Couple of electrical gremlins

[Cloggy Saint]

First up, the brake light warning LED on the check control stays on even after the pedal has been pressed. Both brake lights are working. Secondly, the speedo and econometer keep dropping out. I'm pretty sure it's not the sensor on the diff' as if I give the dash a couple of whacks it comes back to life. Any ideas, Brian ?

[Brianmoooore]

If thumping the dash affects the speedo, check the soldering of the pins that connect the speedo. assembly to the main cluster circuit board. If these are OK, checkk the soldering of the white and blue sockets to the board - these joints aren't weak, but careless attempts at removal of the two plugs can crack any soldering.
Both brake lights may be working, but are they at equal and full brightness? Poor connections at the bulb, out of spec. cheap bulbs, and a poor connection in the earth path of the bulbs can cause the check light to stay on.

[Cloggy Saint]

Cheers Brian. It may be coincidence/my imagination but the brake check light seems to play up when the weather is hot, not just this time but previously as well ...

[Brianmoooore]

E30 bulb check relays are operated by the cold 'inrush' current of the filament, rather than the current that the bulb takes when it's hot. This current will be much higher than the 'lit' current, and will be slightly affected by the ambient temperature. It will be marginally less in hot weather, and, in conjunction with the real problem, might just tip it below the threshold that turns the LED off.
Poor connections could also slightly vary in resistance with temperature.
Someone on here recently cured his similar problem by adding earth wires from the bulb holders to an earth point in the boot. It would have been better if he had made temporary links to prove the point, and then sussed out exactly where the poor connection was, but it solved the problem.

[ChrisHC]

E30 bulb check relays are operated by the cold 'inrush' current of the filament, rather than the current that the bulb takes when it's hot.
Someone on here recently cured his similar problem by adding earth wires from the bulb holders to an earth point in the boot. It would have been better if he had made temporary links to prove the point, and then sussed out exactly where the poor connection was, but it solved the problem.

That was me, and had the additional earth not been effective I would have removed it and looked elsewhere, but I had already attempted to clean up the plug-in connectors at the inboard end of the lighting box, but these are not easy to get to and it had not worked. As other people reported the same problem it seemed to be something of an inherent weakness and when the possibly temporary measure proved effective I saw no reason to remove it, particularly as it was all hidden away behind the boot lining and duplicating the existing wiring was not really a bodge.
I only wish I could find a way to stop similarly false warnings from the brake pad wear indicator! That is the only one of the many electrical problems that came with the car still outstanding, everything else is sorted out with the invaluable help available through this wonderful forum.

[Brianmoooore]

I only wish I could find a way to stop similarly false warnings from the brake pad wear indicator!

Cracked soldered joints on the big 220 ohm resistor inside the instrument cluster.
Wire to sensor(s) rubbing on the inside of the wheel rim.
Poor connection at the sensor plugs.
Poor connection a the plug an socket under the rear seat cushion. This plug and socket ( same one as fuel pump, etc.) is still present on cars with drum rear brakes, where the socket is fitted with a loop of wire to short out the sensor pins.

[paultv]

I got so bored trying to sort my pad wear sensors that I took out the bulb - is was permanently very slightly lit, invisible in the day, annoying in the night - I don't need sensors to tell me when the pads are worn anyway, to be honest.

Paul

[ChrisHC]

I only wish I could find a way to stop similarly false warnings from the brake pad wear indicator!

Cracked soldered joints on the big 220 ohm resistor inside the instrument cluster.
Wire to sensor(s) rubbing on the inside of the wheel rim.
Poor connection at the sensor plugs.
Poor connection a the plug an socket under the rear seat cushion. This plug and socket ( same one as fuel pump, etc.) is still present on cars with drum rear brakes, where the socket is fitted with a loop of wire to short out the sensor pins.

I examined the soldering of the resistor which appeared fine but I applied a soldering iron and melted the solder just in case. A local garage who worked on the car for me said that they had replaced what they described as the looms to the sensors and used tie wraps on the sensor plugs to ensure they were kept tight. The plug and socket under the rear seat is a useful new area to check, I certainly would like to remove the black tape currently over the warning lamp, and an effective warning is clearly a useful feature to have working.

[Brianmoooore]

The bulb is controlled by a BC322 PNP transistor, which has a 4.7k resistor connected between its base and 0V, trying to turn the transistor on and lighting the bulb.
The 220 ohm resistor and the pad sensors (just simple loops of wire) are connected in series and between the base of the transistor and 12V+, holding it and the bulb off.
If the wire of the loop is earthed, by the disc cutting into a sensor, the hold-off voltage on the transistor's base is removed by effectively shorting the 4.7k resistor, and the bulb turns on.
If the wire of the loop is broken, by being cut through by the disc, and is no longer earthed, the voltage on the transistor's base looses the connection to 12V+ via the 220 ohm resistor, and so allows the 4.7k resistor to turn the transistor and bulb on.
For a bulb that constantly glows very dimly, I think I'd look at replacing the transistor, which might have become 'leaky'.
There's a diode connected in series with the transistor's emitter, but I don't know whether it's a plain silicon diode or a zener type. Assuming it's a plain silicon type, then the resistance of the sensor loop will have to rise to an amount that brings the base of the transistor to around 1.2V below the 12V+ supply.
This is around 10% of the total, so the same ratio applies to the difference between the hold on and hold off resistors. Hold on is 4.7k, so hold off will need to be around 470 ohms. This 470 ohms includes the 220 ohm resistor, so around 250 ohms maximum for the sensor loop is required to keep the bulb off.

[Cloggy Saint]

Finally got around to taking the cluster out, and to my untrained eye the solder looks to be in good condition.



Likewise for the points where the blue and white plugs make connection
I'll give it some contact spray and hope for the best, don't really know what else to do.

[Stefan on probation]

Check with a magnifying glass as the solder can sometimes "crack" or lift. You can also check it with a meter.

[Cloggy Saint]

This is really pi$$ing me off now, I can see the next problem will be a broken dash where I keep smacking it to get the speedo to work. Assuming the solder at the connection is ok and the plug is fitted correctly what else could it be?

[Stefan on probation]

https://www.r3vlimited.com/board/showth ... p?t=129004

May help.

[Cloggy Saint]

Cheers, that seems helpful, although all that electrical talk made my head hurt.