Auto-electrical help?

[Myles1970]

High folks,

I'm part-way through re-wiring the lights on an old jdm corolla and I have all working with the exception of the high beam indicator light. It does actually work as it should but it's very dim (the other lights on the dash are all 100%).

When I remove the bulb and put a voltmeter across the terminals it reads 6volts, which appears correct from the wiring diagram as it's in series with the headlamp bulb, ie. 6v + 6v = 12v.
I've swapped bulbs around but the bulb seems fine


Any thoughts?

 

[Eric the chicken]

I'm sure this falls into the grandmother and eggs category, but here goes. Is there a good earth?

 

[Myles1970]

Is there a good earth?

Thanks I'll check the earth, although the headlamps themselves are quite strong. I'll also tighten up the earthing bolt on that particular junction box.

 

[jsf55]

24v lamp used by mistake ?
EDIT Oops... have you looked too see if anything else is lit, i.e. being in series with the lamp giving the lower voltage ?

 

[Myles1970]

.... from the diagram, if the (38 year old) light switch was allowing main beam and dips to illuminate simultaneously, that would reduce the current through the blue dash light significantly.... Step 2, if checking the earth reveals nothing might be to de-pin the dipped beam at the bulbs

 

[Myles1970]

24v lamp used by mistake ?

I've swapped bulbs on the gauges around, but it makes no difference, so I don't think so

 

[Guineafowl]

The indicator works in series with the headlamp bulb, each dropping 6V, so if you remove the indicator, the headlight goes out, is that correct? If so, you should measure battery voltage across the open terminals, since there’s no voltage drop without current flow.

Can you measure the voltage across the bulb when it’s in circuit?

I would switch on the lights, attach the negative MM probe to battery negative, then walk the positive along the earth path to the indicator, then across to the headlight.

You should get 0V up to the indicator, then 6v after it, then 12V after the headlight. Anything else should tell you where the problem is.

 

[carbon]

so if you remove the indicator, the headlight goes out, is that correct?

No the main beam indicator is in series’s with dipped beam filament. The common point on the headlamp is positive switched through a relay and the filaments are switch to chassis ground to illuminate the bulb. When the head lights are on main beam some current passes through the dipped filament and main beam indicator to chassis ground illuminating the indicator bulb. When switched to dipped the current passes through the headlight switch straight to chassis ground.

 

[Keithmac]

It's not in series with headlamp if you follow the diagram, it's in parallel.

If you think about it your 12v 60w headlight bulb would never light up..

For me following the wiring diagram the Relay supplies +12v to the Headlight casing (normally ground but not here), the lighting switch grounds the High beam element).

If the dash bulb was in series with the headlight it would never be supplied enough current to light the High Beam..

 

[Keithmac]

@carbon is correct, +12v through LOW beam filament through High Beam indicator bulb and then to ground.

What wattage bulb have you got for the pilot bulb?, technically should still be a 12v bulb.

 

[carbon]

It's not in series with headlamp if you follow the diagram, it's in parallel.

It is in series with the dipped filaments. The dipped filaments act as a resistor this is why to the main beam indicator isn’t a 12v bulb.

 

[Keithmac]

It is in series with the dipped filaments. The dipped filaments act as a resistor this is why to the main beam indicator isn’t a 12v bulb.

I'd expect to see 12v at the pilot bulb connector across a 55w filament with no load.

 

[Bornfree]

Wouldn't it be designed to be dim ? You don't want a bright light source in your eyes when driving at night.

 

[Guineafowl]

No the main beam indicator is in series’s with dipped beam filament. The common point on the headlamp is positive switched through a relay and the filaments are switch to chassis ground to illuminate the bulb. When the head lights are on main beam some current passes through the dipped filament and main beam indicator to chassis ground illuminating the indicator bulb. When switched to dipped the current passes through the headlight switch straight to chassis ground.

Ah, thanks. I can’t see the circuit diagrams on my screen.

 

[Keithmac]

If you have a read on Potential Divider circuits you should get an idea.

If both the LOW beam filament AND the pilot bulb were the same (55watt rating, same resistance) you would get 6v at the junction.

I would say your pilot bulb should be in the 3 watt range, 12v?.

 

[Myles1970]

OK, thanks lads, I think I see it some watt clearer now
On dipped the high beam indicator light is short circuited. On high beam, the dipping filament is in series with the indicator, and these are both in parallel with the high beam filament.
The indicator lamp is barely visible even with the lights off in the Garage

 

[Myles1970]

If you have a read on Potential Divider circuits you should get an idea.

If both the LOW beam filament AND the pilot bulb were the same (55watt rating, same resistance) you would get 6v at the junction.

I would say your pilot bulb should be in the 3 watt range, 12v?.

I'll have to check (be tomorrow now), but I did see a video on YT where the guy fitted an led to cure the same problem.... and he replaced all the others with w3w bulbs (don't know what voltage)

 

[carbon]

@Myles1970 does disconnecting one headlamp bulb make any difference to the indicator bulb brightness or is the indicator bulb not connected to chassis ground very well?

 

[Keithmac]

If you get stuck an LED replacement pilot bulb would probably do the job (requires very little current).

 

[voipio]

With the headlights on full beam, one end of the indicator light should be at 12V, or slightly below that, since it's in series with the dipped filament when full beam is on, and the other end should be at 0V, with respect to vehicle Earth. It if isn't at 0V, try to find the poor earth connection. Also, make sure the indicator light bulb glass is clear, and not blackened with sputtered tungsten.