angrybob

I have wondered two things about these valves since seeing them:

1. Why is the front brake circuit plumbed into LSPV?
2. Why is the power steering pump plumbed into the Rear ABS?

Last night I had the good fortune to get these two questions answrered. I'm a brake engineer at the Toyota Proving Grounds and have been waiting for my next trip to Japan to find an 'old' brake guy to ask. As luck would have it, he came to me.

We had a brake supplier now named ADVICS (a merger of brake divisions of Aisin and Sumitomo) in town this week. As it turned out, I had dinner with the original Aisin Seiki design engineer who was onsite at Toyota Japan and responsible for the LSPV and Rear ABS when it was later introduced.

He stated the following (not everything was a perfect translation due to the language barrier, but close):

LSPV

The front circuit to the LSPV acts as a safety bypass valve. There is a low pressure plunger or piston that is moved by front brake pressure and redirects fluid to flow into the 'business end' of the LSPV to get proportioned. Since the 4Runner is Front/Rear split its considered a failsafe if the front circuit fails. With no pressure from the front circuit, the plunger stays in place and is simply a though valve allowing full pressure to the rear brakes. Even though the rear brakes suck, they would suck much worse if you lose the front circuit *and* were proportioned by the LSPV. There is a chance though of locking the rears if the vehicle is unloaded I would guess.

Rear ABS
When I first started in brakes in '94 for Kelsey Hayes, I had responsibility for their rear ABS system. It was a simple valve tied to a sensor on the rear diff that read from a fine-toothed ring gear called a tone wheel. These systems were primarily on trucks with light rear ends and prevented the the rear wheels from locking up. All it did was isolate pressure from the master cylinder when the rear wheels locked, and dump pressure and dump pressure to get them spinning again. It was considered passive ABS. There was no reapplying of pressure like any modern 4-whl ABS. It was also a single channel system operating off of a single sensor which meant that the LR and RR wheels acted the same and were not independent of each other while in ABS. Imagine the diff was locked as far as ABS is concerned.

The Toyota Rear ABS is considered active because it can reapply pressure to the rear brakes. Enter the power steering pump circuit into the Toyota Rear ABS - its means by which pressure is reapplied.

When you apply the brakes and lock the rear wheels, pressure is isolated from the master cylinder and then its dumped to get the wheels spinning again. The power steering line is plumbed into a chamber in the ABS valve that has a piston: on one side of the piston is power steering pressure (controlled by a regulator in the ABS valve) and on the other is brake line pressure. When pressure is dumped from the rear brake circuit due to wheel lock, it is reapplied by the pressure from the power steering pump side of the piston pushing the now low pressure brake fluid side of the piston. That piston force from the power steering side of the piston builds brake pressure back up until its dumped again. This whole process is repeated many times per second until the brakes are released.

That's why the power steering pump is plumbed into the Rear ABS.

So now I know...

MudHippy

Very informative.

However, you are only slightly wrong on one thing. A true Load Sensing Proportioning Valve(LSPV) isn't plumbed into the front brake circuit(or vice versa). What you are refering to is a LSPV with a By-Pass circuit(LSP & BV or LSPBV). 86-95 Trucks & 4Runners use both types.

http://personal.utulsa.edu/~nathan-b...12loadsens.pdf

See here for the full definitions of what I mean.
http://autoshop101.com/forms/brake08.pdf

And for the details on the ABS actuators see here.
http://autoshop101.com/forms/brake11.pdf

Toyota technical articles found here.
http://autoshop101.com/

I would have shown you these things earlier. Had you simply asked.

BTW, I work for no one, and have fully trained myself in the arts of automotive mechanics and electronics. And I'll bet I know as much, or more, about it as you do. Maybe I oughta start calling myself an engineer too.

Sorry, had to.

angrybob

Interesting. My '87 FSM only shows the LSP & BV. I had no idea that both were offered.

Wow. Get over yourself.

The purpose for stating my job was to provide the *how/why* I had dinner with the actual designer not some false pretense that a title = knowledge. You probably do know more than me and I assure you that you are in a crowd that is neither small nor elite.

wyoming9

iTrader: (1)

Some people just don`t read very well!

Great write up but I think it was beyond the understanding of most.

Then these ABS brakes are a nightmare for those of us that learned to drive with out them.

then again everybody knows more than me .

I do the best with my 5th grade education.