The cruise contol system consists of four major components.
Vacuum System (including pedal switches and pump)
This is the most common source of problems. For testing, you'll need a way to generate and hold vacuum in the lines. A good tool for this is a miniature Mity-Vac type of device.
Most common wiring problems in this circuit occur in the wires going from the steering column, down the turn signal arm, and into the switch. There is also wiring for the vacuum pump that may need to be checked.
Steering Column Switches
Not usually a problem, but wiring in steering column can be.
This piece rarely goes bad, so it should only be suspect when all other common problems have been ruled out. The easiest way to test it is to put in a known good unit.
First, check the obvious. Blown fuses? Check brake lights -- if both are blown, the system won't work. Also look for any disconnected vacuum lines in the engine compartment. If you can't find the problem, more in-depth troubleshooting is required.
Disconnect the vaccuum line at the pump. With a Mity-Vac apply vaccuum to the system. The bellows should move the throttle and stay until you release vaccuum. Without a Mity-Vac, depress the rubber part of the reservoir and plug the other end of the hose. It should stay put. Repeat this test and depress the clutch pedal, and again with the brake pedal. The vaccuum should always release as soon as you touch the pedal. This will show whether you have a bad switch at the pedal, or a leaking line, or a bad bellows.
To see if the vaccuum pump is operating correctly, it is easiest to remove it. You can also do this from under the dash, but that involves pulling the glove box, a PITA. There should be a three prong electrical connector, a small hose that connects back to the pump and the main hose.
Put all vacuum hoses back in place and disconnect the electrical connector. Using your test leads, connect ground and +12v to the pump and solenoid switch. If you are looking at the connector on the pump like a "U", power is the right upright, the pump is the crossbar, and the valve is the left upright. Look on the electrical connector to make sure: blue/red is +12, brown/white is the pump ground and green/blue is the vent ground.
When you have power to the pump and valve, with the hoses connected; the throttle linkage should move. If you disconnect ground from the pump, the throttle should stay put. Disconnect ground from the vent and the throttle closes. If you get full travel on the throttle, then your pump draws sufficient vaccuum to operate. If all the above checks out, then you have isolated the cruise problem to the electrical side.
If your throttle doesn't move but your pump runs and you have no vacuum leaks, read on. If you have a Mity-Vac or a vaccuum gauge, you can get a numerical value for the pump. A year or so ago, my pump worked fine. This time around it drew 3 inches Hg. I disconnected the hoses, and connected a line to the air intake (the larger black port). I also connected a return line to the exhaust port (the stubby white one under the intake). I plugged the vent port with my finger, stuck the intake hose into some water, and applied power for 3-5 seconds. After this I ran it disconnected to flush the water out. After the flush I got 17 inches Hg out of the pump. I can't guarantee if there are any long term negative effects of this method, but it saved buying a new pump in the short term. (Six months later it still works)
There are two procedures here. The first is from the Bentley manual, the second is an alternative method. The advantage of the Bentley procedure is you may not need to remove the airbag to IDENTIFY the problem. The disadvantage is you will probably have to remove it to FIX the problem. This is in addition to squirming upside down in the footwell.
To test the electrics you need access to the control unit; it's up under the dash on the passenger side. You don't need to remove the dash, but you will need to pull the glove box and a trim panel or two. Push the A/C duct out of the way and remove the connector off the ECU. Contrary to Bentley, there are two: an eight pin connector, and a single ground connector. Pull the eight pin connector down and do the following:
Ignition OFF, power to cruise OFF, multimeter on OHM setting:
Ignition ON, cruise ON, mm to Volts DC:
Multi-meter back to OHMs:
This requires nimble fingers and a little more delicacy, and won't test the wiring from the pump to the ECU or the switch connector to the ECU or the electrical switches at the pedals. It more effectively isolates the switch though and doesn't involve being upside down, on your back.
Everything off, you could pull the negative battery terminal to avoid honking the horn.
Remove the upper cover for the steering wheel. The CC wiring comes down the stalk and comes up over top, left side of the steering wheel switches and goes to a six pin connector. If you remove the steering wheel it's a lot easier; but is not absolutely necessary. I recommend it, anyway because you will probably need to do it to repair the switch wiring.
FOLLOW TO THE LETTER THE AIRBAG REMOVAL PROCEDURES!
Better safe than exploding!!!
You may get lucky and see a broken wire or two in the bend from the stalk to the connector. The wires are wrapped in tape, though; so it may look okay. The following guide will identify which wires go where. You'll be checking the function of the switch. Use the wiring from under the dash to ID which pin is which. These are different pins than at the ECU, but pins 2, 3, and 5 are the other ends of the same wires.
Find pin 4 on the steering column connector, it provides power to the system and distributes it to the others. When testing, be careful not to short the meter at the pins, there's not much room back there.
Multimeter to OHMs:
If you have the Bentley you can see that there are more permutations, but this checks the important ones. You can also see that there are four switch postions in the stalk switch. The electrical positions correspond, from left to right with RES, ON, coast/dump, and OFF; not as posted as RES, OFF, ON. SET is a separate function down on the end.
If you do find a broken wire in the stalk, it is easiest to replace the wiring completely. In an interim, unsuccessful, repair I did successfully leave everthing in the car and added a separate "plug" connector from a computer on top of the column switches. This connector fixed an electrical problem, but I hadn't diagnosed the weak vacuum then. If you do remove the column switches to rewire the switch, take the following advice:
Be careful opening up the switch. There's a spring and a roller bearing-thing that jumps out. Find six strands of very thin test lead. It maintains its flexibility better. Each strand should be at least 4 - 6 inches longer than the original. Wrap all six strands in teflon plumbers tape. Push this bundle through the stalk from the base towards the end. Trying to route the cable the other way is a PITA. Solder (very carefully) the switch portion first. Snug the re-assembled switch in the stalk, then snip the wires to the correct length. Solder the pins for the connector last.
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