PAS fails to cut in - sticky rotary valve?
Discussion
I have an intermittent (though always more regular) problem with the PAS on my S-max. The system is hydraulic with an electric pump.
The symptoms are that when you turn the steering wheel the PAS doesn't work, usually, after a tug or two it will cut in quickly and then keep working for a while. Interestingly, when we took the car to Italy over the summer the problem disappeared - we were there for 6 weeks so long enough to be sure about this. I suspect that the higher ambient temperature helped. Now we're back in Blighty it seems to be worse than ever before.
When the system works it feels absolutely fine - not weak or as though the pump is struggling to create the necessary pressure. I know very little about PAS, but I understand that the rotary valve is what allows the pressure into the rack and therefore gives the assistance (?). If that's the case, then it sounds like a fairly likely culprit in this case.
So... Am I barking up the wrong tree? Is the valve replaceable or is it a new rack job? Will some thinner oil help?
I have checked out some Ford specific sites which indicate either a software update, a new rack, a new PAS pump or a new fluid reservoir. So just change everything then
The symptoms are that when you turn the steering wheel the PAS doesn't work, usually, after a tug or two it will cut in quickly and then keep working for a while. Interestingly, when we took the car to Italy over the summer the problem disappeared - we were there for 6 weeks so long enough to be sure about this. I suspect that the higher ambient temperature helped. Now we're back in Blighty it seems to be worse than ever before.
When the system works it feels absolutely fine - not weak or as though the pump is struggling to create the necessary pressure. I know very little about PAS, but I understand that the rotary valve is what allows the pressure into the rack and therefore gives the assistance (?). If that's the case, then it sounds like a fairly likely culprit in this case.
So... Am I barking up the wrong tree? Is the valve replaceable or is it a new rack job? Will some thinner oil help?
I have checked out some Ford specific sites which indicate either a software update, a new rack, a new PAS pump or a new fluid reservoir. So just change everything then
You need to establish if the pump is operating at the correct speed (too slow and you will get heavy steering). The TRW Gen3 EHPAS system takes:
1) engine rpm
2) vehicle speed
3) Handwheel speed
and from these (data is transmitted to the EHPAS via CAN) is used to determine a suitable pump speed for the conditions (broadly speaking, engine must be running then, if vehicle speed is zero assistance is max, but as vehicle speed increases the pump speed is reduced, unless the handwheel rate is high)
If for some reason these signals are corupt or invalid, the pump goes to a "limp home" mode and runs at a fairly constant mid speed.
The pump returns a CAN message with both its status and the pump load.
Assuming the pump IS running at the correct speed, then it is possible for the rack valving to stick, but pretty unusual.
The first thing is a diagnostic read of the fault memory to see if any pump errors or sensor errors have been stored etc
1) engine rpm
2) vehicle speed
3) Handwheel speed
and from these (data is transmitted to the EHPAS via CAN) is used to determine a suitable pump speed for the conditions (broadly speaking, engine must be running then, if vehicle speed is zero assistance is max, but as vehicle speed increases the pump speed is reduced, unless the handwheel rate is high)
If for some reason these signals are corupt or invalid, the pump goes to a "limp home" mode and runs at a fairly constant mid speed.
The pump returns a CAN message with both its status and the pump load.
Assuming the pump IS running at the correct speed, then it is possible for the rack valving to stick, but pretty unusual.
The first thing is a diagnostic read of the fault memory to see if any pump errors or sensor errors have been stored etc
Steve_D said:
You say giving it a 'Tug' gets it going but you don't say what you are tugging.
If you are refering to the steering wheel then an intermitant electrical fault in the steering angle sensor could be the problem.
So what do you 'Tug'?
Steve
Just tugging the wheel, honest If you are refering to the steering wheel then an intermitant electrical fault in the steering angle sensor could be the problem.
So what do you 'Tug'?
Steve
Thanks for all the information, I wasn't aware that modern PAS systems were so complex that the pump rate actually changed.
In terms of reading the fault code, would one of these do the trick? http://www.tmart.com/Autel-MaxiScan-MS509-OBD2-II-...
Max_Torque said:
You need to establish if the pump is operating at the correct speed (too slow and you will get heavy steering). The TRW Gen3 EHPAS system takes:
1) engine rpm
2) vehicle speed
3) Handwheel speed
and from these (data is transmitted to the EHPAS via CAN) is used to determine a suitable pump speed for the conditions (broadly speaking, engine must be running then, if vehicle speed is zero assistance is max, but as vehicle speed increases the pump speed is reduced, unless the handwheel rate is high)
If for some reason these signals are corupt or invalid, the pump goes to a "limp home" mode and runs at a fairly constant mid speed.
The pump returns a CAN message with both its status and the pump load.
Assuming the pump IS running at the correct speed, then it is possible for the rack valving to stick, but pretty unusual.
The first thing is a diagnostic read of the fault memory to see if any pump errors or sensor errors have been stored etc
Depends on the year, early S-Max should be Gen 3, later ones are gen 6, possibly gen7.1) engine rpm
2) vehicle speed
3) Handwheel speed
and from these (data is transmitted to the EHPAS via CAN) is used to determine a suitable pump speed for the conditions (broadly speaking, engine must be running then, if vehicle speed is zero assistance is max, but as vehicle speed increases the pump speed is reduced, unless the handwheel rate is high)
If for some reason these signals are corupt or invalid, the pump goes to a "limp home" mode and runs at a fairly constant mid speed.
The pump returns a CAN message with both its status and the pump load.
Assuming the pump IS running at the correct speed, then it is possible for the rack valving to stick, but pretty unusual.
The first thing is a diagnostic read of the fault memory to see if any pump errors or sensor errors have been stored etc
If you turn the steering with the vehicle stationary (i know it's not good for the tyres) you should be able to hear the motor speed increasing (pump is in the NS front bumper), this will tell you if the steering angle sensor is working.
If it was a vehicle speed problem, the steering would be light at high speed.
If it is idling and not in limp home mode the motor is very quiet, at 3600rpm, limp home, you can hear the motor, even with no pressure on the system.
It should also indicate an issue by lighting the dash.
If the valve was sticking in the rack, the relief valve won't open as the valve in the rack induces load and the system will stay fairly quiet, just the motor.
If the valve in the rack has failed, it's usually a rack replacement job.
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