Bleeding brakes - 1997 Blazer w/ ABS
#1
11-04-2011, 12:27 PM
Bleeding brakes - 1997 Blazer w/ ABS
Did a search and got few clear results so wanted to get an experienced answer. I had the lines bled for no pressure to rear brakes & know the guy used a vacuum unit to do it. Brakes appeared to be working although it "felt funny". We jacked it up & hit the brakes and it would hold the rear wheels then.
Fast forward 3 months: recently had an emergency stop on the road where the front wheels apparently locked (no ABS noise heard) & the Blazer kept right on heading into traffic with the rear's pushing. (Got lucky everyone was going under 25mph - no hit.) Looks like time for a new master cylinder - no leaks found & reservoir is full.
Do I need some special GM crackhead scan-tool to bleed my Blazer's brakes after a master cylinder / ABS assembly replacement?
There is no proportioning valve outside of the ABS unit as some threads suggest... or at least I can't see one under the hood: just the master cylinder, lines to ABS unit, lines to brakes. I just disconnected the battery for about 1/2 hour to reset the internal proportioning valve as suggested in one thread - we'll see how that goes today.
How often does the ABS unit crap out? Any way to test this - both the hydraulic & electronic parts? Same for wheel sensors - is there an easy test? I'm springing for a new master cylinder but if the ABS unit is bonkers it'll be a boneyard replacement assembly.
This system appears to be problematic & some threads indicate needing some dealer only scan tool... c'mon! Seriously? To bleed the brakes?! Bleeding brakes is easy enough but any vehicle specific methods, tips or fixes would be appreciated.
Fast forward 3 months: recently had an emergency stop on the road where the front wheels apparently locked (no ABS noise heard) & the Blazer kept right on heading into traffic with the rear's pushing. (Got lucky everyone was going under 25mph - no hit.) Looks like time for a new master cylinder - no leaks found & reservoir is full.
Do I need some special GM crackhead scan-tool to bleed my Blazer's brakes after a master cylinder / ABS assembly replacement?
There is no proportioning valve outside of the ABS unit as some threads suggest... or at least I can't see one under the hood: just the master cylinder, lines to ABS unit, lines to brakes. I just disconnected the battery for about 1/2 hour to reset the internal proportioning valve as suggested in one thread - we'll see how that goes today.
How often does the ABS unit crap out? Any way to test this - both the hydraulic & electronic parts? Same for wheel sensors - is there an easy test? I'm springing for a new master cylinder but if the ABS unit is bonkers it'll be a boneyard replacement assembly.
This system appears to be problematic & some threads indicate needing some dealer only scan tool... c'mon! Seriously? To bleed the brakes?! Bleeding brakes is easy enough but any vehicle specific methods, tips or fixes would be appreciated.
Last edited by lunghd; 11-04-2011 at 12:42 PM.
#2
11-04-2011, 08:50 PM
Might not be a master cylinder but, if it is, bench bleed it before installing it.
First, I'd try gravity bleeding each wheel, one at a time, until the fluid comes out clear, (no bubbles) it may take several minutes for each wheel. Don't let the master cylinder reservoir run dry. After all wheels are done, bleed in the normal fashion. If they still feel mushy, you'll need a bi-directional ABS scan tool capable of activating the "Automated Bleed" precedure. Plan on spending at least a couple grand on the scan tool, or take it to a shop and have them do it. It'll be way less than 2K
First, I'd try gravity bleeding each wheel, one at a time, until the fluid comes out clear, (no bubbles) it may take several minutes for each wheel. Don't let the master cylinder reservoir run dry. After all wheels are done, bleed in the normal fashion. If they still feel mushy, you'll need a bi-directional ABS scan tool capable of activating the "Automated Bleed" precedure. Plan on spending at least a couple grand on the scan tool, or take it to a shop and have them do it. It'll be way less than 2K
Last edited by Captain Hook; 11-04-2011 at 08:54 PM.
#3
11-05-2011, 07:55 AM
Thanks for the feedback.
If I'm reading your response right; then I will need access to that scan tool after a master cylinder / ABS assembly replacement??? Replacing both of those is definitely going to let air into the ABS system.
Basic bleeding I can handle plus have access to vacuum bleeder.
Yep - might be just the ABS unit, or the ABS computer, or... so still gotta ask how often does the ABS unit crap out? Any way to test this - both the hydraulic & electronic parts? Same for wheel sensors - is there an easy test?
If I'm reading your response right; then I will need access to that scan tool after a master cylinder / ABS assembly replacement??? Replacing both of those is definitely going to let air into the ABS system.
Basic bleeding I can handle plus have access to vacuum bleeder.
Yep - might be just the ABS unit, or the ABS computer, or... so still gotta ask how often does the ABS unit crap out? Any way to test this - both the hydraulic & electronic parts? Same for wheel sensors - is there an easy test?
Last edited by lunghd; 11-05-2011 at 08:01 AM.
#4
11-05-2011, 08:17 AM
If after gravity bleeding and bleeding normally you still have a mushy pedal, THEN you'll need the scan tool to perform the automated bleed. The purpose of the automated bleed is to purge air from the ABS unit, that's it, that's all it does. When replacing the master cylinder, ABS unit, or the lines between them, the system can easily injest air and the automated bleed should be performed. Once the air is out of the ABS unit, and as long as the master cylinder reservoir is not allowed to run dry, you should never have a problem.
The front wheel speed sensors and the vehicle speed sensor will not affect bleeding. If there is a problem with the ABS system, the amber "ABS" light in the cluster will illuminate and a diagnostic trouble code will be set and you'll need an ABS capable scan tool to retrieve it. This system performs a self test with each ignition cycle, but the DTC's remain in memory until cleared.
EDIT: There are several types of ABS systems, this info pertains to yours.
The front wheel speed sensors and the vehicle speed sensor will not affect bleeding. If there is a problem with the ABS system, the amber "ABS" light in the cluster will illuminate and a diagnostic trouble code will be set and you'll need an ABS capable scan tool to retrieve it. This system performs a self test with each ignition cycle, but the DTC's remain in memory until cleared.
EDIT: There are several types of ABS systems, this info pertains to yours.
Last edited by Captain Hook; 11-05-2011 at 08:28 AM. Reason: Additional info
#5
11-05-2011, 11:11 AM
Thanks! That's the clearest answer I've gotten on this particular system.
Brakes aren't really "mushy" but not having another Blazer to compare it to I can't gauge the feel of the pedal. I do think the pedal should feel firmer. It's not showing any ABS codes & the ABS light only comes on at vehicle start up doing it's diagnostic deal. It does have a "random misfire" issue that has returned but that pops up on the "Check Engine" light. For now I'll "assume" the ABS diagnostics are working & the ABS unit is "ok"... that leaves a bad master cylinder and/or air in the system. (Calipers & wheel cylinders were ok when brakes were vacuum bled 3 months ago. No line leaks found.)
I hope Obama Motors fired every member of the design team connected to this braking system. The bleeding procedure for any ABS system should not require a tool that costs more than the vehicle's resale value (which with bad brakes is about zero...).
Brakes aren't really "mushy" but not having another Blazer to compare it to I can't gauge the feel of the pedal. I do think the pedal should feel firmer. It's not showing any ABS codes & the ABS light only comes on at vehicle start up doing it's diagnostic deal. It does have a "random misfire" issue that has returned but that pops up on the "Check Engine" light. For now I'll "assume" the ABS diagnostics are working & the ABS unit is "ok"... that leaves a bad master cylinder and/or air in the system. (Calipers & wheel cylinders were ok when brakes were vacuum bled 3 months ago. No line leaks found.)
I hope Obama Motors fired every member of the design team connected to this braking system. The bleeding procedure for any ABS system should not require a tool that costs more than the vehicle's resale value (which with bad brakes is about zero...).
Last edited by lunghd; 11-05-2011 at 11:18 AM.
#6
11-05-2011, 12:30 PM
If the ABS light comes on with the ignition, and then goes off and stays off after driving 8MPH, (necessary to check speed sensors) the ABS system is fine.
Vacuum and pressure bleeding are not the preferred method with this system. Both will work but, you'll get the best results by filling the master reservoir, leave the cover off, open one bleeder and let it gravity bleed, don't touch the brake pedal. If you have trouble getting the fluid to flow, a vacuum bleeder can be used to start the flow. Once the flow starts, remove the vacuum bleeder. Usually 2 or 3 minutes does the trick, but I've had them go for up to 20 minutes before all the bubbles stop. Repeat this on each wheel making sure the reservoir does not run dry. If it does.... fill it up and start over. After all 4 wheels are bled, bleed each wheel while having an assistant hold pressure on the pedal. This is a "double check" to make sure your gravity bleed was successful, you shouldn't get any bubbles on the final bleed.
As for the design team, their CEO needs to find a new job.... in Kenya.
Vacuum and pressure bleeding are not the preferred method with this system. Both will work but, you'll get the best results by filling the master reservoir, leave the cover off, open one bleeder and let it gravity bleed, don't touch the brake pedal. If you have trouble getting the fluid to flow, a vacuum bleeder can be used to start the flow. Once the flow starts, remove the vacuum bleeder. Usually 2 or 3 minutes does the trick, but I've had them go for up to 20 minutes before all the bubbles stop. Repeat this on each wheel making sure the reservoir does not run dry. If it does.... fill it up and start over. After all 4 wheels are bled, bleed each wheel while having an assistant hold pressure on the pedal. This is a "double check" to make sure your gravity bleed was successful, you shouldn't get any bubbles on the final bleed.
As for the design team, their CEO needs to find a new job.... in Kenya.
#7
11-06-2011, 10:27 AM
Thanks Captain Hook for the detailed info - with your reply & the following (by Porkfist) I think we've got a great "How To" for this particular system. For darn sure it's the best I've found on the net or in the generic manual at AutoZone.
Drip/gravity bleeding... this had to be invented by someone who had two open beers & refused to set them down while working.
Amen on returning that "CEO" to his 'point of origin'.
.
Sorry it's been a bit since I checked back. As for a trick with the ABS from a '97 blazer ABS, I was talking about my Auto Eginuity Scan Tool, and I think I may be able to trick the scanner into thinking it's a newer blazer (that the scan tool will actuate) and actuate the ABS bleed procedure that way. Without getting into too much detail you can tell the computer what the vehicle actually is (or lie in my case). For that year blazer the scanner can't even tell what the car is when it pulls the VIN number from the car because there is some overlap between Blazers, Vans, and something else in that model year. I don't think there is any overlap in production date though so this theory may not work.
From my limited knowledge it would appear that most 4wd trucks in those 97-01 (could be wider range of years, I haven't checked any further) seem to all have the same abs pump, I think the 4WAL. So I doubt by lieing to the scanner that I'll damage anything, it's just a matter of getting the ECM to initiate the procedure.
As for a trick to actuate the ABS procedure without that scan tool, well I've got none. Just to let you know the thing did work on the Silverado and basically here is what happened. The computer initiates the procedure and tells you to push the brake. Push the brake and the computer turns on the abs pump for a few seconds. The pump stops and you are suppose to release the pedal. The computer tells you to push the pedal again and something else kicks on, I assume it actuates the valves in the abs system. Release the brake pedal again. Press it one more time and you're done. That's all it does.
My father however has been talking to people he knows who do nothing but work on cars for a living and basically what they have told him is that you don't need a computer, you just have to be persistent. Apparently you just crack open the bleed screws and let it run, and run, and run, and run, and when you think you couldn't possibly let it run anymore, keep letting it run because some of these systems especially the Silverados are a pain in the rear to purge of bubbles.
Of course if you've got rear drums, make sure they are adjusted up, because I've already been down that road, where I thought I had air in the system and my rears were just not turned up enough, since they stopped adjusting on their own.
--snip---
From my limited knowledge it would appear that most 4wd trucks in those 97-01 (could be wider range of years, I haven't checked any further) seem to all have the same abs pump, I think the 4WAL. So I doubt by lieing to the scanner that I'll damage anything, it's just a matter of getting the ECM to initiate the procedure.
As for a trick to actuate the ABS procedure without that scan tool, well I've got none. Just to let you know the thing did work on the Silverado and basically here is what happened. The computer initiates the procedure and tells you to push the brake. Push the brake and the computer turns on the abs pump for a few seconds. The pump stops and you are suppose to release the pedal. The computer tells you to push the pedal again and something else kicks on, I assume it actuates the valves in the abs system. Release the brake pedal again. Press it one more time and you're done. That's all it does.
My father however has been talking to people he knows who do nothing but work on cars for a living and basically what they have told him is that you don't need a computer, you just have to be persistent. Apparently you just crack open the bleed screws and let it run, and run, and run, and run, and when you think you couldn't possibly let it run anymore, keep letting it run because some of these systems especially the Silverados are a pain in the rear to purge of bubbles.
Of course if you've got rear drums, make sure they are adjusted up, because I've already been down that road, where I thought I had air in the system and my rears were just not turned up enough, since they stopped adjusting on their own.
--snip---
Drip/gravity bleeding... this had to be invented by someone who had two open beers & refused to set them down while working.
Amen on returning that "CEO" to his 'point of origin'.
.
Last edited by lunghd; 11-06-2011 at 10:33 AM.
#8
11-06-2011, 10:47 AM
When the automated bleed is initiated, the scan tool instructs you to start and idle the engine, and keep constant pressure on the service brake pedal. During the procedure the ABS pump will activate and deactivate several times while operating each of the solenoid controlled valves inside the ABS unit. Takes about 30 seconds to complete.
#9
11-06-2011, 11:37 AM
Definition: 'In Cahoots'
- colloq. a virtual or informal conspiracy between two or more related parties for purpose of mutual gain.
Ex: GM Production Engineering - Service tools vendors - Gm Dealer Service network
'Heartbeat of America', indeed.
And, yes, I'm a conservative and free market advocate
#10
11-07-2011, 07:36 AM
Thanks again - I doubt I find one of those tools available outside of a dealership but it's a good add to this thread & helps to understand the "what's it do?" aspect better. Now if someone would just fab up a simple tool that would activate the proper circuits... surely it doesn't need the entire scan tool's functionality to do that. Bound to be a relatively simple thing to design - trigger circuit A to bleed, trigger circuit B to reset... etc. No diagnostics - just a bleed/reset tool.
BMW has a similar whiz-bang 'reset tool' for their computers. The official dealership multifunction tool... and the $60 reset tool any schmuck like me can buy which does everything needed to zero out the computer after servicing withOUT having to schedule a day off to have your wallet extracted at the dealership.
BMW has a similar whiz-bang 'reset tool' for their computers. The official dealership multifunction tool... and the $60 reset tool any schmuck like me can buy which does everything needed to zero out the computer after servicing withOUT having to schedule a day off to have your wallet extracted at the dealership.
Last edited by lunghd; 11-07-2011 at 07:41 AM.