so i picked up this new to me 98 K1500. i'm trying to get things straight on it, but one thing i'm not sure of is this speedometer issue i'm having.

the needle works and moves. but it is like it is reading double of the actual speed i'm doing. so around 50 mph on the speedometer, the GPS speed on my phone is telling me i'm doing about 25-30. and when driving on the highway, GPS speed might say 55-60 mph. but the speedometer is pegged @ 100.

there are no error codes stored in the computer. so i'm not really sure where to look next. anyone have any pointers?

 

Vehicle speed is data fed to the cluster from the PCM. The PCM receives the speed signal from the vehicle speed sensor which is mounted on the top rear of the transfer case. If it’s a manual transmission, it will also have a speed sensor adapter module (sometimes called a buffer module).

Possible reasons for the higher reading:

1) Larger than factory tires were installed and the change wasn’t programmed. Check the sticker on the door jamb or owners manual to find out what size the tires were from the factory.

2) Drive train components (transmission, transfer case, sensor and/or module) were replaced with the incorrect parts.

3) The cluster is malfunctioning - this is a common problem with these trucks. Several options are available for repair or replacement.

4) Damaged wiring for the related components.

I would start by verifying tire size (if questionable).

Second step is getting an OBD 2 scanner or phone app that allows you to view live data from the PCM. Then, monitor the VSS signal on the data feed as you’re driving - if the signal received by the PCM matches the actual road speed, the cluster is suspect and requires further troubleshooting.

 

awesome. thank you for my next steps. i'm going to run out now and check for the tire size.

if it were incorrect components that were replaced, that wouldn't set any error codes off?

 

It depends on the parts and circumstances of the replacement. Several factors will affect whether or not a code is set.

The speed sensor, for example, will fit (meaning bolt in place) several different trucks over a wide range of years, but it may not read correctly depending on if it’s a 4 or 5 speed transmission.

 

i'm guessing it could be the speedometer itself. the OBII tool with speed shows what i'm sure is more accurate all through acceleration. guess I have to try to find a speedometer.

 

There’s several options for repair or replacement.

Service parts for the cluster are available if you are confident with soldering.

It can be sent for repair by a service, or you can get a replacement unit that’s been programmed for the mileage, whether it’s a reman or from a salvage yard.

circuitboardmedics.com has been a reliable service for my customers that have required cluster repairs.

 

awesome. thank you again. i will check them out.

 

if i can find a speedometer at a junkyard, will they just swap in?

 

If you’re asking about the entire cluster assembly, yes - just make sure the replacement cluster has the same gauges as the existing one.

If you’re asking about changing over just the speedometer assembly, I couldn’t say for sure. I suspect that it would but I’ve never attempted it.

If you’re going to a pull-your-own-parts yard, I’d recommend checking the cluster before you buy it. Remove the cluster lens and manually sweep the speedometer needle using the tip of your finger - if the movement is smooth, it’s good.

If the movement feels choppy, move on because the stepper motor is junk.

 

thank you again for the info.