[Diy_efi] Jetronic - RFI - ABS gotchas

[Lee M. Lemoine]

In my subaru, the proportioning takes place in the ABS unit, which is stand
alone from the master cylinder, and the exact same unit you see on that "
Howstuffworks.com" link.   I can take pictures if you'd like.  Either way, I
can tell you that ABS and non-ABS brake pads are no different...   Brakes
will continue to be brakes, in fact, the only difference between '88 and '90
and '91-94 (in j-body years) was the slightly larger diameter brake rotor,
which was curing the 'warped brake rotor' issues on the V6 RS and Z24
models.  (its .45 inch larger diameter, and 1/4" thicker)   Either way, the
interesting part is that the calipers interchange -- the most effective
braking for that car comes from a slotted (smaller) rotor with the (lerger)
caliper and pad on them.  I have done this swap without incident on my car,
and ABS still functioned...  (until i melted the ABS unit with the radiant
heat from the turbocharger / exhaust piping)

Yes, the REAR-only ABS systems use a different modulation setup...  its more
just a 'dump it if it locks' mechanism...  not very high tech...    Some
GMC3500 trucks built in the mid 90's had this.  I can recall a '96
particularly, with a 454, and REAR only ABS.  Had a regular master
cylinder... not sure where all the recirculation stuff was..  I no longer
work for that company, otherwise i'd go take some pics..



On 3/6/06, Tom Visel <five10man at commspeed.net> wrote:
>
> 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.
>
> TomV
>
> 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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--
Sincerely,

Lee M. Lemoine
http://www.turbochargedsoul.com/
'06 Subaru Legacy GT Limited - 250/250 AWD 5EAT
'93 Chevy Cavalier Z24 Convertible - 300/330 FWD 5MT (Mclaren Turbo!)
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