Why am I dim

[dave.n]

Has anyone got any ideas why my headlights would be so dim?
Replaced the old one's with new units and bulbs because I thought the reflectors had blackened. The earths are good.
Had to slow down last night doing many miles per hour because I couldn't see properly and had a very embarassing experience.
dave.n

[GUMBALL]

Dave,

Take a look at this - modern cars have relays in the lighting circuit to avoid voltage drop to the headlamps. Makes interesting reading.

http://www.madelectrical.com/electri...adlights.shtml

Of course it could be that you have a wiring fault at the headlamp itself.
It can be quite common to mix up two of the three terminals on the back of the headlamp - check there first (I had my wires round the wrong way here when I first built the car and it made a big difference when I got them right - they are great now, but out of interest I don't have relays in the circuit - yet! - they are sitting on the bench waiting)

Hope this helps.

GUMBALL

[GUMBALL]

Dave,

This is interesting too - according to the attachment if you are only getting 11volts at the headlamp (which is possible) your output will be about 50% (i.e. bloody dim!).

http://www.rallylights.com/hella/Relays.htm

I think some manufacturers put relays in their wiring looms and some don't - years ago (i.e. on classic cars) they never bothered - that's why older cars seem to have dim almost yellowish headlamps.

GUMBALL

[paulp]

This will only work if your switches and cable are under rated. The drop in voltage is caused by the resistance of the delivery circuit (switches cable's etc) vs the resistance of the load (the lights). If the delivery circuit is high it acts as a potential divider and the voltage will drop.

A relay is simply an electrically operated switch, so providing your current switch and wiring is up to the job then there should be no voltage drop.

[oglyoodoo]

Dave

Is your problem on both Main Beam and Dip Beam??? Were the Lamps OK before you changed them?? The new units are 12V and not 24V units?? The extra volts won't make them brighter!!

The Headlamp circuit on your car, if it has no Relays, is pretty straightforward and almost certainly will be:

Battery (+Ve) - Fuse - Headlamp Switch - Dip Switch - Headlamps - Chassis (-Ve)

You may have the ignition switch in circuit between the headlamp switch and the battery, it matters not at this stage.

If your Headlamps are pretty Dim something is either wired incorrectly, an earth is missing or high resistance or something has to be getting warm.

Using a test meter on the Volts range I would connect the negative lead direct to the battery and measure the voltage on the battery +Ve as a reference.

Turn the Dip Beam on and measure the voltage on the three terminals of each headlamp.

You should have almost the full battery voltage on 1 of the terminals of each lamp (Probably Blue/Red) and the remaining 2 terminals should have almost zero hopefully less than 200mV.

When you operate the Dip switch to Main Beam you should have almost full battery voltage, this time on a different terminal (Probably Blue/White) and again almost zero volts on the other two.

Depending on what you measure will depend on what to do next.....

Denis

[dave.n]

Thanks everyone
The lights were dim when I bought the car. Thinking that the old lights had seen better days I bought new units. So relays it could be.
dave.n

[route66]

This has prompted me to test our Dax with a volt meter.

My reading are as follows:
Engine running (2500rpm) no lights etc on - 14.1V at alternator.
Engine running (2500rpm) main beam on - 14.1V at alternator - so alternator okay.


Main beam on (engine at 2500 rpm) - 10.2V at headlight!
Main beam on, cooling fan on, wipers on (engine at 2500 rpm) - 9.3V at headlight!

It looks like relays would be a good idea! (or don't drive in the dark)

Steve

[MacGD016]

Originally Posted by route66

This has prompted me to test our Dax with a volt meter.

My reading are as follows:
Engine running (2500rpm) no lights etc on - 14.1V at alternator.
Engine running (2500rpm) main beam on - 14.1V at alternator - so alternator okay.


Main beam on (engine at 2500 rpm) - 10.2V at headlight!
Main beam on, cooling fan on, wipers on (engine at 2500 rpm) - 9.3V at headlight!

It looks like relays would be a good idea! (or don't drive in the dark)

Steve

Steve, With readings like that you may have a voltage drop between the alternator and the battery, are you getting the same reading at the battery as at the alternator.
Mac

[MacGD016]

Originally Posted by dave.n

Thanks everyone
The lights were dim when I bought the car. Thinking that the old lights had seen better days I bought new units. So relays it could be.
dave.n

Dave, Quick and easy way to see whether relays will give any benefit is to get a length of thick cable put a spade connector on one end and plug that into the headlight bulb and put the other end direct onto the battery, if this light is brighter relays will sort the problem if not it could be an earth problem. Sometimes earths only go high resistance when there is a load and they look ok with a meter.
Mac

[oglyoodoo]

I did some measurements on the headlamps for interest this evening. No wonder the Cob is taking so long to finish.. I have the usual H4 Hella headlamps with 60W main beam and 55W dip beam... Also measured was the light output of the Main beam. The Lamp volts were measured directly on the lamp pins not the connector as this will have some volts drop albeit small.

Volts At lamp Dip Beam I Main Beam I Light output at 1 ft
in foot candles
14.1V 5.0A (70.50W) 5.7A (80.37W) 1484 ftc
13.0V 4.8A (62.40W) 5.5A (71.50W) 1168 ftc
12.0V 4.6A (55.20W) 5.25A (63.00W 923 ftc
11.0V 4.4A (44.00W) 5.0A (55.00W 716 ftc
10.0V 4.15A (41.50W) 4.75A (47.50W) 530 ftc
9.0V 3.95A (35.55W) 4.5A (40.50W) 383 ftc

The lamp is specified at 12V and the 55W/60W spec is pretty close to this at 12V. You would normally expect to see quite a bit above 12V at the lamps. The light output falls off pretty dramatically, as you can see, if you haven't got enough volts.

If, as in Steve's case, you have 14V at the Alternator and 10V at the Headlamps, which will be drawing about 4A each, then there is 32W being lost in the switches and wiring. Something is getting warm. A relay is essential for a high curent loads like this but is unlikely to be a cure all. The relay will remove the losses in the Headlamp Switch, Dip switch and possibly the Ignition switch but the wiring also needs to be up to the job as well.

Mac's suggestions is a good starting point using a direct cable as a first check. One final point that I found out the hard way in my youth on my other home assembled car was using cables that were too heavy. ( 60mm Starter cable and short 6mm cables to the lamps). With insufficient cable resistance there is little to limit the inrush current into a cold lamp which can be 10s of amps. This can cause premature lamp failure due to thermal shock and in my case persistent sticking of the 50A relay contacts. You can have too much of a good thing or is it too little?

Hmmmm, my careful formatting hasn't come out too well sorry.... The first column is Lamp volts next Dip Beam current and Watts, then Main beam Current and Watts then finally Light output measured at 1 foot distance

I really must get out more..
Denis