1992 C1500 with the 700R4 here. So I've had an intermittently working speedometer, most days it's just totally dead no matter what speed you're going, and if I'm lucky enough to hit a serious pothole it comes back briefly. Started by replacing the VSS and its pigtail (the old wiring was soaked in ATF from a bad o-ring so I thought it was the issue) and made no difference. Wound up putting the original VSS back in with a new o-ring as the aftermarket one I got was noisy. Still an intermittent speedo after all that.
At this point like an idiot I remembered I have an electronics background and am more than capable of a little troubleshooting rather than throwing money at more parts. I pulled out the glove box and supplied 12VDC+ and - via jumpers to terminals C8 and C9 on the VSSB/DRAC unit (in case anyone should know, the DRAC is behind the glove box on 92-95 but built into the inst. cluster on earlier models). With this jumper setup terribly jerry-rigged up, I went on a test drive and the speedo came back perfectly. While driving, I disconnected the makeshift ground I had connected to terminal C8, and the speedo was still going! So this tells me it is not a bad ground somewhere like I would think and is a 12V+ issue.
The next thing I did for peace of mind was take apart the 12V+ connections on the firewall junction block one by one and clean them real good with a wire brush. Reinstalled each nut TIGHT and ensured each lug of every connector was making good contact. Welp, upon start of the truck and test drive all my electronics worked except the speedo again like usual. I don't understand. To my knowledge there is no dedicated fuse for the speedo or the VSSB, just 1 that serves the whole cluster. Where does the power actually come in to terminal C9 of the VSSB? I would assume it directly leaves the glove box through the firewall, connecting to that junction block via a fusible linked cable like many of the other directly powered electronics on the truck. If that was true though, wouldn't my cleaning of the connections have fixed the issue? I'm PRETTY confident I got all those wire lugs clean, shiny, and bolted down tight.
Not sure if this helps, but the speedo rarely ever operates in cold weather, and will come back pretty reliably upon hitting a decent pothole on a hot summer day. This drives me nuts because it makes me immediately think of a bad ground, but my road test with the jumpers to C8 and C9 likely rules that out.
Also not sure if this helps, but in early 2022 I had somebody swap in a used 350. Truck originally had a 305. This made me think of the engine ground connected to one of the thermostat housing bolts, so I cleaned those up to no avail. At this point I'm pretty sure all the necessary grounds are fine but I could be wrong.
At this point like an idiot I remembered I have an electronics background and am more than capable of a little troubleshooting rather than throwing money at more parts. I pulled out the glove box and supplied 12VDC+ and - via jumpers to terminals C8 and C9 on the VSSB/DRAC unit (in case anyone should know, the DRAC is behind the glove box on 92-95 but built into the inst. cluster on earlier models). With this jumper setup terribly jerry-rigged up, I went on a test drive and the speedo came back perfectly. While driving, I disconnected the makeshift ground I had connected to terminal C8, and the speedo was still going! So this tells me it is not a bad ground somewhere like I would think and is a 12V+ issue.
The next thing I did for peace of mind was take apart the 12V+ connections on the firewall junction block one by one and clean them real good with a wire brush. Reinstalled each nut TIGHT and ensured each lug of every connector was making good contact. Welp, upon start of the truck and test drive all my electronics worked except the speedo again like usual. I don't understand. To my knowledge there is no dedicated fuse for the speedo or the VSSB, just 1 that serves the whole cluster. Where does the power actually come in to terminal C9 of the VSSB? I would assume it directly leaves the glove box through the firewall, connecting to that junction block via a fusible linked cable like many of the other directly powered electronics on the truck. If that was true though, wouldn't my cleaning of the connections have fixed the issue? I'm PRETTY confident I got all those wire lugs clean, shiny, and bolted down tight.
Not sure if this helps, but the speedo rarely ever operates in cold weather, and will come back pretty reliably upon hitting a decent pothole on a hot summer day. This drives me nuts because it makes me immediately think of a bad ground, but my road test with the jumpers to C8 and C9 likely rules that out.
Also not sure if this helps, but in early 2022 I had somebody swap in a used 350. Truck originally had a 305. This made me think of the engine ground connected to one of the thermostat housing bolts, so I cleaned those up to no avail. At this point I'm pretty sure all the necessary grounds are fine but I could be wrong.