Check control lights

[cschollum]

Here's a check control problem I've never seen posted before:

The LCD panel in the instrument panel on the '90 535iM (for sale) has a problem. The display sometimes illuminates faintly, other times not at all. The characters on the display are active and can be seen (fasten belts, light on? etc) if I shine a torch on the display. The lamps themselves check out OK.

What module is responsible for activating the two CC light bulbs?

The SI display lights up OK on startup - it is part of the same LCD as the CC, odometer and auto trans indicator's functions. I'll double check the diagnostic mode later but I think it is dim then as well.

As the car is for sale, I don't want to spend $$ fixing but would prefer it worked as there's less for prospective buyers to gripe/negotiate.

Same problem as always isnt it? bulbs need replacing?

if the bulbs are ok and youre sure(would seem odd) as this is always the problem, perhaps they arent seated properly or maybe a faulty instrument cluster as its that that controls them, presuming that the speedo is illuminated ok - the actual signal comes from the dimmer switch located on your dashboard

as a side note, the lcd section atually plugs into the cluster, if you take it apart its a seperate module, and the bulbs are seated in a ribbon plastic material and if the bulbs have been replaced quite a few times then the ribbon wears away rendering the bulbs inoperative

[Anthony (M5 in Calgary)]

Same problem as always isnt it? bulbs need replacing?

if the bulbs are ok and youre sure(would seem odd) as this is always the problem, perhaps they arent seated properly or maybe a faulty instrument cluster as its that that controls them, presuming that the speedo is illuminated ok - the actual signal comes from the dimmer switch located on your dashboard

as a side note, the lcd section atually plugs into the cluster, if you take it apart its a seperate module, and the bulbs are seated in a ribbon plastic material and if the bulbs have been replaced quite a few times then the ribbon wears away rendering the bulbs inoperative

Lamps are fine. Conductive traces look fine. Instruments are otherwise illuminated fine. As the CC is switched from the dash lights, I'm thinking there's some kind of relay in the CCM that completes the circuit. For sure, the characters are visible and are intermittently dim or un-lit - never fully lit.

A head scratcher?

[cschollum]

Ive had a look at the electrical diagram and the only mention are the 3 speedo illumination bulbs which are controlled via the light switch, it either assumes the lcd bulbs are part of this circuit or they are controlled via another source, which as yet i see not mention of

How do you know the conductive traces are OK, the unit is a sealed unit, is yours different to mine? heres a pic

[cschollum]

Ive had a look at the electrical diagram and the only mention are the 3 speedo illumination bulbs which are controlled via the light switch, it either assumes the lcd bulbs are part of this circuit or they are controlled via another source, which as yet i see not mention of

How do you know the conductive traces are OK, the unit is a sealed unit, is yours different to mine? heres a pic

Ok ive had a look at how mine functions, and the display for the lcd gets iluminated regardless of if you have the lights on or not, when you start the car for a few seconds or if there is a message on the lcd - So my best guess is the instrument cluster illuminates the lcd upon reciept of a message from either the obc or check control or LKM - and seeing as you can see the message on the lcd when shining a torch on it, we know the signal is getting there - I would say the Instrument cluster is responsible for illuminating this, which is why the electrical diagrams dont list it.

Hope that helps

[Anthony (M5 in Calgary)]

Ive had a look at the electrical diagram and the only mention are the 3 speedo illumination bulbs which are controlled via the light switch, it either assumes the lcd bulbs are part of this circuit or they are controlled via another source, which as yet i see not mention of

How do you know the conductive traces are OK, the unit is a sealed unit, is yours different to mine? heres a pic

Mine is different from your picture - it has a PCB protrusion at the center top edge with a single in-line female connector which mates to the main instrument board.

The Check display is lit separately from the instrument lights and doesn't dim on my M5 - I'm assuming for now the 09/89 production instrument cluster is the later type, same as the 11/90 M5 I have.

As for the integrity of the conductive traces, I worked as an Electronics Technician for many years and have done many repairs on printed circuit boards and have a pretty good eye for damage. The traces are clearly visible once the lamps are removed.

[cschollum]

Mine is different from your picture - it has a PCB protrusion at the center top edge with a single in-line female connector which mates to the main instrument board.

The Check display is lit separately from the instrument lights and doesn't dim on my M5 - I'm assuming for now the 09/89 production instrument cluster is the later type, same as the 11/90 M5 I have.

As for the integrity of the conductive traces, I worked as an Electronics Technician for many years and have done many repairs on printed circuit boards and have a pretty good eye for damage. The traces are clearly visible once the lamps are removed.

if you look on your speedo, just above the lcd, you will either see a small round hole or a square one, behind it is an LDR, which detects light levels and adjusts the brightness of the lcd backlights, for example in full sunlight you get a bright display, at night it gets dimmed - so the instrument cluster is responsible for lighting the LCD

Heres a pic:

[Anthony (M5 in Calgary)]

if you look on your speedo, just above the lcd, you will either see a small round hole or a square one, behind it is an LDR, which detects light levels and adjusts the brightness of the lcd backlights, for example in full sunlight you get a bright display, at night it gets dimmed - so the instrument cluster is responsible for lighting the LCD

Heres a pic:

Thanks, but I already checked that photocell. The odometer lighting is fine and does vary with changes in ambient, also when tested with a torch.

[cschollum]

Thanks, but I already checked that photocell. The odometer lighting is fine and does vary with changes in ambient, also when tested with a torch.

try another instrument cluster, any will do 525i and above to test it