[Diy_efi] Jetronic - RFI

Torbjörn Forsman torbjorn.forsman
Thu Mar 2 19:54:27 UTC 2006

I think Volvo normally uses a Bosch ABS system, i haven't heard of any 
serious immunity problems of them before. But i know from other 
businesses that sparking from electric trains can be very nasty. 
Usually, the EMC regulations only require testing of RF immunity from 30 
or 80 MHz and upwards, but the railway sparking has most of its energy 
down around 1 MHz. At so low frequencies, only immunity to conducted 
interference is tested at the agencies.

A such old Bosch ABS system has not much diagnostic capabilities. It 
just switches everything off and illuminates the warning light when it 
thinks there is some malfunction. The test equipment for those systems 
consists in fact of just a break-out box with some switches and an 
integrated multimeter.
I have noted that there is place for decoupling capacitors on the sensor 
inputs to the controller, but they may be fitted in some controllers and 
not in others, obviously depending of the car manufacturer's ideas and 
As the inputs are differential and all wiring to the sensors should be 
either shielded or carefully twisted, the immunity would anyway be 
relatively good. But if the wiring is damaged somewhere, say that the 
outer sheathing of a shielded cable is damaged and the shield is shorted 
to ground, then the system's immunity can be completely destroyed. It 
would be a good idea to check (with disconnected controller) that all 
sensor signals have infinite resistance to ground. Also, check that the 
sensor windings have about the same resistance. Probably all of them 
should be about 1 kohm.
Similar ABS systems are used on many other european cars from the 80's, 
for example Audi, BMW, Opel and Mercedes-Benz. The controller is 
different for each model but peripheral components like sensors and 
valve blocks are standardized.
To prevent fitting the wrong controller to a car and get an ABS system 
that is "almost" working, the sensor signals are connected to different 
pins of various controllers. Although they may use the same PCB, jumpers 
are used to connect the right pin to the right input.

Best regards

Torbj?rn Forsman

Don Sauman wrote:
> Hi Torbj?rn
> On this same topic, I have 1987 Volvo 760 GLE that has a related 
> problem. While waiting at a railway crossing, an electric train passes, 
> and the brake pedal will vibrate and then relax with ABS warning light 
> coming on. The immediate fix is to, at an appropriate place, switch off 
> the ignition and restart.
> I think you have given me a clue as to a solution. Put the ABS 
> controller in a grounded box, and use some ferrite beads on the leads.
> Comment?
> Thankks
> Don
> Torbj?rn Forsman wrote:
>> LE2 and LU2 ECUs are notorious for bad RF immunity. In cars where the 
>> ECU is located in the passenger compartment, it is common that the 
>> engine dies when someone is using a cell phone or portable radio 
>> transmitter in the car. This is not surprising, due to the design of 
>> the ECU. A 2-side PCB with no ground plane and very little decoupling 
>> of incoming signals, and additionally a case with a plastic bottom.
>> On the other hand, due to the analog design, those ECUs do not create 
>> any nasty RF emissions.
>> Best regards
>> Torbj?rn Forsman

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