Trooperbc, yep I have 4wheel ABS.

I too wonder why the engine has to be running. Kind of makes me wonder if people who step on the brakes with the engine off in everyday life are doing damage.

Then again I cannot exactly buy your hypothesis as to why you could damage anything with the engine running. Afterall that is how the vehicle is used 99.9999+% of the time.

Speculating, well at best brainstorming, more like guessing, maybe the rod itself would apply too much concentrated force against the diaphram housing if not helped along by a vacuum assist??? Let me try an analogy. Say you want to push on a car to move it. If you do it with one hand against the sheetmetal on a car on a slight dowhill the car moves and you don't dent the sheetmetal. This is like the rod (hand in the analogy) working with a vacuum assist (slight downhill in the analogy) with the engine running the rod doesn't put too much force on the diaphram. Alternatively, if you push on a car with one hand on a flat surface you might dent the sheetmetal. This is the rod working without a vacuum assist and it could damage the diaphram??? Maybe it is not even an ABS thing just a "new and improved" booster sytem??? Then again this is all really over my head.

To answer the question on my '95, I have rear ABS only - but that doesn't seem to have any impact on bleeding the brakes - still supposed to do it with the engine running, according ot the '95 Helms book I just received - so I guess that's what I will do. Some of you have done it without the engine on with no problem, eh?

Thanks for all the help here!

4x4replybrake bleeding

OK, so it looks like i am wrong (i always figured thereÆd be a second time <img src="/forums/images/graemlins/rolleyes.gif" alt="" />).

IÆve reread the posts here, and reread my 1992 Trooper helms factory workshop manual on bleeding brakesàà.and in the interests of FEA and this thread and in all who read it now and in future searches:

I think my manual follows what yours shows, too. The difference was my reading of it, making some false assumptions based on my past experience, learning and practice.

My helmsÆ factory manual for 1992 trooper has separate sections for the two different systems, yet the introductory section on ôHydraulic Brakesö includes an ôOn-Vehicle Serviceö subheading that includes BLEEDING BRAKE HYDRAULIC SYSTEM which covers both the RWAL rear-only system and the ABS all-wheel system.

As noted, Number 1 instruction is ôFor 4-wheel Antilock Brake System (ABS) equipped vehicle, be sure to remove the ABS main fuse 40A àö. My big mistake, mea culpa, mea culpa, por favor, was to jump ahead, assuming the next 5 steps in that little section applied only to ABS all-wheel systems.

***** ?????Not that i understand the purpose of having the engine running, nor what damage would result if itÆs off. All my experience and learning and practice says the opposite. Does anybody know? Perhaps there is some change in the diaphragm design. Or perhaps it has to do with the RWAL and ABS valve components.?????*****


central, we donÆt operate our vehicles ôthisö way 99% of the time -- with the hydraulic system open, as when bleeding the brakes. My point is that i think/thought this can cause the push rod to extend too far (without the resistive pressure of the hydraulics), and distort/damage the diaphragm. Apparently not.

FEA, i appreciate your following through on this, making sure you had the correct info.

Hope everything went well.


The distinction in terminology between rear-only anti-lock brakes and all 4-wheel anti-lock brakes.
While not the cause for my confusion on this issue, generally i think there is confusion about the two different terminologies and abbreviations for rear-only antilock brake and all-wheel antilock brake systems. So hereÆs a summary that might help someone:

Since the rear-only RWAL uses the terminologyRear Wheel ôAntiLock Brake Systemö in its name it is easy to shortcut it to ABS in oneÆs mind, and perhaps follow the wrong procedures (well, at least this so-called mind).

.