1992 348 TS Alternator Light Function

Yes it should come on when you turn the key with the others you listed. As for troubleshooting this I am sure Mirol will be on here soon and give good advice. He is the man when it comes to things electrical like this.

 

To trouble shoot, first check that the dash bulb is not burned out.
If the bulb is fine, you need to ensure that the signal (called "L" for Lamp) is getting from the alternator to the dash. There are 3 wires that run from the alternator regulator via a circular connector (see picture) they are L (for the dash lamp), IG (for the ignition), and and S (for voltage sense - battery connection). In my case the L wire had melted into the S wire in the wiring harness (passes very close to the exhaust); this was pulling the lamp voltage to battery voltage and hence the dash light never came on (NB: the dash lamp comes on when the alternator pulls L to ground, the other side of the Lamp bulb is wired to 12V in the dash - and this is very normal). A quick check for this would be to unplug the connector from the alternator, then check the wire harness side to verify that the L wire is not connected to S or IG (should read open cct between L and S, and also open cct between L and IG)
But definitely do the easy things first, check the bulb !!!

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Important: NEVER disconnect the battery while the alternator is running. It may cause the alternator output voltage to go very high and damage the car's electronics (you were lucky this time).

Judged by the shape of the connector, your alternator is not original but a Delco from another car (fairly common way of replacing failed 348 alternators). The wire from the alternator light, in the original loom, is yellow/green and should be connected to the "L" (light) pin of the connector. The loom should also have another wire, blue/white, which you can leave unconnected for the time being. The third wire (not inside the loom), red, goes from the large positive post of the alternator to the "S" (voltage sensing) pin of the connector. It appears that the wire, connected to the "L" pin of your connector, is not the yellow/green one. Open up the end of the loom from where your current wire to "L" pin comes and see what wires you have inside.

 

If anyone wants to upgrade the alternator to the last, most powerful Denso version, there is a complete new upgrade kit on Ebay at a reasonable price - https://www.ebay.com/itm/Ferrari-348-Mondial-alternator-Nippondenso-upgrade-kit/222915896875?hash=item33e6d2a62b:g:KD0AAOSwjI5aykXT&vxp=mtr

 

Just compared the alternator on Ebay I posted about above with my 140 Amp Denso alternator which I earlier bought as an upgrade kit. It appears that the upgrade kit on Ebay is not the most powerful version (140 Amp) but a 105 Amp Denso. In any case, if anyone is considering buying it, check with seller what the numbers on the alternator are. They are as follows:

105A Denso, p/n 148471 (or 155785), Denso Number 101211-5070

140A Denso, p/n 156235, Denso Number 101211-7580

 

The only problem with that plan is, if your alternator ever starts to fail, you won't know because the warning lamp doesn't work.

Surely it can't be that much effort to get the wiring sorted and have the car running exactly as it should, with all of the warning lamps working correctly?

 

I didn't think of it earlier - check the 9-pin connector in the engine bay, left side, just in front of the shock tower. The alternator light wire goes through this connector (Pin 6) and the pin/socket may have corroded and lost contact. I had problem with corrosion of pins in this connector and replaced it with a military grade AMP connector (pic).

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You can easily verify whether the wire from the 9-pin connector to the alternator charge bulb is good if you unplug the connector and connect the pin 6 (of the socket side) to the ground. Then switch the ignition on and see if the bulb lights. On the connector diagram I attached above, the pin 6 is shown on the left because it is looking at the face of the plug, should be on the right when looking at the socket. In any case, you can see the pin numbers on the plug face (probably also inside the socket) to identify the pin 6.

 

Yes, that is correct.

 

Here's the circuit:

Ferrari 348 Starter/Alternator Circuit Diagram

The plug is marked as "115". So you want to earth/ground contact (pin?) 6 on the car side, not the alternator side. As Miro says, the ignition must be on.

I don't know if it's possible to see wire colours on the original installation, but the wire is yellow-green.

 

Looking at the diagram, the light should also illuminate with the key in the first position (accessory)
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The light bulb gets +12V on one side from the ignition switch and its other side is connected to the positive side of the alternator exciter coil which, when the alternator is not running, provides ground to the bulb so it lights. When the alternator starts turning, the current flow through the bulb and the exciter coil provides a small magnetic field in the alternator that initiates (primes) the alternator's generation of power. Once the alternator starts generating power, it itself starts supplying power to the positive side of the exciter coil, i.e. it self-excites. At this point, the bulb ends up having positive supply at its both sides (zero current through the bulb) and goes off.

The wire from the female 6 should go straight to the "L" terminal of the alternator's connector. I suspect the wire is broken somewhere (check the back of the female side of the 9-pin connector) or not connected at all to the "L" on the alternator's connector but some other wire goes there.

Note: The alternator will be able to start generating power even if it does not get any initiating current from the light bulb (like when the bulb blows) but, in such a case, it needs to be revved up a bit from idle speed (to about 1500-2000 engine rpm) only once after the engine has been started and, after that, the alternator will function as normal including generating power when the engine is subsequently at idle.

 

It appears that your alternator regulator is partially faulty. Your measured 14.4 V output even at full load is a bit too high which also suggests some problem with the voltage regulator, as if it always (unnecessarily) pushes the alternator to its maximum which can over-strain it in the long run.

How's your battery drain, did you measure it? A faulty regulator can cause fairly high battery drain.

 

One more thing: I see that your S (sensing) pin is not connected. On my 1991 348 the S pin is connected to the big alternator output terminal. The purpose of the S pin on alternators is to sense the voltage delivered by the alternator and maintain it, via the voltage regulator and the exciter coil, at the correct level. On some cars, the S pin is connected to the car's positive line somewhere further downstream from the alternator output terminal to account for the voltage drop along the wires and connectors.

It is possible that your alternator light is not working because the S pin is not connected. And the too high alternator output voltage could be because the regulator senses zero volts and, possibly by some default, goes to the maximum voltage. Did you have a non-functioning alternator light since you bought the car or did it develop it at some point?