I have not seen this problem referenced before, so maybe I'm the only one who has experienced it. My '94 GMC C1500 350/4L60E began having speedometer problems maybe a year ago. Started with the speedo needle bouncing around a little, then progressed to bouncing a lot. Would occasionally read the correct speed, but was usually reading 5-7 mph high. Finally would bounce all over the scale and would sometimes peg at 85 mph. I even saw it run backwards once (only once)!
It seemed to me that maybe a bad or intermittent ground might be the culprit. All other gages and the tachometer were working fine. I did a little research and found that the instrument cluster used a common ground, so that 's not likely the problem.
More investigation and I discovered a small device called the Vehicle Speed Sensor Buffer (VSSB). It takes the single signal from the Vehicle Speed Sensor (VSS) on the transmission and creates two new signals based on the hard-wired jumpers inside the VSSB: one signal for the ECM/Cruise Control and a different signal for the speedo. Seemed like now I was getting somewhere, because the truck ran/shifted fine and the cruise control worked perfectly. I assumed the VSS was good since its lone signal was working for everything but the speedo.
Thinking either the VSSB or the wiring to the instrument cluster was bad, I checked the wiring and found no shorts or high resistance. Doubled checked the instrument cluster ground at the ADLC mount. Used an old Heathkit oscilloscope to verify that there was indeed a signal from the VSSB for the speedo at the instrument cluster, but at this point I had no idea if the signal was good or not.
To make a long story somewhat shorter, I eventually ran out of ideas (and talent) and figured the only thing to try was a new VSSB and/or cluster. Finding a VSSB that was jumpered exactly the same as my original was not likely, so I concentrated on the cluster. Found a '94 at the local junkyard, but it didn't have the tachometer. Thinking maybe it was just the speedometer motor. I bought the cluster and grabbed the VSSB as well (there is info on this forum and elsewhere on re-jumpering the VSSB if you have to).
Got home and plugged the new cluster and VSSB in. Road test. Seemed to work fine. Now I'm thinking "Must be the speedo motor!". Swap the junkyard speedo motor into original cluster and put the original VSSB back in. Note: be sure to note/photograph the position of each gage needle/pointer before you remove them! Now a test drive. SAME PROBLEM! This is starting to piss me off.
Then I remembered there's a circuit board inside the cluster. That's the only thing it can be! Unfortunately, the junkyard cluster does not have the same circuit board in it (no tach). Went to Ebay and looked for just a replacement circuit board. No such luck. Ordered a used cluster w/tach from LKQ (great service!). It arrived yesterday, I put it in, and life is good. Might look into swapping my old odometer into the new cluster, but that's another project and another write-up.
The moral to this story: If your speedo has lost it's mind and everything else is fine...get another cluster because chances are your circuit board is fried!
2nd moral: Factory shop manuals are invaluable (even if at times incomplete). They're expensive, but worth the money.
3rd moral: A fork works great to gently lift the needles off of the cluster face. Thanks YouTube!
I hope this may help someone, but as always, your results may vary.
It seemed to me that maybe a bad or intermittent ground might be the culprit. All other gages and the tachometer were working fine. I did a little research and found that the instrument cluster used a common ground, so that 's not likely the problem.
More investigation and I discovered a small device called the Vehicle Speed Sensor Buffer (VSSB). It takes the single signal from the Vehicle Speed Sensor (VSS) on the transmission and creates two new signals based on the hard-wired jumpers inside the VSSB: one signal for the ECM/Cruise Control and a different signal for the speedo. Seemed like now I was getting somewhere, because the truck ran/shifted fine and the cruise control worked perfectly. I assumed the VSS was good since its lone signal was working for everything but the speedo.
Thinking either the VSSB or the wiring to the instrument cluster was bad, I checked the wiring and found no shorts or high resistance. Doubled checked the instrument cluster ground at the ADLC mount. Used an old Heathkit oscilloscope to verify that there was indeed a signal from the VSSB for the speedo at the instrument cluster, but at this point I had no idea if the signal was good or not.
To make a long story somewhat shorter, I eventually ran out of ideas (and talent) and figured the only thing to try was a new VSSB and/or cluster. Finding a VSSB that was jumpered exactly the same as my original was not likely, so I concentrated on the cluster. Found a '94 at the local junkyard, but it didn't have the tachometer. Thinking maybe it was just the speedometer motor. I bought the cluster and grabbed the VSSB as well (there is info on this forum and elsewhere on re-jumpering the VSSB if you have to).
Got home and plugged the new cluster and VSSB in. Road test. Seemed to work fine. Now I'm thinking "Must be the speedo motor!". Swap the junkyard speedo motor into original cluster and put the original VSSB back in. Note: be sure to note/photograph the position of each gage needle/pointer before you remove them! Now a test drive. SAME PROBLEM! This is starting to piss me off.
Then I remembered there's a circuit board inside the cluster. That's the only thing it can be! Unfortunately, the junkyard cluster does not have the same circuit board in it (no tach). Went to Ebay and looked for just a replacement circuit board. No such luck. Ordered a used cluster w/tach from LKQ (great service!). It arrived yesterday, I put it in, and life is good. Might look into swapping my old odometer into the new cluster, but that's another project and another write-up.
The moral to this story: If your speedo has lost it's mind and everything else is fine...get another cluster because chances are your circuit board is fried!
2nd moral: Factory shop manuals are invaluable (even if at times incomplete). They're expensive, but worth the money.
3rd moral: A fork works great to gently lift the needles off of the cluster face. Thanks YouTube!
I hope this may help someone, but as always, your results may vary.
