Alt light doesn't work

[chrisp]

I am getting charging through system and according to John Walker the circuit must work for the charging system to work.

When I turn the key the light doesn't go on.

How can I check the system and possibly find the source for the light not working.

FWIW the bulb is fine and all of the connections at the back of the light are solid. Also I had the motor rebuilt and the light was out before I pulled the motor and is still out. So, I don't think it's a connection at the back of the alternator because that would have been disconnected and reconnected when the rebuild was done.

Any help would be much appreciated.

[chrisp]

bump

[304065]

Carefully remove the tachometer without scratching the rim.

On the back of the bulb holder for the warning lamp there are two connections, one is a blue wire, the other should be a red with black stripe. With the key on, use a multimeter to verify you have +12v at the red/black wire. Then, disconnect the battery for this next step or you will weld yourself to the car, no kidding. Remove the alternator and disconnect all the connections. Then connect a jumper to the D+/61 blue wire where it connects to the alternator, and run that jumper to one probe of your multimeter. Put the other probe on the blue wire terminal of the bulb holder and check that it says zero ohms. If that wire's broken, report back.

[chrisp]

Did you mean to say "remove the oil pressure and oil temp gauge".

FWIW I read through a prior post where you and John Walker were discussing confirming the circuit isn't broken by using a jumper on the blue wire coming out of the back of the warning lamp (actually you said from the regulator but I improvised). For s---- and grins I ran some wire from the blue (with a color stripe I can't recall) wire to metal and got a decent spark. The oil warning light dimmed at the same time.

However, in that post you guys spoke of how the circuit needs to be in tact for the charging process to take place. I have no problems with the charging circuit.

I was avoiding trying to pull the alternator but I guess it's down to that. I'll let you know.

Thanks John.

-Chris

[304065]

Yes, I have been removing my own tach so much lately that I got locked in the rhythm.

Chris, if you ground out the blue terminal of the bulb holder, all you have proven is that you've got voltage to the "hot" side (the red/black side) which you knew anyway because the light was on!

You really need a multimeter for this kind of testing, they are so inexpensive it's ludicrous, I think I saw one the other day for $7 or something like that, they are available everywhere, auto parts stores, even community hardware stores. I actually carry one in the car they are so inexpensive, compact and lightweight.

Removing the alternator is an easy 20 minute job, the only thing that you have to remember is to disconnect the battery completely before you do it, don't want that positive cable falling onto the engine case and doing an impression of the firebombing of Dresden (Magnesium incendiary bombs). Somebody could have forgotten to tighten the nut on the D+/61 and it fell off or something, or left too much slack and the wires melted against the case, it's really easy to check. You may want to label the wires with a piece of tape to simplify what goes where, it's basically blue to D+/61, brown to D- (your SC may have more than one wire going to ground here) and the fat wire, I don't know what color it is in an SC but it's probably RED, to B+, the 14V output from the battery.

You shouldn't have to pull the alternator from the housing to check this, either. Just remove the fan belt using the tool from your toolkit so you don't break blades, and be sure to make a note of how many shims are on the outside of the pulley and how many are on the inside, you will want to get this right when you put it back together for the correct belt tension.

Good luck!

[chrisp]

Thanks John, the plan is to tackle this tonight.

I've repalced my alt before but apparently forgot how quickly it can be accessed.

Are the contacts labeled as noted on the alt or do I need a schematic?

I got ya on the battery disconnect warning.

The oil light was the one that dimmed so checking for power to the alt light was probably a good thing albeit accidental.

[chrisp]

John,

With the red and brown connected and the blue disconnected, I get spark when I touch the blue wire to chassis. Obviously this is with the battery connected.

With the alternator totally disconnected, a jumper from the blue wire to a multimeter probe and the other multimeter probe to the blue on the back of the warning lamp holder I get 6 ohms. This is with battery connected but key in the off position.

What next?

[Joe Bob]

Might be as simple as the bulb is burnt out.....

[chrisp]

I haven't replaced the bulb but the filament looks fine. Also, I have a piggy-back warning light and that doesn't work either. The bulb in that one is definitely good.

[dentist90]

Before you go nuts taking wires apart, check the bulb for continuity. If the filament is broken there will be no continuity. Or you could try putting 12v across the bulb connectors. It would be prudent to actually see the bulb glow before you start dismantling your electrical system to look for faults. I have replaced a couple of those tiny dash bulbs that looked fine to my eye, too.

[Joe Bob]

Well if the battery is fully charged, read at the terminals

12.5 volts @ rest
13.0 volts @ idle
13.5-14.0 2500 rpms.....

[chrisp]

I'll check it all again just to make sure. Hopefully it's as simple as that. I would have done the bulb swap already if the oil press and alt warning lights didn't use different bulbs. To avoid pulling each gauge, which one uses the same bulb type as the alt warning light?

If I use a jumper between the hot (red w/black) of the alt warning light and the ground (blue w/red)(to simulate a properly functioning bulb) should my piggyback light function normally?

[chrisp]

I jumped between the hot and the ground going into the back of the warning light. With the key on (oil light on) nothing happens. I'm assuming it should light if I jump those terminals.

What next? Thanks guys.

[chrisp]

bump

[304065]

Chris, jumping between the red/black and brown ground will just blow the fuse or light the car on fire. Please do the following.

Disconnect battery
Remove alternator.
Wrap B+ cable to alternator in multiple layers of duct tape and suspend away from engine parts
Reconnect battery
With the light hooked up properly, turn the key on and touch the blue D+/61 wire to the engine case. The light should go on.

If that doesn't happen, follow my original instructions for measuring continuity in the blue wire circuit.

[chrisp]

Thanks John.

I got the light to work by grounding the blue wire at the back of the warning lamp assembly. Battery, alternator, and warning lamp were all fully connected at the time.

I didn't jump the red/blk and brown at the alternator.

As mentioned I did the test you described at the alternator except that I also had the brown disconnected (and taped up). I touched a jumper from blue wire to chassis (as opposed to the case as you describe). Should I redo it with the brown connected?

Thanks.

[304065]

Ok, so the bulb works, but you still don't know if the blue wire is unbroken between where it connects to the bulb holder and where it connects to the alternator. That's what you are trying to figure out.

When you grounded the blue wire to the chassis, did the light come on? It does not matter that the alternator ground was isolated for purposes of that test, only that the key was on and the blue wire was grounded at the alternator end.

[chrisp]

Key on, blue wire grounded: no light

[chrisp]

Also, I am getting 12.5V across the battery with engine off. I get 14V at idle, and 14V at 2000RPM

[john walker's workshop]

wire trace time, from light to alt. a continuity check wouldn't hurt, end to end with a ohmmeter. maybe someone rigged the wiring different than stock.