Comprehending the PPS system

[KMinAF]

After studying the TSM, please tell me if I am understanding the Progressive Power Steering system correctly

The Solenoid is controlled by an ECU which makes adjustments based upon a vehicle speed sensor.

The solenoid opens at higher speed to bleed off pressure therby giving the steering a "heavier" feel due to less hydraulic assistance. This portion of the system does not come into play when parking.

The vacuum switch bleeds air into the throttle body which raises the idle speed causing the pump to spin faster increasing the system pressure to make the steering "lighter".

So.... if the steering becomes lighter only intermittently when parking, would this symptom be indicative of a faulty vacuum valve or a pump that is leaking pressure? The valve does not leak (no smoke upon start up) and it is not "open" with the engine off.

Instead of throwing pumps and racks at it, I would like to diagnose the fault and make the repair. I have removed the solenoid and cleaned the screen, replaced the vacuum valve (it was leaking) and completely flushed the system, all with no change.

Has anyone ever actualy diagnosed this problem?

 

[jetjock]

You've sort of got the vacuum valve thing backwards. It doesn't make the PS system lighter. It's idle up is only to keep the engine from bogging under increased PS load. I suppose in one sense you could say it maintains the pump speed same as what it would be without the engine bogging down but that doesn't effect PS feel much. Anyway, it's not considered part of the PPS system. And maybe I read you wrong but the PPS solenoid is proportional. It just doesn't open at a certain speed. Note in the test where it says not to energize it for more than 30 seconds. Also note the name "progressive" for the system

 

[KMinAF]

Does that mean that the solenoid opens/closes in degrees or does it cycle similar to an injector? The reason I ask because the test is done at full voltage whereas the out-put voltage from the ECU is very low and varies.

Am I on the right diagnostic trail for the described problem or would you suggest looking elsewhere?

 

[Satan]

I once had "jerkyness" in the steering after some mechanic "proactively" replaced the PS VSV. Turned out that one of the feed/return lines to the pump had gotten kinked/bent a bit and the steering would go "from struggling to make a left turn, to all-of-a-sudden full PPS help." Same with switching lanes, or small turns of the steering wheel. DANGEROUS!

I replaced the PS rack on my current Supra, just a few weeks ago. Things were fine while at TX2K6 and until yesterday. Now, feels like no PPS at all. No changes, 'cept I've worked around the lower PS pump area trying to service the A/C.

Looking in my reservoir, fluid's moving well, so I know the pmp's doing it's job.

Another weird thing happened, coincidentally?

My map lights started working again, as well as auto-off for the headlights. They did not work before TX2K6, but the features started working at TX2K9. I had previously assumed bad integration relay, but had not not had a chance to change yet. Related issues/benefits?