Should I install a resistor or not?

pauls81sc · 04-15-2005, 12:38 PM

HI all,

Need a little input. Car is 1981 911SC, alternator is believed to be original w/ external regulator.

Long story short.....

My alternator warning light is not coming on when I start the car. I understand the "special function" the lamp has and the consequences of it not coming on. (I will pull the bulb this weekend and check it and the voltage to it. I also have new bulbs on order.)

In searching this board I've found the "resistor-across-the-lamp" trick recommended for the alternator w/ internal regulator upgrade.

Now I am NOT doing the alternator upgrade at this point so I am sticking with a 2-watt lamp and external voltage regulator.

My question is: Should I still go ahead and put the resistor across the lamp or not?

marcar911 · 04-17-2005, 04:28 PM

Paul,

What exactly is the "special function" of this lamp? I am assuming you are referring to excitation of the alternator.

Tradionally, one side of the alternator lamp will be at 12V, the other side will go to the D+ terminal of the alternator and regulator.

Were you able to fix the problem?

marcar911 · 04-17-2005, 04:32 PM

Paul,

What were the reasons in other posts for the resistor accross the lamp?

In the absence of a bulb, the resistor would provide excitation if required.

Marlin

marcar911 · 04-17-2005, 04:44 PM

Paul,

With the original voltage regulator, the resistor is not required.

It appears more current is required to gain excitation with the internal regulator. Later cars took care of this with a 4W alternator light bulb. On earlier cars, installing the larger bulb is not possible. The fix is to install a resistor in parallel accross the 2W bulb (91 ohms).

The fix on your car is to ensure voltage at the bulb, and that the bulb is working.

Sorry for the multiple posts.

Marlin

pauls81sc · 04-17-2005, 08:30 PM

Hey Marlin, thanx for the replies.

Let's see...I pulled the bulb today and there is continuity across it so that's not the problem. And yes sir, the "special function" I was referring to is the excitation of the alternator.

In a previous post, someone recommended installing the resistor even with the 2-watt lamp...

(I just went looking for it and couldn't find the same one, but I did find this one with info from "Early S Man" who seems like quite an expert on the subject.)

How far can you drive on no charge?

His reasons are listed.

I havn't checked the voltage to the lamp, but that's the next step. The voltage regulator is brand new, so if I don't find any wiring problems, this smells like a bad alternator.

Early_S_Man · 04-18-2005, 01:52 PM

Paul,

Don't replace the alternator just yet!

1. Is the non-lit warning lamp a recent occurence? If the bulb is good, the brushes may be worn out and need replacement. First thing you need to do is troubleshoot the reason for the no-lamp condition. Remove the alternator from the fan housing to inspect the brushes. If badly worn, you may be able to find replacement brushes alone to fit back in the Marchal brush holder locally at a motor/generator rebuild shop. The SEV-Marchal brush holder and new brushes are above $50, which is why I suggest replacing just the brushes themselves! While the alternator is still out ... check for Voltage at the Blue #61 wire to the brush holder with the ignition switch on. Check resistance to ground [alternator case stud] from the brush holder #61 connection tab. An open or intermittent condition is an indication of worn brushes.

2. The so-called 'Zero Ampere speed,' or RPMs below which there is no output available ... increases with age in our 911s because of an increasing buildup of resistance in the alternator warning lamp circuit ... lamp holder tab, power wire Faston connector, 14-pin connector, brush holder Faston connector and tab, brushes to slip rings, etc. Most older 911s and rear-engines VWs suffer from the same malady! One solution, or partial fix ... is to install the 68 - 91 Ohm resistor accross the lamp holder -- to increa0se exitation current. Another is to clean evey connection in the circuit and apply silicone grease to ward off oxidation and corrosion. Typical indication with this problem is faint glow of the warning lamp at night with the lights on ... yet output of the alternator is good ... 13.5 Volts - 14.2 Volts -- per the factory service manual, at cruising speed, or anything above 2000 rpm.

pauls81sc · 04-18-2005, 04:34 PM

Hiya Warren,

Yes....the non-lit warning lamp is a recent occurence. I havn't been watching for it so I'm not quite sure when it started. I sure as heck will watch for it from now on.

I'll pull the alternator and take a look at the brushes. Thanks for the tips. I think that between your response and the info I have from searches / tech bulletins I can tackle this one.

Thanks again for the help.

Paul

JohnJL · 12:49 PM

Guys,
I've replaced the entire wiring harness and am looking to install a "Fan belt warning light" in the form of a dash warning light to illuminate when the alternator stops generating. I have an external regulator.

Based on searches here and some diagrams from some Ford and Corvair bb diagrams posted that a wire in series with the "DF" wire to a bulb on the dash would function correctly.

Marlin, your post above indicates a different connection...however with an external regulator I don't believe I have a "D+" connection at the alt (closed up now) or regulator.

Can anyone verify or perhaps point me to a correct wiring diagram? Perhaps I missed it in the search.

I have scrutinized the Bentley manual, the problem is I only kept the very basic circuits I needed for the track car, so no computer, no nothin'.

thanks

Joe Bob · Joe Bob's Garage

I used a resistor in one case when a high output box replaced the Bosch unit. It was needed to kill power when the ignition was turned off. It was inline on the D+.

Early_S_Man · 05-03-2006, 01:19 PM

John,

Maybe this diagram will help ...

All you need to do now is locate the wire you connected to the D+/61 terminal on the alternator and connect it to a warning lamp like one of these large ones I recommend from CB Performance ... it uses a 4 Watt #1816 bulb:

http://www.cbperformance.com/catalog.asp?ProductID=1118

JohnJL · 05-04-2006, 03:48 AM

Thanks Warren.

ahhh...the blue wire here I think is 61 (no D+ marking tho?).

I see the right diagram now in section 270-10 of the shop manual. Not so clear from the wiring diagram at the back.

So just a 4w bulb in series with the D+/61 wire between the alt and external regulator? Should the bulb be wired a. so its in series in the D+/61 wire or b. with the + side to the D+/61 and the - side to chassis ground?

JohnJL · 05-04-2006, 03:53 AM

Heres the pic of the alt before re-wiring. Blu wire is marked 61 on the alt but no D+ marking...

Early_S_Man · 05-04-2006, 08:26 AM

John,

That is a '75 thru '81 SEV Marchal Alternator that requires an external regulator! Below is the applicable wiring diagram ... below that is a labeled pic with correct wire locations. You need the correct regulator, and a warning lamp connected to the blue wires at both alternator and regulator ... and to a source of switched +12 Volts at the other terminal.

JohnJL · 12:38 PM

Warren,
Thanks, yes I have a new regulator ready to go. Your comment on the wiring of the blue D+/61 from both the regulator and the alt to one side of the bulb and switched power to the other is exactly what I was looking for!

Thanks again, and I'm sure that set of pics above is sure to help many people to come!

John