Dipped beams not coming on

[ihopethatsnotrust]

Hello all,

Before I delve into this I've noticed my dipped beams have stopped coming on when I switch them on/sometimes come on then turn off.

First action is to check the headlight relay?

[Bugjam1999]

Check the fuses first, then investigate the relay.

Cheers

[ihopethatsnotrust]

• Fuses are working ✔️
• Main beam relay clicks and works correctly ✔️
• Dipped beam relay clicks when headlight switch moved to 3rd position✔️

So now I am confused...
I've attached 3 pictures that show my current headlight status

Position 1 has dipped beams on and side lights bright


Position 2 has dipped beams on and side lights dim


Position 3 has dipped beams on and side lights dim


Bad ground? What to check next?

[Othen]

Let's think about this.

The headlamps (neither main nor dipped beams) should not be illuminated in positions I or II. In position I the DRL (bright sidelight bulbs) only should illuminate, in position II only the parking light (dim sidelight bulbs) should be on, so clearly there is an issue on the supply side (this can't be an earth return issue - there should not be an power getting to the headlamp bulbs in positions I and II).

There are a couple of easy checks you could do to help narrow down the fault:

a. Do the DRLs and parking lamps operate properly at the rear of the motor car in positions I, II and III?

b. Is it possible to switch between dipped and main beams in positions I and II, or is it just dipped beam that operates incorrectly in those two regions?

There would seem to be 3 possible scenarios that might produce the effects you describe:

a. The switch on the dash has failed allowing the relay to power up in all 3 positions. It it is quite robust (1970s type design) and comes out really easily (just one nut under plastic knob if I recall) and you could test it with a multi-meter (in a low volts range) in 10 minutes.



b. There is a short in the wiring somewhere between the switch and relay such that some 12v supply is powering up the control side of the relay. I slightly doubt that is the case because one might expect the odd fuse (in the stray supply) to blow from time to time. Let's hope that isn't the issue at the mo because it might be a bugger to find.

c. The relay has failed such that it is allowing power direct from the distribution box - the rail that sits next the relay:



to the headlamps without being switched by the relay. This is where test (b) above might be useful (if it only affects dipped beam this might be more likely). This seems a little unlikely in that it is a permanent live to the battery - so the headlamps would probably stay on with the ignition switched off, but it isn't impossible.

Of the scenarios I'd say the most likely is the first: that the switch is allowing the control side of the relay to be energised in all 3 positions. Fortunately that is easy to access and check with a meter, so I'd suggest doing that first.

The lighting control circuit is really simple on 240s, one just needs to be methodical and eliminate things until whatever is left must be the fault (paraphrasing Sherlock Holmes I think). These graphics should enable you to find the issue, first the headlamp wiring cct (this is for a MY1981 US motorcar, but the principle is the same):



... and the dipped beam cct:



... as you may see, it is pretty simple.

Good fortune, let us know how you get on.



PS. There is a 4th scenario that could manifest itself in the way you describe: a short circuit between a switched live and the supply somewhere between the relay and the dipped beam headlamp bulbs such that they operate permanently with the ignition switched on. Test (b) above might help here. That area is fairly accessible and there aren't so many wires.

[ihopethatsnotrust]

Thanks for the amazing reply, I've taken the headlight switch out before so I'll check that next, I'm also hoping its not a short somewhere that wouldn't be fun to find.

Also great suggestion of checking main beams in position 1 and checking the rear lights too. I'll look at these soon and hopefully that'll be the issue found!

[Othen]

Originally Posted by ihopethatsnotrust

Thanks for the amazing reply, I've taken the headlight switch out before so I'll check that next, I'm also hoping its not a short somewhere that wouldn't be fun to find.

Also great suggestion of checking main beams in position 1 and checking the rear lights too. I'll look at these soon and hopefully that'll be the issue found!

This shouldn’t be too hard to fix, just be methodical.

I’ve just noticed from your initial post that you said the dipped beams were not working, but in the photos they clearly are. Does that mean you did something to switch them on all the time, or is this an intermittent fault such that sometimes they are on all the time, and other times they don’t come on at all? If it is an intermittent fault then it is less likely to be the switch or the relay (both tend to either work or not, nothing in between), and more likely to be a short between some switched live in the two areas I spoke of above. If that is the case Id start looking at the front of the car between the relay and the headlamps first because it is easiest to access.

A thought: if you disconnect the relay (just pull off the connector block) and the headlamp stays on you will know for sure it is a spurious short between the relay and the headlamps. If the lamps go off with the relay disconnected, but come on when you reconnect it then you will know the fault is at or before the relay.

Just be methodical, run as many diagnostic tests (like I suggested) as you can to eliminate issues before you start taking things apart. Write down the results (otherwise you will forget) and logically work out what is wrong, it is a pretty simple system.

Good fortune,

[ihopethatsnotrust]

Yes when I first noticed the issue it was dark and sometimes the dipped beam would come on and then sometimes come off but it's settled down to being on all the time now.

Just checked and the rear lights are on in every position, consistent with the front.
The main beam only stays on when head lights are in position 3.
Haven't had time to check the switch but I will get round to that soon
Thank you for your help

[Othen]

Originally Posted by ihopethatsnotrust

Yes when I first noticed the issue it was dark and sometimes the dipped beam would come on and then sometimes come off but it's settled down to being on all the time now.

Just checked and the rear lights are on in every position, consistent with the front.
The main beam only stays on when head lights are in position 3.
Haven't had time to check the switch but I will get round to that soon
Thank you for your help

Ah, that is interesting (The main beam only stays on when head lights are in position 3). So the relay is still doing more or less what it is supposed to do, and probably isn't getting powered up in positions I and II. That narrows things down a bit.

There would appear to be two likely scenarios:

a. The relay is faulty such that the power side of it is staying switched on all the time, independently of the control side. This seems unlikely because it still switches in position III.

b. There is a short from a 12v switched source somewhere between the relay and the dipped beam bulbs. This would seem more likely, perhaps a cable to the DRL, parking lights, horn (the horn probably isn't ignition switched, so maybe discount that)... other stuff at the front of the engine bay?

I'd suggest trying the isolate the relay test I suggested above; if the dipped beam stays on then that will confirm there is some other source powering up the lights. It should not take long to identify the issue with your multi-meter.

[Othen]

Attached is a better diagram I found in the BofH.

[Othen]

Originally Posted by Othen

Attached is a better diagram I found in the BofH.

I had half an hour to spare this afternoon, so I had a look at the problem, here is a bigger copy of the same diagram:



... as you may see, it is trivially simple.

So far we know:

a. The dipped beams stay on in all 3 positions of switch B.

b. The high/dipped beam relay (A in the diagram) works correctly in position III (only), switching between high and dipped beam.

c. We know A is a latching relay, we may see that it is powered permanently via the red wire from the busbar to terminal A/15 and is actuated by the toggling the indicator stalk (C) via A/31b and then to earth return (thus the headlamp flash will work with the ignition turned off without actuating the latching relay).

The dipped beam bulbs should only power up when:

a. The ignition is switch (J) is on (I think positions I and II) and...

d. The lighting switch (B) is switched to position III (only) such that B/56 powers the control side of relay G (I think this is normally clipped to the bulkhead in the engine compartment by the wiper motor) via G/86.

e. This powers up A/56, the path to A56b (and so to the dipped beam bulbs) is NC, so they illuminate.

The trouble is the dipped beam lights illuminate all the time the ignition is switched on, so either:

f. A/56b is becoming energised when it shouldn't. This could happen if there is a short circuit between a switched live somewhere between the 5 pin relay at G/87 and terminal A/56... or

g. there is a short circuit from a switched live to the blue wire from the latching relay (via the bulb failure switch - which I think we may ignore for the purposes of this discussion) to the bulbs at D.

Fortunately these two area (between G and A, and then between A and the headlamps are fairly accessible (well, better than under the dash). In the light of the evidence I'm fairly confident the OP should be looking for a short circuit between a switched live and either the yellow wire from G/87 and A/56, or the blue wire A/56a to the dipped beam bulbs.

I hope this helps OP, let us know how you get on.