This page covers my experience installing an HID
kit into my E34 M5, particularly the problem of the On Board Computer (OBC)
displaying a "low beam" failure message despite the fact that the
HID low beams were functioning fine. Please maximize your browser screen to
make this page easier to view. I'm not a web guy, and doing this made it easier
to view on my monitor.
USELESS WIRING HARNESS & RELAYS SUPPLIED WITH HID KIT
I bought the HID kit from a seller on eBay, new in box, though there was no brand name on the box. The seller said he'd bought several sets from a car accessory shop gone out of business, and he was kind enough to deliver it to me in person since he wasn't far away. The kit included genuine Philips ballasts and bulbs, and also a wiring harness with it's own 12 volt power leads, relays and 15 amp fuses. The wiring harness was not manufactured by Philips, but by a manufacturer unknown to the seller. The car's low beam female leads would plug into the harness' male connectors, and the harness had female plugs of the same style as the factory plugs that would connect to the HID ballasts. The harness included two independently fused 12Volt N/O (normally open) relays, which would be triggered by the 12 volt current when the lights are turned one, thereby switching the relay to the CLOSED position and causing power from the separate 12 Volt leads, described below, to flow to the HID ballasts.
The + power lead had been attached to the terminal below the + jump start access terminal under the hood. Since the battery on the M5 is under the rear seat, BMW provided this beefy terminal for a jumper cable. Two main + leads are connected beneath the jump start access terminal, one lead coming from the alternator, the other lead going to the battery. The + lead of the kit actually had two red hot wires attached to it, with each hot wire going to its own relay. One hot lead and relay for each low beam. The - lead for the kit was properly grounded to the body, on the inside of the fender well, in front of the strut tower on the driver side.
I followed the installation diagram but got no light from the HIDs, as well as a "low beam" failure message on the OBC. I asked the seller if he could help and he offered to come over in the evening and have a look. While he was there, I was checking the direction of current flow through the harness to figure out what exactly was going on in the harness and relays. I found out that the connectors which the car's low beam leads plug into were installed backwards. The positive lead of the low beam harness was connected to the negative of the harness. I removed incorrectly installed connectors on the harness, and reattached them in the correct orientation, then plugged my low beam leads back into the harness connectors. The HIDs now lit up, but I still had the "low beam" message coming from the OBC.
The seller offered to supply me with another style of harness, actually two of them, one per side, where there would be no independent fused power leads. Basically, this alternative style harness consists of a male connector which accepts power from the car's low beam female plug, sends it through a 15 amp fuse and a coil relay, then on to a female plug which would connect to the ballast. All power for the HIDs would thus come from the car's low beam leads. If this was acceptable, I wondered, why is any harness necessary at all? What could a simple coil relay be doing? My engineer neighbor came over and had a look at this alternative harness and couldn't see how it could possibly do anything. My guess is the manufacturers of these kits are trying to add value, when really, the only thing of value in the kits are the ballasts and bulbs, which are most commonly made by Philips.
KIT INSTALLED CORRECTLY, BUT "LOW BEAM" DISPLAYED ON OBC: FIRST THOUGHTS
My first thought was that I was getting the "low beam" message because the HID kit was pulling the power from the lights from it's own + and - terminals, rather than the low beam leads. I thought that the Light Kontrol Module (LKM) and/or On Board Computer (OBC) recognized that when the low beams were turned on, there was an absence of current draw (since the HID harness pulls the power from an alternate source) and therefore concluded that the low beam bulbs were blown.
Garrick Louie asked me to check whether the low beam message came on immediate the key was turned to the on position, with the headlights turned off, and indeed the OBC displayed the "low beam" message. He also asked if I turned on the headlight switch first, then turn the key to the on position, whether the low beam message appeared, and indeed there was no "low beam" message. It became clear that the LKM/OBC was not checking for the current draw from the low beams. Therefore, the system wouldn't care if the power to the HIDs would come from another source.
What the system does do, is check the low beam bulb filaments immediately the key is turned to the on position. It can do this by either two methods. First, the system may simply check continuity in the low beam lead, where the circuit is completed by the bulb filament. If the bulb filament were broken, or the bulb missing, the circuit would be incomplete and the LKM/OBC would recognize that the low beam is not functioning. The alternative method would be that the system may be looking for more than just continuity, but checking for a particular resistance of the bulb filament.
The most important distinction between a halogen bulb and an HID bulb is that the HID bulb has no filament, very much like a florescent bulb in your garage workshop. The import in that when the low beams are turned off, the system thinks the bulbs are blown because there is no continuity or resistance. However, where the HID bulb is ignited, there is continuity as the electricity is passed through the Xenon gas in the bulb. There's no hard wire, as there is with a filament, but the ignition of the xenon gas has the same effect as an ordinary florescent bulb does, which is to effectively complete the circuit between the positive and negative leads. This is why, when I switch on the lights before turning the key to the on position, there will be no low beam message. As soon as the key is turned to the on position, immediately the ballasts ignite the HID bulbs so that there is continuity and the system therefore recognizes that the low beams are functioning so no "low beam" message is displayed on the OBC.
ELIMINATING THE "LOW BEAM" MESSAGE ON THE OBC
If you were to decide to drive with your HIDs on at all times, switching on your low beams before even turning the key to the on position, you would never have to see the "low beam" message. However, this is not reasonable, and something has to be done to eliminate the "low beam" message when you are not using the headlights. The HID low beams have no filament, so the system will always think the bulbs are burned out when the HIDs are not turned on, so how do we fix this problem?
The solution is to trick the LKM / OBC by making the system check the high beam bulbs (normal halogens with filaments) while it thinks it is checking the low beams. The high beams are normal halogen bulbs with filaments to complete the circuit the system is checking for, and perhaps with the correct resistance in case the system is also looking for that.
The solution is simple, yet brilliant. Garrick Louie came up with it and shared it with me, and I am very grateful to him for this. The solution involves using a 12 Volt N/C (normally closed) relay, wired such that when the lights are turned off, the low beam + lead is shorted to the high beam + lead through the N/C circuit of the relay. Therefore, when the lights are off, the LKM/OBC will find the connection (and perhaps resistance) it is looking for via the high beam halogen filament. Remember that the HID has no filament, so when it is not on, the + lead going to the HID bulb is effectively disconnected.
When the lights are switched on, the 12 volt current passing through the low beam lead triggers the relay to switch to the OPEN position, breaking the connection to the + lead of the high beam so that they LKM / OBC system can only "see" the low beam. Simultaneously, the HID bulbs fire up, so that system recognizes a complete circuit on the low beams.
You will need to replace the fuses for the both of low beam bulbs (one fuse for each low beam) in your fuse box. The standard fuse is a 7.5 amp, but I changed them to 15 amp with original BMW fuses from the dealer.
MODIFICATION TO ONE SIDE ONLY
I cannot explain why, but the procedure described on this page only required me to hook up the relay on the passenger side headlight assembly. I did not install any relay or any special wiring on the driver side headlight assembly. This may be only on E34 cars, or perhaps only on E34 and E32 cars, or only on my 1991 E34 M5.
Maybe there is a good reason for this, but I can't figure it out. When I had the halogen low beam bulbs installed, and removed either the driver side or passenger side low beam bulb, the OBC shows the "low beam" message. Therefore, I would imagine that I should have needed to install the relay setup on the driver side also, since the system knows when one bulb is burned out or missing. However, I checked the function of the HIDs after installing just the passenger side relay setup, just to see what would happen before I cut up the driver side too, and - surprise! The OBC was satisfied. I undid the relay setup on the passenger side, installed the halogen bulbs, and the system worked as normal. To double check, if I removed one of the low beam halogen bulbs, or just the lead to one of them, the OBC displays "low beam". This suggests that did not damage the system.
Again, I can not explain the above, but I'm not complaining. Bottom line? Start with the passenger side, and if that works for your car, don't bother wiring the relay to the driver side.