pjs.oregan

Hi all, another question -- early NA car

My battery/alternator light was disconnected for some time recently (blue wire broken just before the big plastic connector to the gauge). Weirdly the alternator seemed to be working OK prior to this (as I understand it, the circuit runs through the bulb, so a nonfunctioning bulb should stop the alternator working). I've fixed the connection, but now the light is on even when everything seems to be working alright (i.e. - it's off with the key out, on with the key turned 1 notch but engine off, but remains on with then engine running)

I get 12.4V at the (brand new) battery with engine off, still 12.4 when I first start the car, but then 14.2V as soon as I get over about 3000rpm (and stays at 14.2 after the revs drop, until the car is turned off). The battery light remains on regardless of voltage. I've verified those voltage readings with a multimeter (i.e. not just the car's inboard voltmeter)

Any ideas what this might be? The car has recently sat for 8 months (hence the new battery, old one was toast) and I don't know how long the blue wire to the gauge was disconnected before that, so I'm not sure how long the problem has been present. The car charged fine prior to sitting (and probably still does now, given the good voltage reading once the alternator comes online)

pjs.oregan

Checked voltage regulator and brushes look good.

MAGK944

Maybe there is a problem with the exciter wire and that’s why it was disconnected. The charging circuit can work without it as alternators can self-excite but it’s needed for the charging light. Time to get the wiring diagram out and start testing with a multimeter. I’m going to guess that the exciter wire is shorting to ground somewhere along its length. That would cause the light to come on constantly as the other side of the light has battery voltage with the ignition on.

V2Rocket

yes to add what Mike said the alternator will eventually (with some RPM 2000-3000+) self-excite and work normally.
the charging light helps activate the alternator all the way down to idle by providing a small load.

is the bulb correct?


slightly OT, Mike, i seem to recall you had a stock 944 alternator re-wound for more output. if so, what kind of uprating did you get ?

MAGK944

Thats freaky cos I just bought another one and am fitting it today. So I will have these high-output (200Amp) alternators on two of my cars by the end of the day.

I upgraded the first one a while ago when I fitted the IceShark battery/charging cables. It’s a street car and so has decent speakers and a sub plus the AC is constantly on here in FL, the 200A alternator helps somewhat in keeping everything stable especially when stationary in traffic or red lights.

V2Rocket

do you know off hand if the later style alternator is compatible with an early ac/alt bracket?
i know i could just swap brackets, but i bought a new AC compressor for my early car a few years back so i'm sort of married to it now

since high-output alts have a reputation, deserved or not, for giving up low rpm charging capability, have you measured output/battery voltage at idle with the big alt?

MAGK944

Sorry idk about compatibility, I only have experience with late cars. The 200A alternator puts out 13.55v at idle and drops to 12.72v with the AC, headlights, fog lights and audio on. It puts out a steady 14.6v at higher revs. Don’t forget that’s with the upgraded (IceShark) battery/alternator wiring.

Tiger03447

Two things come to mind. first another ground wire from the alt case to the chassis is not a bad thing, either. Second that comes to mind is that if the power lead running from the Alternator is too light a gauge, you will effectively strangle the alt output. I doubt this would be the case with the Ice Shark cabling setup though. Might be worthy of a check. If you don't have 14.6 when running at least at 2000 RPM, w/o load...you will most probably run the battery down. An amp check on the positive cable is a good thing too. You could be getting a low amperage output and still maintain a good voltage output. 12.4 is battery voltage. You must be at least a volt or two above it to keep the battery healthy, and most alternators will do that with all the lights and accessories on.

pjs.oregan

see below

pjs.oregan

Well I think I've got it sorted... I'm not sure what's happened in the past but the PO has made a new exciter wire, connected to the blue exciter wire near the fuse box. The other end of this new wire was disconnected, so I'm not sure whether he/she had it wired up to the gauge, or some other 12v source.

I guess the options are a) connect this new exciter to the 12-pin connector at the back of the gauge, which I think should essentially restore the original circuit or b) go digging into the harness and try to restore the continuity of all the original wiring, and do away with the PO's bypass.

What do you think?

MAGK944

Just keep in mind that dash charging lights operate differently to “regular” electrical circuits as they rely on a difference in +ve voltage rather than a +ve and -ve.

Lighting circuits usually have +12v on one side of the light and ground on the other. Charging lights have +12v on both sides, one +ve from the battery and the other +ve from the alternator (exciter wire). If one circuit supplies less than 12v the charging light will come on, how dim or bright it lights depends on how big the difference in voltage between the circuits is.

So, my point is, don’t just supply the alternator exciter with a 12v supply from a switched battery supply and also connect that straight to the light, it just won’t work.

Hope that all makes sense. GL

pjs.oregan

I think I get it... So if the dash bulb gets it's 'alternator voltage' signal from the blue exciter, I assume it gets it's 'battery voltage' signal from another of the wires to that connector? In which case as long as the blue wire (or rather, pin 11 on that 12-pin connector) is going to the alternator (D+?) it should be hunky dory?

The blue wire is still present between the alternator and where this new wire was spliced in by the fuse box, so it should be easy to connect that back up to the gauge connector.