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.