TCC/Cruise riddle?

After spending the last few weeks fixing stuff around my TS -including the cruise control- I finally got to drive her on a ~350mi trip. And, obviously, the CC stopped working just beginning the trip, forcing me to keep my foot on the pedal for 2/3rds of the trip (it started working again after a long stop, but then it stopped working again). I then realized that the TCC stopped working at the same time the CC did.

Now the interesting thing: the speedometer on this car has been broken from day one (I checked the speed reported by the ECU and it's fine), BUT... when the CC/TCC stop working, the speedo starts working correctly, and vice-versa :screwy:. Essentially, when you start driving and see that the speedo is going nuts, you know you'll have an operating CC and TCC. If everything looks fine with the speed, your right foot is in for some work :shakehead

I don't have the service manuals with me to check the wiring, so I'm hoping someone around here may recognize what the problem is.

As a side note, even without a working TCC or CC, I was able to get 30mpg out of this 3800, at an average speed somewhere between 60 and 65 mph. This car+engine combination is absolutely amazing:iceslolan

WEll the internals of the PCM use the speed signal for the cruise and other controls, and output the signal also for the speedometer. SO it seems all the singularities you describe happen within the PCM....
www.avigex.ca/xport/speedsensor38.jpg

Hello LMP,

Thanks for taking the time to reply.
The weirdest thing about this problem is that I have made several runs with a laptop connected to the car and a GPS, and the speed reported by the ECU is fine (the speed error is undetectable). Also, I made a ~20mi round trip and measured the distance with the car's odometer and the GPS, and the distances are similar, so the ECU seems to be working correctly. This would exclude any problem with the speed sensor as well, right?

I don't know if this is a coincidence, but I managed to correct the problem by moving the shift lever to 3rd and back to OD. Does it make any sense? My guess is a faulty ground somewhere... but where?

PD: I made the return trip with the CC/TCC engaged most of the time and at a constant 65mph (ok... I had a couple of "overtaking-fun" episodes... not too many) and I got 26mpg average. It looks like the "sweet spot" is closer to 60mph, after all :(

...any possibility that the transmission encoder...the contact that tells the ECU which gear..in fact, which lever position is chosen....might be involved ? YOur jiggling with the shift lever makes me think of that....THis should pop a code though.....

Now that you mention it...
I know here's an issue with the switch that tells the ECU which gear is selected, but I thought the problem was only on 1st /2nd... I'll have to check that again.

Another hint/question: is it possible to have crosstalk between the tachometer and the speedo? Would this suggest some other possibility?

The reason I ask this is the following: Under "normal" conditions (CC with TCC engaged) the indicated speed error is close to 5mph, and just slightly dependent on the vehicle speed. However, when the RPMs are changing quickly (when the car is shifting gears, or when you step on the gas when overtaking) the difference between indicated and real speed skyrockets. I think I saw an indicated speed of almost 90 mph today when overtaking from ~60. The needle goes up quite fast you press the pedal, but goes down again relatively fast.

If it helps diagnosing, my TS's fuel gauge is also way off (always indicating more fuel than there really is), but I have seen it go down to what I assume is a normal value a couple of times. I have always blamed the sender unit, but if it's just part of the same issue that would make me a happy person :D The oil pressure gauge is low as well, but I have always thought the engine is just worn (~190K miles). I think the other gauges readings are fine...

Some instrument cluster PCboard issues have been reported by others...cracks...
www.avigex.ca/xport/instrumentpanel.jpg
www.avigex.ca/xport/instrumentpanel1.jpg
Some weirdies do happen on mine, but only slight. I must be cautious with fuel because it starts pumping air now when needle enters the orange section..and a full fillup bumps the needle way past the top. And the oil pressure needle hesitates to low levels at times....no other problems though, and I was thinking the sensors were going haywire....I might reconsider.