The car normally supplies 12 volts to the warning lights with the igniton on. Maybe you're just measuring the voltage across the lamps. It's the display (or your voltmeter) which is providing the earth/ground to turn on the warning lamps. If you earthed those pins with a wire, the lights should turn on.
Oh okay that makes sense. I've just measured the voltage. I haven't tried turning them on with a seperate supply or ground.
Or I could be mistaken.... the other blank slots are called "free". Anyway, looking at the wiring diagram, the reverse button only talks to the TCU. The TCU would then send a signal on that dataline to change the display.
Any idea what tests I could perform on the display unit itself? If I measure resistance between pin 5 and 2 it's about 68k~Ω.
Unfortunately, that's no guarantee of serviceability. Did you have an identical fault before you bought the panel?
Yes I had the same fault with the old display. My old display unit had a tiny bit of corrosion on a resistor and another component. I was able to get those components from an electronics store and soldered and replaced them but still had the same error. So I ordered a new display unit and it's still the same. Also I didn't buy the entire instrument panel. 1200£ was just for the display control unit+ water temp.
By any chance do you have access to a 355?? So that you could just check the voltage on pin 5 of the display.
No the display plug is connected and ignition on. With display plug disconnected and ignition on I get continuity between pins 6,7,8 and pin 55 of TCU with TCU disconnected. With ignition off pins 6,7,8 get continuity with ground.
With display plug disconnected and ignition on there is 12v on 6,7,8 which are the 3 warning lamps auto, low friction and gearbox. When plugged and ignition on there is 12v only in 6 and 7. The gearbox warning lamp is illuminated so as you said there should be a ground contact in that pin to illuminate the lamp. But the weird thing is I don't get any continuity between pin 8(which no longer has 12v because the lamp is illuminated) and chassis ground or even pin 3 which is ground for the display unit.
I don't think I would be making continuity checks with power on the car. You don't want 12 volts (or whatever) going to your ohmmeter. With the ignition on, you would get ignition power on pin 55 of the TCU and pin 1 of the display. I don't know what your ohmmeter would do when measuring across two sources of power (the other source of power being the power on the other side of the warning lamps). When you say "continuity", what is the value? Is it in the low ohms range or just a couple of milliohms? Sometimes when I set my ohmmeter to beep if there is continuity, sometimes it's triggered by a few ohms (not a true short).
So you're measuring the voltage across the lamps (being supplied by the ignition)? Image Unavailable, Please Login What F1 warning lamps normally illuminate when you turn on the ignition, prior to start? Does the red one illuminate briefly?
So I opened up the new display as well to check it out. Found a few things that might or might not be the problem. Image Unavailable, Please Login Image Unavailable, Please Login Image Unavailable, Please Login
The auto warning, low friction and gearbox fault warning lamp illuminate when I turn on the ignition. Auto and low friction turn off after a few seconds but the gearbox fault lamp stays on.
I looked at the datasheet for that LM2903, but I'm not sure what it does. https://www.diodes.com/assets/Datasheets/LM2901-03.pdf In some references, it's called a "comparator", but in others a low current power supply (capable of producing a stable voltage from a wide range of input voltages). " 1. Short-circuits from the output to V CC+ can cause excessive heating and possible destruction. The maximum output current is approximately 20 mA, independent of the magnitude of V CC+ ." Image Unavailable, Please Login The 4949ED also appears to be a voltage regulator.
The input and output pins are right beside each other. Could be that one of the output pins is somehow connected or related to pin5. Obviously this like firing a full auto in the dark but let's see. I'll scrub them down gently with a soft brush and isopropyl and let's see if it makes a difference.
Unfortunately no luck even after cleaning it. The display unit itself is supplying the 9-10v to pin 5 which is causing the error. Now I don't know if it is because of some other reason or because it's not getting a certain signal which is causing this.
Hey @taz355 maybe you could check this out if you have the F1? Would just like to know the voltage, if any, at pin5 of the display connector.
