GM / Workhorse Auto Park Brake System
Notes written and compiled by “OldUsedBear”
Auto Park Chronology
This is an attempt to document (however crudely) the evolution of the Auto Park
system as built and utilized by Chevrolet and Workhorse. The object is to draw
on the collective knowledge of all our forum members regarding the different
versions that have been made. To date, the information we have is based on
various communications with members, as well as a limited number of documents -
- excerpts from a few manuals and/or service bulletins, and very importantly,
the digital pictures that members have sent us. We expect they’re to be
substantial gaps and errors, and in some cases only partial truths in what
little we have gathered so far.
Hopefully, the members can help to fill in the blanks. Information from manuals
as well as personal experiences will be very much welcomed. There is no
expectation that we will get everything perfect - - we're just looking to
supplement the present library of info. Anecdotal experience (what you learned
under your own rig) is very often the best kind.
So - - Follows our chronology - - We fully expect/hope to get lots of
corrections and additions. Even assumptions and guesses may be valuable.
Especially, we would like to hear from people who have various pieces of
documentation they would be willing to share. Anyone with something to
contribute may feel free to PM me and I will gladly exchange email addresses
with them to facilitate the transfer of larger amounts of information. Posts to
this thread (hopefully there will be some), will of course also be catalogued
and included in the editing process.
1989 thru 1995 - - This version (versions?) is characterized by using the power
steering system as a source of hydraulic pressure. There is no separate
reservoir or pump to operate the Auto Park. The system is almost entirely hydraulic
with some rather sophisticated valves doing the job that is later done with
switches and solenoids. We think that all of these units utilize a separate
foot operated brake pedal that applies the same driveline brake that the Auto
Park applies. Our only documentation on this version is a bulletin (or excerpt
from a service manual), which is about 13 pages. It is labeled "5C" -
- "Parking Brakes" All of the pages are labeled like 5C-1, 5C-2 and
so on. We also have some very good digital photos of the different valve
assemblies, the relay lever assembly, the actuator and other components.
A footnote to the above: We now have 3 documented cases of a
"hybrid" version built around 1991. These units do NOT have a foot-applied
brake, but DO have the PARK position on the shift lever, as well as the yellow
push/pull knob on the dashboard - - Just like most the stuff built after about
1998. These hybrids DO run off of the power steering pump like the
typical 1989 to 1995 versions.
The 1989 date is very soft - - there may well be earlier versions and we will
welcome additions and corrections.
1996 thru 1998 - - Both these dates are soft. A big problem with even trying to
establish these time frames is that (as an example), if you say something is a
'96 model, it may very likely have been manufactured in '95. We've never had
any info as to cutoff dates etc. that the manufacturer may have imposed. So at
best, some of these years turn out to be transition periods - - the date thing
just has to be an approximation and best effort. Anyhow, this version still
operates on the same principle as the first one, but it has a dedicated
electrically driven pump and a separate reservoir to power the Auto Park. These
units also have a foot operated parking brake pedal, which applies the same
driveline drum brake that is used on the earlier versions. We have no written
documentation specifically covering this period. The components seem to be a
mix of the earlier units and the '98-99 onward stuff in use today. We could
surely use bulletins, excerpts, hands on or whatever info from those who own,
or have owned or worked on these versions.
1999 thru present. I've read, but not recorded, information about when
Workhorse took over from Chevrolet. Think it was '88 or '89. Also I don't know
what was carried over in terms of design or hardware - - just know there was a
transition. These systems are similar to the previous version in that they
utilize a separate pump and reservoir. The big change however, is that they no
longer incorporate a foot applied (mechanical) means of applying the driveline
brake. Instead, they are equipped with a yellow push-pull button on the
dashboard. This button actuates the parking brake in the same fashion, as does
the gearshift lever when placed in PARK. In 2001, the pump/reservoir was
relocated to the front driver's side from its former location near the frame
rail behind the tranny on the passenger side. We have a CD service manual,
which covers much of this period, but it has some (we believe) errors in it. We
also have a small portion of a 2001 manual which appears to be more accurate
but we only have the first couple of pages - - trying to get the rest as this
is written.
So - - All comments, corrections and additions will be very welcome. We also
will do our best to share whatever we presently have (or get in the future)
with forum members. Just PM me to get the process started. Thanks to all for
past and future contributions.
Auto Park
Warning Lights
Around 1995, the Auto Park system was modified such that it no
longer used the power steering pump as a source of operating pressure.
The new version was fitted with a separate hydraulic pump/motor assembly, which
runs at considerably higher pressure than the power steering/hydroboost system.
The basic principles of operation however, remain much the same
The PARK position on the shift lever is continued, as is the foot
park brake pedal. With this version, the parking brake could
be applied by either putting the shift lever into PARK, or by depressing the
foot pedal. Either one will, by itself, apply the brake. Both must
be released for the vehicle to roll. Both these mechanisms apply the same
drum brake, which is on the driveline right behind the tranny.
There is an indicator light on the dashboard that is
associated with the parking brake system, and it is referred to as the
"Auto Park Warning Light." NOTE: There is also, another light
that indicates when the mechanical foot brake has been applied, but this light
is not related to the Auto Park circuit or configuration. As such, it has
no particular relationship to either the operation or troubleshooting of the Auto
Park system.
Lifted (loosely) from the manual: "The Auto Park
lamp goes on when the parking brake is applied, or whenever the pump motor is
running.
This warning light has two separate switches, which can cause the
light to go ON. One is the pump motor switch - a.k.a. the Rotten Green
Switch - and the other one is a pressure switch mounted on the actuator.
The Rotten Green Switch (RGS) turns on the Auto Park light at the same time it
turns on the pump motor.
The pressure switch on the actuator, closes if the pressure in the
actuator drops below about 450 psi. It is worth noting, that at 450psi,
the park brake will probably still be NOT APPLIED. When the park
brake is applied with the shift lever, the pressure drops to nothing for
practical purposes - - thus turning on the Auto Park "Warning" lamp.
As noted tho, if the pressure was below 450psi for SOME OTHER REASON, the
lamp would also come on and serve as a warning that something is amiss.
Examination of the possible scenarios that might fit "SOME
OTHER REASON," are potentially interesting. Going down the road,
with no leaks in the hydraulic system, one would expect the pressure to be near
the 1600 psi and holding. The AutoPark lamp should be OFF. With
very mild internal "seepage," one could imagine that the pressure
might slowly drop until it dips below the 1200 psi point, at which time the
pump would briefly cycle to build back up to 1600 psi. This
brief cycle would have the AP lamp ON for just the time the pump was running.
The point we pursue here is - - Under what circumstances, would the park brake
low pressure switch (aka pressure indicator switch) turn on the AP warning lamp
while going down the road? In most scenarios, the RGS would have already
turned on the pump AND the AP lamp back around 1200 psi - - LONG BEFORE the
indicator switch got a chance to kick in around 450 psi. ANSWER:
RGS failure where the switch FAILS to turn on at, or below 1200 psi.
So this AutoPark Warning lite can be a useful diagnostic aid.
It should be ON when the shift lever is put into PARK - -
because the low pressure switch on the actuator senses low or no pressure.
It should be ON briefly when the pump runs as you move OUT OF PARK with the
gear shift - - because the pump/motor is running to build pressure. It
may infrequently go ON to signify that the pump is cycling for
pressure maintenance. If it is ON under other situations, or even
flickers (like going down the road, at a stop sign etc.), that could
be an indication of a SERIOUS PROBLEM. Driving the vehicle with these
symptoms runs the considerable risk of a lock up, or the park brake failing to
hold on an incline. Analyzing and addressing these symptoms is best done
on a case by case basis. Suffice to say these are INDICATORS OF POTENTIALLY
DANGEROUS CONDITIONS.
In summary, this lite is useful to indicate brake operating
status, as well as to be an indicator of malfunction or potential
malfunction. Except for infrequent pressure maintenance cycling,
the AutoPark lite should not go on while the vehicle is rolling. If
the lite fails to meet the above conditions, further examination is indicated.
On the later versions of AutoPark, about 1998 and later, the foot
applied park brake disappeared and a yellow push/pull knob on the
dashboard was added. The PARK position on the shift lever remained.
Within this series, the parking brake can be applied by either
putting the shift lever into PARK, or by pulling the yellow knob. Either
one will by itself, apply the brake. Both must be released for the
vehicle to roll. Both these switching mechanisms apply the same drum
brake which is on the driveline right behind the tranny.
There are two indicator lites on the dashboard that are associated
with this parking brake system. One of them is the AutoPark Warning
Lamp, the other is just called the Brake Lamp. Not a particularly clear
distinction, but all the literature we have lists them as such. As
always, a variation would not be surprising.
Lifted (loosely) from the manual: "The AutoPark
lamp goes on when the parking brake is applied, or whenever the pump motor is
running.
In the case of the AutoPark lite (referred to as a WARNING lite),
there are two separate switches which can cause this lite to go ON. One
is the pump motor switch - a.k.a. the Rotten Green Switch, a.k.a. the pressure
maintenance switch, and the other one is a pressure switch mounted on the
actuator. The Rotten Green Switch (RGS) turns on the AutoPark lite at the
same time it turns on the pump motor.
The pressure switch on the actuator, closes if the pressure in the
actuator drops below about 450 psi. It is worth noting, that at 450psi,
the park brake will probably still be NOT APPLIED. When the park brake is
applied with either the shift lever or the yellow knob, the pressure drops to
nothing for practical purposes - - thus turning on the AutoPark
"Warning" lamp. As noted tho, if the pressure was below 450psi
for SOME OTHER REASON, the lamp would also come on and serve as a warning
that something is amiss.
Examination of the possible scenarios that might fit "SOME
OTHER REASON," are potentially interesting. Going down the road,
with no leaks in the hydraulic system, one would expect the pressure to be near
the 1600 psi and holding. The AutoPark lamp should be OFF. With
very mild internal "seepage," one could imagine that the pressure
might slowly drop until it dips below the 1200 psi point, at which time the
pump would briefly cycle to build back up to 1600 psi. This
brief cycle would have the AP lamp ON for just the time the pump was running.
The point we pursue here is - - Under what circumstances, would the park brake
low pressure switch (aka pressure indicator switch) turn on the AP warning lamp
while going down the road? In most scenarios, the RGS would have already
turned on the pump AND the AP lamp back around 1200 psi - - LONG BEFORE the
indicator switch got a chance to kick in around 450 psi. ANSWER:
RGS failure where the switch FAILS to turn on at, or below 1200 psi.
So this AutoPark Warning lite can be a useful diagnostic aid.
It should be ON with application of the shift lever (put in PARK) or the
yellow knob PULLED. It should be ON briefly when the pump runs as you
move OUT OF PARK with either the gear shift or the yellow knob. It would
also go on for a few seconds if/when the pump cycles for routine pressure
maintenance. If it is ON under other situations, or even flickers (like
going down the road, at a stop sign etc.), that could be an
indication of a potentially SERIOUS PROBLEM. Driving the vehicle
with these symptoms runs the considerable risk of a lock up, or the park brake
failing to hold on an incline. Analyzing and addressing these symptoms is
best done on a case by case basis. Suffice to say these are INDICATORS OF POTENTIALLY
DANGEROUS CONDITIONS.
The BRAKE lite is also triggered by two different conditions.
It should go ON when the yellow knob is pulled out - - telling the operator
that the parking brake is ON even tho the shift lever may be OUT OF AUTOPARK
POSITION. It should also go ON if the actuator is in overtravel condition
- - indicating need for parking brake adjustment in most cases.
In summary, both these lites are useful to indicate brake
operating status, as well as to be indicators of malfunction or potential
malfunction. Except for infrequent pressure maintenance cycling,
the AutoPark lite should not go on while the vehicle is rolling. The
Brake lite, should never be on while the vehicle is rolling. If either of
these lites fail to meet the above conditions, further examination is
indicated.
The Rotten Green Switch
This is a sort of "canned" explanation of the Rotten
Green Switch malfunction. We may send you more specific info dealing with
your particular problem once we get into the troubleshooting process a little
farther.
The Rotten Green Switch (RGS for short) is actually called the
"pump motor switch" by Chevvy and Workhorse. It has some bright
green plastic on it, and is by far (maybe 85% of the time) the most frequently
failing component of the AutoPark system with units starting from about 1996
thru present.
Frequently, it fails in stages. Initially, the hydraulic oil
may leak only into the workings of the switch, causing it to malfunction
electronically. Later, as the "disease" progresses, the oil
starts leaking externally - - leading to an empty or low reservoir.
Symptoms may be feeling some drag by the brake while underway, or even total
lockup of the brake. Can happen while rolling or parked. Depending
on the severity of the malfunction, it may happen rather intermittently at
first, and may not be noticed. Then over time, it gets worse until
the fluid is all gone. The problems may also very likely cause the
dashboard warning lites to come on or to flicker on and off.
In the intermittent case, if the brake goes on and off
while travelling, the brake shoes may get worn down to where the AutoPark
doesn't hold the coach from rolling. Sudden and severe leaks are more
likely to cause lockup.
Versions from about 1996 thru 1998, will probably have a foot
applied parking brake along with the AutoPark. Later versions have no
foot pedal, and instead have a yellow knob on the dashboard which can apply the
parking brake. While there are technical differences between these
systems, RGS failure does about the same thing in both cases.
Again, this is sort of a general explanation of the RGS
malfunction. There may well be more, or different considerations in your
particular case. This is for background purposes to help start the
troubleshooting process.
All versions of AutoPark that we know of, have some sort of device
which senses the position of the gear shift selector on the steering column.
Earlier models (about 1989 thru 1994), accomplish this function with a somewhat
sophisticated valve. Later versions (about 1995 thru present) utilize a
switch which in turn, actuates a solenoid - - the combination of which does
just about the same thing that the valve does.
On the earlier style, there is a pretty straightforward mechanical
linkage between the column shift tube (rod?) and the valve. On the later
versions there is a cam on the shift tube which actuates a switch. While
the mechanical configuration of these two devices is quite different, both
share the purpose of "telling the system" when the shift lever is precisely
in the PARK position. This is a critical adjustment - - You don't want
the park brake actuated ANYWHERE except in the PARK position. Similarly,
when the lever IS in PARK, you want the brake to be ON.
It seems that maladjustment of the switch, is more common than
maladjustment of the valve - - nature of the beasts I guess. Failure
of either component, is more rare than improper adjustment.
Switch failure indicates replacement. Valve failure however,
can frequently be cured thru installation of a "rebuild kit."
We have a source for this. The same people will also either rebuild
your valve for you, or sell you a rebuilt - - you get a refund on the core I
believe. New valves have been reported to be scarce as well as expensive.
The service literature we have on these subjects is of little
particular value for those of us who like to do our own repairs. We do
have however, some pretty good digital pictures of both the valve mechanism,
and the switch mechanism - - They can be a big help in identifying the
components. The switch mechanism is reportedly a booger to reach - - sort
of back in a hole. The pictures show this. Nonetheless, most people
have been able to make satisfactory adjustment or repairs to this assembly.
The valve mechanism is more easily reached and worked on we think.
The above is written as a sort of introduction to the switch/valve
adjustment or repair. If you decide to tackle one of these projects,
we'll try to provide additional details and encouragement.
