Thank you. This explains why the all-too common wiring loom issue on the 500 causes some folks so much grief, with reports of multiple seemingly random failures of items (most notably and expensively the airbag ECU) that wouldn't normally be associated with any of the wiring that passes into the rear hatch.
They may be ahead of the world in circuit design, but then they design a rear hatch where inflexible cables are repeatedly bent through a sharp angle, resulting in this:
I didn't say they were perfect, and like every electronic circuit in the world, resistors get brittle, capacitors dry out and the circuit, in normal use, last about 10 years (or they used to). In cars it is a hostile place for electronics where they are exposed to high extremes of tempreture change as well as constant vibration and cables being squashed through the ground plate.
In what you identify about the 500 issue with wires to the rear hatch, Fiat seems to have the same mistake a lot of other car makers make - squashing too many wires through a hole and presuming the rubber grommet will protect them.
To those people who notices a fault with this (i.e. a fault in one or more electrical devices in the car like uncontrolled action on a low speed canbus connected device like windows / central locking / steering position sensor / radio / lights / dash control etc). The high speed canbus is reserved for the engine management, the low speed for auxilary components. Since they all act as transmitter or reciever, all they do is add / read signals to/from the canbus system and behave accordingly. This means that an electric component from a different model fiat should, if the canbus and power cables are connected correctly, behave as if it was made for the car - i.e. a mk3 dash (speedo, clock, computer, lights) is only a display canbus device so it should work in a MK1 or any other Fiat vehicle - but first check the wiring ID codes and values etc to prove it before plugging it in. In theory, you could connect unlimited dash controls to the canbus, and they would all behave - as all they are designed to do is read the canbus signal and display the result.
To fix the issues created by squash / damaged cable similar to the 500 :-
- First disconnect battery and leave it for 2 hours minimum. Good idea is to remove the battery and leave it for a day. You do not want any of the canbus system to retain any charge.
- Since we are going to be working on the canbus system, disconnect the two plugs that plug into the engine management computer. This is sensitive to static electricity that can come from a mains powered soldering iron.
- Then, with a key (central locking / electronic boot release will not work) open the boot and extract the connection similar to the photo above.
- Using an old cloth and a bit of tape OR some masking tape, tape all the wires together.
- Using a fine metal file, file the edges of the hole to get rid of the sharp edges. These are what cut / damaged the cable - so it is useless repairing cable that will recut again.
- Once the hole is smooth (use your finger carefully to feel it), and if it is any way rough, re file.
- Now, for the cables, there will at least be 1 cable being exposed and shorting to the chassis plane and Murphys law it is one of the CANBUS low speed cables which is being grounded. That equates in the canbus signals being shorted and devices that require to 'know' when another device is operating / finished fail.
- If only insulation is frayed and there is no loose wire strands, a piece of cloth insulation tape (similar stuff that they used to use wrapping the handlebars of the old 'racer' drop handlebars. It is called cloth wrapping tape but is also basic insulating tape with a flock / cloth outer.) can be used to reinsulate the wire. If the wire is frayed, then the thickness at that point is reduced. As a result, you will need to cut that wire, strip it back to expose a bit of cable. Use heat shrink rubber on one wire and push it up high enough to be out of the way for now.
- Twist the exposed copper wire together, and place flux on it. Flux is required in auto cables to avoid any possibility of dry joints.
- Now solder the wires.
- When cold, wipe away excess flux and slide down the rubber heatshrink cable ensuring all exposed copper wire is covered. The shrink the heatshrink.
- Before reconnecting anything else, wrap the cable you just soldered in cloth tape. Now you can reinstate the wire, and the grommet in the hole. If it seems loose, that is OK. You may need some rubber cement to bond the grommet back in since the retaining hole is now bigger (since you filed it) This is the intention - it allows the cables to work without being squashed. It also prevents the problem reoccuring since the edges that damaged the cable in the first place is now smooth.
- Once you are sure everything is back and plugged in, you can reinstate the battery. Do not forget to plug in the engine management computer first as if you power up the system without the engine management computer, the body computer will fail to start correctly as it is looking for a signal (clock signal) from the engine management computer. This is another smart move, using the engine management computer to manage timing as it is getting a timing pulse from the camshaft / crankshaft position sensor - so when the engine runs faster, the CANBUS speeds (low & high) change in proportion. This is from an old design that allows items such as wipers (CANBUS Low)to sense the speed of the engine / wheels and adjust the wiper speed dynamically.
CANBUS system is a smart design and since most devices are on CANBUS Low or High, adding functionality / display on any device is relatively easy as compared to the old individual device wiring. It also links all devices to each other. The airbag ECU is connected to CANBUS low, like wipers, rear demister etc, so there is a connection between the cables at the aforementioned grommet and the airbag.
Common problem - CANBUS - is with electric window motors on Renault due to breaking connection in the door connector. It causes the window regulator to 'forget' where top and bottom of the travel is. As a result, on press of open / close of the window, the window does not close/open all the way but instead just takes one step for each press of the button. The expensive fix is to replace the motor / regulator. The cheap fix (can correct fix) is to reprogram the window of top and bottom (step window to top, hold up button for 10 sec. step to bottom, hold down button for ten sec. Press up and window goes all up. Press down and window goes all down. Full solution is to clean the door connector plug and reconnect it correctly.
I have yet to see this issue on FIAT, but with CANBUS controlling all devices, it won't be long before this is an issue.
On some Alfa's, the triptronic gearbox control (steering wheel and/or gear leever) fails due to the same plug socket connection. I know of 2 Alfa 156 triptronic systems that 'locked' which forced the gearbox into 1st while the car was travelling close to 100mph and destroyed the gearbox.
The core issue is that the multi-pin plugs are 'double lock'. i.e., when you are plugging them back in, they click twice. This is to allow the rubber seal to completely seal the contacts away from the air / dampness. A few mechanics when working on a car will disconnect these connectors and forget to double click them back together.
I had an Alfa 156 (triptronic issue), a VW Passat (airbag and electronic brake issue), a Renault 19TXE (starting issue), a Mazda 121 (over reving issue) --- all down to these multiplugs not double clicked properely. Vibrations would cause the connections to seperate which caused the previous multiple issues. Once the plug was clicked together correctly, every electronic problem went away.
P.S. Thanks for the comments, and I will be around for some time yet. BTW, I would guess that the thin black cable is CANBUS (don't have 500 wiring colours to confirm) and the thick black cable is the power to the rear demister. Those two shorting to chassis with the demistor on blows the demister fuse but also sends high current down a data cable to every device in the car before the fuse blows. Those shorting together will kill most of the car electronics.
