1990 Nissan 300ZX hard steering

I believe your vehicle has speed sensitive steering. I did some research and found the following:

Electrical issues that can cause PS problems.-

1. Check your power steering oil pressure switch. The FSM tells you to check the power steering oil pressure switch for 2-3 ohms of resistance between it's terminals when it is disconnected. What it doesn't tell you is that the switch is a normally open switch and that in order to see a resistance of 2-3 ohms it has to be closed. You must check the switch while the power steering system is together and while turning the steering wheel with the car running. The purpose of the switch is to tell the ECU when the power steering pressure has reached a certain point of pressure. When this happens the switch closes. The ECU detects that the switch is closed and raises the engine RPMs to allow the power steering pump to raise the PS oil pressure. This situation usually happens when the car is at a low idle or RPM.

2. Check the power steering solenoid (1990 FSM page ST-37, page ST-88). The power steering solenoid operation is dependant on vehicle speeds. The power steering solenoid reduces the amount of pressure that reaches the steering gear at higher road speeds. If the signal is lost or the solenoid is not working properly you may see a loss of road feel and/or increased steering sensitivity at highway speeds.

3. Check the steering wheel angle sensor (1990 FSM page and 1990 FSM page ST-57). If the sensor is bad or the sensor is loose it can cause erroneous signals to the PS system which will effect it's operation. These signals are sent to the HICAS/Power steering module.

4. Check the steering wheel neutral position switch (1990 FSM page ST-58). The steering wheel must be straight when driving in a straight line. If not, this will send erroneous signals to the PS system which will effect it's operation. These signals are sent to the HICAS/Power steering module.

5. One of the common causes of PS problems is the speed sensor. The speed sensor creates a wave forming pulse which is first sent to the speedometer in the gauge cluster. This signal is then sent to the speed signal input pin 53 at the ECU to calculate the speed. If the speedometer board in the gauge cluster is bad it is possible that although you may see an indicated speed on the speedometer, no signal or an intermittent signal is being sent from the speedometer board to the ECU. If the signal is lost for 10 or more seconds, a hard steering problem will occur. You can eliminate the possibility of a lost signal by using a consult software or a TechTom MDM-100 to monitor the speed signal. Using one of these options to monitor the speed signal monitors the signal after it has left the speedomter in the gauge cluster and has arrived at the ECU. Since the speed sensor is located in the transmission, you will need to monitor the speed signal while driving. You can simulate driving by placing the rear of the vehicle on jack stands so that the tires clear the floor.