I was having a hard time finding a way to test the cruise ECU. After searching the internet, I found this. I don't know where I got it so I can't take credit but this is a great way to find out what's wrong instead of throwing money at it and hoping it works. It worked for me and hopefully it works for anyone else that wants to fix their cruise.
Here's the post:
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"1. Turn ignition on. Turn CC system on. Put transfer case in HI range and gearshift in forward gear. (from this point forward I will say "in drive" to mean the previous sentence). Ensure clutch pedal is released. Disconnect cruise control ECU connector. Connect negative lead of voltmeter to CC ECU harness connector terminal no. 8 (black wire) and positive lead to terminal no. 10 (orange/white wire). If battery voltage exists, go to step 3. If battery voltage does not exist, go to next step.

2. Turn ignition off. Using ohmmeter, check resistance between ECU harness connector term. no. 8 (black wire) and ground. If resistance is less than one ohm, go to next step. if resistance is not less that one ohm, repair black wire between CC ECU and ground.

3. Ensure ignition and CC system are on. Disconnect CC ECU connector. Using voltmeter, check voltage between CC ECU harness connector term. no. 4 (red/yellow wire) and ground. With RESUME/DECEL switch pressed, battery voltage should exist. With RESUME/DECEL switch released, no voltage should exist. If voltage is as specified, go to next step. If voltage is not as specified check circuit from steering wheel switch to CC ECU.

4. Ensure ignition and CC system are on. Ensure CC ECU connector is disconnected. Using voltmeter, check voltage between CC ECU harness term. no. 3 (gray/yellow wire) and ground. With SET/ACCEL switch pressed, batter voltage should exist. With SET/ACCEL switch released, no voltage should exist. If voltage is as specified, go to next step. If voltage is not as specified, check circuit from steering wheel switch to
CC ECU.

5. Ensure ignition is on. Ensure CC ECU connector is disconnected. Using voltmeter, check voltage between CC ECU harness term no. 5 (green/purple wire) and ground. With brake pedal pressed, battery voltage should exist. With brake pedal released, no voltage should exist. If voltage is as specified, go to next step. If voltage is not as specified, check green/purple wire between CC ECU and stoplight switch. Check stoplight switch. Repair as necessary.

6. Turn ignition on. Reconnect CC ECU connector. Using voltmeter, back-probe between CC ECU harness term no. 11 (yellow wire) and ground. Spin a rear tire at about 3 MPH. Voltage should be about 2.5 volts, or vary between zero and 5 volts. If voltage is as specified, go to next step. If voltage is not as specified, check yellow
wire from CC ECU to the vehicle speed sensor. Check ECU. Repair as necessary.

7. Turn ignition on. Turn CC system on. Disconnect CC ECU harness connector. Connect a fused jumper wire between CC ECU harness term No. 1 (Orange/blue wire) and battery voltage. Connect another fused jumper wire between CC ECU harness term no. 7 (orange/red wire) and ground. If vacuum pump operates, go to
next step. If vacuum pump does not operate, check orange/blue and orange/red wires between CC ECU and vacuum pump. Check Vacuum pump. Repair as necessary.

8. Ensure ignition and CC system are on. Ensure CC ECU connector is disconnected. Connect a fuse jumper wire between CC ECU harness term. no. 1 (orange/blue wire) and battery voltage. Connect another fused jumper wire between CC ECU harness term. no. 7 (orange/red wire) and ground. Connect another fused jumper wire between CC ECU harness term. no. 6 (orange/pink wire) and ground. Vacuum pump should operate, valve should close and throttle should open wide. If system operates as specified, repair CC ECU. If system does not operate as specified, check orange/pink wire between CC ECU and vacuum pump. Check vacuum pump. Repair as necessary.

I hope this helps someone. It's a fairly easy way to diagnose any electrical or component problem with this cruise control system. Read: Do steps 7 and 8 to check your vacuum pump."
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Eric

 

Thanks for the posting, now I have somewhere to start.

 

I'm bumping this post since I'm finally going to track down why my CC doesn't work.
Hopefully it'll help others.

 

This is certainly great information but I tend to keep things as simple as possible.
Unless you've had other electrical issues, I wouldnt want to overthink it.
If the vacuum hoses are good - suck test on the cruise diaphragm is good - you have the vacuum pump.
Which is where I ended up - used working pump was $25 and took like 10 mins to swap out.
Taking the cap off of my old pump, the contacts fell off of their connectors and broke.
It was clean inside but heat and age maybe, who knows....

 

I appreciate the work the original poster put into the test steps, now nearly 12 years ago, but as described they are all performed from the cruise control ECU-end of the wiring harness.

Is there any reason I can't/shouldn't be able to test the vacuum pump by applying 12 volts to the correct connector pin on the pump itself (opposite recess 4 on connector C0228)?