Speedometer revs with tach sort of

[VictoryRed]

When i rev high ish my speedometer goes to 10mph while in park is this just normal for these trucks? 1999 tahoe 350 vortec 4L60e

 

[Sabinoerc]

When i rev high ish my speedometer goes to 10mph while in park is this just normal for these trucks? 1999 tahoe 350 vortec 4L60e

I had same once. It was a weak/bad speed/rpm sensor on tailshaft of trans (I forget what it’s called). Speedo seemed ok normally but would run up in park like you mention. I assume that sensor was weak somehow which allowed noise to affect speedo. I don’t understand how or why but a new sensor in trans fixed it.

 

[VictoryRed]

thanks!

 

[Road Trip]

When i rev high ish my speedometer goes to 10mph while in park is this just normal for these trucks? 1999 tahoe 350 vortec 4L60e

Hello VictoryRed,

Your observation is not expected/normal behavior. The good news is that
there are a couple of common sense things we can try to help figure out if the gauges
are the victims of a shared marginal ground allowing 'crosstalk' to occur between these
logically-unrelated gauges.

Before we dive in, here a little background info to keep in mind while we study the
circuits & come up with a troubleshooting theory.

1) From '96-up the speedometer is driven directly by the "VCM" (Vehicle Control Module)
under the hood. (aka Black Box) In turn, the VSS (Vehicle Speed Sensor) is sending 40
pulses per driveshaft revolution up to the Black Box. (NOTE: This '99 Service Manual refers
to the Black Box by 2 different names? Gas engines are VCM, while the Diesel engines are
PCM? {Powertrain Control Module} See bottom of 1st schematic for an example of this.)

Anyway, if the original (stock) computer is under the hood, if you use Black Box everyone
else knows what you are referring to.

2) For gas engines like yours, the tachometer is driven by a wire connected to the Ignition Coil.

****

So, we have 2 gauges being fed discrete signals from 2 different sources. Since the Service Manual treats
the troubleshooting of a inaccurate speedometer as a go/no-go entire instrument cluster 'swap out', there is very
little documentation on the details of how/where the VSS pulses & analog ignition coil pulses are
translated/conditioned to drive the stepper air core motors driving the gauge needles?

...But I digress. I'm going to assume that there are no VSS-related OBD2 codes being reported?
The key to your problem description is that activity on the tach (due to revving the engine) starts
to affect the speedo...although they are driven by completely separate signals.

****

Now that the troubleshooting table is set, let's see what we can figure out. In the electrical schematics
for the instrument panel we can see that these 2 gauges share a common +12V power feed *and*
they also share a ground:

Pink shows +12V feed to Speedo & Tach. Black arrows show ground path from both gauges through Pin 4 of IP connector on 2nd page.

You must be registered for see images attach

(Note: Page 1 of 2.)


The ground path from the previous page is shown by input 'C':

You must be registered for see images attach

(Page 2 of 2)

I read elsewhere that this is your first truck? Or your first (new to you) GMT400 vehicle?
If so, then in order to make an older vehicle like this a reliable daily driver you need to bring
back the weathered connections in your electrical system back to new conductivity/functionality.

Especially all of the grounds, for a ground in poor condition can allow one device to
negatively influence the behavior of another...which is exactly the symptom that you described.

Instead of just telling you to clean *all* the grounds on your vehicle, (which BTW isn't a bad idea)
I've attached 3 more pages from the '99 C/K Service Manual. Each page shows where you can find
the ground associated with your speedo & tach, from Pin 4 on the Instrument Panel Connector, down
to G103 & G104 on the engine. Inspecting & carefully cleaning these grounds may well clear up
the interference/crosstalk between the engine driven tach and the speedo. (which, in your scenario
should be staying right at -0- mph.)

And if it doesn't? Well, now we need to find out if there's one or more bypass capacitors on
the instrument cluster that were implemented using electrolytic capacitors. If so, there's an
excellent chance that they have dried up & are no longer helping to keep these logically unrelated pulses
from interfering with each other's circuitry.

But let's try to fix the obvious first *before* we start focusing on the exotic.

One last thing. You mentioned elsewhere that you are interested in learning how to
fix your own vehicle. I applaud your attitude, it's increasingly rare these days.

Given this, I strongly recommend that you go to this post and download both .pdf files
so that you have your own copies of the '99 C/K Service Manual. The more familiar you
become with the contents of these books, the more you will be large and in charge
of your vehicular destiny. ('99 C/K Manual links)

Good luck. Happy Troubleshooting!

 

[DerekTheGreat]

Some 19 or 20 years ago I had a 1994 Grand Am which did that too. I assumed it was normal..

 

[VictoryRed]

Thank you!

 

[man-a-fre]

Hello VictoryRed,

Your observation is not expected/normal behavior. The good news is that
there are a couple of common sense things we can try to help figure out if the gauges
are the victims of a shared marginal ground allowing 'crosstalk' to occur between these
logically-unrelated gauges.

Before we dive in, here a little background info to keep in mind while we study the
circuits & come up with a troubleshooting theory.

1) From '96-up the speedometer is driven directly by the "VCM" (Vehicle Control Module)
under the hood. (aka Black Box) In turn, the VSS (Vehicle Speed Sensor) is sending 40
pulses per driveshaft revolution up to the Black Box. (NOTE: This '99 Service Manual refers
to the Black Box by 2 different names? Gas engines are VCM, while the Diesel engines are
PCM? {Powertrain Control Module} See bottom of 1st schematic for an example of this.)

Anyway, if the original (stock) computer is under the hood, if you use Black Box everyone
else knows what you are referring to.

2) For gas engines like yours, the tachometer is driven by a wire connected to the Ignition Coil.

****

So, we have 2 gauges being fed discrete signals from 2 different sources. Since the Service Manual treats
the troubleshooting of a inaccurate speedometer as a go/no-go entire instrument cluster 'swap out', there is very
little documentation on the details of how/where the VSS pulses & analog ignition coil pulses are
translated/conditioned to drive the stepper air core motors driving the gauge needles?

...But I digress. I'm going to assume that there are no VSS-related OBD2 codes being reported?
The key to your problem description is that activity on the tach (due to revving the engine) starts
to affect the speedo...although they are driven by completely separate signals.

****

Now that the troubleshooting table is set, let's see what we can figure out. In the electrical schematics
for the instrument panel we can see that these 2 gauges share a common +12V power feed *and*
they also share a ground:

Pink shows +12V feed to Speedo & Tach. Black arrows show ground path from both gauges through Pin 4 of IP connector on 2nd page.

You must be registered for see images attach

(Note: Page 1 of 2.)


The ground path from the previous page is shown by input 'C':

You must be registered for see images attach

(Page 2 of 2)

I read elsewhere that this is your first truck? Or your first (new to you) GMT400 vehicle?
If so, then in order to make an older vehicle like this a reliable daily driver you need to bring
back the weathered connections in your electrical system back to new conductivity/functionality.

Especially all of the grounds, for a ground in poor condition can allow one device to
negatively influence the behavior of another...which is exactly the symptom that you described.

Instead of just telling you to clean *all* the grounds on your vehicle, (which BTW isn't a bad idea)
I've attached 3 more pages from the '99 C/K Service Manual. Each page shows where you can find
the ground associated with your speedo & tach, from Pin 4 on the Instrument Panel Connector, down
to G103 & G104 on the engine. Inspecting & carefully cleaning these grounds may well clear up
the interference/crosstalk between the engine driven tach and the speedo. (which, in your scenario
should be staying right at -0- mph.)

And if it doesn't? Well, now we need to find out if there's one or more bypass capacitors on
the instrument cluster that were implemented using electrolytic capacitors. If so, there's an
excellent chance that they have dried up & are no longer helping to keep these logically unrelated pulses
from interfering with each other's circuitry.

But let's try to fix the obvious first *before* we start focusing on the exotic.

One last thing. You mentioned elsewhere that you are interested in learning how to
fix your own vehicle. I applaud your attitude, it's increasingly rare these days.

Given this, I strongly recommend that you go to this post and download both .pdf files
so that you have your own copies of the '99 C/K Service Manual. The more familiar you
become with the contents of these books, the more you will be large and in charge
of your vehicular destiny. ('99 C/K Manual links)

Good luck. Happy Troubleshooting!

Great Post,thank you!