I have a 98 Blazer LS with 4.3L Vortech Auto 4wd. Occasionally the tach needle would start to bounce erratically full scale and sometimes even the speedo would join in. Never affected the way the truck drove, just hard to tell what your speed was. Cruise still worked fine. Problem came and went after a few minutes and only happened once in a few months. Now it is a permanent affliction and I have no idea what it could be. If the needles are not bouncing, they are reading much higher than they should by almost twice. I tried to ground out the electrical by removing the ground connector from the battery and pressing it to the positive but that did not solve it. The other day we were on a longer drive and after about 2 hours it started reading correct speed and RPM. After we stopped for gas it started again right away. Any thoughts?

 

Sounds like the beginning of a failing gauge cluster to me. You could try removing the cluster and cleaning the electrical contacts on the back. You could also look for cold solders on the circuit board itself.

Another method of diagnosis would involve a high feature scan tool which would be able to monitor the tach and speedo signals from the PCM for signs of the discrepancy. If it is messed up coming from the PCM, then the problem lies elsewhere. If it is not, then there is something wrong with either the communication between the cluster and the PCM or a problem in the cluster itself.

 

Thanks swartlkk for the reply. I thought the problem might be in the ECM so I tried one from another 98 Blazer 4x4 auto. My truck would start but not stay running with the other ECM installed. I tried a dozen times but no luck. I did not notice any excessive tach jump for the second or so it remained running after each attempt. I assumed there was something wrong with the other ECM. Is there something that needs to be done when switching ECMs from identical year and model vehicles that might explain the running issue or can one just swap one for the other.

 

The likely reason the truck would not stay running is due to the security relearn that is required for the replacement PCM. It must learn the key code before it will stay running.

 

Thanks again swartlkk. I tried a start about 10 or 12 times. Can I assume by "re learn key code" that it eventually would have started and run? Does this require dealer intervention?

 

If it is just a security relearn, then it does not require dealer intervention. If it is a seed/key relearn, then yes, it does require a Tech2. You should only need a security relearn. Check out the DIY section. I just posted the security relearn procedure. You have to follow that to a "T".

 

Swarlkk. I checked out your post on the DIY for the security relearn and it worked flawlessly. Started up perfectly. However it also ruled out the ECM as the cause of the bouncing tach/speedo since the problem was still there with the new ECM. As hard as it is for me to believe it could be a backplane or cold solder problem on the cluster I will try that next and see if there is anything evident. Thanks again for your help.

 

If you borrowed that PCM from someone that needs it back, you will have to perform the relearn procedure on it again in the other truck. Yours will still be good.

Below you will find some of the diagnostic charts regarding the tach and speedo. I am not sure if they will really help you much, especially if they require a bi-directional scan tool for some of the diagnostic steps, but they may offer some insight as to what might be causing both of these problems. I guess just look for some similarities.

Inaccurate Tach:

Inoperative Tach:

Inaccurate or Inoperative Speedo: