[DashPro]

I have a 05 suburban. no programming has been done yet.
the speedometer is replicating the tach
when the tach revs up the speedo does the same thing
anyone every seen this?

Im wondering if maybe the alternator is going bad and the frequency is throwing off the speedometer sensor or its picking up the tach signal somehow

Anyone ever seen this or have any ideas for a fix?

thanks in advance to anyone with a idea

[Clusters]

Won't be the alternator.

The VSS circuit (Pin A4) is hardwired from the PCM to the cluster, radio if equipped with SCV, rear steering if equipped, suplemental brake assist if equipped, and the cruise control on a 4.3 V6.

The tach circuit (Pin A5) is hardwired from the PCM to the cluster only.

The rest of the gauges use the Class2 data line to communicate with the PCM.

Unless the wires are shorted together somewhere, the problem could be in the cluster or the PCM. Look for anything that could be shorting A4 and A5 together. If it has any aftermarket garbage in the truck, start by disconnecting it.

Try a different cluster to see if it does the same.

[DashPro]

I dont have any spares ive chopped them all up for parts by now heh

All the other guages function fine which makes sense
but i tell ya the speedometer is moving identical with the rpm
never seen it this is the v8 model i belive 5.3
the truck has no codes on diagnostic
I was told the owner just put speed senor in trans trying to fix before he brought it to me
something isnt making sense here.

What i dont understand is theres no short for sure between pcm and cluster because when you turn truck on all the guages cycle like they should and return to rest position but there the speedo copys to rpm to the teeth

so if there is a problem in wiring i could see it going haywire but not copying the exact pattern of the tach next to it somehow the tach signal is feeding into the speedometer of the class2

[Clusters]

The more I think about it, must be in the cluster, the VSS and Engine speed signals are not going to be the same I don't believe.

Without another cluster to test with, the best way to narrow it down would be to scope the VSS circuit at the cluster connector with the truck running and the cluster unplugged to see if there is a signal present. If there is no signal, then it must be the cluster.

That leaves two possibilities....

The processor is feeding the wrong signal to the stepper driver IC (if it works that way), or the stepper driver IC is NFG. U27 above the tach stepper.

[DashPro]

Thank you for the advice
New discovery that leads me outside of the cluster
im getting codes thru obd of engine speed output error
and trs in the trans for transmission selector valve error
so i believe one of his speed sensors are out the vss pid is reading correctly

[DashPro]

SOLVED transmission range sensor was bad and it was causing havoc at the guages.
thanks for the help clusters

[Reshi]

I agree with cluster its the instrument cluster problem

ive seen it alot
and i change them alot
normal problem in your type of dash
the needle will losen up and start acting crazy like in your case
just change it and job done

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SOLVED transmission range sensor was bad and it was causing havoc at the guages.
thanks for the help clusters

sorry didnt see you solved it
but mostly i face guage issue so whenever ive changed one worked for me
happy you worked it out