Speedometer/Tach. Issues...Help Please!

[diesel920]

So I need to tap into the knowledge vault here. 1998 k1500 z71 ECSB. Reman 5.7 vortec. Reman 4L60e. Manual transfer case. All stock + 33" tires and true duals with Flowmaster 40s and no cats.

I'm having some issues with the gauges.
The speedometer is quite a bit off. When the speedo shows 94, GPS says 70 and the tach. is at 3200-3300. When the speedo shows 74, the GPS says 55 and the tach. is at 2500. I know the tires are bigger than stock but not by much so I don't think it would affect it that much. I also know that it is a gas hog but it gets really low mileage...12-14 mpg on the freeway on a good day.

Cruise control does not work. It has worked in the past for brief stints but shuts off while driving.

(Also the gas gauge is verrrry optimistic (always on full) but I'm assuming that is the pump/sending unit and unrelated.)

I know the cruise control and the VSS are on the same circuit so I was thinking about getting a new VSS to see if that helped the speedo and the cruise. Could that be the problem? Also I don't know what the RPM should be but that seems high to me. (it idles over 1,000). Is that high? Could that be related to this issue or does that seem separate? (IAC valve, vacuum leak, etc.)

I just wanted some direction before I start throwing parts at it.

Thanks for reading all that and thanks in advance for your help!

 

[JOHNGAAA1]

12-14 mpg is normal for these trucks. Cruise control issue can also be related to the multi function switch or a bad brake switch. Notice if it fails on bumps. Gas gauge, check your grounds in the rear first, there are 2. One is under the left rear bumper and the other is on the filler neck.

 

[AK49BWL]

I know the cruise control and the VSS are on the same circuit so I was thinking about getting a new VSS to see if that helped the speedo and the cruise. Could that be the problem? Also I don't know what the RPM should be but that seems high to me. (it idles over 1,000). Is that high? Could that be related to this issue or does that seem separate? (IAC valve, vacuum leak, etc.)

Do you know the entire history of this vehicle to know if the cluster has ever been replaced? Idle over 1000rpm *when warm* is absolutely not normal for a Vortec 5.7 (should be closer to 650-700) - however I would get a scanner on the OBDII to verify the RPM because the PCM won't be wrong even if the gauge is. IF the scanner is showing a lot lower, and there's any possibility the cluster has been replaced in its lifetime, you might have a cluster from a V6 truck in there. The cluster is looking for 6 pulses and is receiving 8, so will show quite significantly higher than actual RPM.

This doesn't change anything relating to the speedometer issue, that's going to be another matter.

 

[diesel920]

Do you know the entire history of this vehicle to know if the cluster has ever been replaced? Idle over 1000rpm *when warm* is absolutely not normal for a Vortec 5.7 (should be closer to 650-700) - however I would get a scanner on the OBDII to verify the RPM because the PCM won't be wrong even if the gauge is. IF the scanner is showing a lot lower, and there's any possibility the cluster has been replaced in its lifetime, you might have a cluster from a V6 truck in there. The cluster is looking for 6 pulses and is receiving 8, so will show quite significantly higher than actual RPM.

This doesn't change anything relating to the speedometer issue, that's going to be another matter.

I don’t know the entire history. Based on some of the work that was done to it before I got it, this is entirely possible. I’ll hook a scanner up to it and report back.

 

[k20chevy]

A 98 is an awesome year because HP tuners supports it. If it is a V8 cluster, it's super easy to recalibrate the speedometer using HP tuners. My cruise control awesome randomly works, but when it shuts off, it's a VIOLENT disengagement. I haven't opened up the box yet but it's usually stripped teeth on the worm gear that drives the unit.

 

[1998_K1500_Sub]

The speedometer is quite a bit off. When the speedo shows 94, GPS says 70 and the tach. is at 3200-3300.

Did an earlier owner replace the OE rear gears (likely 3.73 but please look up your RPO code on the glovebox sticker) with 4.56 or higher? See RPO codes, attached.

With the bigger tires, I wouldn’t be surprised if it’s been re-geared, and this would explain the odd speedo reading (and perhaps the high tach, read on).

Speedo changes can be made by tweaking parameters in the ECU, so you have a solution waiting for you.

Is that 4L60E shifting into 4th and is the lockup torque converter functioning properly? The tach RPM seems a bit high even assuming a re-geared rear.

Related question: If you engage 4x4 while stopped, will it accelerate easily to 20MPH or do you get a sense of binding or resistance to motion? I’m wondering if the front gears match the rear gears, and this test is telltale.

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[AbramF_496]

I would like to know on the tach as well my truck hits 12000 rpm lol

 

[alvaroemg]

on my 99 burb, when engaging 4lo it seems like it binds but than gets freed as i go faster, mayne from 0-5mph it binds