bonnet & trunk open lamps on dashboard always on | FerrariChat

bonnet & trunk open lamps on dashboard always on

Discussion in '456/550/575' started by franschman, Jan 16, 2021.

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  1. franschman

    franschman Formula Junior

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    #1 franschman, Jan 16, 2021
    Last edited: Jan 16, 2021
    Ever since I had my 456GT the warning lamps on the dash for open bonnet and open trunk lid are on. I'm trying to get these go out, but no luck yet.
    The switches themselves work, and I checked the wiring for shorts but can't see anything wrong.

    These two are the only warning light that show when driving.

    Any suggestions? Qavion perhaps?
     
  2. Qavion

    Qavion F1 World Champ
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    Sounds quite bizarre to have both lights on. The circuits should be independent on the 456. I can't figure out why these should illuminate at the same time... unless the previous owner simply let problems accumulate.

    Unfortunately, I just realised I don't have any full wiring diagrams relating to these warning lights for any of the V12's. The best I could do is try to piece together a diagram for the 456M, but there are probably too many differences between the 456 and 456M. The 456M has a multifunction display (or did they fit these to later model 456's?).

    Normally a short to ground will put on the lights, so there has to be some kind of short somewhere. Disconnecting the plugs on the switches simulates doors/lids closed, but since you've checked the switches and they are ok, then this won't help. Do the bonnet and trunk look like they are closing properly?

    Does the 456 have an alarm as standard? Assuming these switches are wired into an alarm system, I'm wondering if that is playing up. i.e. the alarm is generating an earth. But then you would have to ask why isn't the alarm going off :p
     
  3. franschman

    franschman Formula Junior

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    The lamps are on, even if I remove the switches altogether. That would indicate a short to ground in the wire from the dash to the bonnet/trunk switch. But I can't find any.

    I hadn't thought of the alarm system. My car was fitted with a very complicated aftermarket alarm system, which took me weeks to come to grips with (if armed, it shuts off ignition on one bank, and fuel injectors on the other bank). Whoever installed it, cut open many of the looms and changed quite a lot of wiring. Very difficult to fault find...
     
  4. Qavion

    Qavion F1 World Champ
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    Ugh. Sounds like that is the problem. It's the only common link between the two warning lights.
     
  5. Qavion

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    Does the alarm ECU have easy-to-disconnect electrical plugs? If it does and they are easy to access, can you disconnect the alarm ECU to see if it is not an internal ECU problem?

    The 456M bonnet circuit is

    Switch > plug in passenger footwell > splice > instrument panel plug.

    The splice also feeds a specific pin on the alarm ECU (via a plug in the centre console).
     
  6. Qavion

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    The trunk switch on the 456M has a similar setup (but the switch also activates the trunk lighting).

    How did you track the wiring for the switches when you were looking for shorts?

    I believe these plugs are for the instruments on a 456GTA

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    If the wire colours are the same on the 456 and the 456M, the trunk switch wire may be brown/white.
    The bonnet wire may be grey/brown or possibly brown-green, but I can't tell if any of those wires in the photo are these colours.
     
  7. franschman

    franschman Formula Junior

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    With both these under-dash connectors, as well as the switch for the bonnet, disconnected, One of the wires to the bonnet switch is hard to ground. The other measures about 480 Ohm resistance between the wire (measured at the switch end) and the car chassis. This is with the wire 'open' at both ends (?).
    Both wires look black at the switch end but it's difficult to see as there's a sleeve around them very close to the terminals.

    When extending this 'open' 480 Ohm wire aft into the cabin, to see which of the connecor pins it is, I measure zero resistance on both the brown-green, as well as on the white-yellow wire (?). Both of these are on the 10-pin connector. On the other pins, I measure either an infinite resistance (as expected) or this 480 Ohm again. Apparently there's a connection to other wires somewhere down the loom.

    The loom seems to cross to the other (passenger) side of the car, but so far on that side I have only found one brown-green wire and it is connected to main relay panel. Can't sewe where it goes as it disappears into the main loom , but that must be a different wire...
     
  8. franschman

    franschman Formula Junior

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    Can someone with a 456 please confirm:
    If you open the trunk, the trunk light comes on. Now if you push the switch (located in the lid, next to the catch), does the light go out immediately?
    Many thanks!

    Bart
     
  9. Qavion

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    LOL.... White-yellow is typically the output from the speedometer to the Motronics ECUs and power steering module. I have no idea what would happen to these systems if you got a short on the speedo wire. Probably a CEL at least. Maybe heavy power steering at all speeds?

    I guess the only way to move forward is check the wire colours on the trunk lid switch. I imagine that won't be easy to access. Also, the nearest connector probably has 4 wires like the 456M, so it may be hard to identify the wires related to the switch.

    It's interesting that a potential bonnet wire (at the instrument plug) is the colour of the trunk wire on a 456M.

    Why? Is there a delay on your car?

    Just curious... with all the plugs connected, if you simulate a closed trunk (and the trunk light eventually goes out), does it come back on again when you open the bonnet?
     
  10. Qavion

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    #10 Qavion, Jan 17, 2021
    Last edited: Jan 17, 2021
    If you haven't reconnected all the plugs, could you repeat the 480 ohm check with the "Tacho" plug on the power steering module disconnected? It's in the passenger footwell, I believe, above the relay panel. It's not guaranteed, but you may see a resistance change if that white-yellow wire is the speed signal.
     
  11. Qavion

    Qavion F1 World Champ
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    Item #1 if you didn't know:

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    Ignore the red text (I borrowed someone else's diagram)

    Not important. Just in case we run out of ideas.

    Do you have a wire tracker? They aren't good for following wires in big bundles, but may be useful in smaller harnesses and for following simple circuits like these.
     
  12. franschman

    franschman Formula Junior

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    It seems the trunk light is on permanently. I can't make it go out by pressing the switch.
     
  13. Qavion

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    Curiouser and curiouser...

    Not only is there a short on the trunk lamp/switch circuit, but, apparently, a short between that and the bonnet circuit. Does the trunk light go out when you activate the trunk switch (closed) and close the bonnet?
     
  14. Qavion

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    Here's the trunk switch/light circuit on the 456M, although I don't know if the switch has separate contacts for the trunk light and instrument panel trunk warning light. The electric trunk lock release wiring also runs through that 27Q plug.


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    I only drew one half of each connector out of laziness (hope it doesn't cause more confusion). I don't know if there are any diodes in the circuit.

    Earth 22Q is also an earth for the high stop light, the left tail lights and one of the automatic gearbox computers.

    If the trunk light switch is ok and the trunk light is always on, there has to be a short on that brown/green (MV) wire (or whatever colour it is on the 456 non-M). Note that the alarm ECU is in that circuit. The alarm system on your car is still high on my possible cause list.
     
  15. franschman

    franschman Formula Junior

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    It explaines why there was no bulb in the trunk light when I bought the car...

    It gets stranger still.
    After a long and unsatifying day of faultfinding I'm back to square one.
    The 480 Ohm was a resistance in one of the connectors. After fiddling with all of them the resistance has disappeared and I can't reproduce it. Both wires to the bonnet switch now have zero resistance to ground with the dash connectors disconnected. When disconnected from the switch, and dash connectors in, there's still no resistance to ground. With ignition on, there's no 12V on either of the wires. Seems like the whole system is bypassed.
    Wire colors to the bonnet switch, by the way, are black and grey/brown.

    I also fiddled with the alarm system. With the doors locked, alarm armed, I measure no voltage on the bonnet switch wire. Also, releasing the switch (simulating opening the bonnet) doesn't trigger the alarm. Same with the boot.

    I have to remove the trunk padding to see where the light gets it's power from.
     
  16. Qavion

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    Sounds like our cars had the same previous owners :p Lots of dodgy maintenance.

    Ugh... very frustrating.

    By "zero resistance to ground", do you mean zero ohms?

    So they are the same as the 456M. So perhaps at a disconnect plug, the grey/brown wire changes colour before it reaches the instrument panel plug.
     
  17. Qavion

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  18. asgor

    asgor Formula Junior

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    The trunk light in the 456GT (most likely 456M as well) is activated by a ball-type electrical switch right next to the trunk locking mechanism on the lid. It's the one with the two white blade connectors kind of sticking out. When the lid is shut the ball rolls away, opens the circuit and the light shuts off. Conversely, when the lid is open the ball rolls to close the circuit and the light comes on.

    To access it there is no need to remove the lid locking mechanism: you can remove the switch in question on its own: it's attached to the lid with a single screw.
     
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  19. Qavion

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    Ah, so activating the latch won't trigger it? I was working on the assumption that it was latch-activated or proximity activated rather than by trunk lid angle.

    I see the part number is shared by the 456, 456M, 550 and 575

    i.e. 173977

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    Bart, how did you check this switch?
     
  20. Qavion

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  21. franschman

    franschman Formula Junior

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    #21 franschman, Jan 22, 2021
    Last edited: Jan 22, 2021
    Finally had some time to look into this strange issue again, but didn't get far.
    My car doesn't have a ball switch (that I could find) but rather a traditional plunjer type break contact. One wire grey/brown, the other black. Both wires measure 0 Ohm resistance to the chassis. You'd expect the grey/brown to have a resistance to chassis of some 72 Ohm (2W lamp bulb) and +12V?

    The lamp fitting is powered 12V continuously on one terminal, as per the diagram, but I can find no switch in the '-'.

    This part of the car doesn't look like it's been messed with. For a change...


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  22. Qavion

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    So there’s definitely a short to earth on the grey/brown wire. I guess now it’s a matter of breaking up the circuit into sections, but I don’t think that would be easy.

    The baffling thing is why both lights are illuminated.

    Do any of the non-M cars have a digital multifunction display?
     
  23. franschman

    franschman Formula Junior

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    Apparently both brey/brown wires (front and rear) are connected to earth somewhere. AND the black wire of the trunk light is also connected to earth. Whether it should be earthed permanently (switch in the plus) or the other way around, as usual, nobody knows.
    I will just have to follow the wires.
     
  24. Qavion

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    If you disconnect the instrument panel plugs, do you still get a short to ground on the grey/brown wire at the switch? (Switch pushed)

    If there is a single fault, it would have to be near the instrument panel where the wires for the front and rear are closest to each other.


    Black wires are usually directly connected to earth.
     
  25. franschman

    franschman Formula Junior

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    I dug into this again and this time got a bit wiser.
    Firstly, Qavion, your diagram in post 15 was not correct for the 456 NON-M in that the switch and lock are completely independently wired and not connected in the aft part of the car. The lock and switch run forward along the left side of the car, the lamp along the right side. See below diagram for wire colors. The red/black comes from the second wire from the right, on the bottom row of connectors on the main relays panel.

    trunk.jpg

    Then I took a deep breath and unraveled part of the wires going into the alarm, which is a Meta M99T2. If anyone has a wiring diagram for that I would be most grateful! All wires that come out of the alarm unit are black, to add to the confusion. This black is sort of shrink sleeve that can be peeled off and the real color of the wire is exposed.

    thumbnail_IMG_8108.jpg

    While digging, I found a bundle of 5 brownish wires all soldered together and connected to the purple wire of the alarm. According below purple is for alarm on the trunk and bonnet, so I was warm here.

    general wiring diagram.png

    I cut all these wires loose and found following:
    - a brown/white wire which has a resistance to the chassis of about 1,5 Ohm.
    - a brown/green wire, which connected to the chassis makes the trunk light go on.
    - another brown/green wire, which connected to the chassis makes the trunk warning light go out.
    - a brown/grey wire which comes from the bonnet switch
    - a brown wire, which connected to chassis makes the bonnet warning light go on.

    So I connected the brown/grey and brown wires and the warning light comes on as intended.
    I haven't found the brown wire from the trunk switch, that triggers the light and warning light.
    I have no clue what the brown/white wire is for.

    The purple wire on the alarm is connected to ground if the alarm is not armed. That made the trunk/bonnet warning lamps come on. Additionally, a 'false' ground was obtained from the brown/white wire.
    Now that I know the intention of the purple wire, I'll see if I can make it work as intended, rather than just soldering all brown wires together.

    I had promised myself not to touch the alarm system until all other systems work as intended, but I guess here I have to give in.
     

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