[aprssig] BMW electrical systems

Joe Della Barba joe at dellabarba.com
Mon Jan 30 19:28:15 EST 2006

The CAN bus is on the new ones starting this year.
My BMW owner's manual states that radios in excess of 10 watts are not 
approved for use in/on the car. I can tell you that a 7 watt 2 meter 
APRS tracker had no ill effects on my car when I used it on a trip. If 
you start spamming the IBUS with RFI you might get all kinds of bizzar 
malfunctions. For example, I have a circuit to send IBUS info from the 
steering wheel buttons to an Ipod. I know of one of these that 
malfunctioned and the cars turn signals quit working along with the 
climate control until it was removed.
BMW 323iT

Jim Lux wrote:

> Not all BMWs use CAN bus.  Some use what's called the Ibus, which is a 
> multidrop serial bus, at, I think 9600 bps.  A bit of googling will 
> turn up all sorts of information about this (try, for instance, 
> HackTheIBus group on Yahoo) along with very nifty (and inexpensive) PC 
> interfaces so that you can feed your own video into the instrument 
> panel video screen, or display text on the dashboard display, or 
> respond to steeringwheel button presses.  (all it takes is software 
> <grin>)
> As far as BMW compatibility with RF... The manual (and BMW) explicitly 
> say: no way, we don't guarantee EMC, and if you kill the ECU or break 
> the car, tough beans.  However, they DO use BMWs for police cars in 
> Germany, and some of them presumably have radios in them.  In general, 
> all cars meet some sort of EMI/EMC requirements (nobody wants to have 
> their car stop working as you drive by the local radio station).  
> Might be worth tracking down the specs (EC-92 diretive 89/336/EEC? or 
> MIL-STD-464 or some SAE spec)
> http://www.emiguru.com/kgb/kgbv2n4.htm might be interesting reading.  
> It talks about test levels of 200 V/m.  They also mention that typical 
> auto manufacturer requirements are 60dB lower than FCC/VDE limits (no 
> birdies in the FM receiver, e.g.)  It calls out SAE J1113 
> (Electromagnetic Susceptibility Measurement Procedures for Vehicle 
> Components)
> Another article 
> (http://www.evaluationengineering.com/archive/articles/0305/0305using_radar.asp) 
> says that cars have to be immune to fields of 600 V/m in L and S band, 
> and cites GM test standard GMW3097 Rev 4, Dec 2003 and Ford 
> ES-XW7T-12A278-AC Oct 03.
> http://www.evaluationengineering.com/archive/articles/0304/0304emc.htm 
> might be even more interesting. Gives references to documents and 
> talks about general test methods, but no numerical limits.
> http://www.evaluationengineering.com/archive/articles/1203emc.htm 
> gives all the numbers of the specs and cross references, so if you 
> have access to a good library, it might be a good start.
> ------------
> So, here's the deal... I would think that if you keep the field below, 
> say, 100 V/m, you're probably in good shape (assuming you don't have 
> conducted emissions paths).  OET Bulletin 65b tells all about how to 
> figure out V/m for amateur stations, and, I would assume, since you've 
> all done your legally required RF safety surveys, you know what's 
> there, right?  The trick is in realizing that human safety allows time 
> averaging, while equipment compatibility deals with peaks.  No 
> allowance for the 20% duty factor for SSB, for instance.
> 3-30 MHz limit for E field is 1842/f V/m and 61.4 V/m from 30-300 MHz, 
> so if your car is far enough away to meet the limits for "100% keydown 
> carrier", you're probably good to go.
> Just for reference, Page 32 of the OET bulletin has calculations for a 
> 1/4 wave ground plane at 146 MHz, and indicates that at 10W, you'd 
> have to be 0.5 meters away to comply.  At 50 W, 1.1 m.
> For the HF op.. from a 0dBi antenna, running 100W on 20m, 0.7 meters. 
> (that's a field of 131 V/m)
> Good Luck..
> Jim, W6RMK
> > Dave wrote:
> >
> > A lot of modern cars do not have "Always on" power ports any more.  It
> > seems to depend on the philosophy of the individual maker.  The Honda
> > Accord I have now does not.  Other cars I've had in the last 5 years
> > have..
> > ================
> > You think that is bad...
> > BMW's single-wire, CAN-bus electrical system, which greatly reduces 
> the number of wires used in the main harness and dramatically reduces 
> weight and complexity.  Part of the CAN-bus is a new fully electronic 
> instrument cluster featuring Info-Flatscreen for additional system 
> information available at a glance.
> >
> > One would wonder if RF will have any affect on this new system...
> >
> > Regards,
> >
> > Clay AA3JY
> > (via WL2K)
> >__________________________
> I have heard that BMW has tested some cars in a high RF facility similar
> to what the military does.
> I have great faith in the CAN systen. But can not afford one.
> It has been used for over 10 years now. Some one should have heard some
> rumers by now.
> It is not a new system.
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