Progressive Power Steering

Background

The progressive power steering on the MkIV responds differently at high speed to low speed. Changing the valve opening on the hydraulic pump in the following way.

• Low speed = PPS high voltage output = increases the valve opening
  more assistance to help with the steering.

• High speed = PPS low voltage output = decreases the valve opening
  less assistance to help with the steering

This is all well and good but why tell us about it I here you say...................

The problem lies in where the speed signal has come from. (yep, that ole
chestnut again)

The Analysis

The speed signal comes from the Odometer (T7 pin 6, PINK wire). If you have a modded the odometer (reading in MPH) the signal sent to the PPS is therefore reduced in frequency by 5/8. This means that the PPS is providing more assistance at high speed, making steering potentially twitchy. This is more assistance than Toyota intended at high speed.

I'm going to try to route the KPH speed signal to the PPS to see if there is any perceivable change in the response, back to normal. (5/8 is nearly half so I should feel something)

Those without the odometer mod will not be affected and will have normal PPS operation.