MX73 Cressida power steering woes

[toycentury]

Hey guys thought I would chuck this up here to see if anyone could be of any help.

Basically have just acquired an '88 MX73 Cressida with 5M-GE and the power steering is doing some funny things. It has the speed sensitive power steering so has a little ECU box which gets a signal from the trans speed sensor and then modulates a solenoid on the steering rack adjusting the power steering fluid flow/pressure depending on speed the car is travelling.

Well anyway something isn't right with it - it generally has super light and sensitive steering assistance at road speeds but once you slow down to a halt the power steering assistance is severely limited as if it's not quite assisted at all, but you can feel some assistance there. The limited assistance at stand-still is intermittent though, sometimes it has full assistance which is good as that's how its meant to be however the assistance doesn't tone down as the car builds up speed like it should.


Things to note:

- The previous owner had an oscilloscope on the system and could verify the trans was giving the steering ECU a speed signal but the ECU wasn't giving any output to the solenoid on the rack.

- I did find that the solenoid was unplugged from the wiring loom but I figure it was like that because the PO didn't plug it back in after prodding around testing with it. I plugged it back in and it made no difference anyway.

- I have replaced the steering ECU with one from an NZ-spec 85ish MX73 Cressida which had a different part number but looked identical and had the same plug. Again no difference whatsoever with plugging in the replacement ECU.

- The original power steering pump has a little vacuum sensor/switch on the base of it which has been removed and plugged up with a bolt. I am waiting for a replacement power steering pump with sensor/switch to arrive from NZ and will swap it over to see if it makes a difference.

- Worth mentioning is the wheel alignment is out after replacing a rack end which is probably having some effect on it wandering from side to side at road speeds and generally amplifying the affects of the over-assisted steering.

- When taken for a RWC the inspector pulled up the steering dampner as a fail as he felt it was too soft. He explained that it should be like a shock absorber and extend when compressed however I feel if that were the case then the dampner would try and push the steering rack constantly to one side. Having never felt another MX73 dampner I don't know if mine is really soft or not.


Any ideas would be most welcome, thanks!

[neal]

You can put mx83 rack in it and it has no Damper/speed sensitive. Just change the rack ends. I think S2 MX73 did not have damper either.

[toycentury]

mine is an S2 MX73 and definitely has the damper. I found my damper was infact quite soft so found a replacement one which is a bit stiffer and will throw that one and see if that makes a difference with it tracking in a straight line.

I want to do a JZ swap at some stage so was going to swap in a MX83 crossmember but was unsure if I should go MX83 rack or keep the MX73 rack, I understand the MX83 rack is longer and doesn't give as much lock as the MX73 rack does? I often see people put MX73 racks into MX83s for extra lock, just wondering if going opposite putting an MX83 rack into an MX73 will decrease the amount of lock severely.

Cheers!

[CrUZida]

The little vacuum switch on the power steering pump is purely there to increase the idle by 50-100rpm when you turn the steering wheel. It has no other effects on the system. Higher idle = power steering pump spinning faster = more assistance (and prevents the motor from stalling...)

Worth noting that the 80's racks when the PPS solenoid is unplugged default to NO ASSIST, where as the 90's racks default to FULL ASSIST (note this is not a date cut off, just a generalisation).
Probably doesn't help you, but information none-the-less.

I can't really help you, except that if the speed signal to the PPS box stops (it comes directly from the dash, and is driven directly off the speedo), then the assistance should go FULL (ie, the PPS box thinks you are not moving).

Regarding the JZ swap, I'd personally just get some Bonneville JZ mounts and keep the standard crossmember and rack.

[toycentury]

yeah I did wonder if it would go full assist or no assist with the solenoid unplugged, but I figured it must have been full assist as the solenoid was unplugged when I first got the car and it seemed to make no difference plugging it back in.

I have noticed with the replacement PPS box some feel has come back to the steering around corners travelling say 50-60km/h - either that or I'm really getting used to the super-light steering. It still wanders going straight ahead but I think that would be it needing a wheel alignment and perhaps the steering coupler U-joint wearing out a little as steering wheel has what feels like 20mm of free play. Will replace the power steering pump, damper, u-joint and get a wheel alignment and see how it feels then.

[toycentury]

ps you guys have been helpful so far! thanks very much!

[toycentury]

Ok so I replaced the damper because my original one was indeed really soft and weak but it seemed to make no difference to the road stability - it wanders over the place especially from 60km/h+. I thought that would be a wheel alignment issue given a rack end had been replaced and not taken for a wheel alignment afterwards, but I jacked the front up to check the steering column u-joint and found it was quite easy to turn for the first few mms at the rack end and then it would start to have resistance and I didn't see any noticeable change of steering angle happening - so am thinking the rack itself has a bit of free play and is a separate issue to the over-boostedness.

Speaking of the over-boosted steering I notice that first thing after starting it's really over-boosted from the get go at both sitting stationary or at road speeds, but once driving after say 10 mins the steering starts to get heavier again and begins to feel normal. I also noticed on a long drive on the motorway the cruise control is not working - the power and memory LEDs light up when you press the button but tapping the stalk to 'SET' or 'RESUME' does nothing, and this has been travelling over 80km/h.

So now I'm thinking that maybe the instrument cluster isn't sending the speed signal to either the cruise control unit or the power steering ECU.

Gah! Anyone want a Cressida?

[CrUZida]

So same signal also goes to the ECT ECU, which will throw a code if it doesn't receive it.

[toycentury]

Ok it appears to be sorted with a replacement rack fitted. No more excessive free play and steering is nicely weighted and tracks true and straight ahead. Thanks to all who helped out, much appreciated!

[CrUZida]

Nice result!