Finding Shorts on a distributed circuit.

For the last few months the instrument lights in the M3 have been flickering when I hit bumps. Eventually they completely failed. This is how I found the fault:

I assume basic electrical knowledge, which means you have a meter that can read resistance and voltage and that you understand what “continuity” and “shorts” are.

The first resource is the Electrical Trouble Shooting Manual (ETM in the vernacular, grab one for your BMW) Schematics 6300.0-00 through 6300.0-02 show the interior illumination circuitry.

When you look at the schematic for the instrument lights (6300.00) you see they are fed from the headlight switch into junction X196 to the dimmer switch into junction X1019 and out to all the lights (grey/red wires). At first I though I had an open circuit since I had no blown fuzes. Since EVERY light went out at the same time that narrowed it down to a few possibilities:

  1. the headlight switch failed
  2. the connection from the headlight switch to the dimmer failed
  3. the dimmer switch failed
  4. the supply from the dimmer to the splice had opened.

My money was on the dimmer switch since even before the lights failed I had to jimmy the dimmer to get the lights to come up to full brightness, it acted light a rheostat with a dead spot.

I pulled the dimmer switch and headlight switch and checked continuity in the wires, all was fine. The headlight switch had some small pieces rattling around inside, which was not a good sign. The switch continuities checked out, but I ordered a new switched anyway. I disassembled the old switch and found that 18 years of temperature cycles had turned the plastic detent ring inside the switch into a thousand pieces of brittle plastic. I still had hopes that the dimmer switch was too blame and I had no way of directly testing it. When the new switches arrived, I changed them out. Nothing, the problem was still there.

I had all the signs of an open (shorts generally blow fuses) but wire continuities all checked out. This was very confusing until John Firestone on the M3Forum clued me in that the dimmer switch has built in short circuit protection. So, my assumption that a short would blow fuses was wrong. I checked the output wire from the dimmer switch (grey/red) and sure enough it was shorted.

There are 22 individual wires shown in the schematic that could be shorted to the chassis (all the wires from junctions X1019 and X13129). These wires spread completely throughout the car and include two splice junctions. Since they are all spliced together a short on any one of them shows up on every other wire as if it were on that wire. Find a chafing wire by doing a hand over hand search would take weeks and pulling most of the interior out of the car. I needed to more narrowly isolate the fault. The junctions shown in the schematic (X1019 and X13129) are the keys to isolating the ground. In my car there is no junction X13129 since it is a coupe. That left me with junction X1019.

The idea is to get to the splice junction and disconnect all of the wires from the junction. Then you can check each individual wire for short to ground. That narrows the problem down significantly.

Sounds great, except that getting to the junction box is a MAJOR undertaking. Many thanks to John Firestone for pointing me to Jeremy Reyna’s Power Vent DIY which had some critical information.

  1. This requires removal of the drivers seat unless you have no spine.
  2. remove the drivers kick panel and under dashboard panel. The splice box is cliped to the outboard sheet metal above and outboard of the clutch pedal.
  3. First, you have to remove the check control module. Since it is directly above the splice panel it seems like you should pull the splice panel first. There is no clearance to do it in that order. The check control module is held on by two plastic nuts that can be removed using your fingers. The forward nut cannot be seen from beneath the dash, you have to go by feel. Once the nuts are removed you can work the CCM out from above the splice panel.
  4. Now the the CCM is out of the way you can reach above the splice panel and feel a locking tab that is on the top outboard side. With one hand above you can pull the tab, with the other hand you can push up and the splice panel will release.

* Installation is in reverse except you can put the far CCM nut on first since the CCM has a slot instead of a hole. But you must put the splice panel in before the CCM.

Once the the splice panel is accessible you can pop out the splice for the red/grey wires and start pulling the wire off of the fan connector. This is NOT easy, and I had to use needle nose pliers. It is easy to break wires, so think through how to do this in the constrained space.

Once the wires are all free check grounds on all of them. If you are luck you will have only one ground. You can hook the rest back up and recover most of your lighting, then you can hand-over-hand inspect the faulty wire.

In the end I had a very small short caused by chafing on the wire leading to the DC power receptacle light.


Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: