[aprssig] BMW electrical systems

Jim Lux jimlux at earthlink.net
Mon Jan 30 14:17:53 EST 2006

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 
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.

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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