A SHORT INTRODUCTION
The 3 most common problems with the Marks ABS system (forgetting
about brake pad wear, warped rotors etc. which can happen to any vehicle)
occur under the hood.
- ABS pressure switch failure due to contaminated brake fluid (water)
- ABS relay
- Accumulator
If you have a hard pedal, either all the time or intermittently,
chances are that you also have the ABS and Brake warning lights on as well. Associated
with the hard pedal, you generally only have front brakes operating. And
looong stopping
distances. If only the red brake light is on and brakes operate normally,
check for low fluid, sticking parking brake, or faulty parking brake switch.
The pressure switch is located under the master cylinder, inboard,
tilting up at about a 30° angle, six inches or so in front of firewall. There
is a connector on the end of the switch. Beside it and towards the
firewall is the ABS pump motor, and on the outboard side of the master
cylinder is the accumulator, which is charged by the ABS pump to supply
the "power assist" (2000 to 2600 PSI, I think). The accumulator
looks like a sphere. Hard to miss. These 3 assemblies are mounted
to their own assembly, said assembly being attached to the firewall with
one 6mm hex bolt under the accumulator, and a flex line that runs from
the inboard side of the master cylinder (midway along barrel which is under
the fluid reservoir). The flex line runs under the master and attaches
to the accumulator assembly described above with a "banjo fitting."
The ABS relay is on the firewall, above and slightly inboard
of the master and is one of two relays underneath a plastic cover that
is open at the bottom. The ABS relay is the forward one.
Change this out first to see if the problem corrects itself. If not,
just to make sure ABS switch is at fault, break the cover off of relay
so that you can close contacts manually. You will hear the pump run
(kind of a "buzz" sound). The relay is an $8 or so part
at the dealer.
REMOVAL
If you are going to remove any of the hydraulic parts, you
must ensure the brake system is not under any pressure. The pressure
is released by pumping the brake pedal 20 times or so with the key
off.
You will need a
17/16"
thin-wall plumbers socket to remove the ABS pressure switch. These
sockets are sold for example at Home Depot , usually in a package of 5
or so for about $14.00 - apparently they're used to get at fittings within
walls. Anyway, you remove the connector on the end of the switch,
place socket on, and turn. In most cases, the switch comes out, you
install the new one (tighten to 25 ft lbs or so) and you are all set. However,
sometimes, (and it has happened to me) you just cannot get the switch to
turn. Couple of things to try:
- Remove master cylinder reservoir.
- Drain as much fluid from it as you can first with a siphon hose, "turkey
baster"
etc.
Note: there are 2 lines attached to the reservoir, one at rear
inboard and one at front.
Lines are fabric covered neoprene. Just pull them off.
- In front of the reservoir there is a 4mm hex bolt through a tab that
attaches the reservoir to the master cylinder barrel.
- Remove bolt and with a flat blade screwdriver, carefully
pry reservoir straight up and off.
Hopefully the extra room will let you work to better get at
the ABS pressure switch. If you're still unable to remove it, take
off the accumulator (8mm hex wrench) - it may have some fluid in it. If
more than a couple of tablespoons, the accumulator is beginning to fail
and will at some time require replacement.
With the accumulator off, you will see how the assembly is
attached to firewall with one 6mm hex bolt. There is a spacer between
firewall and assembly which prevents the assembly from being hard up against
the firewall. This is so that the vibration from the ABS pump motor
is not heard or felt in the car. Before undoing that 6mm bolt, undo
the steel flex line that was described earlier.
Undo it at the master. Then remove 6mm bolt, slide assembly
inboard and down to clear master cylinder. It is really easy. You will
see that there is a pin attached to master and a hole in the assembly you
are now holding in your hands and away from your car. That is what holds
it on in addition to that 6mm bolt.
Be sure not to lose the spacer from between the assembly and
firewall. Be careful too that brake fluid does not get onto paint
nor
does any debris enter the exposed opening. At this point, set
the assembly in a vice, and if you're lucky, you can undo the switch. If
the ABS pump motor is in the way at this point, remove the 2 bolts and
pull motor away - there will be brake fluid, a round
"O" ring and a screen filter. Motor also engages a slot
in the pump drive.
This sounds more complicated than it really is! Just go slowly and
carefully and you will do A-OK.
REPLACEMENT
Once you have removed the old switch and installed the new
one, reassemble the pump motor to the assembly, slide the assembly from
inboard under the master (assembly will hang on the inboard pin), attach
to the firewall with the 6mm bolt and spacer and you are almost done. Reattach
steel brake line to master cylinder, attach connector to pressure switch,
reattach accumulator (there is an "O" ring at base of sphere),
and tighten sphere to 25 ft lbs or so. I would suggest that you clean out
the reservoir, too. If it's really cruddy, use hot soapy water and
brushes to scrub inside of it (or whatever works), and finally, rinse off
with some brake fluid to displace any water.
Reattach the reservoir to master, lubricating the 2 nipples
under reservoir with brake fluid first.
Attach the 2 neoprene hoses and reinstall 4mm hex bolt. If you detached
the wiring harness at filler cap or at inboard front of master, reattach
them also. You should have at least a gallon of the best DOT 3 or
Dot 4 brake fluid you can find - NOT SYNTHETIC. Fill reservoir.
At this point you will need another pair of hands - preferably attached
to a live person besides yourself!
- Turn on the key. You will hear the pump start and after 30 seconds or
so, the lights described earlier will go out and now you can bleed the
system
- Start with the rears first. With key still on, open a bleeder screw (left
or right side) and just hold brake pedal down about half way. Fluid
will stream out due to the pump pressure.
- Let it run for 15 seconds or so, close bleeder, release pedal, and refill
the master reservoir fluid out of the master cylinder itself as well as
the front to rear brake line.
You want to do this until the fluid color indicates you are
now getting clean (as in new) fluid.
- Go to the other rear caliper and it should flush out in just a few seconds.
Again, key is on, pedal about half way down. Rears are done when
fluid from second caliper is clear.
- Check and refill reservoir.
- With key still on, loosen a front bleeder. What I generally do is hold
my finger over the end of bleeder screw, and have the pedal pumped up and
down (almost to floor), wait about a second between pumps, and do this
until fluid runs clear.
- Repeat for the other side.
- Rinse all of the fluid away with water. If you wish, you can put a hose
over the bleeder, put the other end of hose in a jar or something that
has a bit of fluid to cover end of hose and pump away until no more bubbles
exit hose.
Install wheels and go for a test drive. Hit the brakes hard
enough to engage the ABS anti-skid. If brakes perform as expected, the pedal
will pulse as the car slows down. If the amber light glows and then goes
off, you have a weak accumulator. Replace it. No need to re-bleed the
system. In fact, a "healthy" accumulator should allow you to pump the
pedal at least 3 times before you hear the pump turn on for a few seconds.
Listen carefully, you will be able to hear it.
I hope this answers your questions. To do everything described
above might take as long as 3 hours or as little as 20 minutes. If you
are able to remove the switch without taking the entire assembly out, you
will have a decent brake pedal without bleeding brakes. Bleed 'em anyway
and do so at least every 2 years. Water is the enemy of any brake system.
Dallas Reagh / LASVEGASLSC
