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Discussion Starter #1
Hi, i have a 1998 GMC K3500 6.5 Turbo Diesel
When I go in the interstate the speedometer goes to the top, the GPS tells me 70 MPH and the truck tells me 100 MPH, also the RPM goes to the limit and the engine is not heard at full capacity but the RPM marks 6000. no I know where to start

Thanks
 

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The RPM signal is from the Alternator. The VSS at the TFC is for the Speedometer. Because it is a 4WD it may have multiple VSS sensors. The main one will be on the rear housing in line with the drive shaft or TFC mainshaft. Does the truck shift normally? If it does shift normally it may be a dash ground issue. If you have a friend with a scan tool look at the RPM & MPH signal via the scanner. See if it matches the instrument cluster read out.
 

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Same thing I was thinking. If its an auto and its shifting normally its just about got to be in the dash. The fact that the tach is also wacky points to the dash because like was mentioned above that is run by a separate wire from the alternator that is only used for the dash tach. Engine computer gets its info from crank position sensor.
 

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Discussion Starter #4
Same thing I was thinking. If its an auto and its shifting normally its just about got to be in the dash. The fact that the tach is also wacky points to the dash because like was mentioned above that is run by a separate wire from the alternator that is only used for the dash tach. Engine computer gets its info from crank position sensor.
the Truck is shifting normally, yesterday i checked the dash ground, i all ready order the VSS to check that and keep moving
 

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Pull the gauge cluster and check the electrical conn.
Sometimes just unplugging and re-seating will fix it.
 

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The RPM signal is from the Alternator. The VSS at the TFC is for the Speedometer. Because it is a 4WD it may have multiple VSS sensors. The main one will be on the rear housing in line with the drive shaft or TFC mainshaft. Does the truck shift normally? If it does shift normally it may be a dash ground issue. If you have a friend with a scan tool look at the RPM & MPH signal via the scanner. See if it matches the instrument cluster read out.
Good info I have a Similar issue my speedometer runs 5mph fast at 70mph but I just found the last mechanic put the wrong belt on it was too big and the tensioner was all the way out. Got the right belt now and new tensioner so we shal see
 

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On a 96+ the alternator pulley is the most common one used on gas engines so its not a problem like it was on the 95 and older where it was a larger diesel only pulley. Using the common pulley on a 95- gave you a higher reading than actual. And this is only an issue for the tachometer on the dash. Signal doesn't go anywhere else so it won't effect the transmission or speedometer. Belt length would only be an issue if it was slipping, it can't make it run too fast.

Speedometer signal comes from the Veichle Speed Sensor which is in the tail of the trans on 2x4 or back of the transfer case in 4x4's. Sensor signal first goes to the VSSB DRAC which converts, splits, and calibrates the signal to the computer and dash. If either the speedometer works or the transmission shifts fine the sensor is fine and working. Its when both the speedometer and auto transmission is going wacky that you need to be investigating the sensor, its wiring, and the VSSB DRAC.
 

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Discussion Starter #11
UPDATE...
i replaced all the
Instrument Cluster Dash
and now the speed are perfect until 55MPH then stop there, the truck can go on 66 MPH but the speedometer still in 55 MPH and the trans start to shift wear.

before this the VSS was replaced, the DRAC was replaced, i checked the wires and look good, all rest gauge is working good, The RPMs now show good

remember all star because the truck show speed and RPMs double all the time
 

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Did you replace the VSS on the transfer case?
Follow the wires from the VSS up to the "union" connector and make sure the terminals are not corroded.
Make sure the 2 conns. that plug into the "union" are ok.
I have seen VSS sensors do that when there were metal particles on the sensor, inside the case.
And believe it or not an over filled transfer case will cause that also.
Make sure tranny fluid is not leaking into the transfer case.
 

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Discussion Starter #13
Did you replace the VSS on the transfer case?
Follow the wires from the VSS up to the "union" connector and make sure the terminals are not corroded.
Make sure the 2 conns. that plug into the "union" are ok.
I have seen VSS sensors do that when there were metal particles on the sensor, inside the case.
And believe it or not an over filled transfer case will cause that also.
Make sure tranny fluid is not leaking into the transfer case.
Did you replace the VSS on the transfer case? YES, also the drac module too, i will check out the rest of you suggestion, thanks
 

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Remove the sensor and check it.
 

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Discussion Starter #16
Remove the sensor and check it.
i did, and install the old sensor after cleaned all, no matter what sensor used is the same.

remember the original problem was double speed and RPMs in the Cluster, i replaced the VSS and the Drac and still the same, when i replaced the cluster start to be 100% accuracy until 55 MPH and stay there no matter if the real speed is 70 (by GPS) (the trans is going up and down 1 shift over and over in that moment)

But now with the new Cluster no matter the VSS is new or old, if the Drac is new or old still doing the same, the OBD2 show me error P1643 (i am not sure if this is related)

i checked the wires and grounds and look good, the trans fluid level is in the full site (taked in hot tempeture in park position after shift each D, 3, 2, 1, for 5 seconds)

Thanks for all help
 

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What year truck did the new gauge cluster come from?
They are not all interchangeable - possible cause for P1643.
The transfer case has it's own lube, separate from the transmission.
Remove the fill plug on the transfer case and make sure the fluid level is correct.
 

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T-case uses ATF but it has a separate fluid drain and fill holes.
 
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