What's the purpose of the Ignition Light Resistor?

[Jaroslav]

What's the purpose of the Ignition Light Resistor in my 1987 110 2.5 N/A Diesel?

I'm wondering if this resistor could be reason why the battery warning light doesn't come off by itself when I start the engine. Once I rev the engine up a little bit, the light disappears and the battery seems to be charging OK. I read somewhere that it means that the alternator is not excited properly when it is happening. It's not a big problem, but I'd rather fix it if I could.

I am speculating that with this resistor blown (connected in parallel to the battery warning light), there might not be enough current to be coming through one of the alternator's circuits and it doesn't self excite. Doesn't the manual say that before investigating alternator problems one should check the battery warning light first?

I would check this 100 Ohm resistor - if only I could find it on the truck! I've seen a picture that shows that it has to be under the dash board, on the right side (on my RHD vehicle). Is it supposed to be mounted somehow or simply burrowed among the harness wires?

[western]

it's buried & taped witin the main dash area loom, to get to it means a near full dash strip & uncovering the loom taping,

IIRC it has briwn wires & looks like a in-line single fuse holder of the push & twist to release type.

[jimmy_neutron]

What's the purpose of the Ignition Light Resistor in my 1987 110 2.5 N/A Diesel?

I'm wondering if this resistor could be reason why the battery warning light doesn't come off by itself when I start the engine. Once I rev the engine up a little bit, the light disappears and the battery seems to be charging OK. I read somewhere that it means that the alternator is not excited properly when it is happening. It's not a big problem, but I'd rather fix it if I could.

I am speculating that with this resistor blown (connected in parallel to the battery warning light), there might not be enough current to be coming through one of the alternator's circuits and it doesn't self excite. Doesn't the manual say that before investigating alternator problems one should check the battery warning light first?

I would check this 100 Ohm resistor - if only I could find it on the truck! I've seen a picture that shows that it has to be under the dash board, on the right side (on my RHD vehicle). Is it supposed to be mounted somehow or simply burrowed among the harness wires?

I had that problem last winter ... didn't worry me though and i never even looked into what might be causing it. I fitted a new battery a good marine battery a few months ago as i needed power when the trucks engine isn't running and still wanted to be able to start it ok. Since fitting the new battery, my battery warning light goes off whilst just turning the engine over. Big difference. I wonder if your battery is just starting to fail.

[Aragorn]

Could be a slipping alternator belt?

Belt slips until you rev it a bit after which point it grips and the alternator starts working?

[errol209]

What's the purpose of the Ignition Light Resistor in my 1987 110 2.5 N/A Diesel?

It's there to provide enough current to excite the field coils in the alternator while the alternator isn't generating enough itself and the warning light is blown. When the alty does spin fast enough, it self excites so the alternator terminal goes to +12v and the warning light goes out (if working).

According to the wiring diagram if you're convinced the resistor has blown, you could hack a 100 ohm 5W resistor accross the ignition live (white) and the alternator warning line (brown/yellow) pretty much anywhere you can get to.

[Jaroslav]

I wonder if your battery is just starting to fail.

No, it's a brand-new battery and I had the same problem with my old one.

Could be a slipping alternator belt?

Unlikely, because the battery light doesn't come on back when the engine returns to idling.

[Jaroslav]

It's there to provide enough current to excite the field coils in the alternator while the alternator isn't generating enough itself and the warning light is blown. When the alty does spin fast enough, it self excites so the alternator terminal goes to +12v and the warning light goes out (if working).

According to the wiring diagram if you're convinced the resistor has blown, you could hack a 100 ohm 5W resistor accross the ignition live (white) and the alternator warning line (brown/yellow) pretty much anywhere you can get to.

Aha, I should fix it then, because if the warning light gets blown I'll be stuck on the road.

I can probably test this theory by removing the warning bulb.

Thanks for all replies, guys!

/Jaroslav

[Jaroslav]

I can probably test this theory by removing the warning bulb.

Uh, no, I can't test this theory. If I remove the bulb, how will I know it's functioning properly? I better find the damn resistor.

[David Sparkes]

Uh, no, I can't test this theory. If I remove the bulb, how will I know it's functioning properly? I better find the damn resistor.

Yes you can. Use a voltmeter instead of watching the bulb.

Change nothing, put your meter across the battery.

Start the engine WITHOUT revving it.

Check if the bulb is still on, write down both this fact and the voltage.

Raise the engine speed, try to note the rpm when the bulb goes OFF, write down both 'bulb off' and the higher voltage.

Switch off the engine

Now remove the bulb.

Start the engine WITHOUT revving it.

Write down 'bulb out' and the voltage.

Raise the engine speed, try to note the rpm when the voltage rises, write down both this fact and the higher voltage.

You can also do a static check (engine not running, ignition switched off, battery still connected).

Remove the bulb and do a resistance check across the terminals in the bulb holder.

Note that you should NOT do this test with the ignition switched on, unless you disconnect the battery.

HTH