left it that way. The alt light would usually glow with the ignition key on and would turn off when started. On some occasions the light wouldn't glow at all with the ignition key on. I left it that way...

This morning the starter had a hard time turning the engine over... Lucky i had to head back home and when i tried to start it again it didn't. I checked the voltage on the battery post and have 11volts.

I'll be tracing the blue wire to check if it is opened or not tonight as suggested in some interesting post a while back.

I now want to install a resistance as per the technical bulletin but the only one i found around here are 82ohms 5W or 100ohms 5W. I bought both of them.

Any idea on the one I should try to use between the 82 and the 100ohms unit. They are both 9ohms away from that 91ohm unit!

TIA

Marc-Andre Morin
70 914-6 conversion 3.0L Webers

Jim Sims · 07-12-2002, 10:44 AM

If all you have is the two values I'd install the 82 ohm resistor. Apparently you were close to having enough field excitation current (the light would sometimes go out) before so the 82 ohm should work. Do you know the rules for series and parallel resistors? If you put them in series (one after another) you just add up the individual resistances. If you put them in parallel (side by side) the inverse of the sum of the individual inverses is the total resistance of the combination. So if you added a 10 ohm resistor to the 82 ohm you would be really close (92 ohms) but this is awkward to do as they have to be one after the other and this can get bulky. If you can get a 175 ohm and a 200 ohm resistor and put them in parallel (side by side) this combination will produce 93.3 ohms. Do not go below 5 watt capacity but greater than 5 watts is okay but they may get too big physically. I'm really surprised about the NLA status of the resistor; did you check with Pelican and Performance? Good luck, Jim

Early_S_Man · 07-12-2002, 11:06 AM

Marc-Andre,

You can use the 82 Ohm resistor and it will work! However, since according to the Bosch Automotive Handbook, 4th ed. p. 810 ... this resistance value controls the so-called '0-Ampere speed' of the alternator, you can use a 68 Ohm resistor as well, to give the best alternator performance at idle speeds ... so that is what I would recommend -- a 68 Ohm resistor!

304065 · 07-12-2002, 11:12 AM

Marc:

I have an 84-89 Carrera alternator in my 1971 911E. It only has a 2W bulb for the warning light, and yet the current in the D+/61 circuit (the "Blue Wire") is more than adequate to excite the alternator once rpm reaches about 1200 after startup. So I would think it unlikely that you need the parallel resistor to make the system work. More likely that something else has failed in the charging circuit which is responsible for your low battery voltage.

If the warning light does NOT glow with the ignition on and the engine not running, that suggests to me that your bulb is burned out, or the socket or wires are loose. If the bulb doesn't light, then the D+/61 circuit to the alternator field will not be energized, and the field will not be "excited." The alternator rotor contains very little residual magnetism and is not "self-exciting." If you pull out the oil pressure gauge, you can access the lower bulb holder to test it, and to verify that the connections are tight. If everything's OK, pull off the 14-pin connector on the "console" at the port side of the engine compartment, and ground pin #11 on the MALE portion (you can tell which pin is 11 because they are numbered on the FEMALE side of the connector) and the light should go on. If it doesn't light, the problem is forward of there.

If you are set on having the right resistor, you can buy a 91 ohm, 5w wire-wound resistor from Radio Shack online. Even if you buy five of them, the postage will be more than the resistors. I did this and they are still sitting in my hangar gathering dust.

Recalling the formula for power, where P=EI (watts= volts x amps)
the later cars use a 4w bulb in the circuit. So assuming it's a 14 volt system, you want 0.29 amps in the circuit.

Now, if you have a 2w bulb in there now, the circuit is drawing 0.14 amps. Out comes Ohm's law, which says that E=IR (volts = amps x resistance). So if you lower the resistance in the circuit, you will increase the amperage, voltage remaining constant. So how do you lower the resistance in the circuit to increase the current flow? You put a resistor in PARALLEL with the bulb. You want to DOUBLE the current flow, right?

Resistors in parallel result in a total resistance that is equal to R1 x R2 divided by R1 + R2. We know what resistance value Porsche wants-- 91 ohms. How much resistance is in the bulb? If you combine the equation for power and ohm's law, you can figure out that there's approximately 97 ohms of resistance in the bulb.

97 x 91 = 8827
97 + 91= 188
8827/188 = 47 ohms, so the new resistance of the 2w bulb with the resistor in parallel is 47 ohms.

Into ohm's law this goes, E= I R, so 14 = I x 47, or I = .298 amps, hey, go figure! The EXACT value Porsche wanted.

If you put a BIGGER resistor in, like a 100 ohm, the calculation becomes:

97 x 100 =9700
97 + 100 =197
9700 /197 = 49 ohms

14 = I x 49, or .286 a little under.

SMALLER resistor, like 82

97 x 82 = 7954
97 + 82 = 179
7954/179 = 44 ohms , 0r .318 amps. A little high.

Fun with electricity.

Now I suppose a REAL electrical engineer is going to get on and tell me this is wrong, but it was FUN! (I'm a lawyer by training and an investment banker by profession, which means I love laws and numbers.)

Good luck! Ask me what time it is, I tell you how to build a watch.

si2t3m · 07-12-2002, 11:22 AM

Thanks for the info!

As i'm running a 245mm fan (might be from an 3.2 because of the small holes in it and the internal regulated alt) and running an early pulley like the one that was on my 2.2 with the big holes) with a 9.5x730 belt, my fan speed is probably lower at idle than the regular setup so i'll go and get one of those 68ohms unit.

Just hope it's not the blue wire that came loose at the alt or something to do with the alt as i'll have fun taking it out of the 914

Thanks again

Marc-Andre Morin
70 914-6 conversion 3.0L Webers

si2t3m · 07-14-2002, 04:13 AM

Found that the blue wire was opened (unsoldered at the 14pin male connector).

Redid my soldering job. Still no go. Resistor is on the socket also.

Traced the wire for continuity. It does.

I even checked by pocking the prob in the wire just before it enters the shroud.

I get a reading between a ground point and the blue wire? Is this normal.

I'll have to pull the alt and check...

Marc-Andre Morin
70 914-6 conversion 3.0L Webers