Cruise Control Troubleshooting

Below are high resolution scans of the 1996 Lexus LX450 Electrical Wiring Diagram service manual. They are large jpg files that will print out nicely in high quality on your color printer. Also I added some troubleshooting info that might help you figure out what is going on with your balky cruise control system.

Note: I don't "think" there are any Land Cruiser Series 80 differences that would matter, but use this info with CAUTION unless you happen to own a LX450!

Location diagram shows C18 connector for the cruise control computer - page 24. You will need to access this plug to much of the troubleshooting. You might want to remove the driver's seat so you can stretch out....

Detail of the C18 computer connector and the C4 actuator connector. C4 is located beneath the actuator and faces toward the firewall (see this pic) - lever it off "carefully" with a long flat bladed screwdriver while holding the latch open with the other hand. Note: The drawings show the pins as you are looking into the HARNESS piece, not the mating socket. The mating pins in the computer or actuator are reversed in order (mirror image). The images are drawn up for the factory folks who build the harnesses. Yellow highlights the C18 pins we are worried about.

I really don't intend this to be a "How To Troubleshoot Wiring" guide. If you don't understand this electrical stuff you should find someone who does, so you don't make a mistake or short out a hard-to-replace component. You need a decent digital volt/ ohm meter and lots of patience. Write down your meter readings so you don't get confused!

Following is a recent Private Message from IH8Mud.com that prompted me to make this update:

Quote:

I read your write-up on servicing the cruise control actuator and I had a question for you. What symptoms/problem led you to the actuator? When on cruise my truck (very slightly) surges forward and then coasts instead of holding a constant speed. The change in acceleration is perceptible through the seat-of-the-pants-meter, but nothing odd is visible on the dash gauges or via my Scangauge. I'm curious to know what brought you to refurbish the actuator (and was there anything else you did first?)
Thanks for any help,

The symptoms on my '96 LX450 were that the cruise control would deactivate by itself on the highway at 60-70 mph. First it was just once in a while, then it became more frequent and finally it would not engage at all, even at 25 mph. There were no warning lights at any time.

I used the factory service manual to troubleshoot the system (electrical voltage and resistance checks of the motor, switches and sensor circuits) and checked the electrical connectors for corrosion.

I made sure the cable was free moving and well lubricated.

There is not a whole lot to the system - it just seemed "tired". Once I opened up the housing I knew it was most likely because the grease had turned to hard crud. It was a coin toss if I could gut the box and get it back together correctly, but the fix worked fine and is still working better than ever after several years.

http://www.spokanister.net/images_we.../Cruise_15.JPG

That creamy stuff looks like grease, but it is hard and not at all slippery. The same is true for the gunk on the drive gears..... and the armature bushings were bone dry.

Notice that there are TWO levers, a big black one that attaches to the cable, and (closer to the housing) there is a small silver one

http://www.spokanister.net/images_we.../Cruise_03.JPG

Moving the large lever does nothing internally because the clutch is disengaged. However you CAN move the silver lever and get the interior parts to move - it causes the gear in the above pic to move and that _might possibly_ "clean the contacts", as many here have reported, but it won't fix the basic problem which is dried up grease.

My suggestion is that after doing some preliminary electrical testing, you contact a forum member who lives close by and see if he will agree to a temporary swap of the actuators. It takes little time to remove them (pop the cover, remove the cable, remove the electrical connector, and then the three mount bolts). That will tell you immediately... IF his not screwed up also.

I am sure these units are _very_ pricey from the junkyard.

Good luck, John

If you study the above wiring diagram, you will notice that you can easily check a BUNCH of circuits from the C18 Computer connector, under the dash against the firewall. DO NOT TURN ON THE IGNITION during resistance checks or you can fry your meter's fuse.

From the C 18 computer connector (unplugged): Going left to right you can do these resistance checks with the C4 actuator connector unplugged or plugged in - it doesn't matter:

Those three checks should show that the switches and resistors are working correctly (if you need to go further you can do some more detailed tests between pins 15, 5 and 20 of the C15 switch connector).

Those three checks should show that the wire harness between the computer and actuator is in good condition, and the cruise brake light switch is OK. If the values are different you have a busted or corroded wire, or a bad brake switch.

With the actuator removed and on your workbench - remember that the pins are numbered BACKWARDS (mirror image) from the harness connector shown below!

The Service Hints on Page 139 of the Wiring Diagram Manual have some additional power and resistance checks you can perform to verify that the rest of the power and sensor circuits to the Computer are intact.

The bench power test checks that the motor and internal limit switches are functioning. It does NOT mean that you have a perfectly functioning actuator! The motor may be weak, the brushes sticking or the gear lube may be old and crusty. The only way to tell is to open it up and take a look: Cruise Control Actuator Service