[droopydawg]

When the vehicle just barely starts to move in reverse, the speedo needle goes to 30mph. I stop. needle drops to 0. I put in drive and just barley move forward and the speedo needle moves immediately to 40mph. No matter how slow or fast, needle stays at 40, unless I stop, then it drops down to 0 again.
I replaced the Speed Sensor and even checked for resistance in the wires, nothing changed and resistance is good. I even thought that maybe the speedometer adaptor was put in using the wrong pinon gear reference point when A/T was rebuilt. I adjusted it, and this time, the needle did not move at all when driving, so I adjusted the adapter back to its original position. What next? Could it be something with the speedo gauge instead?

Thank you

 

[Adventure Bob]

Sounds like your sending unit is bad. Which sensor did you replace, the one on the trans or the one on the t case? The speedo is just a receiver for frequency; if it's reading incorrectly, genreally its getting the wrong signal. The fact that you get a fixed MPH reading would lead me to believe that the sending unit on the T case is gone bad, regardless of what gear is in it since you say you've tried both index positions.

 

[droopydawg]

Sounds like your sending unit is bad. Which sensor did you replace, the one on the trans or the one on the t case? The speedo is just a receiver for frequency; if it's reading incorrectly, genreally its getting the wrong signal. The fact that you get a fixed MPH reading would lead me to believe that the sending unit on the T case is gone bad, regardless of what gear is in it since you say you've tried both index positions.

The one I replaced is the one on the T-case, I think. Driver side, just above the tag I mentioned that said 272 ratio. It was the one with a 1/2 inch/11mm bolt that holds it to the adaptor. There is another?

 

[Adventure Bob]

Thats the correct one. Was it doing the 40MPH before you changed sending units? Or was that the reason you changed it?

 

[droopydawg]

Thats the correct one. Was it doing the 40MPH before you changed sending units? Or was that the reason you changed it?

It was reading 40mph when going less than 5, with the original unit on, change it out with a new one, no change. Could it have anything to do with the other thing you mentioned that was attached to the trans?

 

[droopydawg]

This vehicle also has ABS. Would there be an additional speed sensor on the left front hub?

 

[Adventure Bob]

The grand has three ways to sense speed and depending on the difference between them it does some things. But only the speedo sensor on the t case feeds the speedometer. There are the ABS sensors at each wheel, the speedo sending unit at the t case and a speed sensor in the transmission if you have an automatic. The speed sensor on the transmission doesn’t feed the speedometer circuit in any way, neither do the abs sensors. So here’s the logic of the grand: If your speedometer reads greater than the sensor in the tranny, it changes the shift maps depending on what it reads. If your speedo sensor sends a 50 MPH signal and the tranny registers 10 (it’s actually just reading frequencies from the sensors) it tries to help select a gear to limit the wheel slip. If your wheels, as sensed by the abs sensors, are moving faster than the tranny, it tries to do the same thing. If you have a 249 and the back wheels move faster than the front then the VC locks up, the speedo sensor on the t case is moving faster than the tranny sensor and the front wheel abs are slower than the rear. The computer thinks Oh crap! and shifts up and hopes you won’t keep your foot planted.
While that’s a long explanation, the tranny sensor doesn't register speed to the speedometer, it’s only there so the computer can play shifter mom.
Somehow you’re getting a constant signal at a frequency that the computer interprets as 40 MPH, not a variable frequency at a constant voltage as is intended. I would guess there’s a short somewhere.

 

[droopydawg]

Thanks, AdvB. The GC seems to drive properly and shift properly.
A buddy of mine is ASE cert and Master Electro nut. And he has a diagnostic tool at home, so I plan on going over to his house tomorrow to troubleshoot.

 

[droopydawg]

...but now I am thinking more about what you said. The car does take a a bit of time to get up to speed. Sort of like when you start out in 2nd or 3rd gear in a M/T car. Could it be that the car thinks it is in 2nd gear when pulling out from a dead stop? The feeling of torque is not there like it would be when starting out in first.

What gear should I be in, when going about 35-40mph? Just about to shift into 3rd?

Thinking outloud...
Trying to follow your expaination, the way I understand it to work.
Maybe something is telling the transmission I am going 40mph, so I start off in 2nd gear. Maybe it IS the speed sensor in the transmission?
With this reasoning, if I manually put the gear selector in first and start off, wont the engine speed spike up really high, if the computer and the transmission 'thinks' I am going 40mph in first gear?

 

[droopydawg]

Thanks for your advice. Hooked up diagnotic tool and went for a drive. Yep, turned out to be the speedo. small bent connection on circut board and $125 later. Beats $450 for a new unit.

 

[Adventure Bob]

So it was on the actual speedometer circuit board? Wow that seems odd that you'd get damage there without someone having opened up the dash.
Glad you got it fixed.