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Discussion Starter #1 (Edited)
Local shop did all they could but I'm still no closer to a resolution.

Current symptoms:
1. Starts right up every time
2. Upon starting, O/D off light flashes exactly 16 equal-length flashes
3. Speedometer needle stays at 0 mph
4. Odometer reads correct mileage but never increments
5. Shop said they saw a P0500 or P0550 code one time (can't remember which they said)
6. Now the airbag light is on (flashes all the time)
7. When driving, it runs fine for 1/2 mile. Then starts bucking until I let off the gas. After about another 1/2 mile, it starts running ok again. Transmission does engage O/D as it should.

The (independent) shop did the following:
1. Replaced the speed sensor on the back of the transmission
2. Swapped in another cluster
3. Traced every wire in the harness to the transmission

No change (except the airbag light, which hadn't ever lit up before... hoping I can just do the reset procedure to clear it).

Where do I go from here? Replace the ECU? Is it possible that the small gear on the speed sensor is stripped; I read somewhere that someone else had that problem.
 

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Maybe the replacement cluster wasn't any good, either, assuming it was used? I generally advise sending the cluster out to be repaired. If they replaced the speed sensor, they should have seen if the plastic gear was damaged as they would have had to swap it over onto the new sensor. So, what would I do? At this point, I would consider the air bag light as a separate issue for the time being; it may have entered present diagnostic mode accidentally. There is a procedure to reset the mode using the driver's door switch which can be easily found with a little Googling. Vehicle speed sensors are essentially AC voltage generators. The generate about 1 volt per every 10 MPH. I would test for an output signal using the AC voltage function on a multimeter while running the vehicle in gear (wheels supported off of the ground). If there is no signal, the problem lies in the sensor, driven gear or the drive gear inside the transmission. If there is a signal, they I would remove the cluster and test the appropriate wires from the speed sensor at the cluster harness connector to see if there is a signal there. If there is, the cluster is faulty. If not, there is an open or short in the harness between the cluster and speed sensor.
 

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Discussion Starter #3
I agree about sending my original cluster out to Mr. Wizard. I spoke to them and they can rework it. Now, what I want to avoid is getting it refurbished and then plugging it in, only to find that something else took out the original (and would take it out again). So here's a question: are there electronics on the cluster that can be damaged by some other component (say if the ECU is bad) or only electro-mechanical components?

I have a family member who works at AutoZone and we're going to put a high-end scanner on it and take a test drive tonight so we can see live data.

The shop told me (and I spoke to the guy several times during this process) that they rung out every wire coming up to the cluster. So, I think I need to focus on the remainder of that circuit. Can I swap in another ECU (meaning are they just plug & play or do I need to have something programmed?).

Also, does anyone know for sure what the 16 equal-duration flashes of the O/D light means? I found a webpage with the transmission codes but they don't go up to 16.
 

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Do you have a FSM? Your year/engine isn't listed in your sig but for a 2001 V6 it shows how the trouble codes are shown by pulse width. Also, it shows how to check the VSS output at the cluster with a meter as SMJ says. A half a volt output on the AC scale.
 

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Discussion Starter #5
It's a 2002 Desert Runner non-SC.

I don't have a FSM but that's an excellent point; I will order one today. What's the best one to get?

Also, there is no one pulse that's wider than the others - 16 identical-width pulses.

Thank you.
 

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Discussion Starter #7
Click on the FSM link in my post.

It would be helpful to to others to have that info in your signature.
Thank you for the link, that's very helpful! And yes, I will update my profile... good idea.
 

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Discussion Starter #8
Another thought - been reading more about this online today.

The O/D light flashing - from what I have read, the fact that it's flashing for about 8 seconds (not the # of flashes or pulse duration) indicates that the transmission is is failsafe mode (though it shifts up, down, overdrive works, etc.). So now I'm thinking it might be the transmission control unit.

But, how could that wipe out the speedometer and odometer?

Another idea I had was to unplug the cruise control switch pack. This whole issue started when I disengaged the cruise control. Maybe the switch is bad and is shorting out something.
 

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Discussion Starter #9
So took it for a 30-minute drive this morning. Ran fine, no surging or hesitation. Transmission shifting fine. At one point the O/D light did the 8-second flashing routine - but the transmission was not in limp-home mode.

Later, the SES light came on. Went home and put the code scanner on it. P0500 cnf and P0500 (some other 3-letter acronym, can't remember).

Does the ECU control the SES light and O/D light? Since the symptoms are not consistent, I'm beginning to think I should swap in a different ECU and see what that does. Thoughts?
 
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