[Ben M.]

I converted my ‘73 big car idiot light cluster to gauges some time ago. The alternator has since required the engine rev’d to 1200RPM or so to get it to start charging. Investigating why (since I’m tired of it, factory gauge cars didn’t require it) I find all sorts of threads about various car lines explaining resistor wires to limit the current or sticking a lamp on the field line to switched +12V but no real consensus on how the factory did it. Looking at the drawings for this year (and all ‘71-76 big cars) shows a brown wire from the cluster to the field of the alternator. This loses it’s switched voltage in the conversion because the rally gauge cluster doesn’t use this pin, hence the problem. The drawings also show a dashed brown and white wire to the B terminal of the ignition switch that’s only energized in RUN. This is dashed so it’s optional, doesn’t say what causes this wire to be installed but I suspect it’s for gauge cars. The question remains “is it just a wire?” or is there an internal resistor added not shown on the drawings? The factory manual is silent on this, there’s just a single reference in the alternator section of the manual for checking continuity of the ignition switch source on the field wire (lamp for gauge cars) if the car doesn’t automatically charge. I know CS130 & 144 alternators make a big deal out of having a resistor on their lamp wire but I’m unsure on the 10SI the car has on it.

So... can I just hook up switched +12V to the field wire directly or do I need to stick a 150 ohm resistor in series with it? Adding the resistor likely won’t hurt a thing, I just have to add it and wire it all in, while a straight wire is simpler. If the factory engineers felt it was important to have surely they’d have said something about it. But I also don’t want to kill the regulator in my alternator...

[Chief of the 60's]

I take it you went 1 wire? Go to 3 wire. Problem solved.

[Ben M.]

Originally Posted by Chief of the 60's

I take it you went 1 wire? Go to 3 wire. Problem solved.

Wrong. Read the post. The field wire from the bulb is no longer connected due to swapping gauges.

[johnta1]

The field wire from the alternator isn't connected to anything?
(should be)

[Ben M.]

Originally Posted by johnta1

The field wire from the alternator isn't connected to anything?
(should be)

That is correct. When you convert the cluster from a light cluster to gauge cluster with voltmeter you lose the field wire connection inside the car. I know it’s supposed to be hooked up, the concern is if it needs to be current limited with a resistor or diode combo or if I can just run a straight wire from switched +12. The factory drawings do not indicate anything other than a wire.

[johnta1]

How is the voltmeter wired?
I'd either connect to the voltmeter or the ignition switch 'ON'.

[Ben M.]

Originally Posted by johnta1

How is the voltmeter wired?

It uses the stock +12V switch (pink wire) connection on the gauge cluster - this is a factory gauge cluster that replaced the factory idiot light cluster and the wiring on the plugs is the same.

Originally Posted by johnta1

I'd either connect to the voltmeter or the ignition switch 'ON'.

I agree... but should it be resisted with the lamp\lamp equivalent or is a 12ga wire satisfactory enough to not let the smoke out of things?

[johnta1]

Just the wire I would say.
If you had run the field wire straight from the alternator to the power feed by the alternator, then a resistor may be necessary to give it some 'difference' in voltage to regulate?

[Ben M.]

Originally Posted by johnta1

Just the wire I would say.
If you had run the field wire straight from the alternator to the power feed by the alternator, then a resistor may be necessary to give it some 'difference' in voltage to regulate?

No, it's internally regulating. The field wire is to give the rotor proper magnetization and start charging. It can do it on its own but that requires more speed on the shaft to do so. The resistor is because you don't want to cook the wiring if the key sits in the RUN state without the engine actually running.

I checked a later model year manual and they specifically say that this wire is ONLY installed for Rally Gauges or Heated Rear glass. Instead of using an 18ga wire like on the lamp circuit, they use a 24ga wire. I checked an existing harness from a gauge car I found in my stash and yes, they do only have the brown with white wire on the gauge car and not on the idiot lamp car harness. Since the gauge of wire is considerably smaller, I'm presuming they did it for a reason. I'm going to add a 100Ω 2W resistor in series with the wire I add just because better safe than sorry. I'll also tap off of the existing field sense wire near the cluster plug rather than pull the entire harness out.

Amusing thought is that this means if you put an idiot lamp cluster in a gauges car, your alternator lamp won't ever light up... and that despite what the catalogs say, the dash harness for the 1971+ full size line is actually different between gauge and lamp cars (I already found out that the engine harness they sell is wrong for multiple years).

[raggedjim]

I know this is an old post but did the 100ohm resistor fix your problem? Where did you pull your 12v keyed from?

When I did this years ago I think I pulled from a big orange wire in the bundle but added a socketed 1895 bulb in line with the original brown wire. I ended up spray painting the bulb black because every now and then it would glow!

Rg

[george kujanski]

The resistor is a good idea. Internally in the 10SI alt, sure the #1 terminal is used for excitation, but once the alt develops output, the #1 terminal essentially becomes the sense voltage. There may be a difference in voltages between the #1 terminal and the ign wire. There should be some resistance between #1 and the switched ign wire, ... some resistance will isolate the two points. A light bulb performs all 3 functions... excitation, isolation, and indication.

George

[Peter Serio]

Lamp in the wire to allow the alternator to begin to charge.

In the early 1980s I converted a Chevy Van to full gauges (Volt, Fuel, Oil pressure and coolant temperature) and the way I solved this problem was to leave the "GEN" light bulb socket intact and just ty-wrap the wires underneath the dash.

It could also be done using a resistor however the light bulb was a quick, easy solution

[Squidward]

Going through a little of this now. Doing a digital gauge conversion on a 67 Firebird. I run a cs105 alt. My new digital cluster has no provision for a GEN light. I will now use the hi beam indicator bulb as my gen light, because the digital cluster has a built in hi beam indicator.

I thought about using the resistor and ditching the bulb altogether. Kinda torn.