# Bleeding ABS brakes in 92 Pathy SE?



## himilefrontier (Jan 21, 2003)

A friend of mine has a 1992 Pathfinder SE with ABS and traction control and we can never get the brakes to bleed.I was wondering what we are missing, as we have successfully bled other non ABS vehicles many times. Thanks!


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## Animal (Mar 25, 2005)

himilefrontier said:


> A friend of mine has a 1992 Pathfinder SE with ABS and traction control and we can never get the brakes to bleed.I was wondering what we are missing, as we have successfully bled other non ABS vehicles many times. Thanks!


I have a '92 XE with front ABS and haven't had a problem getting them bled. If the master cylinder is replaced, you should bench bleed it before installing, otherwise it is nearly impossible to remove all the air.


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## Animal (Mar 25, 2005)

Animal said:


> I have a '92 XE with front ABS and haven't had a problem getting them bled. If the master cylinder is replaced, you should bench bleed it before installing, otherwise it is nearly impossible to remove all the air.


This may be what you are looking for. The FSM states with rear ABS to turn off the ignition and disconnect the battery ground and actuator connector.


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## X-Traction (Dec 21, 2004)

Just to avoid possible confusion here, most of the first generation ('91 and later?) Pathfinders had rear ABS only. None had traction control, though a posi-traction rear differential was common. The diff would have no bearing on the brake system. Btw, the rear ABS works only in 2wd mode, but it's a very effective and smooth system.

The old Pathfinders also have a brake pressure proportioning valve, that adjusts the amount of brake fluid sent to the front and rear brakes depending on the amount of load and braking severity. All this is quite good to have on a relatively high and short vehicle. It all helps out to minimize braking distance and loss of control during panic stops.


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## Animal (Mar 25, 2005)

X-Traction said:


> Just to avoid possible confusion here, most of the first generation ('91 and later?) Pathfinders had rear ABS only. None had traction control, though a posi-traction rear differential was common. The diff would have no bearing on the brake system. Btw, the rear ABS works only in 2wd mode, but it's a very effective and smooth system.
> 
> The old Pathfinders also have a brake pressure proportioning valve, that adjusts the amount of brake fluid sent to the front and rear brakes depending on the amount of load and braking severity. All this is quite good to have on a relatively high and short vehicle. It all helps out to minimize braking distance and loss of control during panic stops.


I believe front ABS was standard and rear was optional at least in '92.


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## himilefrontier (Jan 21, 2003)

X-Traction said:


> Just to avoid possible confusion here, most of the first generation ('91 and later?) Pathfinders had rear ABS only. None had traction control, though a posi-traction rear differential was common. The diff would have no bearing on the brake system. Btw, the rear ABS works only in 2wd mode, but it's a very effective and smooth system.
> 
> The old Pathfinders also have a brake pressure proportioning valve, that adjusts the amount of brake fluid sent to the front and rear brakes depending on the amount of load and braking severity. All this is quite good to have on a relatively high and short vehicle. It all helps out to minimize braking distance and loss of control during panic stops.


It does have rear wheel anti-lock, but it also has a switch for the traction control, IIRC. We had tried bleeding the system in the past, including the braking force distribution valve, but it would give one or two good squirts, then stop shooting fluid. We'll try disconnecting the battery this week when I get a day off, as I am very curious to see if this works. My Frontier has a very similar brake system, and I would like to change it's fluid next.


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## Animal (Mar 25, 2005)

himilefrontier said:


> It does have rear wheel anti-lock, but it also has a switch for the traction control, IIRC. We had tried bleeding the system in the past, including the braking force distribution valve, but it would give one or two good squirts, then stop shooting fluid. We'll try disconnecting the battery this week when I get a day off, as I am very curious to see if this works. My Frontier has a very similar brake system, and I would like to change it's fluid next.


FSM says to disconnect the battery ground *and actuator connector.* I suspect with reason.


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## X-Traction (Dec 21, 2004)

Animal said:


> I believe front ABS was standard and rear was optional at least in '92.


So I had to go do some research. I have a 1990 "Consumer Reports" review of the 1990 Pathfinder which doesn't mention ABS at all.

I have a real Nissan sales brochure for the 1995 Pathfinder, which lists rear ABS for all versions. If some 1992 model had front ABS, then why would it not be on the 1995 LE, for instance?

I have the Winter 96/97 "Open Road" magazine, which did a retrospective review of the first generation Pathfinder. It states that rear ABS was introduced to all models for 1991.

None of these sources mention front ABS, or traction control. If anyone has information that convincingly contradicts ior supplements these sources, I'd be interested to hear it. 

I also have a factory service manual, which I have pored over extensively during the course of owning our PF for 10 years. Without going through it, I don't recall any mention of traction control or front ABS.

As for a "traction" control" switch mentioned by himilefrontier, I wonder if it's the two-position shock stiffness switch that he's thinking of, which was on the SE but not the XE.


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## Animal (Mar 25, 2005)

X-Traction said:


> So I had to go do some research...


Seems we've been on similar quests. This thread has lead me to attempt to verify what I had assumed, observed, and was initially told by the insurance company. 
Though it appears rear ABS (R-ABS) was an option in early to mid '90s for trucks, all the documentation I have been able to find indicates it as standard equipment on Pathfinders. This brings up several questions. 

First, when I had stock tires and pads, the front brakes would not lock up until the speed had dropped near zero. The documentation says R-ABS is effective above 6mph. Why was I unable to lock the front at higher speeds? 

Second, I'm not sure I understand the pupose of having R-ABS and not front. The intent of ABS is to allow the driver to maintain control in an emergency stop. If the front brakes lock, there is no control. So why prevent the rear, which doesn't provide much forward braking to begin with, from locking? 

I would appreciate anyone insite, opinion, guess (please state which) on this topic.  

BTW, with 31s and high performance pads, I can now lock all 4 tires at 40 mph. I had previously read (somewhere) that any changes from stock can effectively disable ABS - a benefit IMO (I hate ABS and consider it a crutch for those who never learned how to properly brake).


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## X-Traction (Dec 21, 2004)

In response, opinions only.

I agree rear-ABS seems odd at first glance. Probably it was easier and cheaper to implement than a full-on system, esepcially given the Pathfinder's "old fashioned" 1986 design. It was, after all, 1991 when it was introduced.

Bear in mind the rear ABS works in 2wd only. Some of the effects you have seen might be due to the fact it doesn't work in 4wd. I suspect some of the other characteristics you have seen are due to the brake proportioning valve. 

The harder the stop, the more brake force is done by the front wheels. With "set" brake pressures between the front and rear wheels, the balance between them may be right for some speeds, but obviously not for others. So in some cases, and without any ABS, you'd lock the fronts first, and in other cases, the rears first. Brake fluid proportioning, which also is determined by the fore/aft cargo loading, (it's a simple system that determines the angle of a rear control arm) dynamically adjusts the brake fluid between front and rear brakes. Just like you do with your hand brakes while stopping a bicycle quickly without sliding the rear wheel.

I have, ahem, made the odd panic stops with our PF. I've also crept, braked, and engine braked down icy steep narrow, out-sloping roads with a dangerous dropoff, with my heart in my mouth and gingerly trying different ways of slowing down. Indeed, in 2wd and using the brakes, the rear end stayed stable, but the front end locked and slid off course. Engine braking down at low speed in 4wd without locking any tires always worked best.

But in a panic stop, you don't have time to experiment. I've always found the PF to panic stop in a very composed and short manner. The SE's rear disc brakes may contribute to this. 

The rear ABS keeps the rear end behind the front end. Without ABS, and depending on the brake pressure balance between the front and rear brakes, if you lock the back brakes, you risk the rear end sliding past the front end, which amounts to a total loss of control. The proportioning valve seemed kept the front brakes from locking, although I'm not sure since all the panic stops I did were straight-line, not while cornering. So the front end didn't wander off anyway. In a vehicle that's relatively short with a high center of gravity, it may be more important to keep the back end behind the front end (like dragging an anchor) than to keep the front end from locking up. And that proportioning valve is very effective in keeping the front end from locking up. The bottom line is that it's a more effective system than "rear ABS" would suggest.

But those are just my thoughts on the matter. It would be nice for a Nissan engineer to explain all this.


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## Animal (Mar 25, 2005)

X-Traction said:


> In response, opinions only....
> But those are just my thoughts on the matter. It would be nice for a Nissan engineer to explain all this.


Much of what you said does not fit the situations I have been in during which the front brakes provided a smooth controlled stop (which I agree with you). Ice always seems to be the exception. Most systems can not detect the vehicle has any speed when all 4 tire have stopped spinning and there is little or no friction on the tires, as is normal on ice. The condition you described, of the rear passing the front seems more possible when the front locks and the rear does not.
I second the request for a Nissan (or other brake) engineer to explain what we have observed. :cheers:


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## X-Traction (Dec 21, 2004)

It's my understanding that the goal of ABS systems is to prevent wheels from locking during panic stops. This is because it has been found that on most surfaces, a wheel that is being braked to just short of the point where it locks, has more grip, and therefore more braking effect, than a locked wheel. 

The only exception I can think of is on gravel, where a locked wheel "digs" down to more solid material and brakes better without ABS interfering. On steep gravel downhills, this is actually a safety hazard, and so drivers are advised to deactivate ABS in those circumstances. I hadn't heard of the problem with ABS and locking all four wheels on ice, but your explanation makes sense to me. This assumes ABS would allow all the wheels to lock to begin with, which I would think should result in the removal of those systems from the market.

So during a hard stop on ice or pavement, with the front wheels close to locking, but not locked, and the rear wheels locked, the front wheels have more grip than the back ones. The wheels with the least traction will try to be out front, as the vehicle "hangs" from the wheels with the most grip. In addition, locked wheels have no directional stability, while turning wheels have a lot of directional stability. Thus, unless the panic stop is absolutely straight, the rear end will try to pass the front end.

I think that exhausts what I think I know about this.


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