lbs_ft

Hey all, I've got a manual 1990 NA 300ZX with close to 200,000 miles.

A month ago, I went to set the cruise control and realized it would not set. The Cruise Control light comes on, but the Cruise light doesn't illuminate when the Set button is pressed.

Since that time, I've read numerous threads on this forum and many others in an attempt to figure out why mine is not working and how to remedy it.

The following is a list of checks that I've performed using the procedures and schematics outlined in the factory service manual:

Here are the pages from the FSM:

ASCD Schematic

ASCD Diagnostic Procedure

ASCD Actuator

The ASCD module receives 12v to pin 4 when the Cruise Control On/Off button is pressed, and the ASCD hold relay can be heard clicking within the dash.

All pedal switches were removed, tested via multimeter, and reinstalled to the correct positions. Green bumpers are intact and making contact. When the pedals are not pressed, the ASCD module receives 12v to pin 5. When either of the pedals are pressed, pin 5 loses 12v.

When the brake pedal is pressed, pin 11 receives 12v.

When the Set button is pressed, pin 2 receives 12v.

When the Resume button is pressed, pin 1 receives 12v.

When the cancel button is pressed, pins 11 and 2 receive 12v.

Pin 3 on the module has continuity to ground.

Pin 8 on the module has continuity to pin 1 on the actuator.

Pin 9 on the module has continuity to pin 4 on the actuator.

Pin 10 on the module has continuity to pin 2 on the actuator.

Pin 3 on the actuator has continuity to ground.

Hooking power to pin 1 and ground to pin 4 on the actuator powers up the actuator pump.

Hooking power to pins 1, 2, and 3 on the actuator powers up the actuator pump and pulls the throttle cable back.

While powered up, removal of power to pin 1 on the actuator causes the throttle cable to return immediately.

While powered up, removal of ground to pin 4 on the actuator causes the throttle cable to return immediately. (The actuator test procedure in the manual says this should return in 50-60 seconds, but it was almost instantaneous for me.)

Disassembled the set/resume/cancel switch assembly and replaced the tactile microswitches within.

Purchased a used ASCD module from Z1 Motorsports and replaced my factory module.

Tested the speed sensor at the ASCD module. The multimeter recorded pulses when the rear wheels were spun, I believe something around .4v, increasing in frequency as the wheels were spun faster.

Having tested literally every connection leading to and from the ASCD module, as well as replacing the module itself, I'm 100% at a loss as to where to go from here.

Barring replacement of the actuator, the speed sensor, or re-replacement of the ASCD module (it would be shit luck, but its possible), I'm totally lost. I hate throwing money at a problem, especially when I'm not sure that it will fix said problem.

I design electrical schematics by trade, so I've got my hands on the schematics provided in the manual and the concepts of how this should work are second nature to me, but that's really what makes this that much harder. It just doesn't make sense.

Any and all help would be greatly appreciated! Thanks in advance.

EnnEssEnnKAT

Not sure if the 300 is the same as the 240 but I had an issue in my s13 cruise where the vacuum line between the actuator and the tiny compressor was split and leaked.

lbs_ft

There's definitely a vac line between the actuator and cable assembly. It seemed to be in good shape, as the pump was able to pull the throttle open when I ran it off the battery, but I'll replace the line and see if I can take a look at the seals in the pump just to rule that out