• Welcome to The Truck Stop! We see you haven't REGISTERED yet.

    Your truck knowledge is missing!
    • Registration is FREE , all we need is your birthday and email. (We don't share ANY data with ANYONE)
    • We have tons of knowledge here for your diesel truck!
    • Post your own topics and reply to existing threads to help others out!
    • NO ADS! The site is fully functional and ad free!
    CLICK HERE TO REGISTER!

    Problems registering? Click here to contact us!

    Already registered, but need a PASSWORD RESET? CLICK HERE TO RESET YOUR PASSWORD!

1998 GMC K3500 6.5 Turbo Diesel speedometer and RPM issues

Daniel Cassany

New Member
Messages
10
Reaction score
16
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. the truck is shi no I know where to start

Thanks
 

Elliesh7

New Member
Messages
1
Reaction score
1
Automatic vehicles use speed sensors to determine when the gear should be shifted up or down. In the event that a speed sensor doesn't work correctly, your car might not shift gears when it needs to. This can cause high RPM as the engine has to work harder.
 

THEFERMANATOR

FRANKENBURBAN
Staff member
Lead Moderator
Messages
9,992
Reaction score
4,185
Location
ZEPHYRHILLS FL
I would suspect a cluster issue. The tach is ran straight from the alternator, the speedo and radio are on one vssb output(4k ppm) the ECM is on 2 others(2k ppm & 40 pulse per output shaft revolution), and the abs is on another(128k ppm). Either the calibration resistor in your cluster is bad, or you have power backfeeding into the ground circuit.

The VSSB could have an issue on the 4k ppm output circuit, but with the tach doing the same thing I would lean towards a cluster issue. Either in the cluster, or in the power or ground to it. There are ways to test it, but most don't have a signal generator to do it.
 

Daniel Cassany

New Member
Messages
10
Reaction score
16
I would suspect a cluster issue. The tach is ran straight from the alternator, the speedo and radio are on one vssb output(4k ppm) the ECM is on 2 others(2k ppm & 40 pulse per output shaft revolution), and the abs is on another(128k ppm). Either the calibration resistor in your cluster is bad, or you have power backfeeding into the ground circuit.

The VSSB could have an issue on the 4k ppm output circuit, but with the tach doing the same thing I would lean towards a cluster issue. Either in the cluster, or in the power or ground to it. There are ways to test it, but most don't have a signal generator to do it.
i checked an cleaned everithing, i should replace?
 

THEFERMANATOR

FRANKENBURBAN
Staff member
Lead Moderator
Messages
9,992
Reaction score
4,185
Location
ZEPHYRHILLS FL
i checked an cleaned everithing, i should replace?
No, diagnose it. Just replacing parts almost always ends up causing more problems than you started with. The grounds on the back of the intake are grounds for the cluster iirc. If you have a DVOM with a good tachometer in it, you can use that to check the outputs. GM used a signal generator that sent out a calibrated signal to check them.
 
Top