1992 SC400 good Alternator not charging (missing relay?)

[itsaaron14]

Hey I just bought a 1992 Lexus SC400 and plan on fixing it back up to it’s former glory. I’ve been having an issue with the charging system not working properly and I think I’ve narrowed it down to a missing relay. The only problem is I can’t seem to find any documentation on which relay it is and none of the other relays in the car match up to it. I’m suspecting it is the “IG MAIN” relay that’s located in the Relay Block 4 in the interior fuse box. Any help is appreciated.

 

[P Bateman]

There shouldn't be any relays required to make the alternator work.

 

[itsaaron14]

Could the fault be with the ignition switch then? I suspected the IG wire was an issue so I provided voltage to that wire manually upon start and the battery read normal voltage until I stopped and started the car again.

 

[itsaaron14]

Could the fault be with the ignition switch then? I suspected the IG wire was an issue so I provided voltage to that wire manually upon start and the battery read normal voltage until I stopped and started the car again. There is also never a battery light in the instrument cluster and the bulb is good.

 

[P Bateman]

B - High amperage Alternator output to the battery
IG - Switched ignition input that powers up the charging circuit
S - Sensing line - input for measuring the battery voltage
L - Lamp - provides a ground to the charge lamp. It senses when the lamp is on and triggers the circuit to start generating charge

If you think the IG terminal isn't getting 12v when the ignition is on, you'll need to trace the wire back through every connector, junction and fuse until you get to the ignition switch.

Ignition switches are fairly robust and has large contacts - I haven't seen too many fail in my experience.
Plus, other circuits would be inoperative too, if the ignition switch was faulty.

Its more likely a fuse, damaged wire or poor connection. (i.e corrosion, breakage, loose terminal, etc)

 

[blinkerfluid]

check SENSE and IG power - specially the IG, it won't work without power at IG

 

[ivan129]

Could also be a blown charge lamp. You can replace it with a 47 Ohm 10W resistor if you don't care for a lamp.

 

[itsaaron14]

check SENSE and IG power - specially the IG, it won't work without power at IG

I’m suspecting it’s the IG wire. I’ve tested it and there is no continuity between where that wire starts at the alternator and the ENG fuse it’s supposed to connect to. Not sure where exactly it could be though since I’ve tested almost every section and it looks fine to my eyes. The fuses and connector into the fuse box are also perfectly fine, so I’m fairly sure it’s not that. Still have some more areas to check though.

 

[blinkerfluid]

just to check that everything is fine with the alternator and there is not something internally shorted - i would remove the IG wire from the connector, connect another piece of wire and give it 5 amps fused +12v directly, start the car up and see generator behavior, it is certainly possible that something inside the generator caused the IG wire to fail over a long time period


BTW, did you check if there is power at IG wire when ignition is ON?

 

[itsaaron14]

UPDATE:
Got it running, the IG wire was broken by the driver side tire so I replaced that and all is well.

 

[blinkerfluid]

many many congratulations