Tass 928

Can any of you electrical gurus help me with a wiring question? The manuals don’t help because I have no stock wiring.

Back when I was gutting my track car I pulled out all of the wiring except for the engine harness. I then rewired the car by hand, only installing what I absolutely needed. Well, one thing that I did not consider was the alternator exciter circuit. After test driving the car for a day I discovered that it is not charging.

I’m looking for the simplest possible circuit I can install to get it charging again., I started by putting a test jumper from the alternator hot post to the exciter circuit post and it charges perfectly. 14Volts at about 2k rpm and 12.9 at idle, even with 4 fans running.

I suspect that this would kill my battery when the car is not running so I want to connect it to my electrical “power-on” switch that powers up the ignition, fuel etc.

Is it as simple as that? I have done a little research and some opinions say that the exciter circuit needs a diode or a small lamp in it although I don’t understand why.

Am I going to burn up my regulator?

Any opinions appreciated ……

Tass

GlenL

Well, if you dig around you'll find there's a big resistor in parallel with the bulb. Not a high resistor but one that takes lots of current. Apparently the exciter doesn't need a full 12V or maybe it doesn't want it. Here...lemme look...

GlenL

Crap! Can't believe it. I can't find my old 944/928 parts book. The paperback one. In the back is a tech note on what resistor is needed. The PET just says "resistor," of course. Maybe someone else can provide the values for it.

Tass 928

Thanks Glen

I'll stand by with my fingers crossed

GlenL

After an exhaustive search the "Parts & Technical Reference Catalog" has been found. It was stashed in my bedside table along with a Vertex catalog and two 928 catalogs from 2006 and 2007. Perfect bedtime reading, don't you think? And whatever happened to that 928 International operation?

The resistor, P/N 928 641 981 00, is a 68 Ohm and 5 Watt resistor. It's mounted in parallel with the light bulb. The bulb resistance is hard to measure but I _believe_ it's a 1.2W bulb or about 120 Ohm when hot. So the combination is about 44 Ohms. So a 44 Ohm, 5 Watt resistor between a switched 12V and the exciter input should do it.

Tass 928

Awesome !! I'll try that out and let you know what happens

Thanks again !

Alan

It is a 68ohm resistor in parallel with the 1.2W bulb one side to alternator exciter terminal 61 and the other side to ignition switched power.

However even if you don't need it functionally - do add in the bulb too. Filament bulbs are funny things - people naturally think they are simple and easily understandable - but they really aren't. The bulb may have about 120 ohms resistance hot - but when its cold it will be only about 1/10 of that resistance so will dramatically affect the total resistance in that case.

Alan

docmirror

Yep, the strange things an incandescent bulb is good for. Don't substitute a LED here.

Tass 928

Thanks Alan

I don't have any place convenient to put a bulb & socket but I'll tuck it in someplace. Simple enough to do and it will duplicate the original conditions, hopefully letting my alternator and regulator live a long happy life.

GlenL

I suggest trying it with just the resistor. That'll be easy to work into your harness if it works, and I expect that it will. Don't overlook the high wattage needed for that however you proceed.

Alan

Glen the difference is actually substantial in this circuit. Will the resistor get the alternator to excite - probably yes at some RPM but perhaps not as reliably at idle... its easy to add both - in fact the bulb is probably easier than the resistor since there are no thermal issues to consider.

The bulb provides a low resistance and therefore higher current at low voltage differences - it makes the overall resistance curve wrt voltage very very non-linear - this is quite desireable in this case.

Alan

atb

Dave,

Please post pics of the install.

GlenL

I fully understand that. You're guessing at something here to make a point.

Consider that the bulb comes on before the car starts. It's already glowing before the alternator starts turning.

Consider for the moment the tech note. It states to use the replacement resistor, which is a lower resistance, if the alternator doesn't begin charging. This tells me that the current through the resistor is important. And why have a resistor at all? Because the current through the bulb isn't enough.

No thermal issues with a bulb? You're way off there. It's dissapating less power but through a smaller package. The bulb will get hot. The 5W resistor will be in a big ceramic package and will be heated with 3.3W. A bulb is fragile, too. If the wiring is entirely under the hood it'll be exposed to fluids. A bad idea.

Do you think the engineers would rely on an unpredictable, momentary fluctuation in current? I don't. It's the current through the combination of resistor plus bulb that provides the initial excitation. The bulb is only there to provide an idiot light.

Use a 44 Ohm resistor. It'll work and you'll never worry about it.

Tass 928

Forgot to mention....One of the things I did try was going through a bulb alone...it didn't work. I then scrapped the bulb and just jumped the post with twelve volts. The alternator then came to life.

Tass 928

Hey Adam,

I'll take pictures but I don't know how usefull they'll be to anyone that has a "real" electrical system