Headlight problem plus other questions

Headlight problem plus other questions

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Hamish400

Original Poster:

267 posts

221 months

Monday 22nd February
quotequote all
Car is 1989 built 400SE with dim/dip module fitted.
Rectangular headlights with H4 bulbs, sidelights as part of indicator light units and driving lights lower down.

1 Problem
Nearside headlight does not illuminate when main beam is selected.- both by "flash" switch or Dip/Main switch on steering column..
ie
When the lighting switch is set for "headlights" and the column switch set for "main beam";- the driving lights come on (both sides) but ONLY the offside headlamp main beam comes on

2
Following work OK:-
AA
when lighting switch (in centre dash panel) set for "sidelights" - Sidelights and "dim" headlights on (both nearside and offside).

BB
when lighting switch set for "headlights" and column switch set for "dip"
Sidelights on and Dip beam headlights on (both sides)

NOTE
NS Headlamp bulb is OK on both dip and main beam when removed and test connected direct to 12V battery
Swapping the relay for "Main beam" for another similar relay does not change any of the above.

3
I suspect there is a wiring fault somewhere for the NS main beam, but would appreciate thoughts of the experts on here.
Is there a "splice" in the main beam (blue/white coloured) wiring somewhere under the dash?

4
I have a copy of a 400/450 SE schematic wiring diagram that includes the dim/dip module.

5
I have not located the dim/dip relay - i have read that is in the area of the fuse/relay panel under the glove-box but I do not see it there.
Any ideas where it is located?
Does anyone have a photo of it?

6
I have forgotten what the diode at the bottom left of the fuse panel is for. Can someone please remind me?
If I recall correctly it is something to do with the headlight raising operation..

Best regards
Hamish

adam quantrill

11,264 posts

206 months

Monday 22nd February
quotequote all
Hi Hamish, according to the wiring diagram https://thumbsnap.com/f/zR3QcnKD the split off for the nearside/offside headlamp is near the headlamps.

If you have a fault on one side it's likely either where the wire splits off, or in the connector for that headlamp.

Quite often you can get corrosion if water gets into the wire so check the connector.

If you take out both bulbs you can check continuity between the two main filament connections and then trace it along the non-working one till you find low resistance, this will help you pinpoint the fault quickly. (I use a pin for this and poke it through the insulation, but others might disagree with this technique...)

Hamish400

Original Poster:

267 posts

221 months

Monday 22nd February
quotequote all
Hi Adam, thanks for your prompt response.
I will investigate the wiring layout through the engine bay to see where the left/right light headlight wiring splits. I cannot see much from above but should be able to get the car up in the air at a friends workshop later in the week to properly look from underneath.

Lesliehedley

203 posts

224 months

Monday 22nd February
quotequote all
Hi Hamish,
The dim/dip module on my car was tucked up out of the way behind the trim on the left of the passenger footwell, near where the wiring for the electric window goes through to the door. HTH

Hamish400

Original Poster:

267 posts

221 months

Tuesday 23rd February
quotequote all
I will have a look there for the dim/dip module.
Thanks for the info
Hamish

Hamish400

Original Poster:

267 posts

221 months

Tuesday 23rd February
quotequote all
I will have a look there for the dim/dip module.
Thanks for the info
Hamish

Hamish400

Original Poster:

267 posts

221 months

Wednesday 24th February
quotequote all
Near side Headlight fault found.
The Wiring harness runs from behind the dashboard forward within the near side wing structure and appears through a hole in the bulkhead behind the near side headlight, then runs across the front of the car to the offside light area.
At the harness behind the near side headlight there is a redish coloured 3 pin connector, (round pins).
The harness side of this connector forms the T junction for the near side and offside headlights - 3 pairs of wires going in to it from the main harness - blue/red for dip beam, blue/white for main beam and black for earth. Ie one of a pair is the supply from the dashboard, the other of a pair feeds across to the offside of the car.
Between the second part of this connector and the near side headlight unit there is abt 0.5 metre of cable which leads to the 3pin (spade) connection at the headlight unit.

The fault was -
A pin in the near side 3pin connector was dirty.
After cleaning the connector all worked OK.
Offsiide there is a similar arrangement of a 3 pin connector and wire to the headlight unit.

Did not get the chance to hunt for the dim/dip unit - job for another day. It works fine so I will leave it undisturbed for now.

Best regards
Hamish