[Diy_efi] Jetronic - RFI - ABS gotchas

Tom Visel five10man
Mon Mar 6 16:46:11 UTC 2006

Brakes in general don't care about being pulsed, whether by foot or by 
black box.

Pumps are only used on the higher-end systems, the kind that read and 
control all four wheels.  The rear-wheel-only variety tends to use a 
pressure accumulator and isolation and dump valves to control wheel 
pressure.  A rear-ABS equipped vehicle may have a master cylinder with 
more stroke available on its rear-brakes-feeding portion, but many such 
vehicles get the same MCs for ABS and non-ABS.  Most ABS stops just 
don't use that much fluid, unless you drive on ice all day, so the 
easier-to-fit systems don't need a pump.

One gotcha is sensitivity to rust.  One tiny bit of the stuff, perched 
next to the dump solenoid, will give a soft pedal AFTER and ABS stop.  
Since the effect is so far divorced from the cause, one may never know 
the damage one did by simply squeezing nasty brake fluid back up through 
the system when relining one's brakes.  When cars/trucks with this sort 
of difficulty come into my shop, it is a hard sell, trying to explain 
the problem to the customer, unless they have a bit of hydraulic knowledge.


Ian Molton wrote:

> Lee M. Lemoine wrote:
>> The unit that i have is a combination valve unit, master cylinder, 
>> and proportioner.    No, not all the time, but when the motor is 
>> running it will draw enough to make your headlights dim ;)
> Ok, so implementing ABS actually looks to be relatively simple.
> Where are the gotchas ?
> Things Im thinking of include:
> 1) Will non ABS disc brakes put up with being pulsed like ABS ones?
> 2) Evidently the 'path of least resistance' method for my install 
> would be a system where the pump and valve were a single unit, but fed 
> from a seperate master cylinder. The question this leads me to is one 
> of pressures. Do most small car brake systems use pressures within a 
> reasonably similar range, or am I going to be hunting down parts with 
> particular specs? furthermore, is it possible to adjust things like 
> proportioning on ABS setups, or is it hardwired in the ABS computer ?
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